OCR A Level Biology A: Glycolysis, the link reaction and the Krebs cycle (H420) – Revision Notes

These notes condense learning outcomes 5.2.2(a) to 5.2.2(g) of the OCR A Level Biology A (H420) specification, Version 4.1 (April 2026), first assessment 2025. For full explanations read the study guide, then test yourself with the practice questions. Tick off outcomes on the checklist and find the rest of the course on the OCR A Level Biology hub. The free diagnostics are a quick way to find weak spots. Background on ATP is in nucleotides and ATP.

Key definitions

Term	Meaning
Respiration	Transfer of energy stored in complex organic molecules to ATP
Phosphorylation	Adding a phosphate group
Oxidation	Losing electrons; in these pathways, usually losing hydrogen
Reduction	Gain of electrons, or in these pathways usually gain of hydrogen
Decarboxylation	Removal of carbon dioxide from a substrate
Dehydrogenation	Taking hydrogen off a substrate molecule
Substrate-level phosphorylation	Making ATP by passing a phosphate group directly from a substrate to ADP
Coenzyme	Non-protein organic molecule that helps an enzyme by carrying a group or hydrogen

5.2.2(a) Why respire

• ATP is the immediate energy source. Cells store little, so respiration must keep making it.
• Animals: active transport (sodium–potassium pumps), muscle contraction, synthesis of proteins.
• Plants: active transport of ions into root hair cells, active loading of sugars, making cellulose and other macromolecules.
• Microorganisms: growth, division, synthesis, movement.
• Named reaction link: glycolysis starts by using ATP to phosphorylate glucose.
• Plants respire all the time, not only at night.

5.2.2(b) Mitochondrion in one table

Part	Key point
Outer membrane	Smooth, separates organelle from cytoplasm
Intermembrane space	Between the two membranes
Inner membrane	Folded, holds electron carriers and ATP synthase
Cristae	Folds of inner membrane, large surface area
Matrix	Link reaction and Krebs cycle enzymes, mitochondrial DNA, ribosomes
Mitochondrial DNA	Short and circular

Magnification: actual size = image size / magnification. Divide, then convert units at the end (mm × 1000 = µm).

Where each stage happens

Stage	Site	Oxygen needed directly?
Glycolysis	Cytoplasm	No
Link reaction	Matrix	No
Krebs cycle	Matrix	No
Oxidative phosphorylation	Inner membrane and cristae	Yes, final electron acceptor

Method in steps: glycolysis (cytoplasm)

1. Two ATP phosphorylate glucose, giving hexose bisphosphate (6C).
2. Hexose bisphosphate splits into two triose phosphate (3C).
3. Each triose phosphate is oxidised: hydrogen to NAD gives reduced NAD, and two ATP form per triose phosphate.
4. Two pyruvate result.

Net for one glucose: two ATP (four formed, two spent), two reduced NAD and two pyruvate.

Method in steps: link reaction (matrix)

1. Pyruvate (3C) is decarboxylated, releasing CO₂.
2. The remaining part is dehydrogenated: NAD becomes reduced NAD.
3. The two-carbon acetyl group joins coenzyme A to form acetyl CoA.

Per glucose (×2): 2 CO₂, 2 reduced NAD, 2 acetyl CoA, no ATP.

Method in steps: Krebs cycle (matrix)

1. Acetyl CoA (2C) joins oxaloacetate (4C) to form citrate (6C); CoA is released.
2. Citrate is decarboxylated (two CO₂) and dehydrogenated, in enzyme-controlled steps.
3. ATP forms by substrate-level phosphorylation.
4. Oxaloacetate is regenerated, so the cycle continues.

Each turn gives two CO₂, three reduced NAD, one reduced FAD and one ATP. One glucose means two turns.

Totals per glucose

Product	Glycolysis	Link reaction	Krebs cycle	Total
Carbon dioxide	none	2	4	6
Reduced NAD	2	2	6	10
Reduced FAD	none	none	2	2
ATP made directly	2 net	none	2	4

Worked reminders

Magnification. A mitochondrion is 27 mm long on a micrograph at ×15 000. Real length = 27 / 15 000 = 0.0018 mm = 1.8 µm.

Glycolysis in moles. 0.25 mol glucose gives 0.25 × 4 = 1.00 mol ATP made, 0.25 × 2 = 0.50 mol used, so 0.50 mol net. Reduced NAD is 0.25 × 2 = 0.50 mol and pyruvate is 0.50 mol.

After glycolysis. Twelve pyruvate molecules give 12 acetyl CoA and 12 CO₂ in the link reaction. They then give 12 turns of the cycle: 24 CO₂, 36 reduced NAD, 12 reduced FAD and 12 ATP. The total CO₂ is 36, which equals 6 glucose × 6 carbon atoms.

Rule of thumb. Count per pyruvate first, then double for each glucose. This avoids the commonest slip.

