CONCEPTS IN BIOLOGY

PART II. CORNERSTONES: CHEMISTRY, CELLS, AND METABOLISM

 

6. Biochemical Pathways—Cellular Respiration

 

6.5. Anaerobic Cellular Respiration

Although aerobic cellular respiration is the fundamental process by which most organisms generate ATP, some organisms do not have the necessary enzymes to carry out the Krebs cycle and ETS. Most of these are Bacteria or Archaea, but there are certain eukaryotic organisms, such as yeasts, that can live in the absence of oxygen and do not use their Krebs cycle and ETS. Even within multicellular organisms, there are differences in the metabolic activities of cells. For example some of your cells are able to survive for periods of time without oxygen. However, all cells still need a constant supply of ATP. An organism that does not require O2 as its final electron acceptor is called anaerobic (an = without; aerob = air) and performs anaerobic cellular respiration. Although some anaerobic organisms do not use oxygen, they are capable of using other inorganic or organic molecules as their final electron acceptors. The acceptor molecule might be sulfur, nitrogen, or other inorganic atoms or ions. It might also be an organic molecule, such as pyruvic acid (CH3COCOOH). Anaerobic respiration is an incomplete oxidation and results in the production of smaller electron-containing molecules and energy in the form of ATP and heat (figure 6.10).

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FIGURE 6.10. Anaerobic Cellular Respiration in Perspective

This flowchart shows the relationships among the various types of cellular respiration and the descriptive terminology used. Notice that all begin with a molecular source of energy and end with the generation of ATP.

Many organisms that perform anaerobic cellular respiration use the glycolytic pathway to obtain energy. Fermentation is the word used to describe anaerobic pathways that oxidize glucose to generate ATP by using an organic molecule as the ultimate hydrogen electron acceptor. Electrons removed from sugar in the earlier stages of glycolysis are added to the pyruvic acid formed at the end of glycolysis. Depending on the kind of organism and the specific enzymes it possesses, the pyruvic acid can be converted into lactic acid, ethyl alcohol, acetone, or other organic molecules (figure 6.11).

FIGURE 6.11. Fermentations

The upper portion of this figure is a simplified version of glycolysis. Many organisms can carry out the process of glycolysis and derive energy from it. The ultimate end product is determined by the kinds of enzymes the specific organism can produce. The synthesis of these various molecules is the organism’s way of oxidizing NADH to regenerate NAD+ and reducing pyruvic acid to a new end product.

Organisms that produce ethyl alcohol have genes for the production of enzymes that guide electrons onto pyruvic acid. This reaction results in the conversion of pyruvic acid to ethyl alcohol (ethanol) and carbon dioxide. Other organisms have different genes, produce different enzymes, carry out different reactions, and, therefore, lead to the formation of different end products of fermentation. The formation of molecules such as alcohol and lactic acid is necessary to regenerate the NAD+ needed for continued use in glycolysis. It must be done here, because it is not being regenerated by an ETS, as happens in aerobic respiration. Although many products can be formed from pyruvic acid, we will look at only two fermentation pathways in more detail.

Alcoholic Fermentation

Alcoholic fermentation is the anaerobic respiration pathway that yeast cells follow when oxygen is lacking in their environment. In this pathway, the pyruvic acid (CH3COCOOH) is converted to ethanol (a 2-carbon alcohol, CH3CH2OH) and carbon dioxide. Yeast cells then are able to generate only 4 ATPs from glycolysis. The cost for glycolysis is still 2 ATPs; thus, for each glucose a yeast cell oxidizes, it profits by 2 ATPs.

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Although during alcoholic fermentation yeasts get ATP and discard the waste products ethanol and carbon dioxide, these waste products are useful to humans. In making bread, the carbon dioxide is the important end product; it becomes trapped in the bread dough and makes it rise—the bread is leavened. Dough that has not undergone this process is called unleavened. The alcohol produced by the yeast evaporates during the baking process. In the brewing industry, ethanol is the desirable product produced by yeast cells. Champagne, other sparkling wines, and beer are products that contain both carbon dioxide and alcohol. The alcohol accumulates, and the carbon dioxide in the bottle makes them sparkling (bubbly) beverages. In the manufacture of many wines, the carbon dioxide is allowed to escape, so these wines are not sparkling; they are called “still” wines.

