IB Biology: Cell Respiration

For SL, you need glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation in outline — the inputs, outputs, and where each stage happens in the cell. HL adds real detail: chemiosmosis, the electron transport chain, and how ATP synthase actually works. Both levels need anaerobic respiration in yeast and muscle.

Aerobic respiration uses oxygen to fully oxidise glucose through glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation, yielding roughly 30-32 ATP per glucose molecule. Anaerobic respiration skips the mitochondria entirely — yeast produce ethanol and CO2, human muscle produces lactate — yielding just 2 ATP from glycolysis alone.

No — IB Biology doesn't require you to name every intermediate compound in the Krebs cycle. You need the inputs (acetyl-CoA, oxaloacetate), the outputs per turn (2 CO2, 3 reduced NAD, 1 reduced FAD, 1 ATP) and the location (mitochondrial matrix). Naming citrate or succinate structures wastes revision time.

Cell respiration and photosynthesis are taught as a linked pair in Theme C, and the IB loves asking you to compare them directly. Both use electron transport chains and chemiosmosis to generate ATP, but photosynthesis builds glucose using light energy while respiration breaks it down to release energy — essentially the reverse process.

Cell respiration is examined at both SL and HL, but HL students go much deeper into the mechanism. SL covers the four stages in outline; HL adds chemiosmosis, the electron transport chain's protein complexes, and how the proton gradient drives ATP synthase. The extra HL content is one of the trickier AHL topics.

AHL cell respiration examines the electron transport chain in detail: the protein complexes in the inner mitochondrial membrane, the proton gradient they generate, and chemiosmosis driving ATP synthase's rotary mechanism. You're expected to explain why oxygen is the final electron acceptor and what happens to ATP production if it's absent.

Expect command terms like 'Outline' for the four stages, 'Explain' for why anaerobic respiration yields less ATP, and 'Compare' for aerobic versus anaerobic pathways. Paper 2 often pairs cell respiration with data-based questions on oxygen consumption or respirometer experiments, so practising graph interpretation matters as much as recall.

It's the sheer number of linked stages, each with its own inputs, outputs and location, that trips students up — not any single concept being conceptually hard. In fifteen years of teaching this, the students who struggle are the ones who memorise stage names without understanding why each stage needs the one before it.

The biggest one I see is stating an exact ATP total as if it's fixed, or confusing yields between aerobic and anaerobic respiration. The current guide asks for 'approximately 30-32 ATP' from aerobic respiration, not a single hard number — examiners want that qualifier, not false precision.

Cell respiration isn't graded as an isolated topic — it's woven into Theme C alongside photosynthesis and ecosystem energy flow, which together make up a meaningful chunk of both papers. There's no fixed percentage attached to it alone, but a structured question touching cell respiration appears in most Paper 2 sessions.

Don't just memorise a diagram — trace the actual molecule from glucose to CO2 and ask what happens to each carbon and each electron at every stage. Students who can explain the logic, like why pyruvate must become acetyl-CoA before entering the Krebs cycle, retain it far longer than those who just relabel a flowchart.

Past paper questions beat rereading notes for this topic — cell respiration rewards practice applying the stages to unfamiliar contexts, like respirometers, germinating seeds or altitude training, more than pure recall. Work through topical worksheets covering Theme C, then time yourself on a full Paper 2 question under exam conditions.