Cellular respiration is a process that requires oxygen (aerobic conditions). However, many cell types and organisms can carry out energy metabolism when oxygen isn’t available (anaerobic conditions). Given this which of the following is true? a. Under anaerobic conditions the citric acid cycle will be run but oxidative phosphorylation will not be run. b. Under anaerobic conditions NADH must be recycled back to NAD+which can happen within the citric acid cycle. c. In animal cells, and some bacteria, pyruvate can be converted to lactate in order to regenerate NAD+that is needed to run glycolysis. d. Fermentation can only lead to the production of ethanol and CO2? as seen in yeast. e. Glycolysis cannot be run under anaerobic conditions.
The Correct Answer and Explanation is:
The correct answer is:
c. In animal cells, and some bacteria, pyruvate can be converted to lactate in order to regenerate NAD⁺ that is needed to run glycolysis.
Explanation
Cellular respiration is the process by which cells extract energy from glucose. Under aerobic conditions (presence of oxygen), this process involves glycolysis, the citric acid cycle (also known as the Krebs cycle), and oxidative phosphorylation. However, when oxygen is not available (anaerobic conditions), cells cannot use the citric acid cycle and oxidative phosphorylation effectively, as both rely on oxygen directly or indirectly.
Under anaerobic conditions, glycolysis still occurs and serves as the primary method of ATP production. During glycolysis, glucose is broken down into two molecules of pyruvate, generating a small amount of ATP and reducing NAD⁺ to NADH. However, if oxygen is not present to accept electrons during oxidative phosphorylation, NADH cannot be converted back to NAD⁺ through the electron transport chain.
To continue glycolysis (which requires NAD⁺), cells must find an alternative way to regenerate NAD⁺ from NADH. In animal cells and some bacteria, this is achieved through lactic acid fermentation, in which pyruvate is reduced to lactate by lactate dehydrogenase. This reaction oxidizes NADH to NAD⁺, allowing glycolysis to continue producing ATP anaerobically.
This makes option (c) the correct answer.
Let’s briefly look at why the other options are incorrect:
- (a) is incorrect: The citric acid cycle also indirectly depends on oxygen because it needs NAD⁺ and FAD regenerated by the electron transport chain, which only functions in the presence of oxygen.
- (b) is incorrect: NAD⁺ regeneration cannot happen within the citric acid cycle under anaerobic conditions.
- (d) is incorrect: Fermentation does not only lead to ethanol and CO₂; lactic acid fermentation is another pathway.
- (e) is incorrect: Glycolysis can occur under anaerobic conditions—it is the main anaerobic ATP-producing pathway.
Thus, option (c) accurately reflects how cells adapt to anaerobic conditions.