Summary of Alcohol Fermentation

1. Starts with glycolysis

a. Glucose is metabolized to pyruvic acid.

b. A net of 2 ATP is made.

2. During alcoholic fermentation

a. pyruvic acid is reduced to form ethanol.

b. carbon dioxide is released.

3. Yeasts do this in

a. leavened bread.

b. sparkling wine.

Lactic Acid Fermentation

In lactic acid fermentation, the pyruvic acid (CH3COCOOH) that results from glycolysis is converted to lactic acid (CH3CHOHCOOH) by the transfer of electrons that had been removed from the original glucose. In this case, the net profit is again only 2 ATPs per glucose. The buildup of the waste product, lactic acid, eventually interferes with normal metabolic functions and the bacteria die. The lactic acid waste product from these types of anaerobic bacteria are used to make yogurt, cultured sour cream, cheeses, and other fermented dairy products. The lactic acid makes the milk protein coagulate and become pudding-like or solid. It also gives the products their tart flavor, texture, and aroma (Outlooks 6.2).

In the human body, different cells have different metabolic capabilities. Nerve cells must have a constant supply of oxygen to conduct aerobic cellular respiration. Red blood cells lack mitochondria and must rely on the anaerobic process of lactic acid fermentation to provide themselves with energy. Muscle cells can do either. As long as oxygen is available to skeletal muscle cells, they function aerobically. However, when oxygen is unavailable—because of long periods of exercise or heart or lung problems that prevent oxygen from getting to the skeletal muscle cells—the cells make a valiant effort to meet energy demands by functioning anaerobically.

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When skeletal muscle cells function anaerobically, they accumulate lactic acid. This lactic acid must ultimately be metabolized, which requires oxygen. Therefore, the accumulation of lactic acid represents an oxygen debt, which must be repaid in the future. It is the lactic acid buildup that makes muscles tired when we exercise. When the lactic acid concentration becomes great enough, lactic acid fatigue results. As a person cools down after a period of exercise, breathing and heart rate stay high until the oxygen debt is repaid and the level of oxygen in the muscle cells returns to normal. During this period, the lactic acid that has accumulated is converted back into pyruvic acid. The pyruvic acid can then continue through the Krebs cycle and the ETS as oxygen becomes available. In addition to what is happening in the muscles, much of the lactic acid is transported by the bloodstream to the liver, where about 20% is metabolized through the Krebs cycle and 80% is resynthesized into glucose.

Summary of Lactic Acid Fermentation

1. Starts with glycolysis

a. Glucose is metabolized to pyruvic acid.

b. A net of 2 ATP is made.

2. During lactic acid fermentation

a. pyruvic acid is reduced to form lactic acid.

b. no carbon dioxide is released.

3. Muscle cells have the enzymes to do this, but brain cells do not.

a. Muscle cells can survive brief periods of oxygen deprivation, but brain cells cannot.

b. Lactic acid “burns” in muscles.

OUTLOOKS 6.2

Souring vs. Spoilage

The fermentation of carbohydrates to organic acid products, such as lactic acid, is commonly called souring. Cultured sour cream, cheese, and yogurt are produced by the action of fermenting bacteria. Lactic-acid bacteria of the genus Lactobacillus are used in the fermentation process. While growing in the milk, the bacteria convert lactose to lactic acid, which causes the proteins in the milk to coagulate and come out of solution to form a solid curd. The higher acid level also inhibits the growth of spoilage microorganisms. Spoilage, or putrefaction, is the anaerobic respiration of proteins with the release of nitrogen and sulfur-containing organic compounds as products. Protein fermentation by the bacterium Clostridium produces foul-smelling chemicals such as putrescine, cadaverine, hydrogen sulfide, and methyl mercaptan. Clostridium perfringens and C. sporogenes are the two anaerobic bacteria associated with the disease gas gangrene. A gangrenous wound is a foul-smelling infection resulting from the fermentation activities of those two bacteria.

6.5. CONCEPT REVIEW

9. Why are there different end products from different forms of fermentation?