Steps of the Citric Acid Cycle (Krebs Cycle) Explained
Biology · Respiration in Plants · NEET
The Krebs cycle (TCA or citric acid cycle) begins when a 2-carbon acetyl group from acetyl CoA joins the 4-carbon oxaloacetic acid (OAA) to form 6-carbon citric acid. Through one full turn it releases 2 CO2 (2 decarboxylations), makes 3 NADH, 1 FADH2 and 1 GTP, and regenerates OAA so the cycle can repeat. Memory hook: "Citrate Is Krebs' Starting Substrate For Making Oxaloacetate" = Citrate, Isocitrate, alpha-Ketoglutarate, Succinyl CoA, Succinate, Fumarate, Malate, Oxaloacetate.
One turn of the Krebs cycle: acetyl CoA (2C) joins OAA (4C) to form citric acid (6C). Two decarboxylation steps release 2 CO2, and the cycle yields 3 NADH, 1 FADH2 and 1 GTP before regenerating OAA. Occurs in the mitochondrial matrix.
Your doubts, answered
Does the Krebs cycle start with acetyl CoA joining pyruvic acid or oxaloacetic acid?
It starts with the acetyl group (from acetyl CoA) joining oxaloacetic acid (OAA), NOT pyruvic acid. This condensation, plus water, gives 6-carbon citric acid and releases free CoA. NCERT says: 'The TCA cycle starts with the condensation of acetyl group with oxaloacetic acid (OAA) and water to yield citric acid.' A very common NEET trap swaps OAA for pyruvic acid.
What is the correct order of intermediates in one turn?
Oxaloacetic acid + acetyl group gives citric acid. Citric acid is isomerised to isocitrate. Isocitrate is decarboxylated to alpha-ketoglutaric acid, then to succinyl CoA (two decarboxylation steps). Succinyl CoA gives succinic acid (making GTP), then fumaric acid, then malic acid, and finally back to oxaloacetic acid. OAA is regenerated so the cycle continues.
How many CO2 molecules leave in one turn of the cycle?
Exactly 2 CO2 per turn, from 2 successive decarboxylation steps: isocitrate to alpha-ketoglutaric acid, and alpha-ketoglutaric acid to succinyl CoA. The pyruvate to acetyl CoA step (link reaction) also releases CO2, but that is NOT counted as part of the TCA cycle itself. This distinction is directly asked in NEET.
Which step makes GTP, and is it ATP?
GTP is made only at the succinyl CoA to succinic acid step. This is a substrate-level phosphorylation (one per turn). In a coupled reaction, GTP passes its phosphate to ADP to form ATP. So the cycle itself makes GTP directly, which is then converted to ATP.
How many NADH and FADH2 come from one turn?
One turn makes 3 NADH (at isocitrate to alpha-ketoglutarate, alpha-ketoglutarate to succinyl CoA, and malate to OAA) and 1 FADH2 (at succinate to fumarate). These reduced coenzymes carry electrons to the ETS to make most of the ATP. Since one glucose gives 2 acetyl CoA, the cycle turns twice per glucose.
Why is oxaloacetic acid called the first member if it is also the last product?
OAA is both the acceptor at the start and the product at the end, which is why the cycle is a closed loop. Each turn consumes one OAA to grab an acetyl group, and regenerates one OAA at the end. NCERT stresses that continued oxidation of acetyl CoA needs continued replenishment of OAA.
⚠️ The NEET trap ✗ The TCA cycle starts with condensation of the acetyl group with pyruvic acid to yield citric acid. ✓ The TCA cycle starts with condensation of the acetyl group with oxaloacetic acid (OAA) to yield citric acid. 🧠 Acetyl CoA meets OAA, never pyruvic acid. Pyruvic acid is used in the link reaction BEFORE the cycle (pyruvate to acetyl CoA), not inside it. NEET 2017 asked exactly this as the 'wrong statement'.
Real NEET questions
2017
Which statement is wrong for Krebs' cycle?
A · There are three points in the cycle where NAD+ is reduced to NADH + H+
B · There is one point in the cycle where FAD+ is reduced to FADH2
C · During conversion of succinyl CoA to succinic acid, a molecule of GTP is synthesised
D · The cycle starts with condensation of acetyl group (acetyl CoA) with pyruvic acid to yield citric acid ✓
Solution: Option D is the wrong (and hence correct-to-pick) statement. The TCA cycle begins with condensation of the acetyl group with oxaloacetic acid (OAA), not pyruvic acid, to form citric acid. The other three are true per NCERT: 3 NADH points, 1 FADH2 point, and GTP made at the succinyl CoA to succinic acid step.
2023
How many times decarboxylation occurs during each TCA cycle?
A · Once
B · Twice ✓
C · Thrice
D · Many
Solution: Twice. The two successive decarboxylation steps are isocitrate to alpha-ketoglutaric acid, and alpha-ketoglutaric acid to succinyl CoA, each releasing one CO2 (total 2 CO2 per turn). The pyruvate to acetyl CoA decarboxylation is the link reaction and is not counted as part of the TCA cycle.
2023
Match List I with List II: A. Oxidative decarboxylation, B. Glycolysis, C. Oxidative phosphorylation, D. Tricarboxylic acid cycle; I. Citrate synthase, II. Pyruvate dehydrogenase, III. Electron transport system, IV. EMP pathway.
A · A-III, B-IV, C-II, D-I
B · A-II, B-IV, C-I, D-III
C · A-III, B-I, C-II, D-IV
D · A-II, B-IV, C-III, D-I ✓
Solution: A-II, B-IV, C-III, D-I. Oxidative decarboxylation of pyruvate is done by pyruvate dehydrogenase. Glycolysis is the EMP pathway. Oxidative phosphorylation is driven by the electron transport system. The TCA cycle begins with citrate synthase condensing the acetyl group with OAA.
Solved Respiration in Plants NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
In the mitochondrial matrix, the fluid inside the inner mitochondrial membrane. The matrix holds the enzymes of the citric acid cycle. The ETS that follows is on the inner mitochondrial membrane.
How many turns of the cycle occur per glucose molecule?
Two turns per glucose. One glucose gives 2 pyruvate in glycolysis, and each pyruvate becomes one acetyl CoA, so 2 acetyl CoA enter the cycle. So per glucose the cycle yields 6 NADH, 2 FADH2 and 2 GTP.
Which enzyme catalyses the first step of the cycle?
Citrate synthase. It joins the acetyl group with oxaloacetic acid and water to make citric acid, releasing free CoA.
What is the total ATP-equivalent output written for this phase?
NCERT gives the cycle products as 3 NADH, 1 FADH2 and 1 GTP (ATP) per turn. Actual ATP is made later, when NADH and FADH2 pass electrons to the ETS. The cycle itself directly makes only the GTP by substrate-level phosphorylation.
Why must NAD+ and FAD+ be regenerated?
The cycle needs a fresh supply of NAD+ and FAD+ to keep accepting electrons. The ETS regenerates them by re-oxidising NADH and FADH2. Without O2 and the ETS, these run out and the cycle stops.
