NAD of Krebs cycle functions as

Options

A. Acceptor of hydrogen ion and electrons
B. Oxygen acceptor
C. Oxygen donor
D. Donor of phosphate ions.

Answer

### Step-by-Step Solution:

1. **Understanding the Role of NAD in the Krebs Cycle:**
- NAD (Nicotinamide adenine dinucleotide) is a coenzyme that plays a crucial role in cellular respiration. In the Krebs cycle, it is reduced to NADH as it accepts electrons and hydrogen ions during the oxidation of substrates.

2. **Formation of NADH:**
- During the Krebs cycle, various substrates are oxidized, and in this process, NAD+ accepts electrons and hydrogen ions, converting it into NADH. This is a key step in energy production.

3. **NADH in the Electron Transport Chain (ETC):**
- The NADH produced in the Krebs cycle is then utilized in the electron transport chain located in the inner mitochondrial membrane (cristae). Here, NADH donates the electrons it has accepted.

4. **Complex I - NADH-Q Reductase:**
- In the electron transport chain, NADH is first processed by Complex I, also known as NADH-Q reductase. This complex accepts the electrons and protons from NADH.

5. **Role of NADH as an Acceptor:**
- In the context of the Krebs cycle and the electron transport chain, NADH functions as an electron and hydrogen ion acceptor. It does not act as an oxygen acceptor or donor, nor does it donate phosphate ions.

6. **Conclusion:**
- Therefore, the function of NAD in the Krebs cycle is to act as an acceptor of hydrogen ions and electrons, which are crucial for the production of ATP through oxidative phosphorylation.

### Final Answer:
NAD of the Krebs cycle functions as an acceptor of hydrogen ions and electrons.

---