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Biology · Practice book · Senior 6 · Unit 4 of 12

Cellular respiration

Senior 6 Biology · 12 questions with answers. Try each question on paper, then tap it to see the answer.

Unit 4 Cellular respiration

Key unit competence: Assess the structure and significance of ATP and the roles of the coenzymes NAD, FAD and coenzyme A in cellular respiration.

Key ideas

  • Organisms need energy for active transport, muscle contraction, synthesis of macromolecules (anabolism), cell division, nerve impulses, maintaining body temperature and bioluminescence. Cellular respiration releases energy from organic molecules (mainly glucose) and stores it in ATP.
  • ATP (adenosine triphosphate) is a nucleotide: the base adenine, the pentose sugar ribose and three phosphate groups. Hydrolysis by ATPase: ATP + H₂O → ADP + Pᵢ, releasing about 30.5 kJ mol⁻¹. ATP is the universal energy currency: it releases energy in small, manageable amounts, in one step, is easily hydrolysed, is soluble and is made and used constantly. ATP is synthesised by substrate-level phosphorylation (directly in glycolysis and Krebs) and oxidative phosphorylation (via ATP synthase using a proton gradient — chemiosmosis).
  • Coenzymes: NAD (nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide) accept hydrogen atoms (become reduced: NADH and FADH₂) from dehydrogenase reactions and carry them to the electron transport chain. Coenzyme A (CoA) carries acetyl groups from the link reaction into the Krebs cycle. They are derived from B vitamins (niacin, riboflavin, pantothenic acid).
  • Glycolysis (cytoplasm, anaerobic): glucose (6C) is phosphorylated by 2 ATP to hexose bisphosphate, which splits into two triose phosphate (3C); each is oxidised (2 NAD → 2 NADH) and converted into pyruvate (3C), forming 4 ATP. Net: 2 ATP, 2 NADH, 2 pyruvate per glucose.
  • Link reaction (mitochondrial matrix): pyruvate is decarboxylated (CO₂ released) and dehydrogenated (NAD → NADH), and the acetyl group combines with CoA to form acetyl CoA (2C). Per glucose: 2 CO₂, 2 NADH.
  • Krebs cycle (matrix): acetyl CoA (2C) + oxaloacetate (4C) → citrate (6C); citrate is decarboxylated and dehydrogenated back to oxaloacetate. Per turn: 2 CO₂, 3 NADH, 1 FADH₂, 1 ATP; per glucose (two turns): 4 CO₂, 6 NADH, 2 FADH₂, 2 ATP.
  • Oxidative phosphorylation (inner membrane, cristae): NADH and FADH₂ release electrons to the electron transport chain (carriers); energy is used to pump H⁺ into the intermembrane space; H⁺ flows back through ATP synthase, making ATP (chemiosmosis). Oxygen is the final electron acceptor, combining with H⁺ and electrons to form water. Each NADH gives about 2.5 ATP and each FADH₂ about 1.5 ATP; total yield about 30–32 ATP per glucose (older textbooks: 38).
  • Mitochondrion structure: double membrane; inner membrane folded into cristae (large surface area for ETC and ATP synthase); matrix with enzymes of the link reaction and Krebs cycle, 70S ribosomes and circular DNA. Active cells (muscle, sperm, liver) have many mitochondria with many cristae.
  • Anaerobic respiration: without O₂, the ETC stops and NAD must be regenerated so glycolysis can continue. In animals: pyruvate → lactate (lactate dehydrogenase), causing an oxygen debt repaid by oxidising lactate in the liver. In yeast and plants: pyruvate → ethanal + CO₂ → ethanol (alcoholic fermentation). Yield: only 2 ATP per glucose; efficiency of aerobic respiration ≈ 32 × 30.5 ÷ 2 870 ≈ 34%; anaerobic ≈ 2%.
  • Respiratory quotient (RQ) = CO₂ produced ÷ O₂ consumed. Carbohydrate 1.0, protein about 0.9, lipid about 0.7; RQ > 1 shows some anaerobic respiration. A respirometer measures O₂ uptake: soda lime (KOH) absorbs CO₂, so a coloured liquid moves towards the organisms as O₂ is used; a control tube with glass beads corrects for temperature and pressure changes. Factors affecting respiration rate: temperature (enzymes), O₂ concentration, substrate availability, activity level, age, hormones (thyroxine). Other substrates: lipids are hydrolysed to glycerol (→ triose phosphate in glycolysis) and fatty acids (→ acetyl CoA by β-oxidation), releasing more energy per gram; proteins are deaminated and the remaining carbon skeletons enter as pyruvate, acetyl CoA or Krebs intermediates.

Common misconceptions

  • "Respiration is breathing." No: breathing is ventilation; respiration is the chemical release of energy in cells.
  • "Energy is made in respiration." No: energy is transferred from glucose to ATP; it cannot be created.
  • "Glycolysis takes place in the mitochondria." No: glycolysis occurs in the cytoplasm.
  • "Oxygen is used in the Krebs cycle." No: oxygen is used only at the end of the electron transport chain; without it, NAD is not regenerated and Krebs stops indirectly.

