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

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover 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. Learn the content first in the study guide or the revision notes. Course pages: OCR A Level Biology hub and checklist. You can also try the free diagnostics. Where a question gives data, use those values.

Questions

1. (a) State what is meant by respiration. [1] (b) Explain why a root hair cell and a growing yeast cell both need a continuous supply of ATP, naming one process in each. [3]

2. Describe the structure of a mitochondrion, naming four components, and give a function for the cristae and for the matrix. [6]

3. An electron micrograph shows a mitochondrion that measures 42 mm in length. The magnification is ×14 000. Calculate the real length in micrometres. [3]

4. Describe the process of glycolysis, including where it takes place. [5]

5. A suspension of yeast cells breaks down 3.6 g of glucose (Mr 180) by glycolysis only. (a) Calculate the amount of glucose, in mol. [1] (b) Calculate the net amount of ATP formed, in mol. [2] (c) State the amount of reduced NAD formed, in mol. [1]

6. (a) State the site of the link reaction. [1] (b) Describe what happens to each pyruvate molecule in the link reaction. [3]

7. The Krebs cycle in a sample of mitochondria processes 8 molecules of acetyl CoA. (a) Calculate the number of molecules of CO₂, reduced NAD, reduced FAD and ATP formed. [4] (b) Explain why the amount of oxaloacetate in the matrix stays constant while the cycle is running. [1]

8. (a) State the number of carbon atoms in an acetyl group, in oxaloacetate and in citrate. [2] (b) Explain the importance of decarboxylation and dehydrogenation in the Krebs cycle. [4] (c) Name the type of phosphorylation that makes ATP in the Krebs cycle and explain what it means. [2]

9. (a) State where oxidative phosphorylation takes place in a eukaryotic cell. [1] (b) Describe the role of oxygen in oxidative phosphorylation. [2] (c) Explain why the link reaction and the Krebs cycle stop when no oxygen is available. [2]

10. Explain the importance of coenzymes in respiration, with reference to NAD, FAD and coenzyme A. [6]

11. Mitochondria were isolated from liver cells and supplied with pyruvate and oxygen. Inhibitor X, which blocks one enzyme of the Krebs cycle, was added to one sample. Relative concentrations were measured at the start and after 10 minutes.

Measurement	Control: start	Control: 10 min	With X: start	With X: 10 min
Oxaloacetate (arbitrary units)	4.0	3.9	4.0	0.8
Acetyl CoA (arbitrary units)	2.0	2.1	2.0	6.5
CO₂ released in 10 min (µmol)	not applicable	36	not applicable	9

(a) Calculate the percentage decrease in oxaloacetate in the sample with X. [2] (b) Calculate the percentage decrease in CO₂ released caused by X. [2] (c) Use the data to suggest which stage the inhibitor affects, and explain the changes in oxaloacetate and acetyl CoA. [3] (d) Suggest the effect of X on the amount of reduced NAD and reduced FAD formed. [2]

12. (a) Explain why a muscle fibre that contracts repeatedly has many mitochondria with extensive cristae. [3] (b) Describe how the six carbon atoms of one glucose molecule are released as CO₂, naming the stages and where each takes place. [6]

Answers

1. (a) The transfer of energy from complex organic molecules, such as glucose, to ATP [1]. (b) Root hair cell: ATP is used in active transport of mineral ions against a concentration gradient [1]. Yeast: ATP is needed for growth, for example protein synthesis or DNA replication before budding [1]. Cells hold little ATP, so it must be made continuously [1]. [4] Examiner insight: A named process earns more credit than “energy for life processes”; always say what the ATP does.

2. Outer membrane is smooth and surrounds the organelle [1]. Inner membrane is folded into cristae [1]. Matrix is the fluid interior [1]. Mitochondrial DNA (short, circular) is in the matrix [1]. Cristae increase the surface area of inner membrane for electron carriers and ATP synthase [1]. The matrix holds the enzymes for the link reaction and Krebs cycle [1]. [6] Examiner insight: Pair each named part with its function when the question asks for both; a list of names alone usually leaves marks behind.

3. Actual size = image size / magnification [1]. 42 / 14 000 = 0.003 mm [1]. 0.003 × 1000 = 3.0 µm [1]. [3] Examiner insight: Show the formula and the conversion step; always finish with the unit, because a bare number leaves the answer unclear.

4. Takes place in the cytoplasm [1]. Glucose is phosphorylated using two ATP to form hexose bisphosphate [1]. Hexose bisphosphate splits into two triose phosphate [1]. Each triose phosphate is oxidised, with hydrogen accepted by NAD to form reduced NAD, and converted to pyruvate [1]. Four ATP are made and two used, so the net yield is two ATP [1]. [5] Examiner insight: Mention both the site and the order of stages; when a small yield is asked for, give the net value and show how it is reached.

5. (a) 3.6 / 180 = 0.020 mol [1]. (b) ATP made = 0.020 × 4 = 0.080 mol [1]; ATP used = 0.020 × 2 = 0.040 mol, so net = 0.080 − 0.040 = 0.040 mol [1]. (c) 0.020 × 2 = 0.040 mol [1]. [4] Examiner insight: Write the multiplier for each product (2 ATP net, 2 reduced NAD per glucose) so that a slip in arithmetic can still be followed.

