Q.The compound which links glycolysis and Krebs cycle is

Acetyl CoA is the biochemical link between glycolysis (which ends with pyruvate) and the Krebs cycle (which begins by combining acetyl CoA with oxaloacetate).

Glycolysis, occurring in the cytoplasm, breaks down one molecule of glucose into two molecules of pyruvic acid (pyruvate), with a net gain of 2 ATP and 2 NADH. Before the carbon skeleton can enter the mitochondrial Krebs cycle, pyruvate must first be converted into a form the cycle can use.

This conversion — the link reaction — happens in the mitochondrial matrix and is catalysed by the pyruvate dehydrogenase enzyme complex:

Pyruvic acid + NAD+ + CoA → Acetyl CoA + CO2 + NADH

Here, pyruvate (a 3-carbon compound) loses one carbon as CO2 (oxidative decarboxylation) and the remaining 2-carbon acetyl group is picked up by coenzyme A to form acetyl CoA. This acetyl CoA then condenses with the 4-carbon oxaloacetate already present in the Krebs cycle to form 6-carbon citric acid, formally starting the cycle. So acetyl CoA is the essential bridging molecule — not succinic acid or citric acid, which are Krebs-cycle intermediates themselves, and not pyruvic acid, which is glycolysis's own end-product, not yet in a form the Krebs cycle can accept.

Pyruvic acid from glycolysis is oxidatively decarboxylated to Acetyl CoA, which is the actual molecule that enters and links to the Krebs cycle.

Glycolysis takes place in the cytoplasm and converts one molecule of glucose into two molecules of pyruvic acid. Before the carbon skeleton can enter the Krebs (citric acid) cycle, which occurs in the mitochondrial matrix, each pyruvic acid molecule must first undergo oxidative decarboxylation in the link reaction: Pyruvic acid + NAD+ + CoA → Acetyl CoA + CO2 + NADH. This step is catalysed by the pyruvate dehydrogenase complex. The Acetyl CoA formed then combines with oxaloacetic acid (OAA) to form citric acid, the first step of the Krebs cycle. Thus, Acetyl CoA is the direct link connecting glycolysis to the Krebs cycle, not pyruvic acid itself (which is the end product of glycolysis, not the entry molecule of the cycle).

Glycolysis ends with pyruvate; pyruvate is then converted (via oxidative decarboxylation, releasing CO2 and NADH) into Acetyl CoA, which is the actual link molecule that feeds carbon into the Krebs (citric acid) cycle.

Tracing the pathway: Glycolysis (in the cytoplasm) breaks down one glucose molecule into two molecules of pyruvic acid (pyruvate). Under aerobic conditions, pyruvate enters the mitochondrial matrix, where the pyruvate dehydrogenase enzyme complex catalyses its oxidative decarboxylation: pyruvate loses one carbon as CO2, is oxidised, and combines with Coenzyme A to form Acetyl CoA (a 2-carbon acetyl group attached to CoA), generating NADH in the process. It is this Acetyl CoA — not pyruvate itself — that condenses with the 4-carbon oxaloacetate to form 6-carbon citrate, the first step of the Krebs cycle. This is why Acetyl CoA is described as the molecule that links glycolysis to the Krebs cycle.

The other options are intermediates WITHIN the Krebs cycle itself (succinic acid, citric acid) or the end-product of glycolysis (pyruvic acid), not the actual connecting molecule.

Acetyl CoA is the compound that links glycolysis to the Krebs cycle, formed from pyruvic acid by oxidative decarboxylation.

Respiration proceeds through connected stages:

1. Glycolysis (in the cytoplasm) breaks down one molecule of glucose into two molecules of pyruvic acid, with a net gain of 2 ATP and 2 NADH.
2. Link reaction (in the mitochondrial matrix, under aerobic conditions): each pyruvic acid is oxidatively decarboxylated by the pyruvate dehydrogenase complex — it loses a CO2 and is combined with Coenzyme A to form Acetyl CoA (also generating NADH).
3. Acetyl CoA then enters the Krebs cycle by combining with oxaloacetic acid (OAA) to form citric acid, the first intermediate of the cycle.

Pyruvic acid itself is the end product of glycolysis, not the connecting molecule; citric acid and succinic acid are intermediates WITHIN the Krebs cycle itself, not the link between the two pathways. Acetyl CoA is specifically the molecule that carries carbon from glycolysis's product into the Krebs cycle.

Correct answer: (d) Acetyl-Co-A

Pyruvate loses one CO2 and is converted to a 2-carbon acetyl group attached to coenzyme A.

After glycolysis, pyruvate (3-carbon) enters the mitochondrial matrix where the pyruvate dehydrogenase complex catalyses its oxidative decarboxylation: one carboxyl group is removed as CO2, the remaining 2-carbon fragment is oxidised, NAD+ is reduced to NADH, and the resulting acetyl group is attached to coenzyme A to form acetyl-CoA. Acetyl-CoA then enters the TCA (Krebs) cycle by combining with oxaloacetate to form citrate. 2-PGA is a glycolytic intermediate, while alpha-ketoglutarate and succinyl-CoA are intermediates formed later, within the TCA cycle itself, not the direct product of pyruvate oxidation.