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Botany · Respiration in Plants

Krebs Cycle (TCA Cycle)

The Krebs cycle — formally the tricarboxylic acid (TCA) or citric acid cycle — is the central hub of aerobic respiration. Anchored in the mitochondrial matrix, it accepts acetyl-CoA from pyruvate oxidation and drives two rounds of decarboxylation and four rounds of coenzyme reduction per glucose. NEET has examined this subtopic in 2017, 2020, 2023 and 2024, making it one of the highest-yield single topics in Plant Physiology. Mastery requires knowing every intermediate carbon count, every coenzyme reduced, and the single substrate-level phosphorylation step.

NCERT Grounding

NCERT Class XI Biology, Chapter 12 (Respiration in Plants), Section 12.4.1 describes the TCA cycle as starting with the condensation of the acetyl group with oxaloacetic acid (OAA) and water to yield citric acid, catalysed by citrate synthase. The text explicitly lists three points of NAD+ reduction and one point of FAD+ reduction, along with one substrate-level GTP synthesis during succinyl-CoA → succinate conversion.

"The TCA cycle starts with the condensation of acetyl group with oxaloacetic acid (OAA) and water to yield citric acid."

NCERT Class XI Biology, Chapter 12, §12.4.1

The NIOS Biology Chapter 12 corroborates this, summarising the cycle as: 2 pyruvic acid + 8 NAD + 2 FAD + 2 ADP → 6 CO₂ + 8 NADH + 2 FADH₂ + 2 ATP, which counts pyruvate oxidation yields within the mitochondrial segment. Both sources agree that the cycle occurs in the mitochondrial matrix and runs twice per glucose molecule.

Pyruvate to Acetyl-CoA — The Gateway Reaction

Before the Krebs cycle can begin, pyruvate (3C) produced in the cytosol by glycolysis must be transported into the mitochondrial matrix. There it undergoes oxidative decarboxylation — a combined oxidation and decarboxylation — catalysed by the pyruvate dehydrogenase complex (PDH complex), which requires NAD+ and Coenzyme A as cofactors.

Pyruvate → Acetyl-CoA (per molecule)

Occurs in mitochondrial matrix
  1. Start

    Pyruvate (3C)

    Enters mitochondrial matrix from cytosol

  2. Enzyme

    Pyruvate Dehydrogenase

    PDH complex: requires NAD⁺ + CoA

    Mg²⁺ cofactor
  3. Products

    Acetyl-CoA (2C)

    +1 CO₂ released, +1 NADH formed

    Per pyruvate
  4. Per Glucose

    ×2 Turns

    2 CO₂ + 2 NADH from 2 pyruvates

The acetyl group (2C) is handed to Coenzyme A, forming acetyl-CoA — the molecule that enters the Krebs cycle proper. This oxidative decarboxylation is irreversible and represents the committed step toward complete aerobic oxidation of glucose carbon.

TCA Cycle — Step-by-Step Mechanism

The Eight Reactions

The TCA cycle is a closed loop of eight enzymatic reactions. Each turn processes one acetyl-CoA (2C) and regenerates oxaloacetate (4C) to accept the next acetyl group. The intermediate carbon counts and coenzyme yields are fixed and NEET-examined.

Step Reaction Carbons Enzyme Yield
1 Acetyl-CoA + OAA + H₂O → Citrate 2C + 4C → 6C Citrate synthase CoA released
2 Citrate → Isocitrate 6C → 6C Aconitase Isomerisation
3 Isocitrate → α-Ketoglutarate 6C → 5C Isocitrate dehydrogenase 1 NADH + 1 CO₂
4 α-Ketoglutarate → Succinyl-CoA 5C → 4C α-Ketoglutarate dehydrogenase 1 NADH + 1 CO₂
5 Succinyl-CoA → Succinate 4C → 4C Succinyl-CoA synthetase 1 GTP (substrate-level)
6 Succinate → Fumarate 4C → 4C Succinate dehydrogenase* 1 FADH₂
7 Fumarate → Malate 4C → 4C Fumarase H₂O added
8 Malate → OAA 4C → 4C Malate dehydrogenase 1 NADH

*Succinate dehydrogenase is the only TCA enzyme embedded in the inner mitochondrial membrane; all others are soluble in the matrix.

