IGCSE Biology 0610 · Common mistakes
Common mistakes in photosynthesis and respiration
Updated · by the Fahmazing team
Photosynthesis and respiration are mirror images, which makes it easy to mix them up. These are the errors that cost students marks most often.
Swapping the gases
Which gases does a leaf exchange in photosynthesis?
✗ Wrong: It takes in oxygen and gives out carbon dioxide.
✓ Right: It takes in carbon dioxide and gives out oxygen. Aerobic respiration is the other way round.
Free lessons: Photosynthesis · Aerobic respiration
Putting light or energy in the equation as a substance
Write the word equation for photosynthesis
✗ Wrong: carbon dioxide + water + light → glucose + oxygen
✓ Right: carbon dioxide + water → glucose + oxygen, with "light" and "chlorophyll" written over the arrow
Tip: Light is energy, not a raw material. In the same way, do not write "+ energy" as a product of respiration.
Free lessons: Photosynthesis · Aerobic respiration
Saying energy is "produced" or "made"
What is aerobic respiration?
✗ Wrong: Breathing in oxygen to produce energy.
✓ Right: Chemical reactions in cells that use oxygen to break down nutrient molecules to release energy.
Tip: Chlorophyll does not "make" energy either: it transfers energy from light into energy in chemicals.
Free lessons: Aerobic respiration · Photosynthesis
Respiration in the lungs, or only at night
Where and when do plants respire?
✗ Wrong: Plants only respire at night, and respiration happens in the lungs of animals.
✓ Right: All living cells respire all the time, day and night. Aerobic respiration happens in the mitochondria. Lungs are for gas exchange.
Free lesson: Aerobic respiration
Mixing up anaerobic respiration in yeast and muscles
What are the products of anaerobic respiration?
✗ Wrong: Muscles make lactic acid and carbon dioxide; yeast makes lactic acid.
✓ Right: Muscles make lactic acid only. Yeast makes alcohol (ethanol) and carbon dioxide.
Tip: Anaerobic respiration releases much less energy per glucose molecule than aerobic respiration.
Free lesson: Anaerobic respiration
Mixing up the leaf test steps and limiting factors
- Starch is the store; sucrose is the form that travels in the phloem.
- Magnesium ions are needed for chlorophyll; nitrate ions are needed for amino acids.
- Ethanol removes the chlorophyll; boiling water kills the cells. Heat ethanol in a water bath, never over a flame.
- The limiting factor is the one in short supply, not the one that is plentiful.
Free lessons: Photosynthesis · Investigating photosynthesis and limiting factors
Questions students ask
Do plants respire during the day?
Yes. Plants respire all the time. In bright light, photosynthesis is faster than respiration, so overall they take in carbon dioxide.
Is respiration the same as breathing?
No. Breathing moves air in and out of the lungs. Respiration is the chemical reactions in cells that release energy from nutrient molecules.
Why is a plant destarched before a starch test?
It is kept in the dark for about 48 hours to use up starch already in the leaves, so any starch found later was made during the experiment.