The energy releasing metabolic process in which substrate is oxidised without an external electron acceptor is called.

Options

A. Photorespiration
B. Glycolysis
C. Fermentation
D. Aerobic respiration

Answer

To solve the question "The energy releasing metabolic process in which substrate is oxidised without an external electron acceptor is called," we will analyze the options provided and determine which one fits the description.

### Step-by-Step Solution:

1. **Understanding the Question**:
- The question asks for a metabolic process that releases energy through the oxidation of a substrate without the involvement of an external electron acceptor.

2. **Analyzing the Options**:
- **Photorespiration**: This process involves the oxidation of ribulose bisphosphate (RUBP) by the enzyme Rubisco and does not produce energy. Thus, it is not the correct answer.
- **Glycolysis**: This is a metabolic pathway that occurs in both fermentation and aerobic respiration. It involves the breakdown of glucose but does not fit the criteria of being without an external electron acceptor since it can lead to both processes. Therefore, it is also not the correct answer.
- **Fermentation**: This process is characterized by the incomplete oxidation of food substances in the absence of oxygen (no external electron acceptor). It produces either alcohol or lactic acid and releases a small amount of energy (ATP). This fits the description perfectly.
- **Aerobic Respiration**: This process uses oxygen as an external electron acceptor for the oxidation of substrates. Hence, it does not meet the criteria of the question.

3. **Conclusion**:
- Based on the analysis, the correct answer is **Fermentation**. This process occurs without an external electron acceptor and involves the oxidation of substrates, releasing energy.

### Final Answer:
The energy releasing metabolic process in which substrate is oxidised without an external electron acceptor is called **Fermentation**.

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