outline the Krebs cycle, explaining that oxaloacetate (4C) acts as an acceptor of the 2C fragment from acetyl coenzyme A to form citrate (6C), which is converted back to oxaloacetate in a series of small steps

Respiration – The Krebs (Citric Acid) Cycle

1. Where the Cycle Fits into Aerobic Respiration

Aerobic respiration in eukaryotes proceeds through four linked stages. The Krebs cycle is the third stage and links the products of glycolysis to oxidative phosphorylation.

1. Glycolysis (cytosol) – glucose → 2 pyruvate + 2 ATP (substrate‑level) + 2 NADH
2. Link reaction (mitochondrial matrix) – pyruvate → acetyl‑CoA + CO₂ + NADH (see section 2)
3. Krebs cycle (mitochondrial matrix) – oxidation of acetyl‑CoA, generation of NADH, FADH₂ and GTP (see sections 3‑5)
4. Oxidative phosphorylation (inner mitochondrial membrane) – NADH & FADH₂ donate electrons to the electron‑transport chain (ETC); the resulting proton‑motive force drives ATP synthase (see section 6)

The overall yield from one molecule of glucose is approximately 30 ATP equivalents (including the 2 ATP from glycolysis, 2 GTP from the Krebs cycle, and the ATP generated from the 10 NADH + 2 FADH₂ produced).

2. The Link Reaction (Pyruvate → Acetyl‑CoA)

Before entering the Krebs cycle each pyruvate is converted by the pyruvate dehydrogenase complex (PDH):

• Co‑enzyme A (CoA‑SH) provides the thiol group that accepts the 2‑C acetyl fragment.
• One NADH and one CO₂ are produced per pyruvate (i.e. per acetyl‑CoA).
• The reaction is irreversible and is a major point of regulation (inhibited by high NADH/NAD⁺ ratio and acetyl‑CoA).

3. Overview of the Krebs Cycle

Each turn of the cycle processes one acetyl‑CoA (2 C) and regenerates oxaloacetate (4 C), allowing the cycle to continue indefinitely while oxidising the acetyl group to CO₂.

3.1 Net Reaction (per acetyl‑CoA)

3.2 Step‑by‑Step Summary

1. Condensation (citrate synthase)
   

   Oxaloacetate (4 C) + Acetyl‑CoA (2 C) → Citrate (6 C) + CoA‑SH

2. Isomerisation (aconitase)
   

   Citrate ⇌ cis‑Aconitate ⇌ Isocitrate

3. First oxidative decarboxylation (isocitrate dehydrogenase)
   

   Isocitrate + NAD⁺ → α‑Ketoglutarate (5 C) + CO₂ + NADH

4. Second oxidative decarboxylation (α‑ketoglutarate dehydrogenase complex)
   

   α‑Ketoglutarate + NAD⁺ + CoA‑SH → Succinyl‑CoA (4 C) + CO₂ + NADH

5. Substrate‑level phosphorylation (succinyl‑CoA synthetase)
   

   Succinyl‑CoA + GDP + P_i → Succinate + GTP + CoA‑SH

6. Oxidation (succinate dehydrogenase – Complex II of the ETC)
   

   Succinate + FAD → Fumarate + FADH₂

7. Hydration (fumarase)
   

   Fumarate + H₂O → Malate

8. Final oxidation (malate dehydrogenase)
   

   Malate + NAD⁺ → Oxaloacetate + NADH

3.3 Summary Table of Cofactor Production

3.4 Net ATP Yield per Acetyl‑CoA

• 3 NADH × 2.5 ATP = 7.5 ATP
• 1 FADH₂ × 1.5 ATP = 1.5 ATP
• 1 GTP ≈ 1 ATP
• Total ≈ 10 ATP equivalents per acetyl‑CoA (rounded to 10 ATP in the Cambridge syllabus).

4. Oxidative Phosphorylation – How the Reduced Cofactors Generate ATP

The NADH and FADH₂ produced in the Krebs cycle (and in glycolysis) donate their electrons to the electron‑transport chain (ETC) located in the inner mitochondrial membrane.

1. Complex I (NADH → CoQ) – pumps 4 H⁺ per NADH.
2. Complex II (Succinate → FADH₂ → CoQ) – does not pump protons.
3. Complex III (CoQ → Cytochrome c) – pumps 4 H⁺.
4. Complex IV (Cytochrome c → O₂) – pumps 2 H⁺; O₂ is reduced to H₂O.

The proton gradient (≈ 4 H⁺ per ATP) drives ATP synthase (Complex V) to phosphorylate ADP → ATP (chemiosmosis). The overall P/O ratios used in the syllabus are:

• ~2.5 ATP per NADH
• ~1.5 ATP per FADH₂

5. Regulation of the Krebs Cycle

Control is exerted mainly at three irreversible steps, ensuring the cycle matches the cell’s energy demand.

• Citrate synthase – inhibited by high ATP and citrate; activated by ADP.
• Isocitrate dehydrogenase (NAD⁺‑dependent) – activated by ADP and NAD⁺; inhibited by ATP, NADH and low pH.
• α‑Ketoglutarate dehydrogenase complex – activated by Ca²⁺ (in muscle) and NAD⁺; inhibited by ATP, NADH and succinyl‑CoA.

Overall, a high energy state (high ATP, NADH) down‑regulates the cycle, while an energy‑poor state (high ADP, NAD⁺, inorganic phosphate) up‑regulates it.

6. Glossary of Symbols (first use)

• CoA‑SH – co‑enzyme A with a free thiol group.
• P_i – inorganic phosphate.
• NAD⁺ / NADH – oxidised / reduced nicotinamide adenine dinucleotide.
• FAD / FADH₂ – oxidised / reduced flavin adenine dinucleotide.
• GTP / ATP – guanosine / adenosine triphosphate (energy‑currency).

7. Key Points to Remember (Cambridge AO1 & AO2)

• Oxaloacetate (4 C) is the recurring acceptor that combines with acetyl‑CoA (2 C) to form citrate (6 C); the cycle regenerates oxaloacetate each turn.
• Each acetyl‑CoA yields 3 NADH, 1 FADH₂, 1 GTP and 2 CO₂.
• Reduced cofactors feed the ETC, producing ~10 ATP equivalents per acetyl‑CoA; together with glycolysis and the link reaction this gives ~30 ATP per glucose.
• Regulation occurs at citrate synthase, isocitrate dehydrogenase and α‑ketoglutarate dehydrogenase, responding to the cellular ADP/ATP and NAD⁺/NADH ratios.
• Location: mitochondrial matrix in eukaryotes (cytosol in many prokaryotes).