Lactic acid
Explanation
Respiration is the biochemical process by which cells break down glucose to release energy. The first step in all cases is the breakdown of glucose (a 6-carbon molecule) into pyruvate (a 3-carbon molecule) within the cytoplasm. The subsequent fate of pyruvate depends on the presence or absence of oxygen.
Detailed Analysis:
- Anaerobic Respiration in Muscles: During vigorous physical activity, muscle cells may face a shortage of oxygen. In this condition, pyruvate is converted into lactic acid (a 3-carbon molecule) and a small amount of energy. The accumulation of lactic acid in muscle tissues is often responsible for muscle cramps.
- Anaerobic Respiration in Yeast (Fermentation): In organisms like yeast, the absence of oxygen leads to the conversion of pyruvate into ethanol (alcohol) and carbon dioxide. This is distinct from the process in human muscles.
- Aerobic Respiration: In the presence of sufficient oxygen, pyruvate breaks down in the mitochondria to produce carbon dioxide, water, and a large amount of energy.
Option Analysis:
- Alcohol: Produced during fermentation in yeast, not in human muscles.
- Lactic acid Lactic acid: The specific product of anaerobic respiration in muscle cells. (Correct)
- Glucose: This is the substrate (reactant) for respiration, not the product.
- Starch: This is a storage polysaccharide in plants and is not produced during respiration.
Key Takeaway:
The breakdown of glucose in the absence of oxygen yields lactic acid and energy in muscle cells, whereas it yields ethanol, carbon dioxide, and energy in yeast.