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Lactic Acid Pathway

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In order for glycolysis to continue, there must be adequate amounts of NAD available to accept hydrogen atoms. Therefore, the NADH produced in glycolysis must become oxidized by donating its electrons to another molecule. (In aerobic respiration this other molecule is located in the mitochondria and ultimately passes its electrons to oxygen.)

When oxygen is not available in sufficient amounts, the NADH (+ H+) produced in glycolysis is oxidized in the cytoplasm by donating its electrons to pyruvic acid. This results in the re-formation of NAD and the addition of two hydrogen atoms to pyruvic acid, which is thus reduced. This addition of two hydrogen atoms to pyruvic acid produces lactic acid (fig. 5.3).

The metabolic pathway by which glucose is converted to lactic acid is frequently referred to by physiologists as

Fox: Human Physiology, Eighth Edition

5. Cell Respiration and Metabolism

Text

© The McGraw-H Companies, 2003

Chapter Five

Glucose (C6H12O6)

Glucose 6-phosphate

Fructose 6-phosphate

. Fructose 1,6-biphosphate

Dihydroxy-- acetone phosphate À

3-Phosphoglyceraldehyde

3-Phosphoglyceraldehyde

NADH

1,3-Biphosphoglyceric acid A

NADH

1,3-Biphosphoglyceric acid À

3-Phosphoglyceric acid

2-Phosphoglyceric acid

Phosphoenolpyruvic acid

3-Phosphoglyceric acid

2-Phosphoglyceric acid

Phosphoenolpyruvic acid

Phosphoenolpyruvic acid

ADP	k	ADP	A
ATP	®	ATP	®
Pyruvic acid (C3H4O3)		Pyruvic acid (C3H4O3)

â–  Figure 5.2 Glycolysis. In glycolysis, one glucose is converted into two pyruvic acids in nine separate steps. In addition to two pyruvic acids, the products of glycolysis include two NADH and four ATP. Since two ATP were used at the beginning, however, the net gain is two ATP per glucose. Dashed arrows indicate reverse reactions that may occur under other conditions.

Chapter Five

â–  Figure 5.3 The formation of lactic acid. The addition of two hydrogen atoms (colored boxes) from reduced NAD to pyruvic acid produces lactic acid and oxidized NAD. This reaction is catalyzed by lactic acid dehydrogenase (LDH) and is reversible under the proper conditions.

anaerobic respiration. "Anaerobic" describes the fact that oxygen is not used in the process. This is the term that will be used in this text for the pathway leading to lactic acid production. Many biologists, however, prefer the name lactic acid fermentation for this pathway. This is because the lactic acid pathway is basically similar to the way yeast cells convert glucose into ethyl alcohol, a process universally known as fermentation. In both lactic acid and alcohol production, the last electron acceptor is an organic molecule (as opposed to an atom of oxygen, as will be described for aerobic respiration).

The lactic acid pathway yields a net gain of two ATP molecules (produced by glycolysis) per glucose molecule. A cell can survive without oxygen as long as it can produce sufficient energy for its needs in this way and as long as lactic acid concentrations do not become excessive. Some tissues are better adapted to anaerobic conditions than others—skeletal muscles survive longer than cardiac muscle, which in turn survives under anaerobic conditions longer than the brain.

Red blood cells, which lack mitochondria, can use only the lactic acid pathway; therefore (for reasons described in the next section), they cannot use oxygen. This spares the oxygen they carry for delivery to other cells. Except for red blood cells, anaerobic respiration occurs for only a limited period of time in tissues that have energy requirements in excess of their aerobic ability. Anaerobic respiration occurs in the skeletal muscles and heart when the ratio of oxygen supply to oxygen need (related to the concentration of NADH) falls below a critical level. Anaerobic respiration is, in a sense, an emergency procedure that provides some ATP until the emergency (oxygen deficiency) has passed.

It should be noted, though, that there is no real "emergency" in the case of skeletal muscles, where anaerobic respiration is a normal, daily occurrence that does not harm muscle tissue or the individual. Excessive lactic acid production by muscles, however, is associated with pain and muscle fatigue. (The metabolism of skeletal muscles is discussed in chapter 12.) In contrast to skeletal muscles, the heart normally respires only aerobically. If anaerobic conditions do occur in the heart, a potentially dangerous situation may be present.

Fox: Human Physiology, I 5. Cell Respiration and I Text I © The McGraw-Hill

Eighth Edition Metabolism Companies, 2003

Cell Respiration and Metabolism

Ischemia refers to inadequate blood flow to an organ, such that the rate of oxygen delivery is insufficient to maintain aerobic respiration. Inadequate blood flow to the heart, or myorcardial ischemia, may occur if the coronary blood flow is occluded by atherosclerosis, a blood clot, or by an artery spasm. People with myocardial ischemia often experience angina pectoris—severe pain in the chest and left (or sometimes, right) arm area. This pain is associated with increased blood levels of lactic acid which are produced by the is-chemic heart muscle. If the ischemia is prolonged, the cells may die and produce an area called an infarct. The degree of ischemia and angina can be decreased by vasodilator drugs such as nitroglycerin, which improve blood flow to the heart and also decrease the work of the heart by dilating peripheral blood vessels.

Clinical Investigation Clues

Remember that Brenda experienced muscle pain and fatigue during her training, and that she had an episode where she experienced severe pain in her left pectoral region following an intense workout.

What produced her muscle pain and fatigue?

What might have caused the severe pain in her left pectoral region?

Which of these effects are normal?

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