Tag: Notes

  • Meiosis and Gamete Formation

    Do you Know about Meiosis and Gamete Formation?

    Meiosis is a specialized type of cell division that reduces the chromosome number by half, creating four haploid cells, each genetically distinct from the parent cell that gave rise to them. This process occurs in all sexually reproducing single-celled and multicellular eukaryotes, including animals, plants, and fungi. Errors in meiosis resulting in aneuploidy are the leading known cause of miscarriage and the most frequent genetic cause of developmental disabilities.

    In meiosis, DNA replication is followed by two rounds of cell division to produce four potential daughter cells, each with half the number of chromosomes as the original parent cell. The two meiotic divisions are known as Meiosis I and Meiosis II. Before meiosis begins, during S phase of the cell cycle, the DNA of each chromosome is replicated so that it consists of two identical sister chromatids, which remain held together through sister chromatid cohesion. This S-phase can be referred to as “premeiotic S-phase” or “meiotic S-phase.” Immediately following DNA replication, meiotic cells enter a prolonged G2-like stage known as meiotic prophase. During this time, homologous chromosomes pair with each other and undergo genetic recombination, a programmed process in which DNA is cut and then repaired, which allows them to exchange some of their genetic information. A subset of recombination events results in crossovers, which create physical links known as chiasmata (singular: chiasma, for the Greek letter Chi (X)) between the homologous chromosomes. In most organisms, these links are essential to direct each pair of homologous chromosomes to segregate away from each other during Meiosis I, resulting in two haploid cells that have half the number of chromosomes as the parent cell. During Meiosis II, the cohesion between sister chromatids is released and they segregate from one another, as during mitosis. In some cases all four of the meiotic products form gametes such as sperm, spores, or pollen. In female animals, three of the four meiotic products are typically eliminated by extrusion into polar bodies, and only one cell develops to produce an ovum.

    Because the number of chromosomes is halved during meiosis, gametes can fuse (i.e. fertilization) to form a diploid zygote that contains two copies of each chromosome, one from each parent. Thus, alternating cycles of meiosis and fertilization enable sexual reproduction, with successive generations maintaining the same number of chromosomes. For example, diploid human cells contain 23 pairs of chromosomes including 1 pair of sex chromosomes (46 total), half of maternal origin and half of paternal origin. Meiosis produces haploid gametes (ova or sperm) that contain one set of 23 chromosomes. When two gametes (an egg and a sperm) fuse, the resulting zygote is once again diploid, with the mother and father each contributing 23 chromosomes. This same pattern, but not the same number of chromosomes, occurs in all organisms that utilize meiosis.

    Meiosis

    Most plant and animal cells are diploid. The term diploid is derived from the Greek diplos, meaning “double” or “two”; the term implies that the cells of plants and animals have pairs of chromosomes. In human cells, for example, 46 chromosomes are organized in 23 pairs. Hence, human cells are diploid in that they have a pair of 23 individual chromosomes.

    During sexual reproduction, the sex cells of parent organisms unite with one another and form a fertilized egg cell (zygote). In this situation, each sex cell is a gamete. The gametes of human cells are haploid, from the Greek haplos, meaning “single.” This term implies that each gamete contains half of the 46 chromosomes—23 chromosomes in humans. When the human gametes unite with one another, the original diploid condition of 46 chromosomes is reestablished. Mitosis then brings about the development of the diploid cell into a multicellular organism.

    The process by which the chromosome number is halved during gamete formation is meiosis. In meiosis, a cell containing the diploid number of chromosomes is converted into four cells, each having the haploid number of chromosomes. In human cells undergoing meiosis, for instance, a cell containing 46 chromosomes yields four cells, each with 23 chromosomes.

    Meiosis occurs by a series of steps that resemble the steps of mitosis. Two major phases of meiosis occur: meiosis I and meiosis II. During meiosis I, a single cell divides into two. During meiosis II, those two cells each divide again. The same demarcating phases of mitosis take place in meiosis I and meiosis II—prophase, metaphase, anaphase, and telophase—but with some variations contained therein.

    As shown in Figure 1, first, the chromosomes of a cell are divided into two cells. The chromosomes of the two cells then separate and pass into four daughter cells. The parent cell is diploid, while each of the daughter cells has a single set of chromosomes and is haploid. Synapsis and crossing over occur in the prophase I stage.

    Meiosis and Gamete Formation Stages

    Figure 1  The process of meiosis, in which four haploid cells are formed.

    The members of each chromosome pair within a cell are called homologous chromosomes. Homologous chromosomes are similar but not identical. They may carry different versions of the same genetic information. For instance, one homologous chromosome may carry the information for blond hair while the other homologous chromosome may carry the information for black hair.

    Meiosis Phases

    As a cell prepares to enter meiosis, each of its chromosomes has duplicated in the synthesis stage (S) of the cell cycle, as in mitosis. Each chromosome thus consists of two sister chromatids.

    Meiosis I: At the beginning of meiosis, I, a human cell contains 46 chromosomes, or 92 chromatids (the same number as during mitosis). Meiosis I proceeds through the following phases:

    Prophase I: Prophase I is similar in some ways to prophase in mitosis. The chromatids shorten and thicken and become visible under a microscope. An important difference, however, is that a process called synapsis occurs. Synapsis is when the homologous chromosomes migrate toward one another and join to form a tetrad (the combination of four chromatids, two from each homologous chromosome). A second process called crossing over also takes place during prophase I. In this process, segments of DNA from one chromatid in the tetrad pass to another chromatid in the tetrad. These exchanges of chromosomal segments occur in a complex and poorly understood manner. They result in a genetically new chromatid. Crossing over is an important driving force of evolution. After crossing over has taken place, the homologous pair of chromosomes is genetically different.

    Metaphase I: In metaphase I of meiosis, the tetrads align on the equatorial plate (as in mitosis). The centromeres attach to spindle fibers, which extend from the poles of the cell. One centromere attaches per spindle fiber.

    Anaphase I: In anaphase I, the homologous chromosomes or tetrads separate. One homologous chromosome (consisting of two chromatids) moves to one side of the cell, while the other homologous chromosome (consisting of two chromatids) moves to the other side of the cell. The result is that 23 chromosomes (each consisting of two chromatids) move to one pole, and 23 chromosomes (each consisting of two chromatids) move to the other pole. Essentially, the chromosome number of the cell is halved once meiosis I is completed. For this reason, the process is a reduction-division.

    Telophase I: In telophase I of meiosis, the nucleus reorganizes, the chromosomes become chromatin, and the cell membrane begins to pinch inward. Cytokinesis occurs immediately following telophase I. This process occurs differently in plant and animal cells, just as in mitosis.

    Meiosis II: Meiosis II is the second major subdivision of meiosis. It occurs in essentially the same way as mitosis. In meiosis II, a cell contains a single set of chromosomes. Each chromosome, however, still has its duplicated sister chromatid attached. Meiosis II segregates the sister chromatids into separate cells. Meiosis II proceeds through the following phases:

    Prophase II: Prophase II is similar to the prophase of mitosis. The chromatin material condenses, and each chromosome contains two chromatids attached by the centromere. The 23 chromatid pairs, a total of 46 chromatids, then move to the equatorial plate.

    Metaphase II: In metaphase II of meiosis, the 23 chromatid pairs gather at the center of the cell prior to separation. This process is identical to metaphase in mitosis, except that this is occurring in a haploid versus a diploid cell.

    Anaphase II: During anaphase II of meiosis, the centromeres divide and sister chromatids separate, at which time they are referred to as non-replicated chromosomes. Spindle fibers move chromosomes to each pole. In all, 23 chromosomes move to each pole. The forces and attachments that operate in mitosis also operate in anaphase II.

    Telophase II: During telophase II, the chromosomes gather at the poles of the cells and become indistinct. Again, they form a mass of chromatin. The nuclear envelope develops, the nucleoli reappear, and the cells undergo cytokinesis.

    During meiosis II, each cell containing 46 chromatids yields two cells, each with 23 chromosomes. Originally, there were two cells that underwent meiosis II; therefore, the result of meiosis II is four cells, each with 23 chromosomes. Each of the four cells is haploid; that is, each cell contains a single set of chromosomes.

    The 23 chromosomes in the four cells from meiosis are not identical because crossing over has taken place in prophase I. The crossing over yields genetic variation so that each of the four resulting cells from meiosis differs from the other three. Thus, meiosis provides a mechanism for producing variations in the chromosomes. Also, it accounts for the formation of four haploid cells from a single diploid cell.

    Meiosis in Humans

    In humans, meiosis is the process by which sperm cells and egg cells are produced. In the male, meiosis takes place after puberty. Diploid cells within the testes undergo meiosis to produce haploid sperm cells with 23 chromosomes. A single diploid cell yields four haploid sperm cells through meiosis.

    In females, meiosis begins during the fetal stage when a series of diploid cells enter meiosis I. At the conclusion of meiosis, I, the process comes to a halt, and the cells gather in the ovaries. At puberty, meiosis resumes. One cell at the end of meiosis I enters meiosis II each month. The result of meiosis II is a single egg cell per cycle (the other meiotic cells disintegrate). Each egg cell contains 23 chromosomes and is haploid.

    The union of the egg cell and the sperm cell leads to the formation of a fertilized egg cell with 46 chromosomes, or 23 pairs. Fertilization restores the diploid number of chromosomes. The fertilized egg cell, a diploid, is a zygote. Further divisions of the zygote by mitosis eventually yield a complete human being.

    Gamete

    Gametes are the cells that fuse together during sexual reproduction to form a new organism. This lesson covers what these cells are, what they do, and the end result of when they meet.

    Definition of Gamete

    Gametes are the reproductive cells used during sexual reproduction to produce a new organism called a zygote. The gametes in males and females are different. The male gamete is called sperm. It is much smaller than the female gamete and very mobile. It has a long tail, flagellum, that allows it to move towards the female gamete. The female gamete is called an egg or ova. It is much larger than the sperm and is not made to move.

    Formation of Gametes

    Both the male and female gametes are formed during a process of cellular reproduction called meiosis. During meiosis, the DNA is only replicated or copied one time. However, the cells are divided into four separate cells. This means that the new gamete cells have only half of the number of chromosomes as the other cells. So, during meiosis DNA or chromosomes are copied, then split into two cells (with one full set of chromosomes each), then again split into two more cells, leaving only half of the pairs of chromosomes in each new cell.

    These new cells with only half of the chromosomes will mature into the gametes. The gametes are haploid cells because they have only one set of chromosomes. When they unite they will join their single sets of chromosomes to make a complete set, and then they will be considered diploid cells. In the female, the eggs or ova mature in the female’s ovaries. The sperm will mature in the male’s testes.

  • Mitosis and Cell Reproduction

    What do you understand of Mitosis and Cell Reproduction?

    Cell Cycle

    The cell cycle or cell division cycle is the series of events that take place in a cell leading to its division and duplication of its DNA (DNA replication) to produce two daughter cells. In bacteria, which lack a cell nucleus, the cell cycle is divided into the B, C, and D periods. The B period extends from the end of cell division to the beginning of DNA replication. DNA replication occurs during the C period. The D period refers to the stage between the end of DNA replication and the splitting of the bacterial cell into two daughter cells. In cells with a nucleus, as in eukaryotes, the cell cycle is also divided into three periods: interphase, the mitotic (M) phase, and cytokinesis. During interphase, the cell grows, accumulating nutrients needed for mitosis, preparing it for cell division and duplicating its DNA. During the mitotic phase, the chromosomes separate. During the final stage, cytokinesis, the chromosomes and cytoplasm separate into two new daughter cells. To ensure the proper division of the cell, there are control mechanisms known as cell cycle checkpoints.

    Animal cell cycle
    Animal cell cycle

    The cell cycle involves many repetitions of cellular growth and reproduction. With few exceptions (for example, red blood cells), all the cells of living things undergo a cell cycle.

    The cell cycle is generally divided into two phases: interphase and mitosis. During interphase, the cell spends most of its time performing the functions that make it unique. Mitosis is the phase of the cell cycle during which the cell divides into two daughter cells.

    Interphase

    The interphase stage of the cell cycle includes three distinctive parts: The G1 phase, the S phase, and the G2 phase. The G1 phase follows mitosis and is the period in which the cell is synthesizing its structural proteins and enzymes to perform its functions. For example, a pancreas cell in the G1 phase will produce and secrete insulin, a muscle cell will undergo the contractions that permit movement, and a salivary gland cell will secrete salivary enzymes to assist digestion. During the G1 phase, each chromosome consists of a single molecule of DNA and its associated histone protein. In normal human cells, there are 46 chromosomes per cell (except in sex cells with 23 chromosomes and red blood cells with no nucleus and, hence, no chromosomes).

