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Aerobic Anaerobic Respiration in Plants ICSE 9 Biology

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BiologyICSE Class 9 · Respiration in Plants

Aerobic and Anaerobic Respiration in Plants and Related Experiments (ICSE Class 9 Biology)

Updated 17 October 2026 · Fact-checked

Respiration is the release of energy from food in living cells. Aerobic respiration uses oxygen: glucose + oxygen gives carbon dioxide + water + energy (36 to 38 ATP). Anaerobic respiration happens without oxygen: glucose gives alcohol + carbon dioxide + a little energy in yeast and some plant tissues. Both begin with glycolysis in the cytoplasm.

Both routes start with glycolysis in the cytoplasm; with oxygen the energy yield is far higher (36–38 ATP) than without oxygen (2 ATP).

Illustration drawn for QuestionBuddy.

Understand Aerobic and Anaerobic Respiration in Plants and Related Experiments

1. What respiration is. Respiration is a chemical process in all living cells that breaks food down to release energy as ATP. It is not the same as breathing, which is only the physical exchange of gases. It occurs in plants all day and night, in roots, stems, leaves and seeds.

2. Glycolysis and Krebs cycle (outline). *Glycolysis* takes place in the cytoplasm: one glucose (6C) is split into two pyruvate molecules (3C) with a small gain of ATP. In the presence of oxygen the pyruvate enters mitochondria and the *Krebs cycle* completes its breakdown to carbon dioxide and water, releasing most of the energy.

3. Aerobic respiration. C6H12O6 + 6O2 gives 6CO2 + 6H2O + energy (about 36 to 38 ATP). It happens mostly in mitochondria and releases much energy.

4. Anaerobic respiration. Without oxygen, glucose is partly broken down: C6H12O6 gives 2C2H5OH + 2CO2 + a little energy (2 ATP) in yeast and in plant parts like roots in waterlogged soil. Alcohol is formed, so it is also called fermentation. In human muscles lactic acid is formed instead.

5. Experiment: carbon dioxide is given out. Germinating seeds in a flask joined by a delivery tube to lime water turn the lime water milky, showing CO2 is released.

6. Experiment: oxygen is taken in. Seeds in a flask with a small cup of potassium hydroxide (KOH) to absorb CO2; the water level rises in the tube, showing oxygen is used.

7. Experiment: heat is produced. Germinating seeds in a vacuum flask with a thermometer show a rise in temperature compared with boiled (dead) seeds used as control.

Aerobic and Anaerobic Respiration in Plants and Related Experiments: key facts

Aerobic
  • Glucose + oxygen gives carbon dioxide + water + energy
  • Mitochondria
  • 36 to 38 ATP
Anaerobic
  • Glucose gives alcohol + carbon dioxide + little energy
  • Yeast
  • 2 ATP
Steps
  • Glycolysis: cytoplasm
  • Krebs cycle: mitochondria
  • Pyruvate
Tests
  • Lime water turns milky: CO2
  • KOH absorbs CO2
  • Thermometer rises: heat

Aerobic and Anaerobic Respiration in Plants and Related Experiments in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Waterlogged pot

Suppose a house plant is watered too much so that the soil is full of water and the roots turn brown with a sour smell.

Explain with respiration.

Try it first, then show the solutionSolution
  1. Waterlogging fills soil spaces with water, so oxygen is short.
  2. Roots respire anaerobically, giving alcohol and little energy.
  3. Alcohol harms cells and the roots die.

Answer: Roots in waterlogged soil respire anaerobically; alcohol and low energy damage them.

Memory hook: No air, no energy.

Tricks to remember Aerobic and Anaerobic Respiration in Plants and Related Experiments

Short memory aids. Say each one aloud once, then use it on a question the same day.

  • Glyco in cyto

    Glycolysis in the cytoplasm; Krebs in mitochondria.

    Use it when: Site of reactions.

  • KOH eats CO2

    Potassium hydroxide absorbs carbon dioxide.

    Use it when: Experiment 2.

Aerobic and Anaerobic Respiration in Plants and Related Experiments: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Sites · 2 marks · Short answerBoard level

Name the site of (a) glycolysis, (b) the Krebs cycle. [2]

Try it first, then show the solutionSolution
  1. (a) Cytoplasm.
  2. (b) Mitochondria.

Answer: (a) Cytoplasm (b) Mitochondria.

Example 2: Equation · 3 marks · Short answerBoard level

Write the balanced equation for aerobic respiration and state where most energy is released. [3]

Try it first, then show the solutionSolution
  1. C6H12O6 + 6O2 gives 6CO2 + 6H2O + energy.
  2. Count atoms: left 6 C, 12 H, 18 O; right 6 C, 12 H, 12 + 6 = 18 O.
  3. Most energy is released in the mitochondria.

Answer: C6H12O6 + 6O2 gives 6CO2 + 6H2O + energy; mitochondria.

Example 3: Experiment design · 3 marks · Short answerHarder board level

In the experiment to show that CO2 is released, why is a flask of boiled seeds kept as a control? [3]

Try it first, then show the solutionSolution
  1. Boiled seeds are dead and cannot respire.
  2. The lime water in the control stays clear.
  3. It shows that the change is due to living seeds.

Answer: The control shows that only living seeds release CO2.

