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LESSON B6 · EXPLAINER
Aerobic and Anaerobic Respiration
A 100 m sprinter barely breathes during the race — so where does the energy come from?
Narration uses your device’s built-in voice. No sound? Switch to “Captions only” — the captions advance on their own.
Step 2 · Read
Key notes
Copy these into your notebook in your own words — that is how they stick.
Respiration
Enzyme-controlled breakdown of glucose in cells to release energy, stored as ATP.
“Energy is released” — never write “energy is produced/made”.
P1 item: energy from glucose oxidation is “stored in a high-energy bond in ATP”.
For the sprinter question, mention: insufficient O₂ → anaerobic → lactic acid → pain/fatigue → oxygen debt → panting.
Step 2 · Learn from examples
Worked examples & study resources
The kind of support a teacher gives after the lesson: worked examples to attempt first, a board illustration, key terms, common misconceptions and memory aids.
Worked example 1
Comparing the energy from aerobic and anaerobic respiration
Aerobic respiration releases about 38 ATP per glucose molecule; anaerobic respiration in muscle releases 2 ATP. (a) How many times more energy does aerobic respiration release? (b) Explain why. (c) Why can muscles not respire anaerobically for long?
Try it in your notebook first, then reveal the solution one step at a time.
(a) 38 ÷ 2 = 19 times as much energy.
(b) In aerobic respiration glucose is broken down completely to carbon dioxide and water, releasing all its usable energy. In anaerobic respiration it is only partly broken down — much energy stays locked in lactic acid.
(c) Lactic acid builds up in the muscles. It causes fatigue and cramp, and must later be broken down using oxygen — the oxygen debt.
Answer. (a) 19 times. (b) Glucose is completely broken down in aerobic respiration but only partly in anaerobic respiration (energy remains in lactic acid). (c) Lactic acid builds up, causing fatigue/cramp and an oxygen debt.
Examiner’s tip. Respiration RELEASES energy — never write that it “produces” or “creates” energy.
Worked example 2
Explaining the oxygen debt
An athlete’s oxygen uptake was measured before, during and after a 400 m race.
Time
Rest
During race
1 min after
5 min after
10 min after
O₂ uptake (dm³ min⁻¹)
0.3
3.5
2.4
0.9
0.3
Explain why oxygen uptake stays high after the race has ended.
Try it in your notebook first, then reveal the solution one step at a time.
During the race the muscles needed more energy than aerobic respiration could supply, because the heart and lungs could not deliver oxygen fast enough.
So the muscles also respired anaerobically, producing lactic acid.
After the race, extra oxygen is needed to break down (oxidise) the lactic acid — mostly in the liver. This extra oxygen is the oxygen debt.
Breathing rate and heart rate stay high until the debt is repaid, which is why uptake only returns to 0.3 dm³ min⁻¹ after about 10 minutes.
Answer. Lactic acid made by anaerobic respiration during the race must be broken down using extra oxygen afterwards (the oxygen debt), so breathing stays fast and deep until it is removed.
Examiner’s tip. Use the phrase “oxygen debt” and say what the oxygen is FOR (breaking down lactic acid).
On the board · Three types of respiration compared
Aerobic
Anaerobic in muscle
Anaerobic in yeast (fermentation)
Oxygen needed?
Yes
No
No
Word equation
glucose + oxygen → carbon dioxide + water
glucose → lactic acid
glucose → ethanol (alcohol) + carbon dioxide
Energy released
Large (about 38 ATP)
Small (2 ATP)
Small (2 ATP)
Where
Mitochondria (after the cytoplasm stage)
Cytoplasm
Cytoplasm
Example
Most cells, most of the time
Sprinting muscles
Bread making, brewing
The word equations are the most-asked part of this topic.
Key terms
Respiration
The chemical breakdown of food (glucose) in cells to release energy.
Aerobic respiration
Respiration using oxygen; glucose is completely broken down.
Anaerobic respiration
Respiration without oxygen; glucose is partly broken down.
ATP
The molecule that carries the energy released by respiration to where it is needed in the cell.
Lactic acid
The product of anaerobic respiration in muscle.
Oxygen debt
The extra oxygen needed after exercise to break down lactic acid.
Fermentation
Anaerobic respiration by yeast, producing ethanol and carbon dioxide.
Common misconceptions
✗ Respiration is the same as breathing.✓ Breathing moves air in and out of the lungs. Respiration is a chemical reaction in every living cell.
✗ Plants do not respire — they photosynthesise instead.✓ Plants respire all the time; in bright light photosynthesis is faster, so they give out more oxygen than they use.
✗ Anaerobic respiration never produces carbon dioxide.✓ In yeast and plants it does (ethanol + CO₂). Only anaerobic respiration in animal muscle produces no CO₂.
Memory aid
Aerobic = Air (oxygen). Yeast makes Bread rise and Brews drinks: ethanol + CO₂.
Real-life link
Caribbean rum is made by fermenting molasses (from sugar cane) with yeast. The yeast respires anaerobically, turning the sugar into ethanol and carbon dioxide; the ethanol is then distilled.
Try this
Mix one teaspoon of yeast and two of sugar in warm water in a bottle and stretch a balloon over the neck. Watch the balloon inflate over 30 minutes. Which gas is filling it?
Play & practise
Games
3 quick games on this lesson. Earn XP and badges as you play — your best score for each game is saved. Games are for practice and do not change your exam readiness.
Step 3 · Practise
Paper 1 practice — multiple choice
8 items modelled on real CSEC Paper 1 questions. Choose one answer for each, then check. Aim for 80% or better.
Step 4 · Apply
Paper 2 practice — structured questions
Write full answers, then reveal the mark scheme and tick the points you earned (16 marks in total). Your answers are saved on this device.
Based on CSEC P2 May/June 2024, Question 4(a)–(c)
Question 1 [10 marks]
James ran the 100 m at the national sports meet. Afterwards he was gasping for air and felt intense pain in his legs.
(a) Write a balanced chemical equation for aerobic respiration. [2]
MARK SCHEME — (a) (max 2)
(b) State TWO ways in which aerobic respiration differs from anaerobic respiration. [4]
MARK SCHEME — (b) (max 4)
(c) Explain why James felt pain and was gasping for air. [4]
MARK SCHEME — (c) (max 4)
Based on CSEC P2 May/June 2023, Question 5(a), (d)
Question 2 [6 marks]
Respiration and breathing are often confused.
(a) Define “respiration” and “breathing”. [2]
MARK SCHEME — (a) (max 2)
(b) Explain how human beings use anaerobic respiration during exercise. [4]
MARK SCHEME — (b) (max 4)
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Sources
Where this topic appears in CSEC papers
Find these in your own past-paper booklets for extra timed practice. The practice above is adapted from them.