Q.Which step of Krebs cycle operates substrate-level phosphorylation?

The succinyl-CoA -> succinic acid step of the Krebs cycle is the only step that directly generates a high-energy phosphate bond (GTP), via substrate-level phosphorylation.

In the Krebs (citric acid) cycle, succinyl-CoA is converted to succinic acid (succinate) by the enzyme succinyl-CoA synthetase (succinate thiokinase). This reaction couples cleavage of the high-energy thioester bond in succinyl-CoA to the phosphorylation of GDP to GTP (readily interconvertible with ATP) — a substrate-level phosphorylation, distinct from the oxidative phosphorylation carried out later by the electron transport chain. This is the only ATP/GTP-yielding step generated directly within one turn of the Krebs cycle itself.

Options (a) and (b) refer to NAD+/NADH interconversions, but this particular step (succinyl-CoA to succinic acid) does not involve NAD+ at all — those interconversions occur at other Krebs-cycle steps (e.g. isocitrate/α-ketoglutarate/malate dehydrogenase reactions). Option (d), FAD+ reduced to FADH2, actually occurs at the very NEXT step (succinate -> fumarate, catalysed by succinate dehydrogenase), not at this succinyl-CoA -> succinic acid step.

One glucose molecule yields two turns of the Krebs cycle, together producing 24 ATP by the standard textbook tally.

Each turn of the Krebs cycle (per acetyl-CoA entering) produces, directly and indirectly: 1 ATP/GTP (substrate-level phosphorylation at the succinyl-CoA step), 3 NADH, and 1 FADH2. Using the conventional respiratory energy accounting (1 NADH = 3 ATP, 1 FADH2 = 2 ATP via the electron transport chain), one turn yields 1 + (3×3) + (1×2) = 1 + 9 + 2 = 12 ATP. Since glycolysis of one glucose molecule produces two pyruvate molecules, each converted to acetyl-CoA and each entering the Krebs cycle separately, the cycle effectively runs TWICE per glucose, giving a total of 2 × 12 = 24 ATP from the Krebs cycle stage for one glucose molecule.

Option (a), 8 ATP, undercounts (roughly what a single acetyl-CoA's substrate-level phosphorylation plus a partial tally might give, but not the full cycle output). Option (b), 16 ATP, also undercounts the full NADH/FADH2 contribution. Option (d), 38 ATP, is instead the traditionally cited theoretical maximum ATP yield from the COMPLETE aerobic oxidation of one whole glucose molecule (glycolysis + link reaction + Krebs cycle + oxidative phosphorylation combined), not the Krebs cycle alone.

CAC = Citric Acid Cycle, also known as the Krebs cycle or TCA (Tricarboxylic Acid) cycle — the second major stage of aerobic cellular respiration.

After glycolysis produces pyruvate in the cytoplasm, pyruvate enters the mitochondrion and is converted to Acetyl-CoA. This Acetyl-CoA then enters the Citric Acid Cycle in the mitochondrial matrix, where it combines with oxaloacetate (OAA) to form citrate (citric acid) — hence the name. Through a series of enzyme-catalysed steps, the cycle:

1. Completely oxidises the two carbons brought in by Acetyl-CoA, releasing 2 molecules of CO2 per turn.
2. Generates reduced coenzymes NADH and FADH2, which carry electrons to the electron transport chain.
3. Produces one molecule of GTP/ATP (substrate-level phosphorylation) per turn.
4. Regenerates oxaloacetate so the cycle can continue.

The cycle was elucidated by Hans Krebs, so it is also called the Krebs cycle.

Citrate synthase (a condensing enzyme), not a dehydrogenase, catalyses the oxaloacetate-to-citrate step of the citric acid cycle.

Going through the options:

• (a) Oxaloacetic acid + Acetyl-CoA → Citric acid: catalysed by citrate synthase, a condensation reaction, not a dehydrogenation.
• (b) Citric acid → α-ketoglutaric acid (via isocitrate): the isocitrate-to-α-ketoglutarate step is catalysed by isocitrate dehydrogenase, releasing CO₂ and NADH.
• (c) Succinic acid → Fumaric acid: catalysed by succinate dehydrogenase, which is embedded in the inner mitochondrial membrane and reduces FAD to FADH₂.
• (d) Malic acid → Oxaloacetic acid: catalysed by malate dehydrogenase, producing NADH.

