AP BIOLOGY

Detailed explanation-1: -In the third step of the citric acid cycle in which the citrate is converted to ketoglutarate, energy is reduced to NADH₂ and in the fourth step where ketoglutarate is converted into succinyl-CoA in which FAD is reduced to FADH. In the last step of the Krebs cycle, NAD is converted into NADH.

Detailed explanation-2: -These coenzymes can exist in their oxidized (NAD+ and FAD) or reduced forms (NADH and FADH2). NADPH is a close derivatives of NADH that also acts as a redox couple.

Detailed explanation-3: -Answer and Explanation: NAD and FAD are carrier molecules that transport electrons from glycolysis and the tricarboxylic acid cycle into the mitochondrial electron transport chain. Upon receiving electrons, NAD and FAD become reduced and forms NADH and FADH2, respectively.

Detailed explanation-4: -Step 8: In the last step of the Krebs cycle, malate is converted to oxaloacetate. In the process, NAD+ is reduced to form NADH + H+.

Detailed explanation-5: -The oxidation of pyruvic acid results in the reduction of NAD+, production of Ac-S-CoA and a molecule of CO2, as shown below. The Krebs cycle functions during respiration to oxidize Ac-S-CoA and to reduce NAD+ and FAD to NADH and FADH2 (respectively).