Aerobic Respiration

A-Level Biology · Energy Transfers: Respiration and Photosynthesis

Respiration and ATP

Respiration releases energy from glucose to make ATP — the universal energy currency of cells. ATP (adenosine triphosphate) releases energy when its terminal phosphate bond is broken (ATP → ADP + Pi), and is remade using energy from respiration. Aerobic respiration (with oxygen) occurs in four stages.

Stage 1: Glycolysis (in the cytoplasm)

One glucose (6C) is phosphorylated using 2 ATP, then split into two pyruvate (3C) molecules.
Net gain: 2 ATP and 2 reduced NAD (NADH).
Does not require oxygen (so it also occurs in anaerobic respiration).

Stage 2: The link reaction (in the mitochondrial matrix)

Each pyruvate is decarboxylated (loses CO₂) and oxidised, forming acetyl which combines with coenzyme A to form acetyl coenzyme A.
Produces reduced NAD. (Happens twice per glucose.)

Stage 3: The Krebs cycle (in the matrix)

Acetyl CoA (2C) combines with a 4C molecule to form citrate (6C).
A series of reactions releases CO₂, and produces reduced NAD, reduced FAD, and a small amount of ATP (by substrate-level phosphorylation).
Runs twice per glucose.

Stage 4: Oxidative phosphorylation (on the inner mitochondrial membrane / cristae)

Reduced NAD and FAD release electrons to the electron transport chain.
Electrons pass down the chain, releasing energy that pumps H⁺ into the intermembrane space, creating a gradient.
H⁺ flow back through ATP synthase (chemiosmosis), producing the bulk of the ATP (~26–28 ATP).
Oxygen is the final electron acceptor, combining with electrons and H⁺ to form water. Without oxygen this stage stops.

Total: ~30–32 ATP per glucose (theoretical ~38).

Anaerobic respiration

Without oxygen, only glycolysis runs. To keep it going, pyruvate is converted to lactate (animals) or ethanol + CO₂ (yeast/plants), regenerating NAD — but yielding only the 2 ATP from glycolysis.

Worked example

Why does the electron transport chain stop without oxygen?

Oxygen is the final electron acceptor. Without it, electrons (and H⁺) have nowhere to go, so the chain backs up, NAD/FAD can't be re-oxidised, and ATP production by oxidative phosphorylation halts. ✓

Common mistakes

Placing glycolysis in the mitochondria — it's in the cytoplasm.
Forgetting the link reaction and Krebs cycle run twice per glucose.
Saying oxygen is used throughout — it's only needed at the final stage.

Exam tips

Learn the location, inputs and outputs of each of the four stages.
Emphasise oxidative phosphorylation/chemiosmosis produces most of the ATP.
Know that anaerobic respiration regenerates NAD so glycolysis can continue.

Key facts to remember

Respiration makes ATP; aerobic stages: glycolysis (cytoplasm) → link reaction → Krebs cycle (matrix) → oxidative phosphorylation (cristae).
Reduced NAD/FAD feed the electron transport chain; H⁺ drives ATP synthase (chemiosmosis); oxygen = final electron acceptor → water.
~30–32 ATP aerobically; anaerobically only glycolysis runs (2 ATP, + lactate or ethanol to regenerate NAD).