Figure 1 — TCA Cycle Schematic (inline SVG) Krebs Cycle (TCA Cycle) — carbon flow and coenzyme yields KREBS CYCLE OAA (4C) Citrate (6C) Isocitrate (6C) α-KG (5C) Succinyl CoA (4C) Succinate (4C) Fumarate (4C) Malate (4C) Acetyl-CoA (2C) NADH +CO₂ NADH +CO₂ GTP FADH₂ NADH citrate synthase

Figure 1. One turn of the Krebs cycle. Acetyl-CoA (2C, amber arrow) condenses with OAA (4C) to form citrate (6C). Two decarboxylation steps (steps 3 and 4) release 2 CO₂. The single GTP (purple) is the only substrate-level phosphorylation. Three NADH and one FADH₂ are harvested per turn.

What "Per Turn" Means in Practice

Each glucose molecule produces two pyruvates, each yielding one acetyl-CoA. The Krebs cycle therefore runs twice per glucose. The per-turn tally must be doubled to obtain the per-glucose contribution from this stage.

3

NADH per turn

Steps 3, 4, 8. Each NADH feeds the electron transport system, yielding ~2.5 ATP per NADH under modern chemiosmotic accounting (NCERT uses 3).

+
1

FADH₂ per turn

Step 6 (succinate → fumarate). FADH₂ enters ETS at a lower energy level than NADH, yielding ~1.5 ATP per FADH₂ (NCERT uses 2).

Energy Accounting — Per Turn and Per Glucose

Molecule Per Turn (1 Acetyl-CoA) Per Glucose (2 Turns) Note
NADH 3 6 Steps 3, 4, 8
FADH₂ 1 2 Step 6 (succinate dehydrogenase)
GTP 1 2 Substrate-level; Step 5 only
CO₂ 2 4 Steps 3 + 4 (decarboxylations)

Adding the 2 NADH produced during pyruvate oxidation (bridge reaction, before the cycle), the total mitochondrial contribution becomes 8 NADH + 2 FADH₂ + 2 GTP per glucose — a figure NEET questions sometimes test as a combined tally. The grand total including glycolysis NADH is 10 NADH + 2 FADH₂ + 4 ATP (2 from glycolysis + 2 GTP) before ETS.

Substrate-level phosphorylation in the Krebs cycle occurs at exactly one point: succinyl-CoA → succinate, catalysed by succinyl-CoA synthetase, generating 1 GTP (equivalent to 1 ATP). No other step in the cycle directly phosphorylates ADP.

Per Glucose: Krebs Alone

2 ATP

Direct (as GTP) from substrate-level

6 NADH → ETS (18 ATP classical)

2 FADH₂ → ETS (4 ATP classical)

NEET 2020 Q.36

Bridge Reaction (×2)

2 NADH

From pyruvate → acetyl-CoA

2 CO₂ released (not counted in cycle turn)

Enzyme: pyruvate dehydrogenase

NEET 2023 Q.148

Carbon Disposal

4 CO₂

Released per glucose via Krebs

Plus 2 CO₂ from bridge reaction = 6 CO₂ total

Accounts for all 6 carbons of glucose

Enzyme Locations — A NEET-Specific Detail

All Krebs cycle enzymes are soluble proteins in the mitochondrial matrix, with one critical exception: succinate dehydrogenase (the enzyme of Step 6) is embedded in the inner mitochondrial membrane. This positioning is not incidental — it allows FADH₂, which is tightly bound to the enzyme, to directly donate its electrons to ubiquinone (Coenzyme Q) in the ETS without entering the matrix.

Enzyme Location Comparison — Krebs Cycle vs ETS

Mitochondrial Matrix

7 of 8

Krebs cycle enzymes here

  • Citrate synthase (Step 1)
  • Aconitase (Step 2)
  • Isocitrate dehydrogenase (Step 3)
  • α-Ketoglutarate dehydrogenase (Step 4)
  • Succinyl-CoA synthetase (Step 5)
  • Fumarase (Step 7)
  • Malate dehydrogenase (Step 8)
vs

Inner Mitochondrial Membrane

1 of 8

Exception: succinate dehydrogenase

  • Succinate dehydrogenase (Step 6) — the FADH₂-generating enzyme
  • Also the site of all ETS complexes (I–IV) and ATP synthase (Complex V)
  • Proton gradient spans inner membrane (intermembrane space vs matrix)
  • NEET 2024 Q.141 links ETS to inner membrane explicitly

Worked Examples

Worked example 1

A student states: "One turn of the Krebs cycle produces 2 substrate-level ATP." Is this correct? Explain.