6. (a) Mitochondrial matrix [1]. (b) Pyruvate is decarboxylated, releasing CO₂ [1]. It is dehydrogenated, and the hydrogen is accepted by NAD to form reduced NAD [1]. The remaining two-carbon acetyl group combines with coenzyme A to form acetyl CoA [1]. [4] Examiner insight: Use the terms decarboxylated and dehydrogenated and say what is removed in each; “broken down” is too vague.

7. (a) Per turn: two CO₂, three reduced NAD, one reduced FAD and one ATP [1]. CO₂ = 8 × 2 = 16 [1]. Reduced NAD = 8 × 3 = 24 [1]. Reduced FAD = 8 and ATP = 8 [1]. (b) Oxaloacetate is regenerated at the end of each turn, so it is used and made at the same rate [1]. [5] Examiner insight: Counting per turn first, then multiplying, is the safest layout; per-glucose values need a further factor of two.

8. (a) Acetyl group 2 and oxaloacetate 4 [1]; citrate 6 [1]. (b) Decarboxylation removes CO₂, which takes carbon out of the cycle and allows the 6-carbon compound to be converted back to 4-carbon oxaloacetate [1]. This is how the carbon of glucose leaves the body as CO₂ [1]. Dehydrogenation removes hydrogen, which is accepted by NAD and FAD [1]. The reduced coenzymes carry the hydrogen to oxidative phosphorylation, where most of the ATP is made [1]. (c) Substrate-level phosphorylation [1]; a phosphate group is transferred directly from a substrate to ADP, with no electron carriers involved [1]. [8] Examiner insight: Explain each term in terms of what is removed and what the product is used for; naming it without a consequence rarely goes far enough.

9. (a) Inner mitochondrial membrane, including the cristae [1]. (b) Oxygen takes up the electrons at the end of the chain, as the final acceptor [1]; with protons it forms water [1]. (c) Without oxygen the electron carriers cannot pass on electrons, so reduced NAD and reduced FAD are not reoxidised [1]. Oxidised NAD and FAD run short, so the dehydrogenation steps of the link reaction and Krebs cycle cannot continue [1]. [5] Examiner insight: Link the lack of oxygen to the shortage of oxidised coenzymes; saying only that “respiration stops” does not explain why.

10. NAD accepts hydrogen to form reduced NAD [1] in glycolysis, the link reaction and the Krebs cycle [1]. FAD accepts hydrogen to form reduced FAD in the Krebs cycle [1]. The reduced forms carry hydrogen and electrons to the electron carriers on the inner mitochondrial membrane [1], where they are reoxidised, so the coenzymes can be reused [1]. Coenzyme A carries the acetyl group from the link reaction to the Krebs cycle as acetyl CoA, and is released to be reused [1]. [6] Examiner insight: A good answer says what each coenzyme carries, where it is used and that it is recycled; stating only that coenzymes “help enzymes” is not enough.

11. (a) (4.0 − 0.8) / 4.0 × 100 [1] = 80% [1]. (b) (36 − 9) / 36 × 100 [1] = 75% [1]. (c) Oxaloacetate falls sharply while acetyl CoA rises [1]. X probably blocks an enzyme in the Krebs cycle that regenerates oxaloacetate [1]. Without oxaloacetate, acetyl CoA cannot combine to form citrate, so acetyl CoA accumulates and fewer decarboxylation steps run, so less CO₂ is released [1]. (d) Less reduced NAD and reduced FAD are formed [1] because fewer cycles of dehydrogenation take place [1]. [9] Examiner insight: Quote figures from the table when interpreting data, and link each change to the stage of the cycle that explains it.

12. (a) Contraction needs a high rate of ATP use [1]. Many mitochondria provide many sites for the link reaction and Krebs cycle [1]. Extensive cristae mean more inner membrane to hold electron carriers and ATP synthase, so more ATP can be made [1]. (b) In the cytoplasm, glucose is phosphorylated, split into two triose phosphate and oxidised to two pyruvate, with no CO₂ released [1]. Pyruvate enters the matrix [1]. In the link reaction each pyruvate is decarboxylated, giving 2 CO₂ per glucose [1], and forms acetyl CoA [1]. Acetyl CoA joins oxaloacetate to form citrate in the Krebs cycle [1]. Citrate is decarboxylated to regenerate oxaloacetate, releasing 4 CO₂ per glucose over two turns, so 6 CO₂ in total [1]. [9] Examiner insight: In a sequence question, state the site at each stage and count the CO₂; a stage-by-stage layout makes the marking points easy to find.

Where marks are usually lost

• Describing glycolysis as happening in the mitochondrion.
• Quoting 2 ATP for glycolysis without distinguishing the 4 made from the 2 used.
• Forgetting that each glucose produces two pyruvate, so the later stages run twice.
• Saying that the link reaction makes ATP.
• Using “decarboxylation” and “dehydrogenation” without saying what is removed.
• Writing that oxygen is used in the Krebs cycle rather than explaining its indirect effect through the coenzymes.
• Leaving out a function when asked to name and explain mitochondrial parts.
• Not converting units in a magnification calculation, or leaving out the unit.
• Interpreting data without quoting any values from the table.

Next steps

• Go back to the revision notes or the study guide.
• Visit the OCR A Level Biology hub and the checklist.
• Take one of the free 10-minute diagnostics.
• Book a free trial class.

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).