Lactic Acid Fermentation

Glycolysis

Pyruvic acid

NAD+

Lactic acid

Pyruvic acid

Lactic Acid Fermentation figure 7-5

In cheese making, fungi or bacteria are added to large vats of milk. The microorganisms carry out lactic acid fermentation, converting some of the sugar in the milk to lactic acid.

In cheese making, fungi or bacteria are added to large vats of milk. The microorganisms carry out lactic acid fermentation, converting some of the sugar in the milk to lactic acid.

In lactic acid fermentation, an enzyme converts pyruvic acid made during glycolysis into another three-carbon compound, called lactic acid. As Figure 7-4 shows, lactic acid fermentation involves the transfer of one hydrogen atom from NADH and the addition of one free proton (H+) to pyruvic acid. In the process, NADH is oxidized to form NAD+. The resulting NAD+ is used in glycolysis, where it is again reduced to NADH. Thus, the regeneration of NAD+ in lactic acid fermentation helps to keep glycolysis operating.

Lactic acid fermentation by microorganisms plays an essential role in the manufacture of many dairy products, as illustrated in Figure 7-5. Milk will ferment naturally if not refrigerated properly or consumed in a timely manner. Such fermentation of milk is considered "spoiling." But ever since scientists discovered the microorganisms that cause this process, fermentation has been used in a controlled manner to produce cheese, buttermilk, yogurt, sour cream, and other cultured dairy products. Only harmless, active microorganisms are used in the fermentation of dairy products.

Lactic acid fermentation also occurs in your muscle cells during very strenuous exercise, such as sprinting. During this kind of exercise, muscle cells use up oxygen more rapidly than it can be delivered to them. As oxygen becomes depleted, the muscle cells begin to switch from cellular respiration to lactic acid fermentation. Lactic acid accumulates in the muscle cells, making the cells' cytosol more acidic. The increased acidity may reduce the capacity of the cells to contract, resulting in muscle fatigue, pain, and even cramps. Eventually, the lactic acid diffuses into the blood and is transported to the liver, where it can be converted back into pyruvic acid.

Glucose

Ethyl alcohol

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