`NADH_(2)` generated in glycolysis produces ATP in ETS in presence of `O_(2)`. In absence of `O_(2)` this `NADH_(2)` functions as

Options

A. Oxidising agent
B. Phosphorylating agent
C. Reducing agent
D. Carboxylating agent

Answer

To solve the question regarding the role of NADH2 in the absence of oxygen, we can break it down into the following steps:

### Step-by-Step Solution:

1. **Understanding NADH2 Production**:
- NADH2 is produced during glycolysis, which is the process of breaking down glucose to extract energy. This process occurs in the cytoplasm of the cell.

2. **Role of NADH2 in Aerobic Conditions**:
- In the presence of oxygen, NADH2 donates electrons to the electron transport system (ETS), leading to the production of ATP. Each NADH2 can yield approximately three ATP molecules when oxygen is available.

3. **Condition of Absence of Oxygen**:
- When oxygen is not present, the electron transport chain cannot function because it relies on oxygen as the final electron acceptor. This leads to a different metabolic pathway.

4. **Function of NADH2 in Anaerobic Conditions**:
- In the absence of oxygen, NADH2 cannot donate electrons to the ETS. Instead, it must be oxidized back to NAD+ to allow glycolysis to continue. This process is crucial for regenerating NAD+ so that glycolysis can proceed and produce ATP in anaerobic conditions.

5. **Identifying the Role of NADH2**:
- In this context, NADH2 acts as a reducing agent. It donates electrons to other molecules, which allows the conversion of NADH2 back to NAD+. This is essential for the continuation of anaerobic respiration or fermentation.

6. **Types of Fermentation**:
- There are two main types of fermentation processes where NADH2 acts as a reducing agent:
- **Lactic Acid Fermentation**: Occurs in muscle cells and some bacteria, where NADH2 reduces pyruvate to lactic acid.
- **Alcoholic Fermentation**: Occurs in yeast, where NADH2 reduces acetaldehyde to ethanol.

### Final Answer:
In the absence of oxygen, NADH2 functions as a **reducing agent**.

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