Learning objectives
- Write the word and balanced chemical equation for photosynthesis
- Describe the light-dependent and light-independent stages
- Explain and sketch the effect of the three limiting factors
1. The equation
Word equation: carbon dioxide + water → (light energy, chlorophyll) → glucose + oxygen.
Balanced equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Write light energy and chlorophyll above and below the arrow, not as reactants.
2. The two stages
The light-dependent stage happens in the thylakoid membranes of the chloroplast. Light splits water (photolysis), releasing oxygen and producing ATP and NADPH.
The light-independent stage (Calvin cycle) happens in the stroma. ATP and NADPH are used to fix carbon dioxide into glucose. It does not need light directly but stops when the light reaction stops supplying ATP.
3. Limiting factors
The three limiting factors are light intensity, carbon dioxide concentration and temperature. The rate rises as a factor increases until another factor becomes limiting, then the graph plateaus.
Temperature behaves differently: the rate rises to an optimum (about 25–35 °C for most crops) and then falls sharply as the enzymes denature. Draw that curve, not a plateau.
- Light intensity — rate rises then plateaus
- CO₂ concentration — rate rises then plateaus
- Temperature — rises to an optimum, then falls (enzyme denaturation)
4. Standard experiments
Testing a leaf for starch: boil in water (kill tissue), boil in ethanol (remove chlorophyll), rinse and add iodine solution — blue-black means starch, so photosynthesis occurred.
Variegated leaf, destarched plant, and sodium hydroxide (removes CO₂) controls prove that chlorophyll, light and carbon dioxide are each necessary.
Key definitions
- Photolysis
- The splitting of water molecules by light energy during the light-dependent stage.
- Limiting factor
- The factor in shortest supply that directly controls the rate of a process.
- Compensation point
- Light intensity at which the rate of photosynthesis exactly equals the rate of respiration.
Exam tips
- Never write 'plants breathe in CO₂' — use 'take in by diffusion through the stomata'.
- When a graph plateaus, always name the new limiting factor in your answer.
Practice exam questions & model answers
Q1.Write the balanced chemical equation for photosynthesis and name the site of each stage.
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ in the presence of light energy and chlorophyll. The light-dependent stage occurs in the thylakoid membranes and the light-independent (Calvin cycle) stage occurs in the stroma of the chloroplast.
Q2.Explain why the rate of photosynthesis stops increasing at high light intensity.
Light is no longer the limiting factor. Another factor — usually carbon dioxide concentration or temperature — has become limiting, so the rate plateaus until that factor is increased.
Q3.Why is a plant destarched before a photosynthesis experiment?
So any starch detected at the end of the experiment must have been made during the experiment itself, making the result valid.
Continue revising
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