Assertion: NAD⁺ is the most versatile electron acceptor in cellular respiration and functions in several of redox steps during the breakdown of glucose.
Reason: As an electron acceptor, NAD⁺ functions as an oxidizing agent during respiration.

1. Both assertion and reason are true and the reason is the correct explanation of the assertion.
2. Both assertion and reason are true but the reason is not the correct explanation of the assertion.
3. Assertion is true but reason is false.
4. Both assertion and reason are false.

Oxidation of organic compounds by molecular oxygen results in the production of carbon dioxide and water and releases free energy. This is because:

1.High energy covalent bonds in organic molecules are broken to produce weaker covalent bonds in water and carbon dioxide.

2. Electrons are being moved from atoms that have a lower affinity for electrons to atoms with a higher affinity for electrons.

3.The electrons in water and carbon dioxide have higher potential energy than the electrons in organic compounds.

4. The covalent bonds in organic molecules are unstable and stable in water and oxygen.

The mitochondrial proton gradient would be highest leading to ATP synthesis when the following condition is present:
1. pyruvate (present)-oxygen (present)-ATP levels (high)
2. pyruvate (present)-oxygen (present)-ATP levels (low)
3. pyruvate (present)-oxygen (absent)-ATP levels (high)
4. pyruvate (absent)-oxygen (present)-ATP levels (low)