    During the S phase of the cell cycle, the DNA within the nucleus replicates. During this process, each chromosome is faithfully copied, so by the end of the S phase, two DNA molecules exist for each one formerly present in the G1 phase. Human cells contain 92 chromosomes per cell in the S phase.

    In the G2 phase, the cell prepares for mitosis. Proteins organize themselves to form a series of fibers called the spindle, which is involved in chromosome movement during mitosis. The spindle is constructed from amino acids for each mitosis, and then taken apart at the conclusion of the process. Spindle fibers are composed of microtubules.

    Mitosis

    The term mitosis is derived from the Latin stem mito, meaning “threads.” When mitosis was first described a century ago, scientists had seen “threads” within cells, so they gave the name “mitosis” to the process of “thread movement.” During mitosis, the nuclear material becomes visible as threadlike chromosomes. The chromosomes organize in the center of the cell, and then they separate, and 46 chromosomes move into each new cell that forms.

    In cell biology, mitosis is a part of the cell cycle when replicated chromosomes are separated into two new nuclei. In general, mitosis ( the division of the nucleus) is preceded by the S stage of interphase (during which the DNA is replicated) and is often accompanied or followed by cytokinesis, which divides the cytoplasm, organelles and cell membrane into two new cells containing roughly equal shares of these cellular components. Mitosis and cytokinesis together define the mitotic (M) phase of an animal cell cycle the division of the mother cell into two daughter cells genetically identical to each other.

    Mitosis is a continuous process, but for convenience in denoting which portion of the process is taking place, scientists divide mitosis into a series of phases: prophase, metaphase, anaphase, and telophase (see Figure 1):

    Mitosis and Cell Reproduction Process

    Figure 1. The process of mitosis, in which the chromosomes of a cell duplicate and pass into two daughter cells.

    Types of Mitosis

    Types of Mitosis

    The primary result of mitosis and cytokinesis is the transfer of a parent cell’s genome into two daughter cells. The genome is composed of a number of chromosomes complexes of tightly coiled DNA that contain genetic information vital for proper cell function. Because each resultant daughter cell should be genetically identical to the parent cell, the parent cell must make a copy of each chromosome before mitosis. This occurs during the S phase of interphase. Chromosome duplication results in two identical sister chromatids bound together by cohesin proteins at the centromere.

    When mitosis begins, the chromosomes condense and become visible. In some eukaryotes, for example, animals, the nuclear envelope, which segregates the DNA from the cytoplasm, disintegrates into small vesicles. The nucleolus, which makes ribosomes in the cell, also disappears. Microtubules project from opposite ends of the cell, attach to the centromeres and align the chromosomes centrally within the cell. The microtubules then contract to pull the sister chromatids of each chromosome apart. Sister chromatids at this point are called daughter chromosomes. As the cell elongates, corresponding daughter chromosomes are pulled toward opposite ends of the cell and condense maximally in late anaphase. A new nuclear envelope forms around the separated daughter chromosomes, which decondense to form interphase nuclei.

    During mitotic progression, typically after the anaphase onset, the cell may undergo cytokinesis. In animal cells, a cell membrane pinches inward between the two developing nuclei to produce two new cells. In plant cells, a cell plate forms between the two nuclei. Cytokinesis does not always occur; coenocytic (a type of multinucleate condition) cells undergo mitosis without cytokinesis.

    Prophase: Mitosis begins with the condensing of the chromatin to form chromosomes in the phase called prophase. Two copies of each chromosome exist; each one is a chromatid. Two chromatids are joined to one another at a region called the centromere. As prophase unfolds, the chromatids become visible in pairs (called sister chromatids), the spindle fibers form, the nucleoli disappear, and the nuclear envelope dissolves.

    In animal cells during prophase, microscopic bodies called centrioles begin to migrate to opposite sides of the cell. When the centrioles reach the poles of the cell, they produce and are then surrounded by a series of radiating microtubules called an aster. Centrioles and asters are not present in most plant or fungal cells.

    As prophase continues, the chromatids attach to spindle fibers that extend out from opposite poles of the cell. The spindle fibers attach at the region of the centromere at a structure called the kinetochore, an area of protein in the centromere region. Eventually, all pairs of chromatids reach the center of the cell, a region called the equatorial plate.

    Metaphase: Metaphase is the stage of mitosis in which the pairs of chromatids line up on the equatorial plate. This region is also called the metaphase plate. In a human cell, 92 chromosomes in 46 pairs align at the equatorial plate. Each pair is connected at the centromere, where the spindle fiber is attached (more specifically at the kinetochore).

    Anaphase: At the beginning of anaphase, the sister chromatids move apart from one another. The chromatids are called chromosomes after the separation. Each chromosome is attached to a spindle fiber, and the members of each chromosome pair are drawn to opposite poles of the cell by the spindle fibers. During anaphase, the chromosomes can be seen moving. They take on a rough V shape because of their midregion attachment to the spindle fibers. The movement toward the poles is accomplished by several mechanisms, such as an elongation of the spindle fibers, which results in pushing the poles apart.

    The result of anaphase is an equal separation and distribution of the chromosomes. In human cells, a total of 46 chromosomes move to each pole as the process of mitosis continues.

    Telophase: In telophase, the chromosomes finally arrive at the opposite poles of the cell. The distinct chromosomes begin to fade from sight as masses of chromatin are formed again. The events of telophase are essentially the reverse of those in prophase. The spindle is dismantled and its amino acids are recycled, the nucleoli reappear, and the nuclear envelope is reformed.

    Cytokinesis: Cytokinesis is the process in which the cytoplasm divides and two separate cells form. Note that cytokinesis is separate from the four stages of mitosis. In animal cells, cytokinesis begins with the formation of a cleavage furrow in the center of the cell. With the formation of the furrow, the cell membrane begins to pinch into the cytoplasm, and the formation of two cells begins. This process is often referred to as cell cleavage. Microfilaments contract during cleavage and assist the division of the cell into two daughter cells.

    In plant cells, cytokinesis occurs by a different process because a rigid cell wall is involved. Cleavage does not take place in plant cells. Rather, a new cell wall is assembled at the center of the cell, beginning with vesicles formed from the Golgi apparatus (see Bilogy of Cells). As the vesicles join, they form a double membrane called the cell plate. The cell plate forms in the middle of the cytoplasm and grows outward to fuse with the cell membrane. The cell plate separates the two daughter cells. As cell wall material is laid down, the two cells move apart from one another to yield two new daughter cells.

    Mitosis serves several functions in living cells. In many simple organisms, it is the method for asexual reproduction (for example, in the cells of a fungus). In multicellular organisms, mitosis allows the entire organism to grow by forming new cells and replacing older cells. In certain species, mitosis is used to heal wounds or regenerate body parts. It is the universal process for cell division in eukaryotic cells.

    Cell Nucleus

    A distinguishing feature of a living thing is that it reproduces independent of other living things. This reproduction occurs at the cellular level. In certain parts of the body, such as along the gastrointestinal tract, the cells reproduce often. In other parts of the body, such as in the nervous system, the cells reproduce less frequently. With the exception of only a few kinds of cells, such as red blood cells (which lack nuclei when fully mature), all cells of the human body reproduce.

    In eukaryotic cells (see Bilogy of Cells), the structure and contents of the nucleus are of fundamental importance to an understanding of cell reproduction. The nucleus contains the hereditary material (DNA) of the cell assembled into chromosomes. In addition, the nucleus usually contains one or more prominent nucleoli (dense bodies that are the site of ribosome synthesis).

    Anatomy of the Nucleus

    Figure 2: Anatomy of the Nucleus

    The nucleus is surrounded by a nuclear envelope consisting of a double membrane that is continuous with the endoplasmic reticulum. Transport of molecules between the nucleus and cytoplasm is accomplished through a series of nuclear pores lined with proteins that facilitate the passage of molecules out of the nucleus. The proteins provide a certain measure of selectivity in the passage of molecules across the nuclear membrane.

    The nuclear material consists of deoxyribonucleic acid (DNA) organized into long strands. The strands of DNA are composed of nucleotides bonded to one another by covalent bonds. DNA molecules are extremely long relative to the cell; there are approximately 6 feet of DNA in a single human cell. However, in the chromosome, the DNA is condensed and packaged with protein into manageable bodies. The mass of DNA material and its associated protein is chromatin.

    To form chromatin, the DNA molecule is wound around globules of a protein called histone. The units formed in this way are nucleosomes. Millions of nucleosomes are connected by short stretches of histone protein, much like beads on a string. The configuration of the nucleosomes in a coil causes additional coiling of the DNA and the eventual formation of the chromosome.

  • Cellular Respiration

    What is Cellular Respiration?

    Some organisms, such as plants, can trap the energy in sunlight through photosynthesis (see Photosynthesis) and store it in the chemical bonds of carbohydrate molecules. The principal carbohydrate formed through photosynthesis is glucose. Other types of organisms, such as animals, fungi, many protozoa, and a large portion of bacteria, are unable to perform this process. Therefore, these organisms must rely on the carbohydrates formed in plants to obtain the energy necessary for their metabolic processes.

    Animals and other organisms obtain the energy available in carbohydrates through the process of cellular respiration. Cells take the carbohydrates into their cytoplasm, and through a complex series of metabolic processes, they break down the carbohydrates and release the energy. The energy is generally not needed immediately; rather, it is used to combine adenosine diphosphate (ADP) with phosphate ions to form adenosine triphosphate (ATP) molecules. The ATP can then be used for processes in the cells that require energy, much as a battery powers a mechanical device.

    During the process of cellular respiration, carbon dioxide is given off. This carbon dioxide can be used by plant cells during photosynthesis to form new carbohydrates. Also in the process of cellular respiration, oxygen gas is required to serve as an acceptor of electrons. This oxygen is identical to the oxygen gas given off during photosynthesis. Thus, there is an interrelationship between the processes of photosynthesis and cellular respiration, namely the entrapment of energy available in sunlight and the provision of the energy for cellular processes in the form of ATP.

    The overall mechanism of cellular respiration involves four processes: glycolysis, in which glucose molecules are broken down to form pyruvic acid molecules; the Krebs cycle, in which pyruvic acid is further broken down and the energy in its molecule is used to form high-energy compounds, such as nicotinamide adenine dinucleotide (NADH); the electron transport system, in which electrons are transported along a series of coenzymes and cytochromes and the energy in the electrons is released; and chemiosmosis, in which the energy given off by electrons pumps protons across a membrane and provides the energy for ATP synthesis. The general chemical equation for cellular respiration is:

    C6H12O6 + 6 O2 → 6 H2O + 6CO2 + energy

    Figure 1. provides an overview of cellular respiration. Glucose is converted to pyruvic acid in the cytoplasm, which is then used to produce acetyl CoA in the mitochondrion. Finally, the Krebs cycle proceeds in the mitochondrion. Electron transport and chemiosmosis result in energy release; ATP synthesis also occurs in the mitochondrion.

    Glycolysis

    Glycolysis is the process in which one glucose molecule is broken down to form two molecules of pyruvic acid (also called pyruvate). The glycolysis process is a multi-step metabolic pathway that occurs in the cytoplasm of animal cells, plant cells, and the cells of microorganisms. At least six enzymes operate in the metabolic pathway.

    In the first and third steps of the pathway, ATP energizes the molecules. Thus, two ATP molecules must be expended in the process. Further along in the process, the six-carbon glucose molecule converts into intermediary compounds and is then split into two three-carbon compounds. The latter undergo additional conversions and eventually form pyruvic acid at the conclusion of the process.

    During the latter stages of glycolysis, four ATP molecules are synthesized using the energy given off during the chemical reactions. Thus, four ATP molecules are synthesized and two ATP molecules are used during glycolysis, for a net gain of two ATP molecules.

    An overview of cellular respiration

    Figure 1. An overview of cellular respiration.

    Another reaction during glycolysis yields enough energy to convert NAD to NADH (plus a hydrogen ion). The reduced coenzyme (NADH) will later be used in the electron transport system, and its energy will be released. During glycolysis, two NADH molecules are produced.

    Because glycolysis does not require oxygen, the process is considered to be anaerobic. For certain anaerobic organisms, such as some bacteria and fermentation yeasts, glycolysis is the sole source of energy.

    Glycolysis is a somewhat inefficient process because much of the cellular energy remains in the two molecules of pyruvic acid that are created. Interestingly, this process is somewhat similar to a reversal of photosynthesis (see Photosynthesis).