So, of the four options, only the oxaloacetate-to-citrate step does not involve a dehydrogenase enzyme.

Splitting of fructose 1,6-bisphosphate into two 3-carbon sugars is a glycolysis step, not a Krebs cycle reaction, so it is the odd one out.

The Krebs cycle takes place in the mitochondrial matrix and processes the 2-carbon acetyl group (from acetyl-CoA) through a series of oxidative decarboxylations and phosphate transfers. Its reactions include:

• Dephosphorylation from substrates such as succinyl-CoA (substrate-level phosphorylation, forming GTP/ATP) - this IS a Krebs cycle step.
• Shifting of phosphate from a 3-carbon compound to a 2-carbon compound during substrate-level phosphorylation in the cycle - this IS a Krebs cycle step.
• Splitting of fructose 1,6-bisphosphate into two molecules of glyceraldehyde-3-phosphate and dihydroxyacetone phosphate (both 3-carbon compounds) - this is catalysed by the enzyme aldolase and happens in glycolysis, in the cytoplasm, well BEFORE pyruvate ever enters the mitochondrion to form acetyl-CoA. It has no counterpart in the Krebs cycle.

Since (a) belongs to glycolysis and not the Krebs cycle, it is the reaction that is NOT involved in the Krebs cycle, making 'All of the above' incorrect.

Krebs cycle releases CO2.

In the Krebs cycle, acetyl-CoA is oxidised and its carbon is released as CO2 (two molecules per turn), along with NADH, FADH2 and ATP/GTP. Water is formed later in the electron transport chain, and pyruvate is the input from glycolysis. So CO2 is the end product released in the Krebs cycle.

Acetyl coenzyme A (acetyl-CoA) that enters the Krebs cycle contains 2 carbon atoms (the acetyl group, CH3-CO-).

During aerobic respiration, pyruvic acid (a 3-carbon molecule produced by glycolysis) is oxidised inside the mitochondrial matrix. In this step, one carbon is removed as CO2 (oxidative decarboxylation), and the remaining 2-carbon acetyl group combines with coenzyme A to form acetyl-CoA.

This 2-carbon acetyl-CoA then enters the Krebs (citric acid) cycle by combining with the 4-carbon compound oxaloacetic acid (OAA) to form the 6-carbon compound citric acid, which is then processed through the rest of the cycle, eventually releasing the 2 carbons as CO2 over subsequent steps and regenerating OAA.

So, the number of carbon atoms in the acetyl-CoA that enters the Krebs cycle is 2.

Krebs cycle = TCA (Tricarboxylic Acid) cycle, named for citric acid, the three-carboxyl-group compound formed when acetyl-CoA condenses with oxaloacetic acid at the start of the cycle.

The cycle, discovered by Hans Krebs, takes place in the mitochondrial matrix. It begins when the 2-carbon acetyl group (from pyruvate oxidation) combines with 4-carbon oxaloacetic acid to form 6-carbon citric acid -- a tricarboxylic acid -- which then undergoes a series of oxidative decarboxylation steps to regenerate oxaloacetic acid, releasing CO2 and generating NADH/FADH2/ATP along the way. Hence the two names, Krebs cycle and TCA cycle, refer to the same pathway.

The Krebs cycle runs in the mitochondrial matrix, following glycolysis (which occurs in the cytoplasm) and preceding the electron transport chain (on the inner mitochondrial membrane).

Aerobic cellular respiration proceeds through glycolysis in the cytoplasm, producing pyruvate, which is then transported into the mitochondrion. Inside the mitochondrion, pyruvate is oxidatively decarboxylated to acetyl-CoA, which enters the Krebs cycle (also called the citric acid cycle or TCA cycle). This entire cycle of reactions takes place in the mitochondrial matrix, the fluid-filled interior space of the mitochondrion enclosed by the inner membrane, and it generates NADH, FADH2, ATP (GTP), and CO2 — the reduced coenzymes are then used by the electron transport chain located on the inner mitochondrial membrane's cristae.