Answer — Incorrect. Only one substrate-level phosphorylation occurs per turn: the conversion of succinyl-CoA to succinate by succinyl-CoA synthetase yields one GTP (which is equivalent to one ATP). No other step in the cycle directly phosphorylates ADP. This was the correct answer to NEET 2020 Q.36: "The number of substrate-level phosphorylations in one turn of the citric acid cycle is One."

Worked example 2

How many total CO₂ molecules are released when one molecule of glucose undergoes complete aerobic respiration up to and including the Krebs cycle (excluding ETS)?

Answer — 6 CO₂. Pyruvate oxidation (bridge reaction): 2 pyruvates × 1 CO₂ each = 2 CO₂. Krebs cycle: 2 turns × 2 CO₂ per turn (steps 3 and 4 each release 1 CO₂) = 4 CO₂. Total: 2 + 4 = 6 CO₂, which accounts for all six carbons in glucose (C₆H₁₂O₆).

Worked example 3

Identify the first intermediate formed in the Krebs cycle, its carbon count, and the enzyme responsible.

Answer — Citric acid (citrate), 6 carbons. Acetyl-CoA (2C) condenses with oxaloacetate (4C) and water under the catalysis of citrate synthase to yield citrate (6C) and free CoA. This is the committed, first step of the cycle and is the basis of the name "citric acid cycle." Note: the cycle is named after citric acid, a tricarboxylic acid (three –COOH groups), hence also "tricarboxylic acid cycle."

Worked example 4

Why is succinate dehydrogenase considered structurally unique among Krebs cycle enzymes?

Answer. Succinate dehydrogenase (Complex II of ETS) is the only Krebs cycle enzyme that is an integral protein of the inner mitochondrial membrane, not a soluble matrix enzyme. It is directly coupled to the electron transport chain — FADH₂ produced by this enzyme donates electrons directly to ubiquinone (CoQ) without entering the matrix pool. This anatomical integration also explains why FADH₂ enters the ETS at a lower energy point than NADH, producing fewer ATP (2 vs 3 by classical accounting).

Common Confusion & NEET Traps

Pyruvate Oxidation vs Krebs Cycle — Common Confusion Points

Pyruvate Oxidation (Bridge)

Pre-cycle

In mitochondrial matrix

  • Pyruvate (3C) → Acetyl-CoA (2C) + CO₂
  • Enzyme: pyruvate dehydrogenase complex
  • Yields 1 NADH per pyruvate (2 per glucose)
  • Not part of the Krebs cycle proper
  • Irreversible committed step
vs

Krebs Cycle (TCA)

8 steps

Cyclic; starts with OAA

  • Acetyl-CoA (2C) + OAA (4C) → Citrate (6C)
  • First enzyme: citrate synthase
  • Yields 3 NADH + 1 FADH₂ + 1 GTP + 2 CO₂ per turn
  • OAA regenerated at end of each turn
  • 2 turns per glucose

NEET PYQ Snapshot — Krebs Cycle (TCA Cycle)

Four official NEET questions (2017–2024) directly test this subtopic — the highest density for any single Krebs cycle concept cluster.

NEET 2017 · Q.80

Which statement is wrong for Krebs' cycle?

  1. The cycle starts with condensation of acetyl group (acetylCoA) with pyruvic acid to yield citric acid.
  2. There are three points in the cycle where NAD⁺ is reduced to NADH+H⁺
  3. There is one point in the cycle where FAD⁺ is reduced to FADH₂
  4. During conversion of succinyl CoA to succinic acid, a molecule of GTP is synthesised
Answer: (1)

Why: Option (1) is wrong because the Krebs cycle starts with acetyl-CoA condensing with OAA (oxaloacetic acid) — not pyruvic acid — to form citric acid. Pyruvic acid is converted to acetyl-CoA in a separate step (oxidative decarboxylation) before the cycle begins. Options (2), (3), and (4) are all correct statements about the cycle.

NEET 2020 · Q.36

The number of substrate level phosphorylations in one turn of citric acid cycle is:

  1. One
  2. Two
  3. Three
  4. Zero
Answer: (1)

Why: Only one substrate-level phosphorylation occurs per turn — the conversion of succinyl-CoA to succinic acid (succinate) by succinyl-CoA synthetase, which directly synthesises one GTP without going through the ETS. All other energy capture in the Krebs cycle is via coenzyme reduction (NADH, FADH₂).