    Krebs Cycle

    Following glycolysis, the mechanism of cellular respiration involves another multi-step process—the Krebs cycle, which is also called the citric acid cycle or the tricarboxylic acid cycle. The Krebs cycle uses the two molecules of pyruvic acid formed in glycolysis and yields high-energy molecules of NADH and Flavin adenine dinucleotide (FADH2), as well as some ATP.

    Krebs Cycle
    Krebs Cycle

    The Krebs cycle occurs in the mitochondrion of a cell (see Figure 6). This sausage-shaped organelle possesses inner and outer membranes and, therefore, inner and outer compartments. The inner membrane is folded over itself many times; the folds are called cristae. They are somewhat similar to the thylakoid membranes in chloroplasts (see Photosynthesis). Located along the cristae are the important enzymes necessary for the proton pump and for ATP production.

    Prior to entering the Krebs cycle, the pyruvic acid molecules are altered. Each three-carbon pyruvic acid molecule undergoes conversion to a substance called acetyl-coenzyme A, or Acetyl-CoA. During the process, the pyruvic acid molecule is broken down by an enzyme, one carbon atom is released in the form of carbon dioxide, and the remaining two carbon atoms are combined with a coenzyme called coenzyme A. This combination forms Acetyl-CoA. In the process, electrons and a hydrogen ion are transferred to NAD to form high-energy NADH.

    Acetyl-CoA enters the Krebs cycle by combining with a four-carbon acid called oxaloacetic acid. The combination forms the six-carbon acid called citric acid. Citric acid undergoes a series of enzyme-catalyzed conversions. The conversions, which involve up to ten chemical reactions, are all brought about by enzymes. In many of the steps, high-energy electrons are released to NAD. The NAD molecule also acquires a hydrogen ion and becomes NADH. In one of the steps, FAD serves as the electron acceptor, and it acquires two hydrogen ions to become FADH2. Also, in one of the reactions, enough energy is released to synthesize a molecule of ATP. Because for each glucose molecule there are two pyruvic acid molecules entering the system, two ATP molecules are formed.

    Also during the Krebs cycle, the two carbon atoms of Acetyl-CoA are released, and each forms a carbon dioxide molecule. Thus, for each Acetyl-CoA entering the cycle, two carbon dioxide molecules are formed. Two Acetyl-CoA molecules enter the cycle, and each has two carbon atoms, so four carbon dioxide molecules will form. Add these four molecules to the two carbon dioxide molecules formed in the conversion of pyruvic acid to Acetyl-CoA, and it adds up to six carbon dioxide molecules. These six CO2 molecules are given off as waste gas in the Krebs cycle. They represent the six carbons of glucose that originally entered the process of glycolysis.

    At the end of the Krebs cycle, the final product is oxaloacetic acid. This is identical to the oxaloacetic acid that begins the cycle. Now the molecule is ready to accept another Acetyl-CoA molecule to begin another turn of the cycle. All told, the Krebs cycle forms (per two molecules of pyruvic acid) two ATP molecules, ten NADH molecules, and two FADH2 molecules. The NADH and the FADH2 will be used in the electron transport system.

    Electron Transport System

    The electron transport system occurs in the cristae of the mitochondria, where a series of cytochromes (enzymes) and coenzymes exist. These cytochromes and coenzymes act as carrier molecules and transfer molecules. They accept high-energy electrons and pass the electrons to the next molecule in the system. At key proton-pumping sites, the energy of the electrons transports protons across the membrane into the outer compartment of the mitochondrion.

    Each NADH molecule is highly energetic, which accounts for the transfer of six protons into the outer compartment of the mitochondrion. Each FADH2 molecule accounts for the transfer of four protons. The flow of electrons is similar to that taking place in photosynthesis. Electrons pass from NAD to FAD, to other cytochromes and coenzymes, and eventually they lose much of their energy. In cellular respiration, the final electron acceptor is an oxygen atom. In their energy-depleted condition, the electrons unite with an oxygen atom. The electron-oxygen combination then reacts with two hydrogen ions (protons) to form a water molecule (H2O).

    The role of oxygen in cellular respiration is substantial. As a final electron acceptor, it is responsible for removing electrons from the electron transport system. If oxygen were not available, electrons could not be passed among the coenzymes, the energy in electrons could not be released, the proton pump could not be established, and ATP could not be produced. In humans, breathing is the essential process that brings oxygen into the body for delivery to the cells to participate in cellular respiration.

    Chemiosmosis

    The actual production of ATP in cellular respiration takes place through the process of chemiosmosis (see Cells and Energy). Chemiosmosis involves the pumping of protons through special channels in the membranes of mitochondria from the inner to the outer compartment. The pumping establishes a proton (H+) gradient. After the gradient is established, protons diffuse down the gradient through a transport protein called ATP synthase. The flow of hydrogens catalyzes the pairing of a phosphate with ADP, forming ATP.

    The energy production of cellular respiration is substantial. Most biochemists agree that 36 molecules of ATP can be produced for each glucose molecule during cellular respiration as a result of the Krebs cycle reactions, the electron transport system, and chemiosmosis. Also, two ATP molecules are produced through glycolysis, so the net yield is 38 molecules of ATP. These ATP molecules may then be used in the cell for its needs. However, the ATP molecules cannot be stored for long periods of time, so cellular respiration must constantly continue in order to regenerate the ATP molecules as they are used. Each ATP molecule is capable of releasing 7.3 kilocalories of energy per mole.

    Fermentation

    Fermentation is an anaerobic process in which energy can be released from glucose even though oxygen is not available. Fermentation occurs in yeast cells, and a form of fermentation takes place in bacteria and in the muscle cells of animals.

    In yeast cells (the yeast used for baking bread and producing alcoholic beverages), glucose can be metabolized through cellular respiration as in other cells. When oxygen is lacking, however, glucose is still metabolized to pyruvic acid via glycolysis. The pyruvic acid is converted first to acetaldehyde and then to ethyl alcohol. The net gain of ATP to the yeast cell is two molecules—the two molecules of ATP normally produced in glycolysis.

    Yeasts are able to participate in fermentation because they have the necessary enzyme to convert pyruvic acid to ethyl alcohol. This process is essential because it removes electrons and hydrogen ions from NADH during glycolysis. The effect is to free the NAD so it can participate in future reactions of glycolysis. The net gain to the yeast cell of two ATP molecules permits it to remain alive for some time. However, when the percentage of ethyl alcohol reaches approximately 15 percent, the alcohol kills the yeast cells.

    Yeast is used in both bread and alcohol production. Alcohol fermentation is the process that yields beer, wine, and other spirits. The carbon dioxide given off during fermentation supplements the carbon dioxide given off during the Krebs cycle and causes bread to rise.

    In muscle cells, another form of fermentation takes place. When muscle cells contract too frequently (as in strenuous exercise), they rapidly use up their oxygen supply. As a result, the electron transport system and Krebs cycle slow considerably, and ATP production is slowed. However, muscle cells have the ability to produce a small amount of ATP through glycolysis in the absence of oxygen. The muscle cells convert glucose to pyruvic acid. An enzyme in the muscle cells then converts the pyruvic acid to lactic acid. As in the yeast, this reaction frees up the NAD while providing the cells with two ATP molecules from glycolysis. Eventually, however, the lactic acid buildup causes intense fatigue, and the muscle stops contracting.

  • Photosynthesis

    What is Photosynthesis?

    A great variety of living things on Earth, including all green plants, synthesize their foods from simple molecules, such as carbon dioxide and water. For this process, the organisms require energy, and that energy is derived from sunlight.

    Figure 1. shows the energy relationships in living cells. Light energy is captured in the chloroplast of plant cells and used to synthesize glucose molecules, shown as C6H12O6. In the process, oxygen (O2) is released as a waste product. The glucose and oxygen are then used in the mitochondrion of the plant cell, and the energy is released and used to fuel the synthesis of ATP from ADP and P. In the reaction, CO2 and water are released in the mitochondrion to be reused in photosynthesis in the chloroplast.

    Energy relationships in living cells Cycles
    Energy relationships in living cells Cycles

    Energy relationships in living cells

    Figure 1. Energy relationships in living cells.

    The process of utilizing energy to synthesize carbohydrate molecules is called photosynthesis. Photosynthesis is actually two separate processes. in the first process, energy-rich electrons flow through a series of coenzymes and other molecules. This electron energy is trapped. During the trapping process, adenosine triphosphate (ATP) molecules and molecules of nicotinamide adenine dinucleotide phosphate hydrogen (NADPH) are formed. Both ATP and NADPH are rich in energy. These molecules are used in the second process, where carbon dioxide molecules are bound into carbohydrates too form organic substances such as glucose.

    Chloroplast

    The organelle in which photosynthesis occurs (in the leaves and green stems of plants, for example) is called the chloroplast. Chloroplasts are relatively large organelles, containing a watery, protein-rich fluid called stroma. The stroma contains many small structures composed of membranes that resemble stacks of coins. Each stack is a granum (the plural form is grana). Each membrane in the stack is a thylakoid. Within the thylakoid membranes of the granum, many of the reactions of photosynthesis take place. The thylakoids are somewhat similar to the cristae of mitochondria (see Cellular Respiration).

    Photosystems

    Pigment molecules organized into photosystems capture sunlight in the chloroplast. Photosystems are clusters of light-absorbing pigments with some associated molecules—proton (hydrogen ion) pumps, enzymes, coenzymes, and cytochromes (see Cells and Energy). Each photosystem contains about 200 molecules of a green pigment called chlorophyll and about 50 molecules of another family of pigments called carotenoids. In the reaction center of the photosystem, the energy of sunlight is converted to chemical energy. The center is sometimes called a light-harvesting antenna.

    There are two photosystems within the thylakoid membranes, designated photosystem I and photosystem II. The reaction centers of these photosystems are P700 and P680, respectively. The energy captured in these reaction centers drives chemiosmosis, and the energy of chemiosmosis stimulates ATP production in the chloroplasts.

    Process of Photosynthesis

    The process of photosynthesis is conveniently divided into two parts: the energy-fixing reaction (also called the light reaction) and the carbon-fixing reaction (also called the light-independent reaction or the dark reaction).

    Energy-fixing reaction

    The energy-fixing reaction of photosynthesis begins when light is absorbed in photosystem II in the thylakoid membranes. The energy of the sunlight, captured in the P680 reaction center, causes the electrons from P680’s chlorophyll to move to a higher, unstable energy level. These electrons pass through a series of cytochromes in the nearby electron-transport system.

    After passing through the electron transport system, the energy-rich electrons eventually enter Photosystem-I. Some of the energy of the electron is used to pump protons across the thylakoid membrane, and this pumping sets up the potential for chemiosmosis.

    The spent electrons from P680 enter the P700 reaction center in photosystem I. Sunlight activates the electrons, which receive a second boost out of the chlorophyll molecules. There they reach a high energy level. The electrons progress through a second electron transport system, but this time there is no proton pumping. Rather, the energy reduces NADP. This reduction occurs as two electrons join NADP and energize the molecule. Because NADP acquires two negatively charged electrons, it attracts two positively charged protons to balance the charges. Consequently, the NADP molecule is reduced to NADPH, a molecule that contains much energy.

    Because electrons have flowed out of the P680 reaction center, the chlorophyll molecules are left without a certain number of electrons. Electrons secured from water molecules replace these electrons. Each split water molecule releases two electrons that enter the chlorophyll molecules to replace those lost. The split water molecules also release two protons that enter the cytoplasm near the thylakoid and are available to increase the chemiosmotic gradient.

    The third product of the split water molecules is oxygen. Two oxygen atoms combine with one another to form molecular oxygen (O2), which is given off as the by-product of photosynthesis; it fills the atmosphere and is used by all oxygen-requiring organisms, including plant and animal cells.

    Described above are the noncyclic energy-fixing reactions (see Figure 2). Certain plants and autotrophic prokaryotes are also known to participate in cyclic energy-fixing reactions. These reactions involve only photosystem I and the P700 reaction center. Excited electrons leave the reaction center, pass through coenzymes of the electron transport system, and follow a special pathway back to P700. Each electron powers the proton pump and encourages the transport of a proton across the thylakoid membrane. This process enriches the proton gradient and eventually leads to the generation of ATP.

    The energy-fixing reactions of photosynthesis

    Figure 2. The energy-fixing reactions of photosynthesis.

    ATP production in the energy-fixing reactions of photosynthesis occurs by the process of chemiosmosis (explained in Cells and Energy). Essentially, this process consists of a rush of protons across a membrane (the thylakoid membrane, in this case), accompanied by the synthesis of ATP molecules. Biochemists have calculated that the proton concentration on one side of the thylakoid is 10,000 times that of the opposite side of the membrane.

