Question 1

The Krebs cycle (citric acid cycle / TCA cycle) takes place in the:

Accepted Answer

Mitochondrial matrix. The TCA cycle operates in the mitochondrial matrix; the ETS is on the inner membrane.

Question 2

Before entering the Krebs cycle, pyruvate is converted to ______ by oxidative decarboxylation:

Accepted Answer

Acetyl CoA. Pyruvate dehydrogenase converts pyruvate to acetyl CoA, releasing CO₂ and forming NADH.

Question 3

The compound with which acetyl CoA combines to start the Krebs cycle, forming citrate, is:

Accepted Answer

Oxaloacetic acid (OAA). Acetyl CoA (2-C) condenses with OAA (4-C) to form citric acid (6-C), regenerating CoA.

Question 4

For each turn of the Krebs cycle, the number of CO₂ molecules released and NADH produced is:

Accepted Answer

2 CO₂ and 3 NADH. One TCA turn releases 2 CO₂ and yields 3 NADH, 1 FADH₂ and 1 GTP (ATP).

Question 5

Per glucose molecule, complete oxidation via glycolysis + Krebs cycle produces how many NADH in total (before ETS)?

Accepted Answer

10 NADH. Glycolysis (2) + pyruvate→acetyl CoA (2) + Krebs (6) = 10 NADH, plus 2 FADH₂, per glucose.

Question 6

The only ATP produced directly (substrate-level) within the Krebs cycle is at the step:

Accepted Answer

Succinyl CoA → Succinate (forming GTP/ATP). Succinyl CoA synthetase makes one GTP (≈ATP) by substrate-level phosphorylation.

Question 7

The link reaction connecting glycolysis to the Krebs cycle converts pyruvate to acetyl CoA. This conversion involves all of the following EXCEPT:

Accepted Answer

Reduction of FAD to FADH₂. Oxidative decarboxylation of pyruvate by the pyruvate dehydrogenase complex releases CO₂ and reduces NAD⁺ to NADH. No FADH₂ is produced at this step.

Question 8

Where in a plant cell does the pyruvate produced in the cytoplasm undergo its further oxidation during aerobic respiration?

Accepted Answer

It is transported into the mitochondrial matrix. Pyruvate formed by glycolysis in the cytoplasm is moved into the mitochondrial matrix, where the link reaction and Krebs cycle occur.

Question 9

The Krebs cycle is also described as the citric acid cycle. Which enzyme catalyses the very first, condensation step of the cycle?

Accepted Answer

Citrate synthase. Citrate synthase condenses the two-carbon acetyl group of acetyl CoA with the four-carbon oxaloacetate to form the six-carbon citrate, starting the cycle.

Question 10

The respiratory pathway is described as amphibolic because it:

Accepted Answer

Serves both to break down molecules for energy and to provide intermediates for the synthesis (anabolism) of other compounds. Respiratory intermediates (e.g. acetyl CoA, α-ketoglutarate, oxaloacetate) are withdrawn to build fatty acids, amino acids, etc. — so the pathway is catabolic and anabolic, i.e. amphibolic.

Question 11

Oxaloacetate, the acceptor of acetyl CoA, is regenerated at the end of every turn of the Krebs cycle. What is the significance of this regeneration?

Accepted Answer

It allows the cycle to keep running continuously as long as acetyl CoA is supplied. Because oxaloacetate is consumed at the start and regenerated at the end, a single pool can catalyse the oxidation of many acetyl CoA molecules — the defining feature of a cyclic pathway.

Question 12

In one turn of the Krebs cycle there are exactly how many decarboxylation and how many substrate-level phosphorylation events, respectively?

Accepted Answer

Two decarboxylations and one substrate-level phosphorylation. Per turn, isocitrate → α-ketoglutarate and α-ketoglutarate → succinyl CoA each release CO₂ (two decarboxylations), and succinyl CoA → succinate yields one GTP/ATP (one substrate-level phosphorylation).

Question 13

For the complete oxidation of one glucose molecule (both pyruvates), how many NADH molecules are generated specifically within the Krebs cycle proper (excluding glycolysis and the link reaction)?

Accepted Answer

Six. Each turn of the Krebs cycle produces 3 NADH. Two turns occur per glucose (one per pyruvate), so 3 × 2 = 6 NADH from the cycle itself.

Question 14

Statement I: Only one FADH₂ is produced per turn of the Krebs cycle. Statement II: This FADH₂ is generated when succinate is oxidised to fumarate. Which is correct?

Accepted Answer

Both statements are correct. The single FADH₂ of the Krebs cycle is made by succinate dehydrogenase as it oxidises succinate to fumarate; both statements are true.

Question 15

Counting the link reaction and both turns of the Krebs cycle together, how many CO₂ molecules are released per glucose after glycolysis?

Accepted Answer

Six. The link reaction releases 1 CO₂ per pyruvate (2 total), and each Krebs turn releases 2 CO₂ (4 total for two turns), giving 6 CO₂ — accounting for all six carbons of glucose.

Question 16

Arrange the following four-carbon Krebs-cycle intermediates in the correct order of their formation: (i) malate (ii) succinate (iii) fumarate (iv) oxaloacetate.

Accepted Answer

(ii) → (iii) → (i) → (iv). After succinyl CoA the sequence is succinate → fumarate → malate → oxaloacetate, which then condenses with the next acetyl CoA to restart the cycle.

Question 17

Which among the following steps in cellular respiration do not need molecular oxygen (O2)?

Accepted Answer

Both glycolysis and TCA cycle. Neither glycolysis nor the TCA cycle uses molecular O2 directly; O2 is used only in the ETC.

Question 18

Complete the reaction: Pyruvic acid + (i) --enzyme--> Acetyl CoA + (iii) + NADH + H+, with (ii) as a cofactor. Identify (i), (ii) and (iii).

Accepted Answer

(i) CoA, (ii) Mg2+, (iii) CO2. Pyruvate combines with CoA (enzyme pyruvate dehydrogenase needs Mg2+) to give acetyl CoA, releasing CO2 and forming NADH + H+.

Question 19

Pyruvate enters the mitochondrial matrix and undergoes

Accepted Answer

Oxidative decarboxylation. Pyruvate undergoes oxidative decarboxylation to form acetyl CoA in the mitochondrial matrix.

Question 20

Which enzyme catalyses the conversion of pyruvate to acetyl CoA in the mitochondrial matrix?

Accepted Answer

Pyruvate dehydrogenase. Pyruvate dehydrogenase catalyses the oxidative decarboxylation of pyruvate to acetyl CoA.

Aerobic Respiration & Krebs Cycle — NEET Biology MCQs with Solutions

Questions with solutions