Lesson 6: Cellular Respiration — Glycolysis to Oxidative PhosphorylationAP

If photosynthesis stores energy, cellular respiration is how the cell spends it. The CED (Topic 3.6) frames respiration as a controlled, stepwise release of the energy in glucose — controlled, because burning glucose in one step would waste most of its energy as heat, whereas passing its electrons down a chain of carriers lets the cell bank the energy in about 30–32 ATP per glucose. The electrons are the currency: glucose is progressively oxidised (loses electrons), oxygen is reduced (gains them) at the very end, and the energy difference between those two states is what powers ATP production. Learn this lesson as an electron journey and the stages will never blur together again.

Stage 1: Glycolysis — the universal first step

Glycolysis runs in the cytosol, needs no oxygen, and is found in nearly every living organism — evidence it evolved before free oxygen existed. The cell invests 2 ATP to prime glucose, then splits the 6-carbon sugar into two 3-carbon pyruvate molecules, harvesting 4 ATP (by substrate-level phosphorylation, where an enzyme hands a phosphate directly to ADP) and loading 2 NADH with electrons. Net gain per glucose: 2 ATP and 2 NADH. Everything else in aerobic respiration happens to squeeze out the energy still stored in pyruvate and in those carriers.

Stages 2 and 3: Pyruvate oxidation and the Krebs cycle

Pyruvate enters the mitochondrial matrix, where each pyruvate loses a CO₂ and its remaining 2-carbon acetyl group is attached to coenzyme A (acetyl CoA), generating one NADH per pyruvate. The Krebs cycle (citric acid cycle) then oxidises the acetyl group completely: per glucose (two turns), it yields 4 CO₂, 6 NADH, 2 FADH₂ and 2 ATP (again by substrate-level phosphorylation). Notice the pattern: the carbon is fully accounted for — 6 CO₂ released for the 6 carbons of glucose, mostly as waste. The cycle's real harvest is electrons: 8 NADH and 2 FADH₂ per glucose so far (counting glycolysis and pyruvate oxidation) now queue up to cash in.

Stage 4: Oxidative phosphorylation — where most ATP is made

NADH and FADH₂ deliver their electrons to the electron transport chain, a series of protein complexes in the inner mitochondrial membrane. As electrons pass from complex to complex towards oxygen, the released energy pumps protons (H⁺) from the matrix into the intermembrane space, creating an electrochemical gradient — a dam of potential energy. Oxygen is the final electron acceptor: it combines with the spent electrons and protons to form water. Protons flow back into the matrix only through ATP synthase, whose spinning channel couples that flow to ADP + Pi → ATP. This is chemiosmosis, the same mechanism as photophosphorylation in Lesson 5. Typical modern yields: each NADH from the matrix ≈ 2.5 ATP, each FADH₂ ≈ 1.5 ATP, giving a total of roughly 30–32 ATP per glucose under ideal conditions (textbooks differ slightly; the exam rewards the reasoning, not one magic number).

When there is no oxygen: fermentation

Fermentation solves one problem only: glycolysis consumes NAD⁺ to make NADH, and without a way to unload NADH, glycolysis halts and the cell gets zero ATP. Alcoholic fermentation (yeast) converts pyruvate to ethanol + CO₂, regenerating NAD⁺. Lactic acid fermentation (muscle cells, lactobacilli) converts pyruvate directly to lactate, also regenerating NAD⁺. The yield is just the 2 net ATP from glycolysis — a poor return, but fast and oxygen-independent. Respiration of other fuels connects here too: fatty acids are broken to acetyl CoA (which is why fats yield more energy per gram — they feed more electrons into the chain), and amino acids enter at several cycle points after losing their nitrogen (deamination).

Worked example 1: the full ATP ledger

Build the per-glucose account, using 2.5 ATP per mitochondrial NADH, 1.5 per FADH₂, and assuming cytosolic NADH from glycolysis is shuttled in at a value of 1.5 ATP each.

Total ≈ 30 ATP. Two lessons from the ledger: oxidative phosphorylation provides the overwhelming majority (≈26 of 30), and the exact total depends on shuttle costs and conditions — which is why a cell poisoned at the electron transport chain loses far more ATP than one missing a single glycolytic enzyme step.

Worked example 2: the respirometer experiment

In the classic AP lab, a respirometer containing germinating peas and a KOH pellet (which absorbs CO₂) shows its fluid marker moving towards the peas by 0.8 mL in 20 minutes. A second respirometer with glass beads only moves 0.05 mL; a third, with germinating peas but no KOH, moves 0.3 mL.

Interpret each. The beads-only control measures gas changes from temperature and pressure drift alone: 0.05 mL, which must be subtracted as correction. With KOH, all CO₂ produced is absorbed, so gas volume change = oxygen consumed: corrected respiration rate = 0.8 − 0.05 = 0.75 mL O₂ per 20 minutes. Without KOH, the peas simultaneously consume O₂ and release CO₂, so the net movement (0.3 − 0.05 = 0.25 mL) reflects the difference between the two. For carbohydrate respiration the CO₂ released ≈ O₂ consumed (respiratory quotient ≈ 1), so near-zero net movement without KOH is expected at steady state — the small net fall suggests slightly more O₂ consumed than CO₂ released. The exam loves the control correction and the KOH logic; state both explicitly.