    In photosynthesis, the protons pass back across the membranes through channels lying alongside sites where enzymes are located. As the protons pass through the channels, the energy of the protons is released to form high-energy ATP bonds. ATP is formed in the energy-fixing reactions along with the NADPH formed in the main reactions. Both ATP and NADPH provide the energy necessary for the synthesis of carbohydrates that occurs in the second major set of events in photosynthesis.

    Carbon-fixing reaction

    Glucose and other carbohydrates are synthesized in the carbon-fixing reaction of photosynthesis, often called the Calvin cycle after Melvin Calvin, who performed much of the biochemical research (see Figure 3). This phase of photosynthesis occurs in the stroma of the plant cell.

    A carbon-fixing reaction or the Calvin cycle
    A carbon-fixing reaction or the Calvin cycle

    Figure 3. A carbon-fixing reaction, also called the Calvin cycle.

    In the carbon-fixing reaction, an essential material is carbon dioxide, which is obtained from the atmosphere. The carbon dioxide is attached to a five-carbon compound called ribulose bisphosphate. Ribulose bisphosphate carboxylase catalyzes this reaction.

    After carbon dioxide has been joined to ribulose bisphosphate, a six-carbon product forms, which immediately breaks into two three-carbon molecules called phosphoglycerate. Each phosphoglycerate molecule converts to another organic compound, but only in the presence of ATP. The ATP used is the ATP synthesized in the energy-fixing reaction. The organic compound formed converts to still another organic compound using the energy present in NADPH. Again, the energy-fixing reaction provides the essential energy. Each of the organic compounds that results consists of three carbon atoms. Eventually, the compounds interact with one another and join to form a single molecule of six-carbon glucose. This process also generates additional molecules of ribulose bisphosphate to participate in further carbon-fixing reactions.

    Glucose can be stored in plants in several ways. In some plants, the glucose molecules are joined to one another to form starch molecules. Potato plants, for example, store starch in tubers (underground stems). In some plants, glucose converts to fructose (fruit sugar), and the energy is stored in this form. In still other plants, fructose combines with glucose to form sucrose, commonly known as table sugar. The energy is stored in carbohydrates in this form. Plant cells obtain energy for their activities from these molecules. Animals use the same forms of glucose by consuming plants and delivering the molecules to their cells.

    All living things on Earth depend in some way on photosynthesis. It is the main mechanism for bringing the energy of sunlight into living systems and making that energy available for the chemical reactions taking place in cells.

  • Cells and Energy

    What are Cells and Energy?

    The Laws of Thermodynamics; Life can exist only where molecules and cells remain organized. All cells need energys to maintain organization. Physicists define energy as the ability to do work; in this case, the work is the continuation of life itself.

    Energy has been expressed in terms of reliable observations known as the laws of thermodynamics. There are two such laws. The first law of thermodynamics states that energy can neither be created nor destroyed. This law implies that the total amount of energy in a closed system (for example, the universe) remains constant. Energys neither enters nor leaves a closed system.

    Cells and Energy 1

    Within a closed system, energy can change, however. For instance, the chemical energy in gasoline is released when the fuel combines with oxygen and a spark ignites the mixture within a car’s engine. The gasoline’s chemical energy is changed into heat energy, sound energy, and the energy of motion.

    The second law of thermodynamics states that the amount of available energy in a closed system is decreasing constantly. Energy becomes unavailable for use by living things because of entropy, which is the degree of disorder or randomness of a system. The entropy of any closed system is constantly increasing. In essence, any closed system tends toward disorganization.

    Unfortunately, the transfers of energy in living systems are never completely efficient. Every body movement, every thought, and every chemical reaction in the cells involves a shift of energy and a measurable decrease of energy available to do work in the process. For this reason, considerably more energy must be taken into the system than is necessary to carry out the actions of life.

    Chemical Reactions

    Most chemical compounds do not combine with one another automatically, nor do chemical compounds break apart automatically. The great majority of the chemical reactions that occur within living things must be energized. This means that the atoms of a molecule must be separated by energy put into the system. The energy forces apart the atoms in the molecules and allows the reaction to take place.

    To initiate a chemical reaction, a type of “spark,” referred to as the energy of activation, is needed. For example, hydrogen and oxygen can combine to form water at room temperature, but the reaction requires activation energy.

    Any chemical reaction in which energy is released is called an exergonic reaction. In an exergonic chemical reaction, the products end up with less energy than the reactants. Other chemical reactions are endergonic reactions. In endergonic reactions, energy is obtained and trapped from the environment. The products of endergonic reactions have more energy than the reactants taking part in the chemical reaction. For example, plants carry out the process of photosynthesis, in which they trap energy from the sun to form carbohydrates (see Photosynthesis).

    The activation energy needed to spark an exergonic or endergonic reaction can be heat energy or chemical energy. Reactions that require activation energy can also proceed in the presence of biological catalysts. Catalysts are substances that speed up chemical reactions but remain unchanged themselves. Catalysts work by lowering the required amount of activation energy for the chemical reaction. For example, hydrogen and oxygen combine with one another in the presence of platinum. In this case, platinum is the catalyst. In biological systems, the most common catalysts are protein molecules called enzymes. Enzymes are absolutely essential if chemical reactions are to occur in cells.

    Enzymes

    The chemical reactions in all cells of living things operate in the presence of biological catalysts called enzymes. Because a particular enzyme catalyzes only one reaction, there are thousands of different enzymes in a cell catalyzing thousands of different chemical reactions. The substance changed or acted on by an enzyme is its substrate. The products of a chemical reaction catalyzed by an enzyme are end products.

    All enzymes are composed of proteins. (Proteins are chains of amino acids; see The Chemical Basis of Life.) When an enzyme functions, a key portion of the enzyme, called the active site, interacts with the substrate. The active site closely matches the molecular configuration of the substrate. After this interaction has taken place, a change in shape in the active site places a physical stress on the substrate. This physical stress aids the alteration of the substrate and produces the end products. During the time the active site is associated with the substrate, the combination is referred to as the enzyme-substrate complex. After the enzyme has performed its work, the product or products are released from the enzyme’s active site. The enzyme is then free to function in another chemical reaction.

    Enzyme-catalyzed reactions occur extremely fast. They happen about a million times faster than uncatalyzed reactions. With some exceptions, the names of enzymes end in “–ase.” For example, the enzyme that breaks down hydrogen peroxide to water and hydrogen is catalase. Other enzymes include amylase, hydrolase, peptidase, and kinase.

    The rate of an enzyme-catalyzed reaction depends on a number of factors, such as the concentration of the substrate, the acidity and temperature of the environment, and the presence of other chemicals. At higher temperatures, enzyme reactions occur more rapidly, but only up to a point. Because enzymes are proteins, excessive amounts of heat can change their structures, rendering them inactive. An enzyme altered by heat is said to be denatured.

    Enzymes work together in metabolic pathways. A metabolic pathway is a sequence of chemical reactions occurring in a cell. A single enzyme-catalyzed reaction may be one of multiple reactions in a metabolic pathway. Metabolic pathways may be of two general types: catabolic and anabolic. Catabolic pathways involve the breakdown or digestion of large, complex molecules. The general term for this process is catabolism. Anabolic pathways involve the synthesis of large molecules, generally by joining smaller molecules together. The general term for this process is anabolism.

    Many enzymes are assisted by chemical substances called cofactors. Cofactors may be ions or molecules associated with an enzyme and are required in order for a chemical reaction to take place. Ions that might operate as cofactors include those of iron, manganese, and zinc. Organic molecules acting as cofactors are referred to as coenzymes.

    Adenosine Triphosphate (ATP)

    Adenosine Triphosphate (ATP)

    The chemical substance that serves as the currency of energy in a cell is adenosine triphosphate (ATP). ATP is referred to as currency because it can be “spent” in order to make chemical reactions occur. The more energy required for a chemical reaction, the more ATP molecules must be spent.

    Virtually all forms of life use ATP, a nearly universal molecule of energy transfer. The energy released during catabolic reactions is stored in ATP molecules. In addition, the energy trapped in anabolic reactions (such as photosynthesis) is trapped in ATP molecules.

    An ATP molecule consists of three parts. One part is a double ring of carbon and nitrogen atoms called adenine. Attached to the adenine molecule is a small five-carbon carbohydrate called ribose. Attached to the ribose molecule are three phosphate units linked together by covalent bonds.

    Adenosine Triphosphate Structure
    Adenosine Triphosphate Structure

    The covalent bonds that unite the phosphate units in ATP are high-energy bonds. When an ATP molecule is broken down by an enzyme, the third (terminal) phosphate unit is released as a phosphate group, which is an ion. When this happens, approximately 7.3 kilocalories of energy are released. (A kilocalorie equals 1,000 calories.) This energy is made available to do the work of the cell.

    The adenosine triphosphatase enzyme accomplishes the breakdown of an ATP molecule. The products of ATP breakdown are adenosine diphosphate (ADP) and a phosphate ion. Adenosine diphosphate and the phosphate ion can be reconstituted to form ATP, much like a battery can be recharged. To accomplish this, synthesis energy must be available. This energy can be made available in the cell through two extremely important processes: photosynthesis.

    ATP Production

    ATP is generated from ADP and phosphate ions by a complex set of processes occurring in the cell. These processes depend on the activities of a special group of coenzymes. Three important coenzymes are nicotinamide adenine dinucleotide (NAD), nicotinamide adenine dinucleotide phosphate (NADP), and Flavin adenine dinucleotide (FAD).

    NAD and NADP are structurally similar to ATP. Both molecules have a nitrogen-containing ring called nicotinic acid, which is the chemically active part of the coenzymes. In FAD, the chemically active portion is the Flavin group. The vitamin riboflavin is used in the body to produce this Flavin group.

    All coenzymes perform essentially the same work. During the chemical reactions of metabolism, coenzymes accept electrons and pass them on to other coenzymes or other molecules. The removal of electrons or protons from a coenzyme is oxidation. The addition of electrons to a molecule is reduction. Therefore, the chemical reactions performed by coenzymes are called oxidation-reduction reactions.

    The oxidation-reduction reactions performed by the coenzymes and other molecules are essential to the energy metabolism of the cell. Other molecules participating in this energy reaction are called cytochromes. Together with the coenzymes, cytochromes accept and release electrons in a system called the electron transport system. The passage of energy-rich electrons among cytochromes and coenzymes drains the energy from the electrons to form ATP from ADP and phosphate ions.

    The actual formation of ATP molecules requires a complex process called chemiosmosis. Chemiosmosis involves the creation of a steep proton (hydrogen ion) gradient. This gradient occurs between the membrane-bound compartments of the mitochondria of all cells and the chloroplasts of plant cells. A gradient is formed when large numbers of protons (hydrogen ions) are pumped into the membrane-bound compartments of the mitochondria. The protons build up dramatically within the compartment, finally reaching an enormous number. The energy released from the electrons during the electron transport system pumps the protons.

    After large numbers of protons have gathered within the compartments of mitochondria and chloroplasts, they suddenly reverse their directions and escape back across the membranes and out of the compartments. The escaping protons release their energy in this motion. This energy is used by enzymes to unite ADP with phosphate ions to form ATP. The energy is trapped in the high-energy bond of ATP by this process, and the ATP molecules are made available to perform cell work. The movement of protons is chemiosmosis because it is a movement of chemicals (in this case, protons) across a semipermeable membrane. Because chemiosmosis occurs in mitochondria and chloroplasts, these organelles play an essential role in the cell’s energy metabolism. Photosynthesis explains how energy is trapped in the chloroplasts in plants, while Cellular Respiration explains how energy is released in the mitochondria of plant and animal cells.

  • What is The Chemical Basis of Life?

    What is The Chemical Basis of Life?

    Understanding of Chemical; A chemical substance is a form of matter that has the constant chemical composition and characteristic properties. It cannot be separated into components by physical separation methods, i.e., without breaking chemical bonds. Chemical substances can be chemical elements, chemical compounds, ions or alloys.

    Chemical substances are often called ‘pure’ to set them apart from mixtures. A common example of a chemical substance is pure water; it has the same properties and the same ratio of hydrogen to oxygen whether it is isolated from a river or made in a laboratory. Other chemical substances commonly encountered in pure form are the diamond (carbon), gold, table salt (sodium chloride) and refined sugar (sucrose). However, in practice, no substance is entirely pure, and chemical purity is specified according to the intended use of the chemical.

    Chemical substances exist as solids, liquids, gases, or plasma, and may change between these phases of matter with changes in temperature or pressure. Chemical substances may be combined or converted to others by means of chemical reactions. Now you will understanding of The Chemical Basis of Life.

