Krebs cycle — steps, enzymes, and energy yield per glucose molecule

Question

Describe the steps of the Krebs cycle (citric acid cycle). Name the key enzymes involved and calculate the total energy yield from one glucose molecule through aerobic respiration.

(NEET 2022, similar pattern)

Solution — Step by Step

Before the Krebs cycle begins, pyruvate (from glycolysis) is converted to Acetyl CoA in the mitochondrial matrix:

\text{Pyruvate (3C)} \xrightarrow{\text{Pyruvate dehydrogenase}} \text{Acetyl CoA (2C)} + \text{CO}_2 + \text{NADH}

This is called the link reaction or oxidative decarboxylation of pyruvate. One glucose produces 2 pyruvate molecules, so this step occurs twice per glucose.

Acetyl CoA (2C) enters the cycle in the mitochondrial matrix:

Acetyl CoA (2C) + OAA (4C) → Citrate (6C) — Enzyme: citrate synthase
Citrate (6C) → Isocitrate (6C) — Enzyme: aconitase
Isocitrate → $\alpha$ -Ketoglutarate (5C) — Enzyme: isocitrate dehydrogenase; releases $\text{CO}_2$ + NADH
$\alpha$ -Ketoglutarate → Succinyl CoA (4C) — Enzyme: $\alpha$ -ketoglutarate dehydrogenase; releases $\text{CO}_2$ + NADH
Succinyl CoA → Succinate (4C) — Enzyme: succinyl CoA synthetase; produces GTP (equivalent to 1 ATP)
Succinate → Fumarate (4C) — Enzyme: succinate dehydrogenase; produces $\text{FADH}_2$
Fumarate → Malate (4C) — Enzyme: fumarase
Malate → OAA (4C) — Enzyme: malate dehydrogenase; produces NADH

OAA is regenerated, ready to accept the next Acetyl CoA — hence it is a cycle.

Per one Acetyl CoA (one turn):

3 NADH (steps 3, 4, and 8)
1 $\text{FADH}_2$ (step 6)
1 GTP (= 1 ATP) (step 5)
2 $\text{CO}_2$ released (steps 3 and 4)

Since one glucose gives 2 Acetyl CoA, multiply by 2 for the total Krebs cycle output per glucose.

Stage	NADH	FADH2	ATP (direct)
Glycolysis	2	0	2
Link reaction (x2)	2	0	0
Krebs cycle (x2)	6	2	2
Total	10	2	4

In oxidative phosphorylation (ETC):

Each NADH → 2.5 ATP (some textbooks say 3)
Each $\text{FADH}_2$ → 1.5 ATP (some textbooks say 2)

Total ATP = 4 (direct) + (10 x 2.5) + (2 x 1.5) = 4 + 25 + 3 = 32 ATP

Using the older values (3 per NADH, 2 per $\text{FADH}_2$ ): 4 + 30 + 4 = 38 ATP

NEET uses the older value of 36-38 ATP per glucose (based on NCERT). Write 36 ATP if glycolytic NADH loses energy during shuttle transport, or 38 ATP if shuttle transport is 100% efficient. For safety, write 36 ATP as the standard NEET answer.

Why This Works

The Krebs cycle completely oxidises the 2-carbon acetyl group to $\text{CO}_2$ , extracting all remaining chemical energy as NADH, $\text{FADH}_2$ , and GTP. These electron carriers then feed into the electron transport chain, where the real ATP production happens via chemiosmosis (oxidative phosphorylation).

The cycle is amphibolic — it is both catabolic (breaks down acetyl groups for energy) and anabolic (provides intermediates for amino acid synthesis, fatty acid synthesis, and gluconeogenesis).

To remember the Krebs cycle intermediates in order: Citrate, Isocitrate, Alpha-ketoglutarate, Succinyl CoA, Succinate, Fumarate, Malate, Oxaloacetate. Mnemonic: Can I Always See Someone Finding My Oranges.

Common Mistake

The most frequent error: counting the energy yield only from the Krebs cycle and forgetting the link reaction. The 2 NADH from pyruvate → Acetyl CoA conversion (link reaction) contribute 5 ATP via ETC. Students who forget this get 30 instead of 36 ATP.

Another common mistake: writing that the Krebs cycle occurs in the cytoplasm. It occurs in the mitochondrial matrix. Glycolysis is the only step that occurs in the cytoplasm. The link reaction and Krebs cycle are both mitochondrial.

Question

Solution — Step by Step

Why This Works

Common Mistake

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