NEET 2023 · Q.148

Match the following and choose the correct option:
A. Oxidative decarboxylation — i. Pyruvate dehydrogenase
B. Glycolysis — ii. EMP pathway
C. Oxidative phosphorylation — iii. ETS
D. TCA cycle — iv. Citrate synthase

  1. A-i, B-ii, C-iii, D-iv
  2. A-ii, B-i, C-iv, D-iii
  3. A-iv, B-iii, C-ii, D-i
  4. A-iii, B-iv, C-i, D-ii
Answer: (1)

Why: Oxidative decarboxylation is catalysed by the pyruvate dehydrogenase complex; glycolysis is the Embden-Meyerhof-Parnas (EMP) pathway; oxidative phosphorylation occurs at the electron transport system (ETS); and the key/committed enzyme of the TCA cycle is citrate synthase (Step 1).

NEET 2024 · Q.141

Match the following and choose the correct option:
A. Citric acid cycle — i. Mitochondrial matrix
B. Glycolysis — ii. Cytoplasm
C. ETS — iii. Inner mitochondrial membrane
D. Proton gradient — iv. Intermembrane space

  1. A-ii, B-i, C-iv, D-iii
  2. A-i, B-ii, C-iii, D-iv
  3. A-iii, B-iv, C-i, D-ii
  4. A-iv, B-iii, C-ii, D-i
Answer: (2)

Why: Citric acid (Krebs) cycle → mitochondrial matrix; Glycolysis → cytoplasm; ETS (electron transport system) → inner mitochondrial membrane; Proton gradient is maintained in the intermembrane space (high [H⁺]) relative to the matrix. This is the standard NCERT location assignment for all four processes.

FAQs — Krebs Cycle (TCA Cycle)

Frequently examined concepts, common confusions, and location-specific facts for NEET aspirants.

Where does the Krebs cycle take place?

The Krebs cycle (TCA cycle) takes place in the mitochondrial matrix. The exception is succinate dehydrogenase, which is embedded in the inner mitochondrial membrane. This is confirmed by NEET 2024 Q.141.

What is the starting molecule of the Krebs cycle?

The Krebs cycle starts with the condensation of acetyl-CoA (2C) with oxaloacetic acid or OAA (4C) and water to form citric acid (6C), catalysed by citrate synthase. It does NOT start with pyruvic acid — a classic NEET 2017 trap.

How many NADH, FADH₂, GTP and CO₂ are produced per turn of the Krebs cycle?

Per single turn: 3 NADH, 1 FADH₂, 1 GTP (equivalent to 1 ATP), and 2 CO₂. Since two turns occur per glucose molecule, the total from the cycle alone is 6 NADH, 2 FADH₂, 2 GTP, and 4 CO₂.

How many substrate-level phosphorylations occur in one turn of the Krebs cycle?

Only one substrate-level phosphorylation occurs per turn: the conversion of succinyl-CoA to succinic acid by succinyl-CoA synthetase, which generates one GTP (equivalent to ATP). This was tested in NEET 2020 Q.36.

What is oxidative decarboxylation and which enzyme catalyses it?

Oxidative decarboxylation is the conversion of pyruvate (3C) to acetyl-CoA (2C) with the release of one CO₂ and one NADH. It is catalysed by the pyruvate dehydrogenase complex and occurs in the mitochondrial matrix before the Krebs cycle begins.

What is the role of oxaloacetate in the Krebs cycle?

Oxaloacetate (OAA, 4C) is both the entry acceptor and the final regenerated product of the Krebs cycle. It accepts the acetyl group from acetyl-CoA to form citrate, and is regenerated at the end of each turn (from malate in Step 8) to keep the cycle running continuously.

Which enzyme catalyses the first step of the Krebs cycle?

Citrate synthase catalyses the condensation of acetyl-CoA (2C) with OAA (4C) and water to form citrate (6C). This is the first and committed step of the TCA cycle. NEET 2023 Q.148 identifies citrate synthase as the key enzyme of the TCA cycle.

Related Topics
Respiration in Plants Back to the full chapter notes hub. Glycolysis The cytoplasmic EMP pathway that feeds pyruvate to the mitochondria. Electron Transport System How NADH and FADH₂ from the Krebs cycle are converted into ATP. Oxidative Phosphorylation Chemiosmosis and ATP synthase — the ATP payoff stage. Respiratory Balance Sheet Net ATP tally across glycolysis, Krebs cycle, and ETS. Fermentation Anaerobic alternative when the Krebs cycle cannot operate. Amphibolic Pathway How Krebs intermediates serve both catabolism and anabolism. Do Plants Breathe? Overview of gaseous exchange and the context of cellular respiration.