Nad and fad work by loading and unloading and during energy transfers.

Hydrogen and electrons

Explanation:

Nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD) serve as reducing power during energy transfers. One NAD+ accepts one hydrogen ions and two electrons and becomes reduced into NADH. Likewise, FAD accepts two hydrogen ions and two electrons and is reduced into FADH2. Oxidation of NADH and FADH2 into NAD+ and FAD respectively releases both hydrogen ions and electrons.  

For example, some of the energy of glucose released during glycolysis and Kreb's cycle is temporarily stored in the form of NADH and FADH2. Oxidation of NADH and FADH2 via electron transfer chain of mitochondria releases both electrons and hydrogen ions (protons). The protons are pumped across the inner mitochondrial membrane to generate the proton concentration gradient.  

1. b.

2. d.

3. a.

5. d.

6. c.

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