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 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.
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).
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.
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.
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.
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 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.
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