How do organisms generate energy when oxygen is not available?

• A. By photosynthesis
• B. Through oxidative phosphorylation
• C. By fermentation
• D. Through aerobic respiration

Answer: (C)

Verified step by step guidance

1. Understand that organisms generate energy through different metabolic pathways depending on the availability of oxygen.
2. Recognize that when oxygen is not available, organisms cannot use aerobic respiration, which relies on oxygen to produce ATP through oxidative phosphorylation.
3. Identify that fermentation is a metabolic process that allows cells to generate energy in the absence of oxygen by converting glucose into ATP and byproducts like lactic acid or ethanol.
4. Note that fermentation is less efficient than aerobic respiration, producing only 2 ATP molecules per glucose molecule compared to up to 38 ATP molecules in aerobic conditions.
5. Conclude that in anaerobic conditions, organisms rely on fermentation to meet their energy needs, allowing them to survive and function without oxygen.