NEET Respiration in Plants Previous Year Questions with Solutions

Glycolysis occurs in the cytoplasm of all living cells. A single molecule of hexose sugar (glucose, 6 carbons) undergoes a 10-step enzymatic sequence ending with cleavage into two molecules of triose keto-acid (pyruvic acid, 3 carbons).

• • Stoichiometric yield: 2 pyruvic acid molecules per 1 glucose molecule consumed.

Multiply the input glucose quantity by the stoichiometric conversion factor of 2.

• • 206 × 2 = 412 molecules of pyruvic acid.

Looking at the available options: (1) 206, (2) 309, (3) 103, (4) 412. Option (4) is the exact calculated value.

• • Matches official NTA final answer key Option D / (4).

Metabolic oxidation of NADH + H⁺ and FADH₂ generated during glycolysis and the TCA cycle transfers electrons through a series of enzyme complexes embedded in the inner mitochondrial membrane:

• • Complex I transfers electrons from NADH to ubiquinone (coenzyme Q) with concomitant proton pumping.

Complex II functions simultaneously as an enzyme of the Krebs cycle and a component of the electron transport chain:

• • Electrons from FADH₂ are transferred to ubiquinone to generate ubiquinol (QH₂).
• • Unlike Complexes I, III, and IV, Complex II does not pump protons across the inner mitochondrial membrane.

Contrast Complex II with the other options:

• • Thus Complex II is specifically Succinate dehydrogenase, matching Option (2).

Oxidation of substrate in the TCA cycle is coupled with the reduction of electron acceptors ($NAD^+$ or $FAD$): - Isocitrate to $\alpha$-ketoglutarate: Dehydrogenation coupled with $NAD^+ \rightarrow NADH + H^+$. - $\alpha$-ketoglutarate to Succinyl-CoA: Dehydrogenation coupled with $NAD^+ \rightarrow NADH + H^+$. - Succinate to Fumarate: Dehydrogenation coupled with $FAD \rightarrow FADH_2$. - Malate to Oxaloacetate: Dehydrogenation coupled with $NAD^+ \rightarrow NADH + H^+$.

During the conversion of succinyl-CoA to succinic acid, a molecule of GTP is synthesized by substrate-level phosphorylation: $$\text{Succinyl-CoA} + \text{GDP} + P_i \rightarrow \text{Succinate} + \text{GTP} + \text{CoA-SH}$$ This is a cleavage/hydrolysis of a high-energy thioester bond; it does not involve oxidation or reduction.

Conversion of succinyl-CoA to succinic acid does not involve oxidation of the substrate.

Glycolysis takes place in the cytoplasm of the cell and is present in all living organisms (B -> I). The citric acid cycle takes place in the matrix of mitochondria where soluble dehydrogenases reside (A -> II).

The electron transport system (ETS) is embedded in the inner mitochondrial membrane (cristae) (C -> IV). As electrons move through the ETS complexes, protons are pumped from the matrix into the intermembrane space, generating the electrochemical proton gradient driving ATP synthesis via Complex V (D -> III).

A-II, B-I, C-IV, D-III corresponds to Option (4).

During the initial phase of the Embden-Meyerhof-Parnas (EMP) pathway, glucose is primed for cleavage via two irreversible kinase-catalyzed phosphorylation reactions:

NCERT states verbatim: "ATP is utilised at two steps: first in the conversion of glucose into glucose 6-phosphate and second in the conversion of fructose 6-phosphate to fructose 1, 6-bisphosphate."

Assertion states that ATP is used at two steps. Reason identifies the exact two enzymatic reactions responsible for this consumption. Therefore, R is the direct and complete explanation of A.

Analyzing the metabolic pairings in aerobic respiration:

Matching A-II and B-IV immediately isolates Options (1) and (3). Checking C-III and D-I unambiguously confirms Option (1).

Electrons from $NADH + H^+$ are transferred to $O_2$ via Complexes I, III, and IV, pumping 10 protons across the inner mitochondrial membrane, whereas electrons from $FADH_2$ enter at Complex II, pumping 6 protons. As established in the canonical NCERT text:

Statement (2) asserts that "one molecule of $NADH + H^+$ gives rise to $2\text{ ATP}$ molecules, and one $FADH_2$ gives rise to $3\text{ ATP}$ molecules". This swaps the canonical stoichiometry, making it the incorrect statement requested by the question.

- Statement (1): Oxygen acts as the final hydrogen/electron acceptor only at the very end of the ETS (Complex IV) to form metabolic water. - Statement (3): ATP synthase is universally designated as Complex V. - Statement (4): The free energy liberated during redox electron transitions drives active proton pumping into the intermembrane space, generating the electrochemical proton gradient.

Substrate level phosphorylation is the direct synthesis of ATP/GTP from ADP/GDP coupled to the cleavage of a high-energy metabolic intermediate, occurring independently of the electron transport chain and ATP synthase.

During the conversion of Succinyl-CoA to succinic acid, a molecule of GTP is synthesized by substrate level phosphorylation: $$\text{Succinyl-CoA} + \text{GDP} + \text{P}_i \xrightarrow{\text{Succinyl-CoA synthetase}} \text{Succinate} + \text{CoA-SH} + \text{GTP}$$ In a coupled reaction, GTP transfers its phosphate to ADP, yielding ATP.

In ONE turn of the citric acid cycle (for each Acetyl-CoA entering the cycle), there are: - 3 steps of NADH formation - 1 step of FADH2 formation - Exactly 1 step of substrate-level phosphorylation (GTP/ATP).

Per molecule of glucose (which yields 2 molecules of Acetyl-CoA), there are two turns and two substrate-level phosphorylations. But the question explicitly specifies "in ONE turn of citric acid cycle", which is exactly One.