    Acids and Bases

    Acids are chemical compounds that release hydrogen ions (H+) when placed in water. For example, when hydrogen chloride is placed in water, it releases its hydrogen ions and the solution becomes hydrochloric acid.

    Bases are chemical compounds that attract hydrogen atoms when they are placed in water. An example of a base is sodium hydroxide (NaOH). When this substance is placed in water, it attracts hydrogen ions, and a basic (or alkaline) solution results as hydroxyl (–OH) ions accumulate.

    Molecule

    Most of the compounds of interest to biologists are composed of units called molecules. A molecule is a precise arrangement of atoms held together by chemical bonds, and a compound is a molecule that contains atoms of more than one element. A molecule may be composed of two or more atoms of the same element, as in oxygen gas (O2), or it may be composed of atoms from different elements. The arrangements of the atoms in a molecule account for the properties of a compound. The molecular weight is equal to the atomic weights of the atoms in the molecule.

    The atoms in molecules may be joined to one another by various linkages called bonds. One example of a bond is an ionic bond, which is formed when the electrons of one atom transfer to a second atom. This creates electrically charged atoms called ions. The electrical charges cause the ions to be attracted to one another, and the attraction forms the ionic bond.

    A second type of linkage is a covalent bond. A covalent bond forms when two atoms share one or more electrons with one another. For example, as shown in Figure 1, oxygen shares its electrons with two hydrogen atoms, and the resulting molecule is water (H2O). Nitrogen shares its electrons with three hydrogen atoms, and the resulting molecule is ammonia (NH3). If one pair of electrons is shared, the bond is a single bond; if two pairs are shared, it is a double bond.

    Formation of a covalent bond in water and ammonia molecules

    Figure 1. Formation of a covalent bond in water and ammonia molecules. In each molecule, the second shell fills with eight electrons.

    Organic Compound: The chemical compounds of living things are known as organic compounds because of their association with organisms and because they are carbon-containing compounds. Organic compounds, which are the compounds associated with life processes, are the subject matter of organic chemistry. Among the numerous types of organic compounds, four major categories are found in all living things: carbohydrates, lipids, proteins, and nucleic acids.

    Carbohydrates

    Almost all organisms use carbohydrates as sources of energy. In addition, some carbohydrates serve as structural materials. Carbohydrates are molecules composed of carbon, hydrogen, and oxygen; the ratio of hydrogen atoms to oxygen and carbon atoms is 2:1.

    Simple carbohydrates, commonly referred to as sugars, can be monosaccharides if they are composed of single molecules, or disaccharides if they are composed of two molecules. The most important monosaccharide is glucose, a carbohydrate with the molecular formula C6H12O6. Glucose is the basic form of fuel in living things. In multicellular organisms, it is soluble and is transported by body fluids to all cells, where it is metabolized to release its energy. Glucose is the starting material for cellular respiration, and it is the main product of photosynthesis

    Three important disaccharides are also found in living things: maltose, sucrose, and lactose. Maltose is a combination of two glucose units covalently linked. The table sugar sucrose is formed by linking glucose to another monosaccharide called fructose. (Figure 2 shows that in the synthesis of sucrose, a water molecule is produced. The process is therefore called a dehydration reaction. The reversal of the process is hydrolysis, a process in which the molecule is split and water is added.) Lactose is composed of glucose and galactose units.

    Glucose and fructose molecules combine to form the disaccharide sucrose

    Figure 2. Glucose and fructose molecules combine to form the disaccharide sucrose.

    Complex carbohydrates are known as polysaccharides. Polysaccharides are formed by linking innumerable monosaccharides. Among the most important polysaccharides is starch, which is composed of hundreds or thousands of glucose units linked to one another. Starch serves as a storage form for carbohydrates. Much of the world’s human population satisfies its energy needs with starch in the form of rice, wheat, corn, and potatoes.

    Two other important polysaccharides are glycogen and cellulose. Glycogen is also composed of thousands of glucose units, but the units are bonded in a different pattern than in starch. Glycogen is the form in which glucose is stored in the human liver. Cellulose is used primarily as a structural carbohydrate. It is also composed of glucose units, but the units cannot be released from one another except by a few species of organisms. Wood is composed chiefly of cellulose, as are plant cell walls. Cotton fabric and paper are commercial cellulose products.

    Lipids

    Lipids are organic molecules composed of carbon, hydrogen, and oxygen atoms. The ratio of hydrogen atoms to oxygen atoms is much higher in lipids than in carbohydrates. Lipids include steroids (the material of which many hormones are composed), waxes, and fats.

    Fat molecules are composed of a glycerol molecule and one, two, or three molecules of fatty acids (see Figure 3). A glycerol molecule contains three hydroxyl (–OH) groups. A fatty acid is a long chain of carbon atoms (from 4 to 24) with a carboxyl (–COOH) group at one end. The fatty acids in a fat may all be alike or they may all be different. They are bound to the glycerol molecule by a process that involves the removal of water.

    Certain fatty acids have one or more double bonds in their molecules. Fats that include these molecules are unsaturated fats. Other fatty acids have no double bonds. Fats that include these fatty acids are saturated fats. In most human health situations, the consumption of unsaturated fats is preferred to the consumption of saturated fats.

    Fats stored in cells usually form clear oil droplets called globules because fats do not dissolve in water. Plants often store fats in their seeds, and animals store fats in large, clear globules in the cells of adipose tissue. The fats in adipose tissue contain much concentrated energy. Hence, they serve as a reserve energy supply to the organism. The enzyme lipase breaks down fats into fatty acids and glycerol in the human digestive system.

    A fat molecule is constructed by combining a glycerol molecule with three fatty acid molecules

    Figure 3. A fat molecule is constructed by combining a glycerol molecule with three fatty acid molecules. (Two saturated fatty acids and one unsaturated fatty acid are shown for comparison.) The constructed molecule is at the bottom.

    Protein

    Proteins, among the most complex of all organic compounds, are composed of amino acids (see Figure 4), which contain carbon, hydrogen, oxygen, and nitrogen atoms. Certain amino acids also have sulfur atoms, phosphorus, or other trace elements such as iron or copper.

    The structure and chemistry of amino acids

    Figure 4. The structure and chemistry of amino acids. When two amino acids are joined in a dipeptide, the –OH of one amino acid is removed, and the –H of the second is removed. So, water is removed. A dipeptide bond (right) forms to join the amino acids together

    Many proteins are immense and extremely complex. However, all proteins are composed of long chains of relatively simple amino acids. There are 20 kinds of amino acids. Each amino acid (see the left illustration in Figure 4) has an amino (–NH2) group, a carboxyl (–COOH) group, and a group of atoms called an –R group (where R stands for radical). The amino acids differ depending on the nature of the –R group, as shown in the middle illustration of Figure 4. Examples of amino acids are alanine, valine, glutamic acid, tryptophan, tyrosine, and histidine.

    The removal of water molecules links amino acids to form a protein. The process is called dehydration synthesis, and a by-product of the synthesis is water. The links forged between the amino acids are peptide bonds, and small proteins are often called peptides.

    All living things depend on proteins for their existence. Proteins are the major molecules from which living things are constructed. Certain proteins are dissolved or suspended in the watery substance of the cells, while others are incorporated into various structures of the cells. Proteins are also found as supporting and strengthening materials in tissues outside of cells. Bone, cartilage, tendons, and ligaments are all composed of proteins.

    One essential function of proteins is as an enzyme. Enzymes catalyze the chemical reactions that take place within cells. They are not used up in a reaction; rather, they remain available to catalyze succeeding reactions.

    Every species manufactures proteins unique to that species. The information for synthesizing the unique proteins is located in the nucleus of the cell. The so-called genetic code specifies the amino acid sequence in proteins. Hence, the genetic code regulates the chemistry taking place within a cell. Proteins also can serve as a reserve source of energy for the cell. When the amino group is removed from an amino acid, the resulting compound is energy-rich.

    Nucleic acids: Like proteins, nucleic acids are very large molecules. The nucleic acids are composed of smaller units called nucleotides. Each nucleotide contains a carbohydrate molecule (sugar), a phosphate group, and a nitrogen-containing molecule that, because of its properties, is a nitrogenous base.

    Living organisms have two important nucleic acids. One type is deoxyribonucleic acid, or DNA. The other is ribonucleic acid, or RNA. DNA is found primarily in the nucleus of the cell, while RNA is found in both the nucleus and the cytoplasm, a semiliquid substance that composes the volume of the cell.

    DNA and RNA differ from one another in their components. DNA contains the carbohydrate deoxyribose, while RNA has ribose. In addition, DNA contains the base thymine, while RNA has uracil.

    Elements and Atoms

    For many centuries, biology was the study of the natural world. Biologists searched for unidentified plants and animals, classified them, and studied their anatomy and how they acted in nature. Then in the 1700s, scientists discovered the chemical and physical bases of living things. They soon realized that the chemical organization of all living things is remarkably similar.

    Elements: All living things on Earth are composed of fundamental building blocks of matter called elements. More than 100 elements are known to exist, including those that are man-made. An element is a substance that cannot be chemically decomposed. Oxygen, iron, calcium, sodium, hydrogen, carbon, and nitrogen are examples of elements.

    Atoms: Each element is composed of one particular kind of atom. An atom is the smallest part of an element that can enter into combinations with atoms of other elements.

    Atoms consist of positively charged particles called protons surrounded by negatively charged particles called electrons. A third type of particle, a neutron, has no electrical charge; it has the same weight as a proton. Protons and neutrons adhere tightly to form the dense, positively charged nucleus of the atom. Electrons spin around the nucleus.

    The electron arrangement in an atom plays an essential role in the chemistry of the atom. Atoms are most stable when their outer shell of electrons has a full quota. The first electron shell has a maximum of two electrons. The second and all other outer shells have a maximum of eight electrons. Atoms tend to gain or lose electrons until their outer shells have a stable arrangement. The gaining or losing of electrons, or the sharing of electrons, contributes to the chemical reactions in which an atom participates.

  • What is Cells Biology?

    What is Cells Biology?

    Cells biology is the study of cell structure and function, and it revolves around the concept that the cell is the fundamental unit of life. Focusing on the cell permits a detailed understanding of the tissues and organisms that cells compose. Some organisms have only one cell, while others are organized into cooperative groups with huge numbers of cells. On the whole, cell biology focuses on the structure and function of a cell, from the most general properties shared by all cells, to the unique, highly intricate functions particular to specialized cells.

    Cells Defined: One of the basic tenets of biology is that all living things are composed of one or more cells. Some organisms consist of a single cell, while others have multiple cells organized into tissues, and tissues organized into organs. In many living things, organs function together as an organ system. However, even in these complex organisms, the basic biology revolves around the activities of the cell.

    Cells Biology

    The starting point for this discipline might be considered the 1830s. Though scientists had been using microscopes for centuries, they were not always sure what they were looking at. Robert Hooke’s initial observation in 1665 of plant-cell walls in slices of cork was followed shortly by Antonie van Leeuwenhoek’s first descriptions of live cells with visible moving parts. In the 1830s two scientists who were colleagues Schleiden, looking at plant cells, and Schwann, looking first at animal cells provided the first clearly stated definition of the cell. Their definition stated that all living creatures, both simple and complex, are made out of one or more cells, and the cell is the structural and functional unit of life a concept that became known as cell theory.

    As microscopes and staining techniques improved over the nineteenth and twentieth centuries, scientists were able to see more and more internal detail within cells. The microscopes used by van Leeuwenhoek probably magnified specimens a few hundredfold. Today high-powered electron microscopes can magnify specimens more than a million times and can reveal the shapes of organelles at the scale of a micrometer and below. With confocal microscopy a series of images can be combined, allowing researchers to generate detailed three-dimensional representations of cells. These improved imaging techniques have helped us better understand the wonderful complexity of cells and the structures they form.

    There are several main subfields within cell biology. One is the study of cell energy and the biochemical mechanisms that support cell metabolism. As cells are machines unto themselves, the focus on cell energy overlaps with the pursuit of questions of how energy first arose in original primordial cells, billions of years ago. Another subfield of cell biology concerns the genetics of the cell and its tight interconnection with the proteins controlling the release of genetic information from the nucleus to the cell cytoplasm. Yet another subfield focuses on the structure of cell components, known as subcellular compartments. Cutting across many biological disciplines is the additional subfield of cell biology, concerned with cell communication and signaling, concentrating on the messages that cells give to and receive from other cells and themselves. And finally, there is the subfield primarily concerned with the cell cycle, the rotation of phases beginning and ending with cell division and focused on different periods of growth and DNA replication. Many cell biologists dwell at the intersection of two or more of these subfields as our ability to analyze cells in more complex ways expands.