Example 4: Energy comparison · 3 marks · Short answerHarder board level

Explain why yeast grown in sealed bottles without air produces much less energy than yeast grown with air. [3]

Try it first, then show the solutionSolution
  1. Anaerobic respiration gives 2 ATP per glucose.
  2. Aerobic gives about 36 to 38 ATP.
  3. Without air, glucose is only partly broken down.

Answer: Anaerobic respiration is less efficient because glucose is only partly oxidised.

Example 5: Case: the warm heap · 6 marks · Case-basedHOTS / competency

Case. A heap of wet germinating seeds is found to be warm in the middle after two days, while a heap of dry seeds is cool. (i) Name the process. [1] (ii) Which gas is also released? [1] (iii) Why is the dry heap cool? [2] (iv) Why does the warm heap feel warm? [2]

Try it first, then show the solutionSolution
  1. (i) Respiration.
  2. (ii) Carbon dioxide.
  3. (iii) Dry seeds respire very slowly.
  4. (iv) Energy released in respiration appears partly as heat.

Answer: (i) Respiration (ii) CO2 (iii) Dry seeds respire little (iv) Some energy is lost as heat.

Where students lose marks in Aerobic and Anaerobic Respiration in Plants and Related Experiments

The slips examiners see most often, and what to write instead.

  • Mistake: Equations not balanced

    Oxygen counts are wrong.

    Do this: Check 6 CO2 and 6 H2O.

    Usually costs: 1 mark.

  • Mistake: Writing that glycolysis needs oxygen

    It does not.

    Do this: State that it is anaerobic and in the cytoplasm.

    Usually costs: 1 mark.

  • Mistake: Skipping the control in experiments

    Control not mentioned.

    Do this: State what the control shows.

    Usually costs: 1 mark.

Exam tips

  • Learn equations with state symbols not required but balanced.
  • Always state the site and the ATP count.
  • Describe each experiment with apparatus, observation and conclusion.
  • Use boiled seeds as the control.

Practice questions on Aerobic and Anaerobic Respiration in Plants and Related Experiments

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

  1. Q1 · 1 mark · MCQBoard level

    Germinating seeds kept in a closed flask turn the lime water in a connected tube milky. This shows the release of:
    (a) oxygen
    (b) carbon dioxide
    (c) nitrogen
    (d) hydrogen

    Hint

    Test.

    Answer

    (b) carbon dioxide.

  2. Q2 · 2 marks · Very short answerBoard level

    Why is respiration different from breathing? [2]

    Hint

    Cell process.

    Answer

    Respiration is the release of energy from food in cells; breathing is the exchange of gases.

  3. Q3 · 2 marks · Very short answerBoard level

    Name the two products formed when yeast respires glucose without oxygen. [2]

    Hint

    Fermentation.

    Answer

    Alcohol and carbon dioxide.

  4. Q4 · 1 mark · Assertion-reasonHarder board level

    Assertion (A): Plants respire at night as well as in the day.
    Reason (R): Respiration is needed to release energy in all living cells.
    (a) Both A and R are true and R is the correct explanation of A.
    (b) Both A and R are true but R is not the correct explanation of A.
    (c) A is true but R is false.
    (d) A is false but R is true.

    Hint

    Always.

    Answer

    (a).

  5. Q5 · 4 marks · Long answerHarder board level

    A yeast culture is kept first with air and then sealed. List four changes in the way glucose is broken down. [4]

    Hint

    Oxygen/energy.

    Answer

    Oxygen is no longer used; breakdown is partial; ATP falls from 36-38 to 2 per glucose; products change from CO2 and water to alcohol and CO2.

  6. Q6 · 2 marks · Very short answerHarder board level

    Explain the use of KOH in the experiment showing oxygen uptake. [2]

    Hint

    Absorb.

    Answer

    It absorbs the CO2 formed, so the fall in gas volume shows oxygen used.

  7. Q7 · 2 marks · Short answerHarder board level

    Compare respiration and photosynthesis on two points. [2]

    Hint

    Opposite.

    Answer

    Respiration releases energy and occurs always, while photosynthesis stores energy and occurs in light.

  8. Q8 · 3 marks · Long answerHOTS / competency

    A student sets up seeds, KOH and a coloured liquid in a tube. The liquid rises. Explain the result and what would happen if KOH were absent. [3]

    Hint

    Gas volume.

    Answer

    Seeds use oxygen and release CO2 which KOH absorbs, so the volume falls and the liquid rises; without KOH the CO2 released replaces the oxygen and the liquid stays almost level.

  9. Q9 · 4 marks · Case-basedHOTS / competency

    Case. A farmer finds that a flooded field kills his crop roots in a few days. (i) State the type of respiration in the roots. [1] (ii) Give two harmful effects. [2] (iii) Suggest a remedy. [1]

    Hint

    No oxygen.

    Answer

    (i) Anaerobic. (ii) Little energy for growth and alcohol builds up. (iii) Drain the field.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

  1. 1.Energy molecule?Show answer

    ATP.

  2. 2.Site of Krebs cycle?Show answer

    Mitochondria.

  3. 3.Anaerobic product in yeast?Show answer

    Alcohol and CO2.

  4. 4.Where does glycolysis occur?Show answer

    Cytoplasm.

  5. 5.Gas used in respiration?Show answer

    Oxygen.

Aerobic and Anaerobic Respiration in Plants and Related Experiments: frequently asked questions

Do plants breathe?

They exchange gases through stomata and lenticels.

Is fermentation the same as anaerobic respiration?

In yeast, yes.

Where does heat come from?

Part of the energy released in respiration.