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Anaerobic Respiration for AP Biology

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By — McGraw-Hill Professional
Updated on Oct 24, 2011

Practice problems for these concepts can be found at:  Respiration Review Questions for AP Biology

 

Anaerobic respiration, or fermentation, occurs when oxygen is unavailable or cannot be used by the organism. As in aerobic respiration, glycolysis occurs and pyruvate is produced. The pyruvate enters the Krebs cycle, producing NADH, FADH2, and some ATP. The problem arises in the ETC—because there is no oxygen available, the electrons do not pass down the chain to the final electron acceptor, causing a buildup of NADH in the system. This buildup of NADH means that the NAD+ normally regenerated during oxidative phosphorylation is not produced, and this creates an NAD+ shortage. This is a problem because in order for glycolysis to proceed to the pyruvate stage, it needs NAD+ to help perform the necessary reactions. Fermentation is the process that begins with glycolysis and ends when NAD+ is regenerated. A glucose molecule that enters the fermentation pathway produces two net ATP per molecule of glucose, representing a tremendous decline in ATP efficiency.

Under aerobic conditions, NAD+ is recycled from NADH by the movement of electrons down the electron transport chain. Under anaerobic conditions, NAD+ is recycled from NADH by the movement of electrons to pyruvate, namely, fermentation. The two main types of fermentation are alcohol fermentation and lactic acid fermentation. Refer to Figures 7.5 and 7.6 for the representations of the different forms of fermentation. Alcohol fermentation (Figure 7.5) occurs in fungi, yeast, and some bacteria. The first step involves the conversion of pyruvate into two 2-carbon acetaldehyde molecules. Then, in the all-important step of alcohol fermentation, the acetaldehyde molecules are converted to ethanol, regenerating two NAD+ molecules in the process.

Anaerobic Respiration

Anaerobic Respiration

Lactic acid fermentation (Figure 7.6) occurs in human muscle cells when oxygen is not available. This is a simpler process than alcoholic fermentation—the pyruvate is directly reduced to lactate by NADH to regenerate the NADneeded for the resumption of glycolysis. Have you ever had a cramp during exercise? The pain you felt was the result of lactic acid fermentation. Your muscle was deprived of the necessary amount of oxygen to continue glycolysis, and it switched over to fermentation. The pain from the cramp came from the acidity in the muscle.

 

Practice problems for these concepts can be found at:  Respiration Review Questions for AP Biology

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