Figure 10.14 A summary of respiration. In the first phase, glycolysis, which takes place in the cytoplasm, a sugar molecule is converted to two pyruvic acid molecules. The subsequent phases take place within the matrix of a mitochondrion. In aerobic respiration, the pyruvic acid is broken down in the citric acid cycle, and energy is transferred to compounds such as NADH, ATP, and FADH2. Carbon dioxide and hydrogen are also released. The released hydrogen is carried by an electron transport system and combined with oxygen, forming water. In anaerobic respiration and fermentation, pyruvic acid is converted in the absence of oxygen gas to ethyl alcohol or lactic acid with little release of energy. Inset from Sylvia S. Mader, Biology, 7th edition. © 2001 The McGraw-Hill Companies. All rights reserved.
are picked up and temporarily held by an acceptor molecule, NAD (nicotinamide adenine dinucleotide). What happens to them next depends on the kind of respiration involved: aerobic respiration, true anaerobic respiration, or fermentation.
In aerobic respiration (the most common type of respiration), glycolysis is followed by two major stages: the citric acid cycle and electron transport. Both stages occur in the mitochondria and involve many smaller steps, each of which is controlled by enzymes (see Fig. 10.14).
The Citric Acid (Krebs) Cycle The citric acid cycle was originally named the Krebs cycle after Hans Krebs, a British biochemist who received a Nobel Prize in 1953 for his unraveling of many of the complex reactions that take place in respiration. The name citric acid cycle, or tricarboxylic acid (TCA) cycle, reflects the important role played by several organic acids during the process.
Before entering the citric acid cycle, which takes place in the fluid matrix located within the compartments formed by the cristae of mitochondria (see Fig. 3.14), carbon dioxide is released from pyruvic acid that was produced by glycoly-sis. What remains is restructured to a 2-carbon acetyl group. This acetyl group combines with an acceptor molecule called coenzyme A (CoA). This combination (called acetyl CoA) then enters the citric acid cycle, which is a series of biochemical reactions that are catalyzed by enzymes. Little of the energy originally trapped in the glucose molecule is released during glycolysis. As the citric acid cycle proceeds, however, high-energy electrons and hydrogen are successively removed. This removal takes place from a series of organic acids and, after transfer, ultimately produces compounds such as NADH (reduced nicotinamide adenine dinucleotide) and FADH2 (reduced flavin adenine dinucleotide), as well as a small amount of ATP. Carbon dioxide is produced as a byproduct while the cycle is proceeding.
Electron Transport Much of the energy originally in the glucose molecule now has been transferred to the acceptors NAD and FAD, which became NADH and FADH2, respectively. NADH and FADH2 are electron donors to an electron transport system consisting of special acceptor molecules arranged in a precise sequence on the inner membrane of mitochondria. The electrons flow through the system down an energy gradient. Energy is extracted from the high-energy electrons to pump protons across the inner mitochon-drial membrane, creating an electrochemical gradient. With the aid of an enzyme, the gradient can then be harvested to produce ATP. The production of ATP stops if there are no electron donors or electron acceptor oxygen.
The acceptor molecules include iron-containing proteins called cytochromes. Energy is released in small increments at each step along the system, and ATP is produced from ADP and P. As the final step in aerobic respiration (see Fig. 10.14), oxygen acts as the ultimate electron acceptor, producing water as it combines with hydrogen.
By the time the process is complete, the recoverable energy locked in a molecule of glucose has been released and is stored in ATP molecules. This stored energy is then available for use in the synthesis of other molecules and for growth, active transport, and a host of other metabolic processes. Aerobic respiration produces a net gain of 36 ATP molecules from one glucose molecule, using up six molecules of oxygen and producing six molecules of carbon dioxide and a net total of six molecules of water. For each mole (180 grams) of glucose aerobically respired, 686 Kcal of energy is released, with about 39% of it being stored in ATP molecules and the remainder being released as heat.
In living organisms, glucose molecules often may undergo glycolysis without enough oxygen being available to complete aerobic respiration. In such cases, the hydrogen released during glycolysis is simply transferred from the hydrogen acceptor molecules back to the pyruvic acid after it has been formed, creating ethyl alcohol in some organisms, and lactic acid or similar substances in others. A little energy is released during either fermentation or true anaerobic respiration, but most of it remains locked up in the alcohol, lactic acid, or other compounds produced.
In true anaerobic respiration, the hydrogen removed from the glucose molecule during glycolysis is combined with an inorganic ion, as, for example, when sulfur bacteria (discussed in Chapter 17) convert sulfate (SO4) to sulfur (S) or another sulfur compound or when certain cellulose bacteria produce methane gas (CH4) by combining the hydrogen with carbon dioxide.
Oxygen gas is not required to make these compounds, but few organisms can live long without oxygen, and many that carry on fermentation can also respire aerobically. If oxygen becomes available, the remaining energy can be released by further breakdown of these compounds. About 7% of the total energy in a glucose molecule is removed during anaerobic respiration or fermentation. So much of that energy goes into the making of the alcohol or the lactic acid or is dissipated as heat that there is a net gain of only two ATP molecules (compared with 36 ATP molecules produced in aerobic respiration). The forms of anaerobic respiration are adaptive to the organisms that have them in that they recycle NAD and allow glycolysis to continue.
Living cells can tolerate only certain concentrations of alcohol. In media in which yeasts are fermenting sugars, for example, once the alcohol concentration builds up beyond 12%, the cells die and fermentation ceases. This is why most wines have an alcohol concentration of about 12% (24 proof).
Many bacteria carry on both fermentation and true anaerobic respiration simultaneously, making it difficult to distinguish between the two processes. Some texts use the terms anaerobic respiration and fermentation interchangeably to designate respiration occurring in the presence of little or no oxygen gas.
Plant Metabolism 189
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