Where this shows up in the real world

Poisons target this pathway with terrible precision. Cyanide blocks the final complex of the electron transport chain, so electrons back up, the proton gradient collapses and ATP production stops within minutes — cells die of energy failure even though the blood is full of oxygen. Conversely, the uncoupler 2,4-dinitrophenol (once sold as a diet drug) makes the inner membrane leaky to protons: the gradient dissipates as heat instead of ATP, which is why it caused fatal overheating. Brown fat in infants uses a natural uncoupling protein (thermogenin) for exactly this purpose — burning the gradient for warmth.

Common mistakes

• Placing glycolysis in the mitochondria. It is cytosolic; only pyruvate oxidation and the Krebs cycle are in the matrix, and the chain is in the inner membrane.
• Saying the Krebs cycle makes most of the ATP. Directly it makes 2 per glucose; its NADH/FADH₂ products fund oxidative phosphorylation, which makes most of it.
• Claiming fermentation produces 36 ATP, or any ATP beyond glycolysis. Fermentation's own steps make none; the total is the 2 net ATP from glycolysis, and its true purpose is regenerating NAD⁺.
• Describing oxygen as being used in glycolysis or the Krebs cycle. Oxygen acts only at the end of the electron transport chain, forming water.
• Treating "30–32 ATP" as exact. Yield varies with shuttles, leaks and cell type; the exam accepts a range and rewards correct accounting.
• Forgetting that CO₂ comes from pyruvate oxidation and the Krebs cycle, not from the electron transport chain. The carbon leaves before the electrons reach oxygen.

AP Exam Experience

Question 1. During cellular respiration, most ATP is produced by

• (A) glycolysis in the cytosol
• (B) substrate-level phosphorylation in the Krebs cycle
• (C) the flow of protons through ATP synthase, driven by the electron transport chain
• (D) fermentation of pyruvate to lactate

Question 2. Muscle cells switch to lactic acid fermentation during intense exercise primarily because they must

• (A) produce more ATP per glucose than aerobic respiration allows
• (B) regenerate NAD⁺ so glycolysis can continue making ATP without oxygen
• (C) convert accumulated CO₂ into glucose
• (D) stop glycolysis until oxygen returns

Question 3. A mitochondrion's inner membrane is made freely permeable to protons by an uncoupling chemical, while the electron transport chain continues to run. The immediate result is that

• (A) oxygen consumption stops and ATP production rises
• (B) electrons stop flowing because the chain requires a gradient
• (C) the proton gradient dissipates, ATP production falls sharply, and energy is released as heat
• (D) NADH accumulates because glycolysis stops

Answer key

• 1. (C) — Oxidative phosphorylation produces roughly 26 of the ~30 ATP per glucose; glycolysis and the Krebs cycle contribute only 2 each by substrate-level phosphorylation.
• 2. (B) — Fermentation yields only 2 ATP but regenerates the NAD⁺ glycolysis consumes; without it, glycolysis would stall and ATP output would be zero.
• 3. (C) — Uncouplers collapse the gradient that ATP synthase needs. The chain keeps running (oxygen is still consumed) but the energy escapes as heat instead of ATP.

Free-response question. Scientists measure oxygen consumption in isolated mitochondria. Adding glucose alone produces little oxygen uptake. Adding ADP and inorganic phosphate (Pi) causes a rapid increase in oxygen consumption, which slows again when the ADP is used up. Adding a small amount of cyanide stops oxygen consumption entirely. (a) Explain why mitochondria consume little oxygen until ADP and Pi are added. (b) Explain why oxygen consumption slows once the added ADP is exhausted, even though fuel remains. (c) Explain the effect of cyanide in terms of electron flow and ATP production. (d) The scientists then add a chemical that makes the inner membrane permeable to protons. Predict the effect on oxygen consumption and on ATP production, and justify each prediction.

Scoring guideline

• (a) 1 point: ATP synthase needs ADP + Pi as substrates; without them, protons cannot flow back through the synthase, the gradient builds to a maximum, and the electron transport chain stalls — so oxygen (the final acceptor) is not consumed.
• (b) 1 point: once ADP is converted to ATP, the same back-pressure re-establishes: the steep gradient resists further proton pumping, electron flow slows, and oxygen uptake falls (this coupling is called respiratory control).
• (c) 1 point: cyanide blocks the final electron transfer to oxygen; electrons back up along the chain (carriers stay reduced), proton pumping stops, the gradient collapses, and ATP synthesis by chemiosmosis ceases.
• (d) 1 point: predicts oxygen consumption continues or increases (chain runs freely without back-pressure). 1 point: predicts ATP production falls toward zero because the gradient dissipates as heat rather than driving ATP synthase (uncoupling).