Which of the following statements about anaerobic respiration is incorrect ?

Options

A. In animal cells, like muscles during exercise, when oxygen is inadequate for cellular respiration pyruvic acid is reduced to lactic acid by lactate dehydrogenase .
B. The net gain of ATP is 2 molecules.
C. In this process , less than seven percent of the energy in glucose is released .
D. If the concentration of alcohol is seven percent, it becomes toxic to yeast .

Answer

To determine which statement about anaerobic respiration is incorrect, let's analyze each option step by step.

### Step-by-Step Solution:

1. **Understanding Anaerobic Respiration**:
Anaerobic respiration occurs in the absence of oxygen and involves the conversion of glucose into energy. The end products can vary, including lactic acid in animals and ethanol in yeast.

2. **Analyzing the Statements**:
- **Statement 1**: "In animal cells, like the muscles during exercise when oxygen is inadequate for cellular respiration, the pyruvic acid is reduced to lactic acid by lactate dehydrogenase."
- **Analysis**: This statement is true. When oxygen levels are low, pyruvic acid is converted to lactic acid in muscle cells through the action of the enzyme lactate dehydrogenase.

- **Statement 2**: "Net gain of the ATP is two molecules."
- **Analysis**: This statement is also true. During anaerobic respiration (specifically glycolysis), there is a net gain of 2 ATP molecules produced from one molecule of glucose.

- **Statement 3**: "In this process, less than 7% of the energy in glucose is released."
- **Analysis**: This statement is true. Anaerobic respiration is less efficient than aerobic respiration, and typically, less than 7% of the energy in glucose is captured as ATP.

- **Statement 4**: "If the concentration of alcohol is 7%, it becomes toxic to the yeast."
- **Analysis**: This statement is incorrect. The concentration of alcohol that becomes toxic to yeast is generally around 13%, not 7%. At 7%, yeast can still survive and continue fermentation.

3. **Conclusion**:
The incorrect statement about anaerobic respiration is **Statement 4**.

### Final Answer:
The incorrect statement about anaerobic respiration is: **"If the concentration of alcohol is 7%, it becomes toxic to the yeast."**

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