Respiration is a process in which organic compounds or reduced inorganics (such as hydrogen or ferrous iron) are oxidized by inorganic electron acceptors for the production of energy. Eukaryotes can only use oxygen as a final electron acceptor, in what is called aerobic respiration. Microorganisms can also use nitrate, sulfate, some metals, and even carbon dioxide, in what is generally called anaerobic respiration. In environmental applications the term anoxic respiration is generally used for nitrate reduction, and anaerobic respiration is limited to the other forms.
Strictly speaking, glycolysis is not a part of respiration, although it is the first step leading to respiration for glucose. Nevertheless, when discussing respiration, many scientists and engineers are referring to the overall conversion of glucose or other carbohydrates to carbon dioxide and water:
Respiration occurs in two phases. The first is the Krebs cycle [also called the citric acid cycle or tricarboxylic acid (TCA) cycle]. The Krebs cycle completes the job of oxidizing the carbon that originated with glucose, forming CO2 and ATP. However, much of the energy is left in the reducing power of NADH2 or FADH2. These are converted to ATP in the second phase, called the electron transport system or cytochrome system. The cytochrome system is also responsible for reducing oxygen to water, the other product of the overall reaction for the oxidation of glucose. Both the Krebs cycle and the cyto-chrome system are cyclic because intermediates involved in each reaction are regenerated by other reactions.
Both of these process are mediated by enzymes bound to membranes and require the presence of the membranes in order to function. In eukaryotes this occurs within the mitochondria. Pyruvate must diffuse into the mitochondria to enter the process. Prokaryotes do not have internal membrane structures. Their respiratory enzymes are bound to their cell membrane. The discussion here focuses on eukaryotes, but the process is similar in pro-karyotes. Mitochondria consist of an inner and an outer membrane, forming an inner and an outer compartment. The inner membrane is folded extensively (Figure 5.6). The Krebs cycle occurs within the inner compartment. The cytochrome system is integral to the inner membrane and involves reactants in both compartments.
Glycolysis forms two three-carbon pyruvate molecules for each glucose. When oxygen or another suitable electron acceptor is available, the pyruvate enters into respiration instead of fermentation. Respiration begins when the pyruvate diffuses into the inner compartment of the mitochondria. There, each pyruvate becomes covalently bonded through a sulfhydryl bond with a coenzyme, called coenzyme A CoA, which itself is a derivative of
ADP. The process is an oxidation and requires an NAD. It results in a compound called acetyl-CoA:
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