Final answer:
When oxygen is not present, many cells can metabolize pyruvate and generate ATP via the process of fermentation. option d is correct.
Step-by-step explanation:
In the presence of oxygen, all cells synthesize ATP via glycolysis. However, when oxygen is not present, many cells can metabolize pyruvate via the process of fermentation. Fermentation does not involve an electron transport system and does not produce additional ATP beyond what is generated during glycolysis. Instead, it allows for the regeneration of NAD+ from NADH, enabling glycolysis to continue and produce ATP.
There are different types of fermentation, including lactic acid fermentation and alcoholic fermentation, both of which start with glycolysis. These processes are crucial for ATP production in cells where oxygen is scarce or during intensive activities like exercise when oxygen supply cannot meet demand.
In the presence of oxygen, cells undergo glycolysis, generating pyruvate. If oxygen is scarce or absent, cells can metabolize pyruvate through the process of fermentation (option d). Fermentation involves the conversion of pyruvate to various byproducts, such as lactic acid or ethanol, to regenerate NAD+ for glycolysis to continue.
Unlike aerobic respiration (options a and c), which includes oxidative phosphorylation and electron transport in the presence of oxygen, fermentation is an anaerobic process that allows cells to sustain energy production when oxygen is limited, albeit with less efficiency than aerobic pathways.