Question

Coenzyme $NA{D^ + }$ and $FAD$ are connected with respiratory reactions as they
A. Are involved in each step of ATP synthesis.
B. Function in Krebs cycle and terminal oxidative phosphorylation.
C. Act as a hydrogen carrier.
D. Are reducing agents.

Answer 1

Hint: NAD and FAD are two cofactors which are involved in cellular respiration. Cellular respiration is a process in which energy is derived from the food (glucose). Cellular respiration includes following steps- Glycolysis, Krebs cycle and electron transport chain. The electrons and hydrogen atoms released from glycolysis and Kreb’s cycle combine with $NA{D^ + }$ and $FAD$ molecules to form NADH and $FAD{H_2}.$

Complete Answer:
1) $NA{D^ + }$: Nicotinamide Adenine Dinucleotide in its oxidized state is called $NA{D^ + }$, after being reduced, it is referred to as$NADH.$
2) FAD: Flavin adenine in its oxidized state is called FAD. After being reduced, it is called $FAD{H_2}.$
3) $NA{D^ + }$and FAD are two coenzymes that are involved in cellular respiratory reactions. They are responsible for accepting electrons and carrying electrons or hydrogen to the electron transport chain. They are thus involved in the reactions which finally synthesize ATP.
Electron transport chain refers to a group of chemical reactions in which electrons from high energy molecules like NADH and $FAD{H_2}$are shifted to low energy molecules like oxygen.
When these electron carrier molecules accept the electrons, they are reduced into NADH and $FAD{H_2}.$
4) Cellular respiration is a biological process in which glucose is converted into $C{O_2}, {H_2}O$ and energy.
$\mathop {{C_6}{H_{12}}{O_6} + {O_2}}\limits_{Glu\cos e\,\:oxygen} \to C{O_2} + {H_2}O + Energy$
5) This energy is released in the form of ATP.
6) Role of $NA{D^ + }$ and FAD is crucial during respiratory reactions as they act as electron carriers and convert energy stored in glucose into ATP.

The correct option is (C).

Note: Some enzymes use the coenzymes $NA{D^ + }$ or FAD which act as electron/hydrogen carriers in reactions. $NA{D^ + }$ receives two electrons and two \[{H^ + }\]forming $NADH + {H^ + }$. FAD receives two electrons and two ${H^ + }, $forming $FAD{H_2}$. They act as an oxidizing agent. Only a small amount of $NAD{H^ + }$ molecules is used over and over again.