Form 4 · Common mistakes

Cellular Respiration, common mistakes

The mistakes SPM students make on Cellular Respiration, why each one loses marks, and the correct version.

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The mistakes, why they lose marks, and the fix

Common mistake	Why it loses marks	What earns the mark
Saying respiration is the same as breathing.	Breathing is the physical movement of air, while respiration is a chemical reaction inside cells, so treating them as one loses the distinction the question is testing.	State that breathing (ventilation) moves air in and out of the lungs, while respiration releases energy from glucose inside every cell.
Writing that anaerobic respiration releases more energy than aerobic respiration.	It reverses the real comparison; anaerobic respiration only partly breaks glucose down, so it releases less energy.	Aerobic respiration releases far more energy per glucose molecule because glucose is broken down completely into carbon dioxide and water.
Giving lactic acid as a product of yeast fermentation.	Lactic acid is the product in human muscle, not in yeast, so this confuses the two anaerobic pathways.	Yeast produces ethanol and carbon dioxide; only human muscle produces lactic acid.
Leaving oxygen out of the aerobic respiration equation.	Without oxygen the equation describes an anaerobic pathway, so the answer is no longer aerobic respiration.	Write glucose + oxygen → carbon dioxide + water + energy, with oxygen clearly on the reactant side.
Saying respiration happens only in the mitochondria.	Glycolysis, the first stage, takes place in the cytoplasm, so the statement ignores where respiration actually begins.	State that glycolysis happens in the cytoplasm of every cell and only the later aerobic stages take place in the mitochondria.
Assuming anaerobic respiration happens only when oxygen is completely absent.	A muscle can respire anaerobically while oxygen is still present but insufficient for its demand.	State that anaerobic respiration occurs whenever the oxygen supplied is not enough to meet the cell's demand.
Writing that respiration creates energy.	Energy cannot be created; respiration only transfers energy already stored in glucose.	State that respiration releases (transfers) the chemical energy already stored in the bonds of glucose into ATP.
Believing the oxygen debt is repaid during the exercise itself.	The extra oxygen is taken in after exercise stops, when breathing and heart rate remain raised.	State that the oxygen debt is repaid after exercise, which is why breathing stays heavy once the activity has stopped.
Treating ATP as the cell's long-term energy store.	ATP is made and broken down continuously for immediate use; glucose is the stored fuel, not ATP.	State that glucose is the fuel and ATP is the immediate, usable form of energy that is constantly recycled.
Forgetting the control when describing a respiration experiment.	Without a control using dead (boiled) seeds, a result cannot be attributed to respiration rather than the apparatus.	Include an identical control with boiled, cooled seeds that do not respire, and keep all other conditions the same.
Writing that yeast fermentation produces carbon dioxide and water.	Water is a product of aerobic respiration, not of fermentation, so this mixes the two pathways.	State that yeast fermentation produces ethanol and carbon dioxide, with no water formed.
Saying limewater 'turns clear' to show carbon dioxide is present.	Carbon dioxide makes limewater turn cloudy (milky); clear limewater shows no carbon dioxide.	State that limewater turns cloudy when carbon dioxide from respiration is bubbled through it.

How to avoid these mistakes

Memorise the word equations for aerobic and both types of anaerobic respiration, including the correct reactants and products.
Compare human anaerobic respiration and yeast fermentation side by side, noting the different end products.
Learn an experiment showing carbon dioxide release or heat from respiration, including how a control is set up.
Learn where each stage of respiration takes place inside the cell, from glycolysis in the cytoplasm to the aerobic stages in the mitochondria.
List the everyday uses of the energy released by respiration, such as movement, active transport, growth and maintaining body temperature.
Practise explaining oxygen debt and muscle fatigue using the correct sequence: exercise, lactic acid buildup, oxygen debt, repayment after exercise.

Frequently asked questions

What is the difference between respiration and breathing?

Breathing (ventilation) is the physical movement of air in and out of the lungs. Respiration is a chemical process inside every cell that releases energy from glucose. Breathing supplies the oxygen that aerobic respiration needs and removes the carbon dioxide it produces.

How does anaerobic respiration differ in humans and yeast?

In humans, glucose is broken down to lactic acid with a small energy release. In yeast, glucose is broken down to ethanol and carbon dioxide, also with a small energy release. Both happen without oxygen, but the products are different.

Why does aerobic respiration release more energy?

Aerobic respiration fully breaks glucose down to carbon dioxide and water using oxygen, releasing a large amount of energy. Anaerobic respiration only partly breaks glucose down, so much of the energy stays locked in the lactic acid or ethanol, giving a much smaller yield.