NAD functions in cell respiration as an co-enzyme.

Explanation

**Step-by-Step Text Solution:**

1. **Understanding NAD**: NAD stands for Nicotinamide Adenine Dinucleotide. It is a coenzyme found in all living cells and is essential for various metabolic processes.

2. **Role of NAD in Metabolism**: NAD plays a crucial role in cellular metabolism. It is involved in oxidation-reduction (redox) reactions, which are fundamental processes in cellular respiration.

3. **Oxidation-Reduction Reactions**: During cellular respiration, glucose is oxidized to produce energy. NAD acts as an electron carrier, accepting electrons during the oxidation of glucose and becoming reduced to NADH.

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