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Glycolysis and the Lactic Acid Pathway 104

I. Glycolysis refers to the conversion of glucose to two molecules of pyruvic acid.

A. In the process, two molecules of ATP are consumed and four molecules of ATP are formed. Thus, there is a net gain of two ATP.

B. In the steps of glycolysis, two pairs of hydrogens are released. Electrons from these hydrogens reduce two molecules of NAD.

II. When respiration is anaerobic, reduced NAD is oxidized by pyruvic acid, which accepts two hydrogen atoms and is thereby reduced to lactic acid.

A. Skeletal muscles use anaerobic respiration and thus produce lactic acid during exercise. Heart muscle respires anaerobically for just a short time, under conditions of ischemia.

B. Lactic acid can be converted to glucose in the liver by a process called gluconeogenesis.

Aerobic Respiration 108

I. The Krebs cycle begins when coenzyme A donates acetic acid to an enzyme that adds it to oxaloacetic acid to form citric acid.

A. Acetyl CoA is formed from pyruvic acid by the removal of carbon dioxide and two hydrogens.

B. The formation of citric acid begins a cyclic pathway that ultimately forms a new molecule of oxaloacetic acid.

C. As the Krebs cycle progresses, one molecule of ATP is formed, and three molecules of NAD and one of FAD are reduced by hydrogens from the Krebs cycle.

II. Reduced NAD and FAD donate their electrons to an electron-transport chain of molecules located in the cristae.

A. The electrons from NAD and FAD are passed from one cytochrome of the electron-transport chain to the next in a series of coupled oxidation-reduction reactions.

B. As each cytochrome ion gains an electron, it becomes reduced; as it passes the electron to the next cytochrome, it becomes oxidized.

C. The last cytochrome becomes oxidized by donating its electron to oxygen, which functions as the final electron acceptor.

D. When one oxygen atom accepts two electrons and two protons, it becomes reduced to form water.

E. The energy provided by electron transport is used to form ATP from ADP and Pi in the process known as oxidative phosphorylation.

III. Thirty to thirty-two molecules of ATP are produced by the aerobic respiration of one glucose molecule. Of these, two are produced in the cytoplasm by glycolysis and the remainder are produced in the mitochondria.

IV. The formation of glycogen from glucose is called glycogenesis; the breakdown of glycogen is called glycogenolysis.

A. Glycogenolysis yields glucose 6-phosphate, which can enter the pathway of glycolysis.

B. The liver contains an enzyme (which skeletal muscles do not) that can produce free glucose from glucose 6-phosphate. Thus, the liver can secrete glucose derived from glycogen.

V. Carbohydrate metabolism is influenced by the availability of oxygen and by a negative feedback effect of ATP on glycolysis and the Krebs cycle.

Metabolism of Lipids and Proteins 114

I. In lipolysis, triglycerides yield glycerol and fatty acids.

A. Glycerol can be converted to phosphoglyceraldehyde and used for energy.

B. In the process of P-oxidation of fatty acids, a number of acetyl CoA molecules are produced.

C. Processes that operate in the reverse direction can convert glucose to triglycerides.

II. Amino acids derived from the hydrolysis of proteins can serve as sources of energy.

A. Through transamination, a particular amino acid and a particular keto acid (pyruvic acid or one of the Krebs cycle acids) can serve as substrates to form a new amino acid and a new keto acid.

B. In oxidative deamination, amino acids are converted into keto acids as their amino group is incorporated into urea.

III. Each organ uses certain blood-borne energy carriers as its preferred energy source.

A. The brain has an almost absolute requirement for blood glucose as its energy source.

B. During exercise, the needs of skeletal muscles for blood glucose can be met by glycogenolysis and by gluconeogenesis in the liver.

122 Chapter Five

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