State where each of the four stages in aerobic respiration occurs in eukaryotic cells: glycolysis in the cytoplasm, link reaction in the mitochondrial matrix, Krebs cycle in the mitochondrial matrix, oxidative phosphorylation on the inner membrane of

Respiration – Cambridge IGCSE/A‑Level Biology (9700) – Topic 12

Learning Outcomes

• 12.1 – Energy need & ATP features

  • Explain why cells need a continual supply of energy.
  • Describe the structural features of ATP that make it the universal energy carrier.

• 12.2 – Location of the four stages of aerobic respiration

  • State where each stage of aerobic respiration occurs in a eukaryotic cell.

Why Cells Need Energy (12.1)

Cellular activities that require a continual supply of ATP include:

• Active transport of ions and nutrients across membranes.
• Synthesis of macromolecules – proteins, nucleic acids and polysaccharides.
• Cell division and DNA replication.
• Maintenance of cell shape, movement and, in poikilotherms, temperature regulation.

ATP is the universal energy carrier because its high‑energy phosphoanhydride bonds can be hydrolysed to release usable energy, and the molecule can be regenerated repeatedly (mainly by oxidative phosphorylation).

Key Features of ATP (12.1)

Location of the Four Stages of Aerobic Respiration (12.2)

Candidates must be able to state where each stage of aerobic respiration occurs in eukaryotic cells.

• Glycolysis – cytoplasm (cytosol)
• Link reaction (pyruvate oxidation) – mitochondrial matrix
• Krebs (citric‑acid) cycle – mitochondrial matrix
• Oxidative phosphorylation (electron‑transport chain + chemiosmosis) – inner mitochondrial membrane (cristae)

Summary Table – Main Products & Approximate ATP Yield

¹Each NADH yields ~2.5 ATP and each FADH₂ yields ~1.5 ATP in the electron‑transport chain (Cambridge values often use 3 ATP per NADH and 2 ATP per FADH₂ – both conventions are accepted).

Key Points for Each Stage

1. Glycolysis (Cytoplasm)

• One glucose (C₆H₁₂O₆) is split into two molecules of pyruvate.
• Energy investment: 2 ATP are used.
• Energy payoff: 4 ATP are produced → net gain = 2 ATP.
• 2 NAD⁺ are reduced to 2 NADH, which later feed the electron‑transport chain.

2. Link Reaction – Pyruvate Oxidation (Mitochondrial Matrix)

• Each pyruvate is transported into the mitochondrion and decarboxylated.
• Products per pyruvate: 1 CO₂, 1 NADH, 1 acetyl‑CoA, 1 H⁺ (2 × per glucose).

3. Krebs (Citric‑Acid) Cycle (Mitochondrial Matrix)

• Acetyl‑CoA (2‑C) combines with oxaloacetate (4‑C) to form citrate (6‑C); a series of reactions regenerates oxaloacetate.
• For each acetyl‑CoA: 3 NADH, 1 FADH₂, 1 GTP (≈ ATP) and 2 CO₂ are released.
• All reactions occur in the mitochondrial matrix.

4. Oxidative Phosphorylation (Inner Mitochondrial Membrane)

• Protein complexes I–IV of the electron‑transport chain (ETC) are embedded in the inner membrane.
• NADH and FADH₂ donate electrons to the ETC; electron flow powers proton pumps that create a H⁺ gradient across the inner membrane.
• ATP synthase uses the return flow of protons (chemiosmosis) to synthesise ≈ 30 ATP per glucose.
• O₂ is the final electron acceptor, forming H₂O.

Overall Equation for Aerobic Respiration

C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + ≈ 38 ATP (≈ 30 ATP from oxidative phosphorylation)

Respiratory Quotient (RQ)

• Definition: Ratio of carbon dioxide produced to oxygen consumed during metabolism.
• Formula: RQ = CO₂ produced / O₂ consumed

Worked Example – Glucose

1. From the overall equation: 1 mol glucose → 6 mol CO₂ + 6 mol O₂.
2. RQ = 6 mol CO₂ / 6 mol O₂ = 1.00.
3. Interpretation: an RQ of 1 indicates that the organism is primarily oxidising carbohydrates.