    In line with the continually increasing interdisciplinary study, the recent emergence of systems biology has affected many biological disciplines; it is a methodology that encourages the analysis of living systems within the context of other systems. In the field of cell biology, systems biology has enabled the asking and answering of more complex questions, such as the interrelationships of gene regulatory networks, evolutionary relationships between genomes, and the interactions between intracellular signaling networks. Ultimately, the broader a lens we take on our discoveries in cell biology, the more likely we can decipher the complexities of all living systems, large and small.

    One of the first scientists to observe cells was Englishman Robert Hooke. In the mid-1600s, Hooke examined a thin slice of cork through the newly developed microscope. The microscopic compartments in the cork impressed him and reminded him of rooms in a monastery, known as cells. He therefore referred to the units as cells. Later in that century, Anton Van Leeuwenhoek, a Dutch merchant, made further observations of plant, animal, and microorganism cells. In 1838, German botanist Matthias Schleiden proposed that all plants are composed of cells. A year later, his colleague, anatomist Theodor Schwann, concluded that all animals are also composed of cells. In 1858, biologist Rudolf Virchow proposed that all living things are made of cells and that all cells arise from preexisting cells. These premises have come down to us as the cell theory.

    Cells

    Movement Through the Plasma Membrane

    In order for the cell cytoplasm to communicate with the external environment, materials must be able to move through the plasma membrane. This movement occurs through several mechanisms.

    Diffusion: One method of movement through the membrane is diffusion. Diffusion is the movement of molecules from a region of higher concentration to one of lower concentration. This movement occurs because the molecules are constantly colliding with one another. The net movement of the molecules is away from the region of high concentration to the region of low concentration.

    Diffusion is a random movement of molecules down the pathway called the concentration gradient. Molecules are said to move down the concentration gradient because they move from a region of higher concentration to a region of lower concentration. A drop of dye placed in a beaker of water illustrates diffusion as the dye molecules spread out and color the water.

    Osmosis: Another method of movement across the membrane is osmosis. Osmosis is the movement of water from a region of higher concentration to one of lower concentration. Osmosis occurs across a membrane that is semipermeable. A semipermeable membrane lets only certain molecules pass through while keeping other molecules out. Osmosis is really a type of diffusion involving only water molecules.

    Facilitated diffusion: A third mechanism for movement across the plasma membrane is facilitated diffusion. Certain proteins in the membrane assist facilitated diffusion by permitting only certain molecules to pass across the membrane. The proteins encourage movement in the direction that diffusion would normally take place, from a region with a higher concentration of molecules to a region of lower concentration.

    Active transport: A fourth method for movement across the membrane is active transport. When active transport is taking place, a protein moves a certain material across the membrane from a region of lower concentration to a region of higher concentration. Because this movement is happening against the concentration gradient, the cell must expend energy that is usually derived from a substance called adenosine triphosphate, or ATP (see Chapter 4). An example of active transport occurs in human nerve cells. Here, sodium ions are constantly transported out of the cell into the external fluid bathing the cell, a region of high concentration of sodium. (This transport of sodium sets up the nerve cell for the impulse that will occur within it later.)

    Endocytosis and exocytosis: The final mechanism for movement across the plasma membrane into the cell is endocytosis, a process in which a small patch of plasma membrane encloses particles or tiny volumes of fluid that are at or near the cell surface. The membrane enclosure then sinks into the cytoplasm and pinches off from the membrane, forming a vesicle that moves into the cytoplasm. When the vesicle contains solid particulate matter, the process is called phagocytosis. When the vesicle contains droplets of fluid, the process is called pinocytosis. Along with the other mechanisms for transport across the plasma membrane, endocytosis ensures that the internal cellular environment will be able to exchange materials with the external environment and that the cell will continue to thrive and function. Exocytosis is the reverse of endocytosis, where internally produced substances are enclosed in vesicles and fuse with the cell membrane, releasing the contents to the exterior of the cell.

    The Structure of Prokaryote and Eukaryote Cells

    During the 1950s, scientists developed the concept that all organisms may be classified as prokaryotes or eukaryotes. The cells of all prokaryotes and eukaryotes possess two basic features: a plasma membrane, also called a cell membrane, and cytoplasm. However, the cells of prokaryotes are simpler than those of eukaryotes. For example, prokaryotic cells lack a nucleus, while eukaryotic cells have a nucleus. Prokaryotic cells lack internal cellular bodies (organelles), while eukaryotic cells possess them. Examples of prokaryotes are bacteria and archaea. Examples of eukaryotes are protists, fungi, plants, and animals (everything except prokaryotes).

    Plasma membrane: All prokaryote and eukaryote cells have plasma membranes. The plasma membrane (also known as the cell membrane) is the outermost cell surface, which separates the cell from the external environment. The plasma membrane is composed primarily of proteins and lipids, especially phospholipids. The lipids occur in two layers (a bilayer). Proteins embedded in the bilayer appear to float within the lipid, so the membrane is constantly in flux. The membrane is therefore referred to as a fluid mosaic structure. Within the fluid mosaic structure, proteins carry out most of the membrane’s functions.

    The “Movement through the Plasma Membrane” section later in this chapter describes the process by which materials pass between the interior and exterior of a cell.

    Cytoplasm and organelles: All prokaryote and eukaryote cells also have cytoplasm (or cytosol), a semiliquid substance that composes the volume of a cell. Essentially, cytoplasm is the gel-like material enclosed by the plasma membrane.

    Within the cytoplasm of eukaryote cells are a number of membrane-bound bodies called organelles (“little organs”) that provide a specialized function within the cell.

    One example of an organelle is the endoplasmic reticulum (ER). The ER is a series of membranes extending throughout the cytoplasm of eukaryotic cells. In some places, the ER is studded with submicroscopic bodies called ribosomes. This type of ER is called rough ER. In other places, there are no ribosomes. This type of ER is called smooth ER. The rough ER is the site of protein synthesis in a cell because it contains ribosomes; however, the smooth ER lacks ribosomes and is responsible for producing lipids. Within the ribosomes, amino acids are actually bound together to form proteins. Cisternae are spaces within the folds of the ER membranes.

    Another organelle is the Golgi apparatus (also called Golgi body). The Golgi apparatus is a series of flattened sacs, usually curled at the edges. In the Golgi body, the cell’s proteins and lipids are processed and packaged before being sent to their final destination. To accomplish this function, the outermost sac of the Golgi body often bulges and breaks away to form drop like vesicles known as secretory vesicles.

    An organelle called the lysosome (see Figure) is derived from the Golgi body. It is a drop like sac of enzymes in the cytoplasm. These enzymes are used for digestion within the cell. They break down particles of food taken into the cell and make the products available for use; they also help break down old cell organelles. Enzymes are also contained in a cytoplasmic body called the peroxisome.

    Diagram of an Animal Cells Biology
    Diagram of an Animal Cells Biology

    Figure The components of an idealized eukaryotic cell. The diagram shows the relative sizes and locations of the cell parts.

    The organelle that releases quantities of energy to form adenosine triphosphate (ATP) is the mitochondrion (the plural form is mitochondria). Because mitochondria are involved in energy release and storage, they are called the “powerhouses of the cells.”

    Green plant cells, for example, contain organelles known as chloroplasts, which function in the process of photosynthesis. Within chloroplasts, energy from the sun is absorbed and transformed into the energy of carbohydrate molecules. Plant cells specialized for photosynthesis contain large numbers of chloroplasts, which are green because the chlorophyll pigments within the chloroplasts are green. Leaves of a plant contain numerous chloroplasts. Plant cells not specializing in photosynthesis (for example, root cells) are not green.

    An organelle found in mature plant cells is a large, fluid-filled central vacuole. The vacuole may occupy more than 75 percent of the plant cell. In the vacuole, the plant stores nutrients, as well as toxic wastes. Pressure within the growing vacuole may cause the cell to swell.

    The cytoskeleton is an interconnected system of fibers, threads, and interwoven molecules that give structure to the cell. The main components of the cytoskeleton are microtubules, microfilaments, and intermediate filaments. All are assembled from subunits of protein.

    The centriole organelle is a cylinder like structure that occurs in pairs. Centrioles function in cell division.

    Many cells have specialized cytoskeletal structures called flagella and cilia. Flagella are long, hair like organelles that extend from the cell, permitting it to move. In prokaryotic cells, such as bacteria, the flagella rotate like the propeller of a motorboat. In eukaryotic cells, such as certain protozoa and sperm cells, the flagella whip about and propel the cell. Cilia are shorter and more numerous than flagella. In moving cells, the cilia wave in unison and move the cell forward. Paramecium is a well-known ciliated protozoan. Cilia are also found on the surface of several types of cells, such as those that line the human respiratory tract.

    Nucleus: Prokaryotic cells lack a nucleus; the word prokaryotic means “primitive nucleus.” Eukaryotic cells, on the other hand, have a distinct nucleus.

    The nucleus of eukaryotic cells is composed primarily of protein and deoxyribonucleic acid, or DNA. The DNA is tightly wound around special proteins called histones; the mixture of DNA and histone proteins is called chromatin. The chromatin is folded even further into distinct threads called chromosomes. Functional segments of the chromosomes are referred to as genes. Approximately 21,000 genes are located in the nucleus of all human cells.

    The nuclear envelope, an outer membrane, surrounds the nucleus of a eukaryotic cell. The nuclear envelope is a double membrane, consisting of two lipid layers (similar to the plasma membrane). Pores in the nuclear envelope allow the internal nuclear environment to communicate with the external nuclear environment.

    Within the nucleus are two or more dense organelles referred to as nucleoli (the singular form is nucleolus). In nucleoli, submicroscopic particles known as ribosomes are assembled before their passage out of the nucleus into the cytoplasm.

    Although prokaryotic cells have no nucleus, they do have DNA. The DNA exists freely in the cytoplasm as a closed loop. It has no protein to support it and no membrane covering it. A bacterium typically has a single looped chromosome.

    Cell Wall

    Many kinds of prokaryotes and eukaryotes contain a structure outside the cell membrane called the cell wall. With only a few exceptions, all prokaryotes have thick, rigid cell walls that give them their shape. Among the eukaryotes, some protists, and all fungi and plants, have cell walls. Cell walls are not identical in these organisms, however. In fungi, the cell wall contains a polysaccharide called chitin. Plant cells, in contrast, have no chitin; their cell walls are composed exclusively of the polysaccharide cellulose.

    Cell walls provide support and help cells resist mechanical pressures, but they are not solid, so materials are able to pass through rather easily. Cell walls are not selective devices, as plasma membranes are.

  • What is Cells?

    What is Cells?

    The cell (from Latin cella, meaning “small room”) is the basic structural, functional, and biological unit of all known living organisms. A cell is the smallest unit of life that can replicate independently, and cells are often called the “building blocks of life”. The study of cells is called cell biology.

    Cells

    Cells consist of cytoplasm enclosed within a membrane, which contains many biomolecules such as proteins and nucleic acids. Organisms can be classified as unicellular (consisting of a single cell; including bacteria) or multicellular (including plants and animals). While the number of cells in plants and animals varies from species to species, humans contain more than 10 trillion (1012) cell. Most plant and animal cells are visible only under a microscope, with dimensions between 1 and 100 micrometers.

    The cell was discovered by Robert Hooke in 1665, who named the biological unit for its resemblance to cell inhabited by Christian monks in a monastery. Cell theory, first developed in 1839 by Matthias Jakob Schleiden and Theodor Schwann, states that all organisms are composed of one or more cell, that cell are the fundamental unit of structure and function in all living organisms, that all cell come from preexisting cell, and that all cell contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells. Cells emerged on Earth at least 3.5 billion years ago.

    Types of Cells

    Eukaryote and Prokaryote Cells
    Eukaryote and Prokaryote

    Prokaryote Cells

    Prokaryotic cells were the first form of life on Earth, characterized by having vital biological processes including cell signaling and being self-sustaining. They are simpler and smaller than eukaryotic cells and lack membrane-bound organelles such as the nucleus. Prokaryotes include two of the domains of life, bacteria, and archaea. The DNA of a prokaryotic cell consists of a single chromosome that is in direct contact with the cytoplasm. The nuclear region in the cytoplasm is called the nucleoid. Most prokaryotes are the smallest of all organisms ranging from 0.5 to 2.0 µm in diameter.

