NEET Practice Questions (MCQs) with Answers & Solutions

(d) Succinate dehydrogenase enzyme is present on inner membrane of mitochondria and catalysed the oxidation of succinate to fumarate.

(a) Glycolysis, Krebs cycle and electron transport system are meant for ATP synthesis in different steps. ATP is the energy currency of cell.

One mole of ATP liberates 12 kcal of energy. So 686 kcal will liberate 686/12 = 57.1 ATP molecules.

(c): TCA cycle is the cyclic biochemical catabolic pathway for the oxidation of fuel molecule through acetyl CoA which is completely oxidized to carbon dioxide and water. Kreb's cycle operates in the mitochondrial matrix because it contains complex mixture of soluble enzymes that catalyse the respiration of pyruvic acid and other smali molecules. The cycle consists of 9 enzymatic steps, four of which are dehydrogenations

(a) : In Krebs cycle, GTP is formed during the conversion of succinyl CoA to succinic acid or succinate. The enzyme required for this reaction is succinyl thiokinase. The phosphorylation in this reaction is substrate level phosphorylation where ATP synthesis is directly linked to the liberation of energy.

(b) In respiraion, from 180gm of glucose 264 gm CO₂+ 108gm H₂O + 686 Kcal energy is formed.

${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6 + 6{\mathrm{O}}_2 \stackrel{ }{\to }6{\mathrm{CO}}_2 + 6{\mathrm{H}}_2\mathrm{O} \mathrm{Kcal}$

Atomic weight of C =12, H = 1, O=16

$$\begin{gathered} {\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6 = (12 \times 6 + 12 \times 1 + 16 \times 6) \\ = 180 \mathrm{gm} \\ 6{\mathrm{CO}}_2 = 6(12 + 16 \times 2) \\ = 264 \mathrm{gm} \\ 6{\mathrm{H}}_2\mathrm{O} =6(2 \times 1 + 16) \\ = 108 \mathrm{gm} \\ \therefore \mathrm{From} 1 \mathrm{molecule} \mathrm{or} 180 \mathrm{gm} \mathrm{of} \mathrm{glucose}, \mathrm{through} \\ \mathrm{respiration}, 264 \mathrm{gm} {\mathrm{CO}}_2, 108\mathrm{gm} {\mathrm{H}}_2\mathrm{O} \mathrm{and} 686 \mathrm{Kcal} \mathrm{energy} \mathrm{are} \mathrm{formed}. \end{gathered}$$

(c) : In glycolysis one molecule of hexose sugar (glucose/fructose), a 6C-compound is splitted to form
two molecules of a 3C-compound, pyruvic acid (CH₃COCOOH).

(a) : Glycolysis is the first step of respiration in which glucose is broken down to pyruvate. During glycolysis 1 molecule of glucose is converted to 2 molecule of pyruvic acid in the cytoplasm. These two molecule of pyruvic acid is the converted to acetyl CoA by decarboxylation. This acetyl CoA is the connecting link betwwen glycolysis and Krebs' cycle and functions as substrate entrant for Krebs' cycle. Acetyl CoA thus formed participates in Krebs' cycle and combines with oxaloacetic acid to form citrate.

(c): Oxalosuccinate is decarboxylated to form α-ketoglutarate through the enzyme decarboxylase. It is one of the step involved in Krebs' cycle. It releases one molecule of CO₂.

$\mathrm{Oxalosuccinate} \underset{{\mathrm{Mg}}^{2+}}{\overset{\mathrm{Decarboxylase}}{\to }} \mathrm{\alpha }-\mathrm{ketoglutarate} + {\mathrm{CO}}_2$

Transfer of phosphate group from ATP to a carbohydrate is referred to as phosphorylation and is carried out in presence of phosphorylase enzymes. E.g., glucose (a carbohydrate) is phosphorylated to glucose-6-phosphate by ATP in the presence of enzyme hexokinase or glucokinase and Mg^₂₊ .