Figure 419

Glycolytic pathway. Under anaerobic conditions, there is a net synthesis of two molecules of ATP for every molecule of glucose that enters the pathway. Note that at the pH existing in the body, the products produced by the various glycolytic steps exist in the ionized, anionic form (pyruvate, for example). They are actually produced as acids (pyruvic acid, for example) that then ionize.

reactions produce a net gain of two molecules of ATP and four atoms of hydrogen, two of which are transferred to NAD+ and two are released as hydrogen ions:

These 10 reactions, none of which utilizes molecular oxygen, take place in the cytosol. Note (Figure 4-19) that all the intermediates between glucose and the end product pyruvate contain one or more ionized phosphate groups. As we shall learn in Chapter 6, plasma membranes are impermeable to such highly ionized

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

PART ONE Basic Cell Functions molecules, and thus these molecules remain trapped within the cell.

Note that the early steps in glycolysis (reactions 1 and 3) each use, rather than produce, one molecule of ATP, to form phosphorylated intermediates. In addition, note that reaction 4 splits a six-carbon intermediate into two three-carbon molecules, and reaction 5 converts one of these three-carbon molecules into the other so that at the end of reaction 5 we have two molecules of 3-phosphoglyceraldehyde derived from one molecule of glucose. Keep in mind, then, that from this point on, two molecules of each intermediate are involved.

The first formation of ATP in glycolysis occurs during reaction 7 when a phosphate group is transferred to ADP to form ATP. Since, as stressed above, two intermediates exist at this point, reaction 7 produces two molecules of ATP, one from each of them. In this reaction, the mechanism of forming ATP is known as substrate-level phosphorylation since the phosphate group is transferred from a substrate molecule to ADP. As we shall see, this mechanism is quite different from that used during oxidative phosphorylation, in which free inorganic phosphate is coupled to ADP to form ATP.

A similar substrate-level phosphorylation of ADP occurs during reaction 10, where again two molecules of ATP are formed. Thus, reactions 7 and 10 generate a total of four molecules of ATP for every molecule of glucose entering the pathway. There is a net gain, however, of only two molecules of ATP during glycolysis because two molecules of ATP were used in reactions 1 and 3.

The end product of glycolysis, pyruvate, can proceed in one of two directions, depending on the availability of molecular oxygen, which, as we stressed earlier, is not utilized in any of the glycolytic reactions themselves. If oxygen is present—that is, if aerobic conditions exist—pyruvate can enter the Krebs cycle and be broken down into carbon dioxide, as described in the next section. In contrast, in the absence of oxygen (anaerobic conditions), pyruvate is converted to lac-tate (the ionized form of lactic acid) by a single enzymemediated reaction. In this reaction (Figure 4-20) two hydrogen atoms derived from NADH + H+ are transferred to each molecule of pyruvate to form lactate, and NAD+ is regenerated. These hydrogens had originally been transferred to NAD+ during reaction 6 of glycolysis, so the coenzyme NAD+ shuttles hydrogen between the two reactions during anaerobic glycoly-sis. The overall reaction for anaerobic glycolysis is

Reaction 6

As stated in the previous paragraph, under aerobic conditions pyruvate is not converted to lactate but rather enters the Krebs cycle. Therefore, the mechanism

Reaction 6

Glucose

COO-Pyruvate

^(aerobic)

Glucose

(anaerobic)

COO-Pyruvate

^(aerobic)

COO-Lactate

Krebs cycle

Essentials of Human Physiology

Essentials of Human Physiology

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