state that ATP is synthesised by: transfer of phosphate in substrate-linked reactions, chemiosmosis in membranes of mitochondria and chloroplasts

Energy – Synthesis of ATP

ATP (adenosine‑triphosphate) is the universal energy‑currency of the cell. In Cambridge International AS & A Level Biology (9700) students must be able to state that ATP is synthesised by:

  • Transfer of a phosphate group in substrate‑level (substrate‑linked) phosphorylation reactions.

  • Chemiosmosis in specialised membranes – oxidative phosphorylation in mitochondria and photophosphorylation in chloroplasts.

1. Substrate‑level (substrate‑linked) phosphorylation

High‑energy phosphate groups are transferred directly from a phosphorylated metabolic intermediate to ADP. No membrane gradient is involved.

Pathway (location)Reaction (high‑energy donor → ADP)
Glycolysis – cytosol1,3‑Bisphosphoglycerate + ADP → 3‑Phosphoglycerate + ATP
Citric‑acid cycle – mitochondrial matrixSuccinyl‑CoA + GDP + Pi → Succinate + CoA‑SH + GTP
GTP + ADP → ATP + GDP

2. Chemiosmotic synthesis of ATP (chemiosmosis)

Both oxidative phosphorylation and photophosphorylation obey Peter Mitchell’s chemiosmotic theory: energy released by electron transport is first stored as an electrochemical proton gradient (Δp). The return flow of H⁺ through ATP synthase drives ADP + Pi → ATP.

2.1 Oxidative phosphorylation – mitochondria

Location: inner mitochondrial membrane (IMM); the gradient is between the inter‑membrane space and the matrix.

StepComplex / ComponentKey event (per NADH/FADH₂)
Electron donationNADH → Complex I ; FADH₂ → Complex IIElectrons enter the electron‑transport chain (ETC).
Electron transfer & proton pumpingComplex I, Complex III, Complex IV

  • ≈10 H⁺ pumped per NADH.

  • ≈6 H⁺ pumped per FADH₂.

Final electron acceptorO₂ (Complex IV)O₂ + 4 e⁻ + 4 H⁺ → 2 H₂O.
ATP synthesisATP synthase (F₁F₀‑ATPase)H⁺ flow back into the matrix drives ADP + Pi → ATP.

Overall simplified reaction for one NADH:

NADH + H⁺ + ½O₂ + ADP + Pi → NAD⁺ + H₂O + ATP

2.2 Photophosphorylation – chloroplasts

Location: thylakoid membrane; the gradient is across the thylakoid (lumen ↔ stroma).

The electron flow follows the classic Z‑scheme (non‑cyclic) and can also run cyclically around PSI.

ComponentFunction
Photosystem II (PSII)Absorbs light (~680 nm); H₂O → O₂ + 2 e⁻ + 2 H⁺; electrons enter the ETC.
Plastoquinone (PQ) poolCarries electrons from PSII to the cytochrome b₆f complex.
Cytochrome b₆f complexPumps ≈3 H⁺ from the stroma into the lumen per electron transferred.
Photosystem I (PSI)Re‑excites electrons (~700 nm); reduces NADP⁺ → NADPH.
ATP synthase (CF₁CF₀)H⁺ flow from lumen → stroma drives ADP + Pi → ATP.

Non‑cyclic (linear) flow produces both ATP and NADPH; cyclic flow (PSI only) produces ATP alone and helps balance the ATP/NADPH ratio.

Overall simplified photophosphorylation reaction (per 3 ADP):

2 H₂O + 2 NADP⁺ + 3 ADP + 3 Pi → O₂ + 2 NADPH + 3 ATP (light‑driven)

3. Stoichiometry – How many ATP are made?

  • ≈4 H⁺ are required by ATP synthase to generate 1 ATP (including the transport of ADP/Pi).

  • From the proton‑pumping data:

    • ≈10 H⁺ per NADH → 10 ÷ 4 ≈ 2.5 ATP per NADH.

    • ≈6 H⁺ per FADH₂ → 6 ÷ 4 ≈ 1.5 ATP per FADH₂.

  • Typical yield from one molecule of glucose (Cambridge A‑Level approximation):

    • Glycolysis (substrate‑level): 2 ATP.

    • Pyruvate → Acetyl‑CoA (link reaction): 2 NADH → 5 ATP.

    • Citric‑acid cycle: 6 NADH → 15 ATP, 2 FADH₂ → 3 ATP, 2 GTP → 2 ATP.

    • Total ≈ 32 ATP per glucose.

4. ATP in cellular processes (AO1/AO2)

ATP provides the energy for virtually all energy‑requiring activities in the cell.

  • Active transport – e.g. Na⁺/K⁺‑ATPase pumps 3 Na⁺ out and 2 K⁺ in per ATP hydrolysed.

  • Biosynthesis – polymerisation of macromolecules (DNA, RNA, proteins, polysaccharides) requires ATP‑dependent ligases or synthetases.

  • Cell division & motility – spindle formation, cytokinesis, and flagellar/ciliary beating are all ATP‑driven.

5. Linking ATP synthesis to the rest of the syllabus

  • Respiration (12.2) – oxidative phosphorylation is the final stage of aerobic respiration; the amount of ATP generated explains why aerobic metabolism yields far more energy than anaerobic glycolysis.

  • Photosynthesis (13.1) – photophosphorylation supplies the ATP required for the Calvin‑Benson cycle (CO₂ fixation) and for the regeneration of RuBP.

  • Understanding the quantitative yields of ATP allows students to compare the energy budgets of aerobic respiration versus photosynthesis.

6. Calculating the Respiratory Quotient (RQ) (AO3)

RQ formula

RQ = (moles of CO₂ produced) ÷ (moles of O₂ consumed)

Worked example – oxidation of glucose:

C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O

CO₂ produced = 6 mol, O₂ consumed = 6 mol → RQ = 6 ÷ 6 = 1.0. (A value of 1.0 indicates a carbohydrate fuel.)

7. Practical tip (AO3)

  • Measuring ATP production – the luciferin‑luciferase assay gives a rapid, quantitative read‑out of ATP in cell extracts (luminescence ∝ ATP concentration).

  • Respiratory experiments – a closed‑system respirometer (water‑filled syringe) can compare O₂ consumption (and thus oxidative phosphorylation) in resting vs. active tissue samples.

  • Photosynthetic rate – oxygen evolution can be measured with a Clark‑type electrode; the rate correlates with photophosphorylation activity.

Suggested diagram: comparative schematic of (i) substrate‑level phosphorylation, (ii) oxidative phosphorylation (mitochondrion) and (iii) photophosphorylation (chloroplast), showing locations, electron flow and proton gradients.