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A biochemical process whereby specific substrates are oxidized with a subsequent release of carbon dioxide, CO2. There is usually conservation of energy accompanying the oxidation which is coupled to the synthesis of energy-rich compounds, such as adenosine triphosphate (ATP), whose free energy is then used to drive otherwise unfavorable reactions that are essential for physiological processes such as growth. Respiration is carried out by specific proteins, called enzymes, and it is necessary for the synthesis of essential metabolites, including carbohydrates, amino acids, and fatty acids, and for the transport of minerals and other solutes between cells. Thus respiration is an essential characteristic of life itself in plants as well as in other organisms.
Overall aerobic respiration is the end result of a sequence of many biochemical reactions that ultimately lead to O2 uptake and CO2 evolution. In the absence of O2, as may occur in bulky plant tissues such as the potato tuber and carrot root and in submerged plants such as germinating rice seedlings, the breakdown of hexose does not go to completion. The end products are either lactic acid or ethanol, which are produced by anaerobic glycolysis or fermentation.
The sequence of reactions of anaerobic glycolysis or fermentation is shown in the illustration. The enzymes associated with anerobic glycolysis have been isolated from many plant tissues, but more often ethanol and not lactic acid is the final product.
In aerobic tissues, pyruvic acid produced during glycolysis is completely oxidized with the accompanying synthesis of much more ATP than in anaerobic glycolysis. Pyruvic acid oxidation takes place in the mitochondria by means of a cyclic sequence of reactions, the Krebs cycle (also known as the citric acid cycle) which begins when the first product of pyruvate oxidation, acetyl coenzyme A, reacts with oxaloacetic acid to produce citric acid. Oxaloacetic acid is eventually regenerated. Thus the cycle can be repeated. In terms of conservation of chemical energy, the Krebs cycle is about 12 times more efficient than anaerobic glycolysis per mole of glucose oxidized. See Citric acid cycle
In addition to anaerobic glycolysis and the Krebs cycle, there are two other sequences of biochemical reactions related to respiration that are important in plant tissues: (1) The pentose phosphate pathway permits an alternate mechanism for converting hexose phosphate to pyruvate, and (2) in germinating fatty seeds the reactions of the Krebs cycle are modified so that acetyl coenzyme A is converted to succinic acid and then to hexose by a pathway called the glyoxylate cycle. See Photorespiration, Photosynthesis, Plant growth, Plant metabolism