Lesson 7.2.3

7.2.3 Link reaction and Krebs cycle Quiz: Pearson Edexcel Biology A (Salters-Nuffield), Unit 7

20 questions

Written by Revision Ninja · Last reviewed 11 October 2026

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Lesson 7.2.3, Link reaction and Krebs cycle: 20 multiple choice questions for the Pearson Edexcel Biology A (Salters-Nuffield) (9BI0), Unit 7: Run for your Life, written with Revision Ninja.

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The 20 questions

  1. The link reaction converts pyruvate into:

    • Citrate, releasing water and producing reduced FAD in the inner membrane
    • Glucose, releasing oxygen and reducing ATP in the mitochondrial matrix
    • Acetyl CoA, releasing carbon dioxide and producing reduced NAD
    • Lactate, releasing carbon dioxide and regenerating NAD+ in the cytoplasm

  2. Where do the link reaction and the Krebs cycle take place?

    • The mitochondrial matrix
    • The inner mitochondrial membrane, where the electron transport chain is located
    • The outer mitochondrial membrane, where the proteins for the Krebs cycle are made
    • The cytoplasm, alongside glycolysis, using the same enzymes as the first stage

  3. How many molecules of carbon dioxide are released in one turn of the Krebs cycle?

    • 6, since the cycle releases one carbon dioxide for each carbon in the acetyl group
    • 2, one for each acetyl group entering the cycle
    • 4, since each turn releases four carbon dioxide molecules from the acetyl group
    • 1, since the cycle releases one carbon dioxide per acetyl CoA only at the first step

  4. Per acetyl CoA entering the Krebs cycle, which set of products is formed?

    • 2 CO2, 3 reduced NAD, 1 reduced FAD and 1 ATP
    • 1 CO2, 3 reduced NAD, 2 reduced FAD and 1 water
    • 3 CO2, 2 reduced NAD, 1 reduced FAD and 2 ATP
    • 2 CO2, 1 reduced NAD, 3 reduced FAD and 1 ATP

  5. How many reduced NAD molecules are produced by the link reaction and Krebs cycle from one glucose molecule in total?

    • 8, since 2 from the link reaction and 6 from the Krebs cycle
    • 10, since 2 from the link reaction and 8 from the Krebs cycle
    • 4, since 2 from the link reaction and 2 from the Krebs cycle
    • 6, since 2 from the link reaction and 4 from the Krebs cycle

  6. Why does the Krebs cycle require a continuous supply of oxygen to continue operating?

    • The Krebs cycle needs oxygen to convert pyruvate to lactate, which is the first step
    • The Krebs cycle produces oxygen as a product, which must be removed to stop the cycle
    • The Krebs cycle directly consumes oxygen as a substrate in the first step of the cycle
    • Reduced NAD and reduced FAD must be reoxidised by the electron transport chain

  7. Why is the Krebs cycle described as 'cyclical'?

    • Because the cycle repeats only once per glucose molecule and then stops permanently
    • Because the cycle produces glucose at the end of each turn, which becomes the input for the next
    • Because the starting molecule, oxaloacetate, is regenerated at the end of each turn to accept another acetyl group
    • Because the cycle returns carbon dioxide to the atmosphere in a closed loop without any chemistry

  8. Which of these is the best description of the purpose of the link reaction?

    • To split glucose into two pyruvate molecules, releasing ATP for use in the cytoplasm
    • To convert oxygen into water so that the electron transport chain can run to completion
    • To prepare pyruvate for the Krebs cycle by converting it into acetyl CoA and releasing carbon dioxide
    • To produce lactate from pyruvate so that the cell can regenerate NAD in the absence of oxygen

  9. Which statement describes the overall role of the Krebs cycle in complete oxidation of glucose?

    • It oxidises acetyl groups to carbon dioxide and transfers energy to reduced coenzymes for later ATP production
    • It reduces carbon dioxide to glucose, storing energy in the bonds of the sugar
    • It produces lactate from pyruvate, so that the cell can continue without oxygen
    • It oxidises glucose directly to water, producing most of the ATP in the cytoplasm

  10. Why do the link reaction and Krebs cycle occur in the mitochondria rather than the cytoplasm?

    • The cytoplasm contains too much oxygen, which would destroy the enzymes needed for the cycle
    • The enzymes and the carriers that pass electrons to the chain are located in the mitochondria
    • Glucose cannot be taken into the cytoplasm, so the cycle must happen in the mitochondria
    • Mitochondria are the only organelles that contain ribosomes, which are needed for the Krebs cycle