    A prokaryotic cell has three architectural regions:

    I. Enclosing the cell is the cell envelope generally consisting of a plasma membrane covered by a cell wall which, for some bacteria, may be further covered by a third layer called a capsule. Though most prokaryotes have both a cell membrane and a cell wall, there are exceptions such as Mycoplasma (bacteria) and Thermo-plasma (archaea) which only possess the cell membrane layer. The envelope gives rigidity to the cell and separates the interior of the cell from its environment, serving as a protective filter. The cell wall consists of peptidoglycan in bacteria and acts as an additional barrier against exterior forces. It also prevents the cell from expanding and bursting (cytolysis) from osmotic pressure due to a hypotonic environment. Some eukaryotic cells (plant cells and fungal cells) also have a cell wall.

    II. Inside the cell is the cytoplasmic region that contains the genome (DNA), ribosomes and various sorts of inclusions. The genetic material is freely found in the cytoplasm. Prokaryotes can carry extrachromosomal DNA elements called plasmids, which are usually circular. Linear bacterial plasmids have been identified in several species of spirochete bacteria, including members of the genus Borrelia notably Borrelia burgdorferi, which causes Lyme disease. Though not forming a nucleus, the DNA is condensed in a nucleoid. Plasmids encode additional genes, such as antibiotic resistance genes.

    III. On the outside, flagella and pili project from the cell’s surface. These are structures (not present in all prokaryotes) made of proteins that facilitate movement and communication between cells.

    The Structure of Prokaryote and Eukaryote Cells
    The Structure of Prokaryote and Eukaryote Cells

    Eukaryote Cells

    Plants, animals, fungi, slime moulds, protozoa, and algae are all eukaryotic. These cells are about fifteen times wider than a typical prokaryote and can be as much as a thousand times greater in volume. The main distinguishing feature of eukaryotes as compared to prokaryotes is compartmentalization: the presence of membrane-bound organelles (compartments) in which specific metabolic activities take place. Most important among these is a cell nucleus, an organelle that houses the cell’s DNA. This nucleus gives the eukaryote its name, which means “true kernel (nucleus)”. Other differences include:

    I. The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup. Cell walls may or may not be present.

    II. The eukaryotic DNA is organized in one or more linear molecules, called chromosomes, which are associated with histone proteins. All chromosomal DNA is stored in the cell nucleus, separated from the cytoplasm by a membrane. Some eukaryotic organelles such as mitochondria also contain some DNA.

    III. Many eukaryotic cell are ciliated with primary cilia. Primary cilia play important roles in chemosensation, mechanosensation, and thermosensation. Cilia may thus be “viewed as a sensory cellular antenna that coordinates a large number of cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation.”

    IV. Motile cells of eukaryotes can move using motile cilia or flagella. Motile cells are absent in conifers and flowering plants. Eukaryotic flagella are less complex than those of prokaryotes.

  • What is Biology?

    What is Biology?

    The study of living organisms, divided into many specialized fields that cover their morphology, physiology, anatomy, behavior, origin, and distribution; The plants and animals of a particular area, and the physiology, behavior, and other qualities of a particular organism or class of organisms.

    Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, evolution, distribution, identification and taxonomy. Modern biology is a vast and eclectic field, composed of many branches and sub disciplines. However, despite the broad scope of biology, there are certain general and unifying concepts within it that govern all study and research, consolidating it into single, coherent field. In general, biology recognizes the cell as the basic unit of life, genes as the basic unit of heredity, and evolution as the engine that propels the synthesis and creation of new species. It is also understood today that all the organisms survive by consuming and transforming energy and by regulating their internal environment to maintain a stable and vital condition known as homeostasis.

    History of Biology:

    The term biology is derived from the Greek word βίος, bios, “life” and the suffix -λογία, -logia, “study of.” The Latin-language form of the term first appeared in 1736 when Swedish scientist Carl Linnaeus (Carl von Linné) used biologi in his Bibliotheca botanica. It was used again in 1766 in a work entitled Philosophiae Naturalis sive physicae: tomus III, Continens Geologian, biologian, phytologian generalis, by Michael Christoph Hanov, a disciple of Christian Wolff. The first German use, Biologie, was in a 1771 translation of Linnaeus’ work. In 1797, Theodor Georg August Roose used the term in the preface of a book, Grundzüge der Lehre van der Lebenskraft. Karl Friedrich Burdach used the term in 1800 in a more restricted sense of the study of human beings from a morphological, physiological and psychological perspective (Propädeutik zum Studien der GE Samm ten Heilkunst). The term came into its modern usage with the six-volume treatise Biologie, oder Philosophie der lebenden Natur (1802–22) by Gottfried Reinhold Treviranus, who announced; The objects of our research will be the different forms and manifestations of life, the conditions and laws under which these phenomena occur, and the causes through which they have been effected. The science that concerns itself with these objects we will indicate by the name biology [Biologie] or the doctrine of life [Lebenslehre].

    Biology is the study of living things. It encompasses the cellular basis of living things, the energy metabolism that underlies the activities of life, and the genetic basis for inheritance in organisms. Biology also includes the study of evolutionary relationships among organisms and the diversity of life on Earth. It considers the biology of microorganisms, plants, and animals, for example, and it brings together the structural and functional relationships that underlie their day-to-day activities. Biology draws on the sciences of chemistry and physics for its foundations and applies the laws of these disciplines to living things.

    Biology living world scale
    Biology – Living World Scale

    Many sub disciplines and special areas of biology exist, which can be conveniently divided into practical and theoretical categories. Types of practical biology include plant breeding, wildlife management, medical science, and crop production. Theoretical biology encompasses such disciplines as physiology (the study of the function of living things), biochemistry (the study of the chemistry of organisms), taxonomy (classification), ecology (the study of populations and their interactions with each other and their environments), and microbiology (the study of microscopic organisms).

    Their fascination with biology has a long history. Even early humans had to study the animals that they hunted and know where to find the plants that they gathered for food. The invention of agriculture was the first great advance of human civilization. Medicine has been important to us from earliest history as well. The earliest known medical texts are from China (2500 B.C.), Mesopotamia (2112 B.C.), and Egypt (1800 B.C.).

    In classical times, Aristotle is often considered to be the first to practice scientific zoology. He is known to have performed extensive studies of marine life and plants. His student, Theophrastus, wrote one of the West’s earliest known botanical texts in 300 B.C. on the structure, life cycle and uses of plants. The Roman physician Galen used his experience in patching up gladiators for the arena to write texts on surgical procedures in A.D. 158.

    During the Renaissance, Leonardo da Vinci risked censure by participating in human dissection and making detailed anatomical drawings that are still considered among the most beautiful ever made. Invention of the printing press and the ability to reproduce woodcut illustrations meant that information was much easier to record and disseminate. One of the first illustrated biology books is a botanical text written by German botanist Leonhard Fuchs in 1542. Binomial classification was inaugurated by Carolus Linnaeus in 1735, using Latin names to group species according to their characteristics.

    Microscopes opened up new worlds for scientists. In 1665, Robert Hooke, used a simple compound microscope to examine a thin sliver of cork. He observed that the plant tissue consisted of rectangular units that reminded him of the tiny rooms used by monks. He called these units “cells.” In 1676, Anton von Leeuwenhoek published the first drawings of living single celled organisms. Theodore Schwann added the information that animal tissue is also composed of cells in 1839.

    During the Victorian era, and throughout the 19th century, “Natural Science” became something of a mania. Thousands of new species were discovered and described by intrepid adventurers and by backyard botanists and entomologists alike. In 1812, Georges Cuvier described fossils and hypothesized that Earth had undergone “successive bouts of Creation and destruction” over long periods of time. On Nov. 24, 1859, Charles Darwin published “On the Origin of Species,” the text that forever changed the world by showing that all living things are interrelated and that species were not separately created but arise from ancestral forms that are changed and shaped by adaptation to their environment.

    While much of the world’s attention was captured by biology questions at the macroscopic organism level, a quiet monk was investigating how living things pass traits from one generation to the next. Gregor Mendel is now known as the father of genetics although is papers on inheritance, published in 1866, went largely unnoticed at the time. His work was rediscovered in 1900 and further understanding of inheritance rapidly followed.

    The 20th and 21st centuries may be known to future generations as the beginning of the “Biological Revolution.” Beginning with Watson and Crick explaining the structure and function of DNA in 1953, all fields of biology have expanded exponentially and touch every aspect of our lives. Medicine will be changed by development of therapies tailored to a patient’s genetic blueprint or by combining biology and technology with brain-controlled prosthetics. Economies hinge on the proper management of ecological resources, balancing human needs with conservation. We may discover ways to save our oceans while using them to produce enough food to feed the nations. We may “grow” batteries from bacteria or light buildings with bioluminescent fungi. The possibilities are endless; biology is just coming into its own.

    Characteristics of Living Things:

    Defining a living thing is a difficult proposition, as is defining “life”—that property possessed by living things. However, a living thing possesses certain properties that help define what life is.

    Biology Human Lifecycle
    Biology Human Life-cycle

    Complex organization: Living things have a level of complexity and organization not found in lifeless objects. At its most fundamental level, a living thing is composed of one or more cells. These units, generally too small to be seen with the naked eye, are organized into tissues. A tissue is a series of cells that accomplish a shared function. Tissues, in turn, form organs, such as the stomach and kidney. A number of organs working together compose an organ system. An organism is a complex series of various organ systems.

    Metabolism: Living things exhibit a rapid turnover of chemical materials, which is referred to as metabolism. Metabolism involves exchanges of chemical matter with the external environment and extensive transformations of organic matter within the cells of a living organism. Metabolism generally involves the release or use of chemical energy. Nonliving things do not display metabolism.

    Responsiveness: All living things are able to respond to stimuli in the external environment. For example, living things respond to changes in light, heat, sound, and chemical and mechanical contact. To detect stimuli, organisms have means for receiving information, such as eyes, ears, and taste buds.

    To respond effectively to changes in the environment, an organism must coordinate its responses. A system of nerves and a number of chemical regulators called hormones coordinate activities within an organism. The organism responds to the stimuli by means of a number of effectors, such as muscles and glands. Energy is generally used in the process.

    Organisms change their behavior in response to changes in the surrounding environment. For example, an organism may move in response to its environment. Responses such as this occur in definite patterns and make up the behavior of an organism. The behavior is active, not passive; an animal responding to a stimulus is different from a stone rolling down a hill. Living things display responsiveness; nonliving things do not.

    Growth: Growth requires an organism to take in material from the environment and organize the material into its own structures. To accomplish growth, an organism expends some of the energy it acquires during metabolism. An organism has a pattern for accomplishing the building of growth structures.

    During growth, a living organism transforms material that is unlike itself into materials that are like it. A person, for example, digests a meal of meat and vegetables and transforms the chemical material into more of himself or herself. A nonliving organism does not display this characteristic.

    Reproduction: A living thing has the ability to produce copies of itself by the process known as reproduction. These copies are made while the organism is still living. Among plants and simple animals, reproduction is often an extension of the growth process. More complex organisms engage in a type of reproduction called sexual reproduction, in which two parents contribute to the formation of a new individual. During this process, a new combination of traits can be produced.

    Asexual reproduction involves only one parent, and the resulting cells are generally identical to the parent cell. For example, bacteria grow and quickly reach maturity, after which they split into two organisms by a process of asexual reproduction called binary fission.

    Evolution: Living organisms have the ability to adapt to their environment through the process of evolution. During evolution, changes occur in populations, and the organisms in the population become better able to metabolize, respond, and reproduce. They develop abilities to cope with their environment that their ancestors did not have.

    Evolution also results in a greater variety of organisms than existed in previous eras. This proliferation of populations of organisms is unique to living things.

    Ecology: The environment influences the living things that it surrounds. Ecology is the study of relationships between organisms and their relationships with their environment. Both biotic factors (living things) and abiotic factors (non-living things) can alter the environment. Rain and sunlight are non-living components, for example, that greatly influence the environment. Living things may migrate or hibernate if the environment becomes difficult to live in.

  • Do You Really Want to your Own a Business?

    Do You Really Want to your Own a Business?

    If “Yes” So, this article of post little help you How to Start. “Hope springs eternal in the human breast,” said English poet and essayist Alexander Pope several centuries ago. He wasn’t describing people expanding or starting a business, but he may as well have been. Everyone who goes into business for themselves hopes to meet or surpass a set of personal goals.

    A business (also known as an enterprise, a company or a firm) is an organizational entity involved in the provision of goods and services to consumers. Businesses serve as a form of economic activity and are prevalent in capitalist economies, where most of them are privately owned and provide goods and services allocated through a market to consumers and customers in exchange for other goods, services, money, or other forms of exchange that hold intrinsic economic value. Businesses may also be social non-profit enterprises or state-owned public enterprises operated by governments with specific social and economic objectives. A business owned by multiple private individuals may form an incorporated company or jointly organize as a partnership. Countries have different laws that may ascribe different rights to the various business entities.

