BIOLOGY Volume 1 - A Guide to General Biology - 2004

9. ENERGY UTILIZATION

9.3. Cellular Respiration

9.3.6. Anaerobic Respiration

Anaerobic Respiration is frequently referred to as Fermentation. Many microorganisms derive the bulk of their ATP through anaerobic respiration. For certain Bacteria, oxygen—even in the normal amounts present in the atmosphere—is outright toxic, forcing them to inhabit oxygen-free environments. Such organisms are termed obligate anaerobes (an example being Clostridium tetani, the CAUSATIVE AGENT OF tetanus).

Other organisms are also known, such as Yeasts and intestinal parasites (tapeworms, among others), which can survive both in the absence and presence of oxygen. They are called facultative anaerobes: when necessary, they switch to anaerobic respiration, yet utilize the aerobic pathway when oxygen is available. Certain Cells temporarily experiencing an oxygen deficit (Skeletal Muscle cells, for instance) also possess the capacity for anaerobic respiration.

Glycolysis serves as the initial phase of anaerobic respiration as well. For each glucose molecule, it yields two molecules of pyruvic acid, two molecules of ATP, and two molecules of reduced NAD (see Table 9.1). During aerobic respiration, the hydrogen attached to NAD is ultimately transferred to oxygen and oxidized to Water following a series of energy-releasing reactions. This proves impossible in anaerobic respiration due to the absence of oxygen. Instead, hydrogen is reattached to pyruvic acid, meaning that a portion of the energy stored within the glucose molecule remains unextracted (trapped in the final fermentation product). Below, we will examine in greater detail how this process occurs in Fungi and animal cells.

Anaerobic respiration in fungi, such as Yeast

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The final Stages of the process known as Alcoholic Fermentation are illustrated here. During alcoholic fermentation, ATP is generated exclusively during its early stages—specifically, during The breakdown of glucose into pyruvic acid. Alcoholic fermentation is utilized in The production of beer, wine, and other alcoholic beverages. In baking, the CO₂ released by yeast during alcoholic fermentation is harnessed—bubbles of this gas cause the dough to rise. The final product of alcoholic fermentation, ethanol, still contains a substantial amount of energy (in Brazil, for instance, it is used to produce motor fuel for automobiles). However, in the absence of oxygen, energy cannot be extracted from ethanol.

The overall ATP yield in alcoholic fermentation amounts to two ATP molecules per glucose molecule.

Anaerobic respiration in animal cells, such as Muscle tissue

Unlike alcoholic fermentation, neither CO₂ nor ethanol is produced during Lactic acid fermentation. The end product in this case is lactic acid, the accumulation of which in Muscles induces fatigue and occasionally cramps. The oxygen debt incurred during intense muscular exertion will be discussed in Section 9.3.8.

In lactic acid fermentation, much like in alcoholic fermentation, two ATP molecules are formed per glucose molecule. A significant amount of energy remains conserved in its end product, lactic acid.

The general pathway of anaerobic respiration is shown in Fig. 9.9.

Fig. 9.9. Pathway of anaerobic respiration.