*At this point in the cycle, the hydrogen carrier is FAD (flavine adenine dinucleotide).

Naczynia Krwiono

Figure D.2

Chemical reactions of the citric acid cycle. NADH molecules carrying hydrogens are highlighted.

Figure 4.12, shows that NADH can release the electrons and hydrogen nucleus. Since this reaction removes electrons, the resulting NAD+ is said to be oxidized. Oxidation results from the removal of electrons, often as part of hydrogen atoms; it is the opposite of reduction. The two electrons this reaction releases pass to a series of electron carriers. The regenerated NAD+ can once again accept electrons, and is recycled.

The molecules that act as electron carriers comprise an electron transport chain described in

Chapter 4 (pp. 118—119). As electrons are passed from one carrier to another, the carriers are alternately reduced and oxidized as they accept or release electrons. The transported electrons gradually lose energy as they proceed down the chain.

Among the members of the electron transport chain are several proteins, including a set of iron-containing molecules called cytochromes. The chain is located in the inner membranes of the mitochondria (see chapter 3, p. 77). The folds of the inner mitochondrial membrane provide surface area

Shier-Butler-Lewis: I Back Matter I Appendix D: A Closer Look I I © The McGraw-Hill

Human Anatomy and at Cellular Companies, 2001

Physiology, Ninth Edition Respiration/Nucleic Acids on which the energy reactions take place. In a muscle cell, the inner mitochondrial membrane, if stretched out, may be as much as forty-five times as long as the cell membrane!

The final cytochrome of the electron transport chain (cytochrome oxidase) gives up a pair of electrons and causes two hydrogen ions (formed at the beginning of the sequence) to combine with an atom of oxygen. This process produces a water molecule:

Thus, oxygen is the final electron acceptor. In the absence of oxygen, electrons cannot pass through the electron transport chain, NAD+ cannot be regenerated, and aerobic respiration halts.

Note in figure 4.12 that as electrons pass through the electron transport chain, energy is released. Some of this energy is used by a mechanism involving the enzyme complex ATP synthase to combine phosphate and ADP by a high-energy bond (phosphorylation), forming ATP. Also note in fig ures D.1 and D.2 that twelve pairs of hydrogen atoms are released during the complete breakdown of one glucose molecule—two pairs from glycolysis, two pairs from the conversion of pyruvic acid to acetyl coenzyme A (one pair from each of two pyru-vic acid molecules), and eight pairs from the citric acid cycle (four pairs for each of two acetyl coen-zyme A molecules).

High-energy electrons from ten pairs of these hydrogen atoms produce thirty ATP molecules in the electron transport chain. Two pairs enter the chain differently and form four ATP molecules. Because this process of forming ATP involves both the oxidation of hydrogen atoms and the bonding of phosphate to ADP, it is called oxidative phos-phorylation. Also, there is a net gain of two ATP molecules during glycolysis, and two ATP molecules form by direct enzyme action in two turns of the citric acid cycle. Thus, a maximum of thirty-eight ATP molecules form for each glucose molecule metabolized.

Shier-Butler-Lewis: I Back Matter I Appendix D: A Closer Look I I © The McGraw-Hill

Human Anatomy and at Cellular Companies, 2001

Physiology, Ninth Edition Respiration/Nucleic Acids

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