7.6Energy Production System
The energy production system, one of a number of basic metabolic systems (see 7.4), is discussed here in more detail.
A metabolic system consisting of the degradation (glycolysis) and synthesis (gluconeogenesis) of sugars is shown in the middle part of Figure. 7-3. Glucose, which consists of six carbons (C6), is activated by phosphorylation to become fructose 1,6-bisphosphate (FBP), which is then degraded by aldolase to two C3 (i.e., three carbons) compounds. The compounds are oxidized by NAD+ to generate pyruvic acid. In aerobic respiration, acetyl-CoA is also generated by oxidization along with decarboxylation by NAD+. Under anaerobic conditions, pyruvic acid is used for ethanol production by fermentation as well as for the production of lactic acid. Glycolysis is also used as a carbohydrate synthesis pathway (gluconeogenesis) by replacing some irreversible reactions with other enzymatic reactions (Fig. 7-4). Glycolysis, which converts glucose to pyruvic acid, occurs in the cytoplasm. During this process, two ATP molecules are used for the activation of carbohydrates, and four ATP molecules are obtained later in the process. The NADH generated can also be used for ATP synthesis under aerobic conditions.
Citric Acid Cycle (Tricarboxylic acid)
The citric acid cycle is shown in the lower-middle part of Figure. 7-3. This cycle starts by generating citric acid (C6) through the condensation of acetyl-CoA (the acetyl group is C2) with oxaloacetic acid, and CoA is recycled. This is essentially a reaction that has two decarboxylation processes in the early stage and recycles oxaloacetic acid in the later stage. The carbon dioxide we exhale is generated by pyruvic acid dehydratase through these two decarboxylation stages. NADH and FADH2 generated during these processes reduce the electron transport chain (discussed in Chapter 8), are subsequently oxidized by oxygen (oxidative phosphorylation), and are used for the production of large amounts of ATP (34 molecules per glucose molecule). In other words, at this stage, glucose as a nutritional carbon compound has been eliminated, whereas most of the free energy it had is stored in the form of these reducing substances.