Worked examples

Example 1. A seed used 25 cm³ of O₂ and released 18 cm³ of CO₂ in an hour. Calculate the RQ and identify the likely substrate.

Solution

RQ = CO₂ ÷ O₂ = 18 ÷ 25 = 0.72, close to 0.7, so the seed is respiring mainly lipid (e.g. an oil-rich groundnut or sunflower seed).

Example 2. Aerobic respiration of one glucose molecule (2 870 kJ) gives 32 ATP at 30.5 kJ each. Calculate the efficiency.

Solution

Energy in ATP = 32 × 30.5 = 976 kJ. Efficiency = 976 ÷ 2 870 × 100 = 34%. The rest is lost as heat.

Practice questions

  1. Describe the structure of ATP and explain why it is suitable as an energy currency.

    Show answer

    Adenine, ribose, three phosphates. Releases energy in small usable amounts; one-step hydrolysis; soluble and easily transported; quickly regenerated; universal.

    Common mistake: Saying ATP stores energy long term.

  2. Explain the roles of NAD, FAD and coenzyme A.

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    NAD and FAD accept hydrogen from dehydrogenation in glycolysis, link reaction and Krebs; carry it to ETC to make ATP. CoA carries acetyl groups into the Krebs cycle.

    Common mistake: Mixing up the roles.

  3. Outline glycolysis.

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    Glucose phosphorylated by 2 ATP; splits into 2 triose phosphate; oxidised with NAD reduced; converted to 2 pyruvate; 4 ATP made, net 2 ATP and 2 NADH.

    Common mistake: Placing glycolysis in mitochondria.

  4. Describe the link reaction and the Krebs cycle.

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    Link: pyruvate decarboxylated and dehydrogenated, combines with CoA to form acetyl CoA. Krebs: acetyl CoA + oxaloacetate → citrate; decarboxylations and dehydrogenations regenerate oxaloacetate; producing CO₂, NADH, FADH₂, ATP.

    Common mistake: Saying oxygen is used in Krebs.

  5. Explain oxidative phosphorylation.

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    NADH and FADH₂ release electrons to the ETC on cristae; energy pumps H⁺ into intermembrane space; H⁺ diffuses back through ATP synthase making ATP; O₂ final acceptor forms water.

    Common mistake: Leaving out the proton gradient.

  6. Compare aerobic and anaerobic respiration.

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    Aerobic: needs O₂, in cytoplasm and mitochondria, products CO₂ and water, about 32 ATP. Anaerobic: no O₂, cytoplasm only, products lactate or ethanol and CO₂, 2 ATP.

    Common mistake: Saying anaerobic produces no ATP.

  7. Define RQ and give values for three substrates.

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    RQ = volume CO₂ produced ÷ volume O₂ used. Carbohydrate 1.0; protein 0.9; lipid 0.7.

    Common mistake: Inverting the ratio.

  8. Explain how temperature affects the rate of respiration.

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    Rate increases with temperature up to an optimum as molecules gain kinetic energy and collide more; above optimum, enzymes denature and rate falls.

    Common mistake: Saying enzymes are killed.

Matching and drag-and-drop

  1. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. The final electron acceptor in aerobic respiration is:a. pyruvate
    2. The end product of glycolysis is:b. lactate
    3. Anaerobic respiration in muscles produces:c. oxygen
    4. Yeast fermentation produces:d. ethanol and carbon dioxide
    Show answer

    1 → c (oxygen) · 2 → a (pyruvate) · 3 → b (lactate) · 4 → d (ethanol and carbon dioxide)

    • Forms water.
    • Two per glucose.
    • Causes oxygen debt.
    • Used in brewing.

  2. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    absorb carbon dioxide  ·  1.0  ·  0.7  ·  ribose
    1. The RQ of carbohydrate is .
    2. In a respirometer, soda lime is used to .
    3. The sugar in ATP is .
    Show answer

    1: 1.0 · 2: absorb carbon dioxide · 3: ribose

    • Equal CO₂ and O₂.
    • So O₂ uptake is measured.
    • A pentose.

  3. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. The coenzyme that carries acetyl groups is:a. cytoplasm
    2. Net ATP gain from glycolysis per glucose is:b. coenzyme A
    3. The enzyme that makes ATP in mitochondria is:c. ATP synthase
    4. Glycolysis takes place in the:d. 2
    Show answer

    1 → b (coenzyme A) · 2 → d (2) · 3 → c (ATP synthase) · 4 → a (cytoplasm)

    • Into the Krebs cycle.
    • 4 made, 2 used.
    • Driven by H⁺ flow.
    • No oxygen needed.

  4. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    link reaction  ·  citrate  ·  chemiosmosis  ·  Calvin cycle
    1. Acetyl CoA is formed in the .
    2. Acetyl CoA combines with oxaloacetate to form .
    3. Making ATP using a proton gradient is .
    Show answer

    1: link reaction · 2: citrate · 3: chemiosmosis

    • Pyruvate is decarboxylated.
    • A 6C compound.
    • Through ATP synthase.

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