Cellular respiration includes all the processes by which a cell uses food molecules such as glucose to produce energy in the form of adenosine triphosphate (ATP) for use by all the energy-consuming activities carried out by the cell (Kimball, 2004). This occurs in the cytoplasm of all eukaryotic cells. Cells which respire in the presence of oxygen do so aerobically whereas those which do it in the absence of oxygen are said to respire anaerobically. Many aerobic cells are also capable of anaerobic respiration when temporarily deprived of oxygen, such as the human fetus just prior to delivery (Devlin, 1997, 269).
The Krebs cycle, also known as the tricarboxylic acid cycle, or the citric acid cycle, takes place mainly in the mitochondria (Devlin, 1997, 231; Kimball, 2004). It takes place when enough oxygen is present to support aerobic respiration after glycolysis within the mitochondrion (Anderson, 1999). A primary function of the Krebs cycle is to produce reducing equivalents which can be utilized to generate energy, ATP, in the electron transport-oxidative phosphorylation sequence which takes place exclusively in the mitochondria (Devlin, 1997, 231; Kimball, 2004). The inner membrane of mitochondria contains five integral membrane proteins: NADH dehydrogenase, succinate dehydrogenase, cytochrome c reductase (also known as the cytochrome b-c1 complex), cytochrome c oxidase, and ATP synthase. The soluble matrix contains a mixture of enzymes which catalyze the respiration of pyruvi9c acid and other small organic molecules. In the Kreb's cycle, each of the three carbon atoms present in pyruvate generated by glycolysis leaves the mitochondrion as a molecule of carbon dioxide. A pair of electrons are removed in the process and transferred to NAD+ to form NADH + H+, and a pair of electrons are removed from FAD to give FADH2. These electrons are transferred to the respiratory chain.
In a sequence of reactions in the Kreb...