Atp

Energy

Energy

Electrical work Mechanical work Molecular activation Tissue building Other work

Carbon dioxide

Water

Heat

Electrical work Mechanical work Molecular activation Tissue building Other work

Figure 4. The role of ATP in body energy metabolism.

mol of glucose to 2 mol of pyruvic acid generates four hydrogen atoms and 8 mol of ATP. The hydrogen atoms released are eventually converted to water; the details are discussed below. Before the pyruvic acid can be converted to carbon dioxide, water, and energy, it must be transformed into a highly versatile metabolite, the two-carbon substance acetyl-CoA (Fig. 6). This transformation is irreversible.

Citric Acid Cycle. The citric acid cycle is a series of chemical reactions that metabolizes acetyl-CoA to carbon dioxide and hydrogen atoms, as indicated in Figures 7 and 8. In each cycle, two carbon dioxide molecules and four pairs of hydrogen atoms are put through the respiratory chain, together with the electrons, to generate 12 mol of

ATP and 4 mol of water from oxygen. This cycle is the major link, or common path, in the transformation of carbohydrate, fat, and protein to carbon dioxide and water. Metabolites of the three nutrients enter the cycle at different strategic points. Because the cycle requires the respiratory chain to complete its work, it will not function in the absence of oxygen (anaerobically).

As indicated above, pyruvic acid may be removed from the glycolysis process by being converted to lactic acid. If so, there must be a source of hydrogen atoms, which are normally obtained from the production of phosphoglycer-aldhyde (Fig. 6). In this case, glucose metabolism and energy (ATP) production can continue for a while without oxygen (that is, without going through the citric acid cycle). This anaerobic respiration occurs in muscle where an oc-

Table 4. Definitions of Some Metabolic Terms

Term

Glycolysis

Glycogenesis Glycogenolysis

Gluconeogenesis

Citric acid cycle

Respiratory chain

Definition

The breaking down of hexoses (six carbon sugars), mainly glucose, into three-carbon substances (pyruvic or lactic acid); the process is sometimes termed the Embden-Meyerhof pathway The formation of glycogen from glucose The breaking down of glycogen into glucose and its metabolites The synthesis of glucose (and thus glycogen) from noncarbohydrate sources, such as lactate, glycerol, and amino acids Also termed the Krebs Cycle or tricarboxylic acid cycle; the process whereby carbohydrate, fat, and protein is completely oxidized to carbon dioxide, water, and energy; accomplished with the assistance of the respiratory chain The transport of hydrogen atoms from biological oxidation for acceptance by oxygen atoms to form water molecules

One mole of glucose (six carbons)

Figure 5. The overall result of glycolysis.

One mole of pyruvic acid (three carbons)

One mole of pyruvic acid (three carbons)

Figure 5. The overall result of glycolysis.

casional burst of energy is needed. The lactic acid that accumulates is converted back to pyruvic acid when the oxygen supply is restored, in which case the citric acid cycle is reactivated. The soreness of muscle from heavy work or exercise results from the presence of a large amount of lactic acid.

Respiratory Chain. Biological oxidation, or the respiratory chain, is a very complicated process whereby hydrogen atoms released by substances through oxidation are transported by a number of intermediates until the hydrogen atoms are accepted by oxygen to produce water. The respiratory chain involves both oxidation and reduction. Ox-

Glucose 4 I ATP--ADP

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