Cambridge Notes, Past Papers, Revision Questions

Cambridge A-Level Biology – Energy: ATP Synthesis

Energy – Synthesis of ATP

ATP (adenosine triphosphate) is the universal energy‑currency of the cell. In A‑Level Biology it is essential to know the two principal mechanisms by which ATP is formed:

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

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

1. Substrate‑linked (substrate‑level) phosphorylation

In this process a high‑energy phosphate is transferred directly from a phosphorylated metabolic intermediate to ADP.

Typical example in glycolysis:
\$\text{1,3‑Bisphosphoglycerate} + \text{ADP} \rightarrow \text{3‑Phosphoglycerate} + \text{ATP}\$

Typical example in the citric acid cycle:
\$\text{Succinyl‑CoA} + \text{GDP} + \text{Pi} \rightarrow \text{Succinate} + \text{CoA‑SH} + \text{GTP}\$
(GTP can readily donate its phosphate to ADP to form ATP.)

These reactions occur in the cytosol (glycolysis) or mitochondrial matrix (citric acid cycle) and do not involve a membrane gradient.

2. Chemiosmotic synthesis of ATP

Both oxidative phosphorylation and photophosphorylation rely on the same principle: an electrochemical proton gradient (Δp) across a membrane drives the synthesis of ATP by the enzyme ATP synthase.

2.1 Oxidative phosphorylation (mitochondria)

Electrons from NADH and FADH₂ pass through the electron transport chain (ETC) in the inner mitochondrial membrane. Energy released pumps protons from the matrix into the inter‑membrane space, creating a proton‑motive force.

Step	Location	Key Event
Electron transfer	Inner mitochondrial membrane	Complexes I–I \cdot pass electrons to O₂, forming H₂O.
Proton pumping	Inner mitochondrial membrane	Complexes I, III, I \cdot pump H⁺ into the inter‑membrane space.
ATP synthesis	Matrix (ATP synthase)	H⁺ flow back through ATP synthase drives ADP + Pi → ATP.

Overall reaction (simplified):

\$\text{NADH} + \text{H}^+ + \frac{1}{2}\text{O}2 + \text{ADP} + \text{Pi} \rightarrow \text{NAD}^+ + \text{H}2\text{O} + \text{ATP}\$

2.2 Photophosphorylation (chloroplasts)

Light energy excites electrons in photosystem II. The electrons travel through the thylakoid membrane electron transport chain, generating a proton gradient across the thylakoid membrane.

Component	Function
Photosystem II	Absorbs light, splits water, releases O₂ and electrons.
Cytochrome b₆f complex	Pumps H⁺ into the thylakoid lumen.
Photosystem I	Re‑excites electrons; NADP⁺ is reduced to NADPH.
ATP synthase (CF₁CF₀)	Uses H⁺ flow back to the stroma to form ATP.

Overall photophosphorylation reaction (simplified):

\$2\text{H}2\text{O} + 2\text{NADP}^+ + 3\text{ADP} + 3\text{Pi} + \text{light} \rightarrow \text{O}2 + 2\text{NADPH} + 3\text{ATP}\$

Key Points to Remember

ATP can be formed directly by transferring a phosphate from a high‑energy substrate (substrate‑level phosphorylation).

In chemiosmosis, the energy from electron transport is first stored as a proton gradient, then used by ATP synthase to phosphorylate ADP.

Oxidative phosphorylation occurs in the inner mitochondrial membrane; photophosphorylation occurs in the thylakoid membrane of chloroplasts.

Both processes are essential for supplying the cell with the ATP required for biosynthesis, transport, and movement.

Suggested diagram: Comparative schematic of ATP synthesis by substrate‑level phosphorylation, oxidative phosphorylation (mitochondria) and photophosphorylation (chloroplast).