  11. Which of the following is produced by the link reaction and not by glycolysis?

    • ATP, which is made in the first step of glycolysis by substrate-level phosphorylation
    • Reduced NAD, which is made in both glycolysis and the link reaction
    • Carbon dioxide, released when the pyruvate loses a carbon atom
    • Pyruvate, which is the product of glycolysis and is used by the link reaction

  12. The Krebs cycle begins with acetyl CoA combining with:

    • Oxygen, forming water directly so that no carbon dioxide is released in the cycle
    • Pyruvate, forming a three-carbon molecule that is reduced to ethanol in the cycle
    • Oxaloacetate, forming citrate
    • Glucose, forming a six-carbon molecule that is oxidised to lactate in the cycle

  13. Which statement is accurate about the ATP produced in the Krebs cycle?

    • It is formed by the release of calcium from the reticulum, with two ATP per cycle
    • It is formed by substrate-level phosphorylation, with one ATP per acetyl CoA
    • It is formed by chemiosmosis through ATP synthase, with many ATP per acetyl CoA
    • It is formed by the photolysis of water, with one ATP per carbon dioxide released

  14. A student measures carbon dioxide output from mitochondria with and without a Krebs cycle inhibitor. Which result would be expected with the inhibitor present?

    • No change in carbon dioxide output, because carbon dioxide is released only in glycolysis
    • A rise in oxygen output, because the inhibitor forces the cycle to release oxygen instead
    • A fall in carbon dioxide output, because the cycle that releases it is blocked
    • A rise in carbon dioxide output, because the inhibitor speeds up the cycle that releases it

  15. Which statement describes the role of reduced NAD and reduced FAD from the Krebs cycle?

    • They are used as energy stores in the cytoplasm, from which ATP is later released
    • They are converted into carbon dioxide in the matrix, which is then released from the cell
    • They carry electrons and hydrogen to the electron transport chain in the inner mitochondrial membrane
    • They are used to make glucose, which is then transported out of the mitochondria

  16. Which statement evaluates the claim that the Krebs cycle makes most of the ATP from one glucose?

    • The claim is accurate, since the cycle makes all of the ATP for aerobic respiration without any other step
    • The claim is accurate, since the cycle is the main ATP-producing step and makes most of the ATP directly
    • The claim is inaccurate, since the cycle makes no ATP at all and only produces carbon dioxide
    • The claim is inaccurate, since the cycle makes only a small amount of ATP directly and most ATP comes from oxidative phosphorylation

  17. Which statement about the decarboxylation steps in aerobic respiration is correct?

    • They add carbon to glucose so that the molecule becomes larger and more energy rich
    • They release carbon dioxide as carbon atoms are removed from intermediates of the pathway
    • They release lactate as the carbon atoms are removed from pyruvate in the matrix
    • They release oxygen as the carbon atoms are combined with hydrogen to form water

  18. Which feature of the mitochondrial matrix is important for the Krebs cycle?

    • It contains the proton pumps that move hydrogen ions across the inner membrane
    • It contains the glucose transporters that move glucose into the mitochondria for use
    • It contains the enzymes needed to catalyse each step of the cycle
    • It contains the calcium stores that trigger the release of acetyl CoA from pyruvate

  19. How many carbon atoms does citrate contain, given that oxaloacetate has four carbons and acetyl CoA contributes two?

    • 4, since oxaloacetate alone contributes all of the carbon atoms in citrate
    • 2, since only the acetyl group is retained in citrate after the first step
    • 3, since citrate is a three-carbon compound in the same way that pyruvate is
    • 6, since the four-carbon oxaloacetate combines with the two-carbon acetyl group

  20. Why must the link reaction happen before the Krebs cycle can start?

    • Because the Krebs cycle needs acetyl CoA as its entry compound, which the link reaction produces from pyruvate
    • Because the Krebs cycle needs lactate as its substrate, which the link reaction makes from pyruvate
    • Because the Krebs cycle needs oxygen as its entry compound, which the link reaction releases from water
    • Because the Krebs cycle needs glucose as its entry compound, which the link reaction releases from glycogen

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