Linking the stages together

• Glycolysis feeds pyruvate to the link reaction, which feeds acetyl CoA to the Krebs cycle.
• The carbon atoms leave as CO₂ in the link reaction and Krebs cycle, not in glycolysis.
• The hydrogen atoms end up on reduced NAD and reduced FAD, which are used in oxidative phosphorylation.
• Direct ATP formation (substrate-level phosphorylation) happens in glycolysis and the Krebs cycle only.
• Each stage depends on oxidised NAD or FAD being available, so a shortage of oxygen has effects on all of them.

Coenzymes

Coenzyme	Carries	Used in
NAD	Hydrogen	Glycolysis, link reaction, Krebs cycle
FAD	Hydrogen	Krebs cycle
Coenzyme A	Acetyl group	Link reaction and delivery to the Krebs cycle

Reduced NAD and reduced FAD deliver hydrogen and electrons to oxidative phosphorylation. They are recycled: oxidised NAD and FAD must be available again for the earlier stages to carry on.

5.2.2(g) Oxidative phosphorylation: what to say

• Site: inner mitochondrial membrane, including cristae.
• Reduced NAD and reduced FAD give up hydrogen, and electrons pass along electron carriers.
• Energy is released step by step along the chain.
• Oxygen takes up the electrons at the end of the chain and, with protons, becomes water.
• No oxygen: chain stops, reduced coenzymes build up, link reaction and Krebs cycle slow.
• Cristae give a large surface area for carriers and ATP synthase.

Must-know distinctions

Pair	Difference
Decarboxylation and dehydrogenation	CO₂ removed, compared with hydrogen removed
Oxidation and reduction of NAD	Losing hydrogen turns reduced NAD into NAD; gaining hydrogen turns NAD into reduced NAD
Gross and net ATP in glycolysis	4 made, 2 used, net 2
Substrate-level and oxidative phosphorylation	Direct phosphate transfer, compared with ATP made using the electron carriers on the inner membrane
Acetyl group and acetyl CoA	Two-carbon group, compared with that group attached to coenzyme A
Citrate and oxaloacetate	6C product, compared with the 4C compound regenerated

Quick self-test

1. State the site of glycolysis.
2. Name the three-carbon compound formed when hexose bisphosphate splits.
3. Nine glucose molecules go through glycolysis. How many reduced NAD form?
4. Name the two types of reaction that happen to pyruvate in the link reaction.
5. State what coenzyme A carries.
6. How many CO₂ molecules leave the Krebs cycle for each acetyl CoA that enters?
7. A cell oxidises 9 glucose molecules completely as far as the end of the Krebs cycle. How many CO₂ molecules form in total?
8. Name the 4C compound that accepts the acetyl group.
9. A mitochondrion is 16 mm long on a micrograph at ×8000. State its real length in µm.
10. State the role of oxygen in oxidative phosphorylation.
11. Explain why cristae help oxidative phosphorylation.
12. Which coenzyme is reduced only in the Krebs cycle, of NAD and FAD?

Answers

1. Cytoplasm.
2. Triose phosphate.
3. 18 (2 per glucose).
4. Decarboxylation and dehydrogenation (followed by combination with coenzyme A).
5. The acetyl group.
7. 54 (6 per glucose).
8. Oxaloacetate.
9. 16 / 8000 = 0.002 mm = 2.0 µm.
10. It accepts the electrons at the end of the chain and forms water with protons.
11. Their folding gives a large inner membrane surface area for many electron carriers and ATP synthase enzymes.
12. FAD (NAD is also reduced in glycolysis and the link reaction).

Exam phrasing to practise

• “Describe” answers need the stages in order with the site of each.
• “Explain” answers need a reason after each statement, for example why oxygen shortage stops the Krebs cycle.
• “Calculate” answers need working shown, including the multiplier per glucose.
• Data questions need figures quoted from the table or graph, with percentage changes calculated from the starting value.

Where marks are usually lost

• Placing glycolysis in the mitochondrion instead of the cytoplasm.
• Writing “makes ATP” for the link reaction, which makes none directly.
• Stating 2 ATP for glycolysis without showing that 2 are used and 4 made.
• Forgetting to double the link reaction and Krebs cycle numbers per glucose.
• Writing “hydrogen” and “oxygen” interchangeably in oxidation and reduction answers.
• Describing the Krebs cycle as a straight chain, with no regeneration of oxaloacetate.
• Saying oxygen is “used in the Krebs cycle” instead of explaining its role at the end of the electron carriers.
• Giving cristae as “where respiration happens” without saying they increase the surface area.
• Not converting mm to µm in a magnification calculation.

Next steps

Work through the practice questions, or return to the study guide.

Official syllabus

OCR, A Level Biology A (H420) specification, Version 4.1 (April 2026), first assessment 2025: section 5.2.2, learning outcomes (a) to (g).