Figure 420

Under anaerobic conditions, the coenzyme NAD+ utilized in the glycolytic reaction 6 (see Figure 4-19) is regenerated when it transfers its hydrogen atoms to pyruvate during the formation of lactate.

just described for regenerating NAD+ from NADH + H+ by forming lactate does not occur. (Compare Equations 4-1 and 4-2.) Instead, as we shall see, H+ and the hydrogens of NADH are transferred to oxygen during oxidative phosphorylation, regenerating NAD+ and producing H2O.

In most cells, the amount of ATP produced by gly-colysis from one molecule of glucose is much smaller than the amount formed under aerobic conditions by the other two ATP-generating pathways—the Krebs cycle and oxidative phosphorylation. There are special cases, however, in which glycolysis supplies most, or even all, of a cell's ATP. For example, erythrocytes contain the enzymes for glycolysis but have no mitochondria, which, as we have said, are required for the other pathways. All of their ATP production occurs, therefore, by glycolysis. Also, certain types of skeletal muscles contain considerable amounts of glycolytic enzymes but have few mitochondria. During intense muscle activity, glycolysis provides most of the ATP in these cells and is associated with the production of large amounts of lactate. Despite these exceptions, most cells do not have sufficient concentrations of gly-colytic enzymes or enough glucose to provide, by gly-colysis alone, the high rates of ATP production necessary to meet their energy requirements and thus are unable to function for long under anaerobic conditions.

Our discussion of glycolysis has focused upon glucose as the major carbohydrate entering the glycolytic pathway. However, other carbohydrates such as fructose, derived from the disaccharide sucrose (table sugar), and galactose, from the disaccharide lactose

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Protein Activity and Cellular Metabolism CHAPTER FOUR

Protein Activity and Cellular Metabolism CHAPTER FOUR

TABLE 4-5 Characteristics of Glycolysis

Entering substrates

Glucose and other monosaccharides

Enzyme location

Cytosol

Net ATP production

2 ATP formed directly per molecule of glucose entering pathway Can be produced in the absence of oxygen (anaerobically)

Coenzyme production

2 NADH + 2 H+ formed under aerobic conditions

Final products

Pyruvate—under aerobic conditions Lactate—under anaerobic conditions

Net reaction

Aerobic:

2 pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H2O

Anaerobic:

Glucose + 2 ADP + 2 Pi--» 2 lactate + 2 ATP + 2 H2O

(milk sugar), can also be catabolized by glycolysis since these carbohydrates are converted into several of the intermediates that participate in the early portion of the glycolytic pathway.

In some microoganisms (yeast cells, for example), pyruvate is converted under anaerobic conditions to carbon dioxide and alcohol (CH3CH2OH) rather than to lactate. This process is known as fermentation and forms the basis for the production of alcohol from cereal grains rich in carbohydrates.

Table 4-5 summarizes the major characteristics of glycolysis.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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