    The word “business” can refer to a particular organization or to an entire market sector (for example: “the financial sector”) or to the sum of all economic activity (“the business sector”). Compound forms such as “agribusiness” represent subsets of the concept’s broader meaning, which encompasses all activity by suppliers of goods and services.

    Sole Ownership: A sole proprietorship (ownership), also known as a sole trader, is owned by one person and operates for their benefit. The owner operates the business alone and may hire employees. A sole proprietor has unlimited liability for all obligations incurred by the business, whether from operating costs or judgments against the business. All assets of the business belong to a sole proprietor, including, for example, a computer infrastructure, any inventory, manufacturing equipment, or retail fixtures, as well as any real property owned by the sole proprietor.

    Do You Really Want to your Own a Business - Sole Ownership

    While your particular configuration is sure to be unique, perhaps you will agree with some of the ones I have compiled over the years from talking to hundreds of budding entrepreneurs.

    Independence: A search for freedom and independence is the driving force behind many businesspeople. Wasn’t it Johnny Paycheck who wrote the song “Take This Job and Shove It?”

    Personal Fulfillment: For many people, owning a business is a genuinely fulfilling experience, one that lifetime employees never know.

    Lifestyle Change: many people find that while they can make a good income working for other people, they are missing some of life’s precious moments. With the flexibility of small business ownership, you can take time to stop and smell the roses.

    Respect: Successful small business owners are respected, both by themselves and their peers.

    Money: You can get rich in a small business, or at least do very well financially. most entrepreneurs don’t get wealthy, but some do. If money is your motivator, admit it.

    Power: When it is your business, you can have your employees do it your way. There is a little Ghengis Khan in us all, so don’t be surprised if power is one of your goals. If it is, think about how to use this goal in a constructive way.

    Right Livelihood: From natural foods to solar power to many types of service businesses, a great many cause-driven small businesses have done very well by doing good.

    If owning a small business can help a person accomplish these goals, it’s small wonder that so many are started. Unfortunately, while the potential for great success exists, so do many risks. Running a small business may require that you sacrifice some short-term comforts for long-term benefits. It is hard, demanding work that requires a wide variety of skills few people are born with. But even if you possess (or more likely acquire) the skills and determination you need to successfully run a business, your business will need one more critical ingredient: money.

    You need money to start your business, money to keep it running, and money to make it grow. This is not the same thing as saying you can guarantee success in your small business if you begin with a fat wallet. now, let me confess to one major bias here. I believe that most small business owners and founders are better off starting small and borrowing, or otherwise raising, as little money as possible. Put another way, there is no such thing as “raising plenty of capital to ensure success.” Unless you, as the prospective business founder, learn to get the most mileage out of every dollar, you may go broke and will surely spend more than you need to. But that doesn’t mean that you should try to save money by selling cheap merchandise or providing marginal services. In today’s competitive economy, your customers want the best you can give them at the best price. They will remember the quality of what they get from you long after they have forgotten how much they paid.

    In practical terms, that means you must buy only the best goods for your customers. Anything that affects the image your business has in your customer’s mind should be first-rate. It also means that you shouldn’t spend money on things that don’t affect the customer. For example, unless you’re a real estate broker your customers probably won’t care if you drive an old, beat-up car to an office in a converted broom closet, as long as you provide them an honest product or service for an honest price. Save the nice car, fancy office, and mobile telephone until after your business is a success.

    Self-Evaluation Exercises

    Here’s a question to ponder: Are you the right person for your business? Because running a business is a very demanding endeavor that can take most of your time and energy, your business probably will suffer if you’re unhappy. Your business can become an albatross around your neck if you don’t have the skills and temperament to run it. Simply put, I’ve learned that no business, whether or not it has sound financial backing, is likely to succeed unless you, as the prospective owner, make two decisions correctly:

    • You must honestly evaluate yourself to decide whether you possess the skills and personality needed to succeed in a small business.
    • You must choose the right business.

    A small business is a very personal endeavor. It will honestly reflect your opinions and attitudes, whether or not you design it that way. Think of it this way: The shadow your business casts will be your shadow. If you are sloppy, rude, crafty, or naively trusting, your business will mirror these attributes. If your personal characteristics are more positive than those, your business will be more positive, too. To put this concretely, suppose you go out for the Sunday paper and are met by a newsie who is groggy from a hangover and badmouths his girlfriend in front of you. chances are that next Sunday will find you at a different newsstand.

    I’m not saying you need to be psychologically perfect to run a small business. But to succeed, you must ask people for their money every day and convince a substantial number of them to give it to you. By providing your goods or services, you will create intimate personal relationships with a number of people. It makes no difference whether you refer to people who give you money as clients, customers, patients, members, students, or disciples. It makes a great deal of difference to your chances of ultimate success if you understand that these people are exchanging their money for the conviction that you are giving them their money’s worth.

    The following self-evaluation exercises will help you assess whether you have what it takes to successfully run a small business. Take out a blank sheet of paper or open a computer file.

    Your Strong and Weak Points

    Take a few minutes to list your personal and business strengths and weaknesses. Include everything you can think of, even if it doesn’t appear to be related to your business. For instance, your strong points may include the mastery of a hobby, your positive personality traits, and your sexual charisma, as well as your specific business skills. Take your time and be generous.

    To provide you with a little help, I include a sample list for Antoinette Gorzak, a personal friend who has what she hopes is a good business idea: a slightly different approach to selling women’s clothing. You’ll get to know her better as we go along. Her strengths, weaknesses, fantasies, and fears are surely different from yours. So, too, almost certainly, is the business she wants to start. So be sure to make your own lists—don’t copy Antoinette’s.

    Your list of strong and weak points will help you see any obvious conflicts between your personality and the business you’re in or want to start. For example, if you don’t like being around people but plan to start a life insurance agency with you as the primary salesperson, you may have a personality clash with your business. The solution might be to find another part of the insurance business that doesn’t require as much people contact.

    Unfortunately, many people don’t realize that their personalities will have a direct bearing on their business success. An example close to the experience of folks at nolo involves bookstores. In the years since nolo began publishing, they have seen all sorts of people, from retired librarians to unemployed Ph.D.’s, open bookstores. A large percentage of these stores have failed because the skills needed to run a successful bookstore involve more than a love of books.

    General and Specific Skills Your Business Needs

    Businesses need two kinds of skills to survive and prosper: Skills for business in general and skills specific to the particular business. For example, every business needs someone to keep good financial records. on the other hand, the tender touch and manual dexterity needed by glassblowers are not skills needed by the average paving contractor. Next, take a few minutes and list the skills your business needs. don’t worry about making an exhaustively complete list, just jot down the first things that come to mind. make sure you have some general business skills as well as some of the more important skills specific to your particular business.

    If you don’t have all the skills your business needs, your backers will want to know how you will make up for the deficiency. For example, let’s say you want to start a trucking business. You have a good background in maintenance, truck repair, and long distance driving, and you know how to sell and get work. Sounds good so far—but, let’s say you don’t know the first thing about bookkeeping or cash flow management and the thought of using a computer makes you nervous. Because some trucking businesses work on large dollar volumes, small profit margins, and slow-paying customers, your backers will expect you to learn cash flow management or hire someone qualified to handle that part of the business.

    Your Likes and Dislikes

    Take a few minutes and make a list of the things you really like doing and those you don’t enjoy. Write this list without thinking about the business—simply concentrate on what makes you happy or unhappy.

    If you enjoy talking to new people, keeping books, or working with computers, be sure to include those. Put down all the activities you can think of that give you pleasure. Antoinette’s list is shown as an example.

    As a business owner, you will spend most of your waking hours in the business, and if it doesn’t make you happy, you probably won’t be very good at it. If this list creates doubts about whether you’re pursuing the right business, I suggest you let your unconscious mind work on the problem. most likely, you’ll know the answer after one or two good nights’ sleep.

    Specific Business Goals

    Finally, list your specific business goals. Exactly what do you want your business to accomplish for you? Freedom from 9 to 5? money—and if so, how much? more time with the children? making the world or your little part of it a better place? It’s your wish list, so be specific and enjoy writing it.

    How to Use the Self-evaluation Lists?

    After you’ve completed the four self-evaluation lists, spend some time reading them over. Take a moment to compare the skills needed in your business to the list of skills you have. do you have what it takes?

    Show them to your family and, if you’re brave, to your friends or anyone who knows you well and can be objective. of course, before showing the lists to anyone, you may choose to delete any private information that isn’t critical to your business. If you show your lists to someone who knows the tough realities of running a successful small business, so much the better. You may want to find a former teacher, a fellow employee, or someone else whose judgment you respect.

    What do they think? do they point out any obvious inconsistencies between your personality or skills and what you want to accomplish? If so, pay attention. Treat this exercise seriously and you will know yourself better. oh, and don’t destroy your lists. Assuming you go ahead with your business and write your business plan, the lists can serve as background material or even become part of the final plan.

    You have accomplished several things if you have followed these steps. You have looked inside and asked yourself some basic questions about who you are and what you are realistically qualified to do. As a result, you should now have a better idea of whether you are willing to pay the price required to be successful as a small businessperson. If you are still eager to have a business, you have said, “Yes, I am willing to make short-term sacrifices to achieve long-term benefits and to do whatever is necessary—no matter the inconvenience— to reach my goals.”

    Reality Check: Banker’s Analysis

    Banks and institutions that lend money have a lot of knowledge about the success rate of small businesses. Bankers are often overly cautious in making loans to small businesses. For that very reason it makes sense to study their approach, even though it may seem discouraging at first glance.

    Do You Really Want to your Own a Business - Bankers

    Banker’s Ideal

    Bankers look for an ideal loan applicant, who typically meets these requirements:

    • For an existing business, a cash flow sufficient to make the loan payments.
    • For a new business, an owner who has a track record of profitably owning and operating the same sort of business.
    • An owner with a sound, well-thought-out business plan.
    • An owner with financial reserves and personal collateral sufficient to solve the unexpected problems and fluctuations that affect all businesses.

    Why does such a person need a loan, you ask? He or she probably doesn’t, which, of course, is the point. People who lend money are most comfortable with people so close to their ideal loan candidate that they don’t need to borrow. However, to stay in business themselves, banks and other lenders must lend out the money deposited with them. To do this, they must lend to at least some people whose creditworthiness is less than perfect.

    Measuring Up to the Banker’s Ideal

    Who are these ordinary mortals who slip through bankers’ fine screens of approval? And more to the point, how can you qualify as one of them? Your job is to show how your situation is similar to the banker’s ideal.

    A good bet is the person who has worked for, or preferably managed, a successful business in the same field as the proposed new business. For example, if you have profitably run a clothing store for an absentee owner for a year or two, a lender may believe you are ready to do it on your own. All you need is a good location, a sound business plan, and a little capital. Then, watch out Neiman-Marcus!

    Further away from a lender’s ideal is the person who has sound experience managing one type of business, but proposes to start one in a different field. let’s say you ran the most profitable hot dog stand in the Squaw valley ski resort, and now you want to market computer software in the Silicon Valley of California. In your favor is your experience running a successful business. on the negative side is the fact that computer software marketing has

    no relationship to hot dog selling. In this situation, you might be able to get a loan if you hire people who make up for your lack of experience. At the very least, you would need someone with a strong software marketing background, as well as a person with experience managing retail sales and service businesses. naturally, both of those people are most desirable if they have many years of successful experience in the software marketing business, preferably in California.

    Use the Banker’s Ideal

    It’s helpful to use the bankers’ model in your decision-making process. Use a skeptical attitude as a counterweight to your optimism to get a balanced view of your prospects. What is it that makes you think you will be one of the minority of small business owners who will succeed? If you don’t have some specific answers, you are in trouble. most new businesses fail, and the large majority of survivors do not genuinely prosper.

    Many people start their own business because they can’t stand working for others. They don’t have a choice. They must be either boss or bum. They are more than willing to trade security for the chance to call the shots. They meet a good chunk of their goals when they leave their paycheck behind. This is fine as far as it goes, but in my experience, the more successful small business owners have other goals as well.

    A small distributor we know has a well thought-out business and a sound business plan for the future. Still, he believes that his own personal commitment is the most important thing he has going for him. He puts it this way: “I break my tail to live up to the commitments I make to my customers. If a supplier doesn’t perform for me, I’ll still do everything I can to keep my promise to my customer, even if it costs me money.” This sort of personal commitment enables this successful business owner to make short-term adjustments to meet his long-range goals. And while it would be an exaggeration to say he pays this price gladly, he does pay it.

    Note: This article of “Do You Really Want to your Own a Business?” from Internet and book of How to Write a Business Plan, only for share knowledge with help.