CB6.1 — Photosynthesis (Edexcel 1SC0)
The equation
$$6CO_2 + 6H_2O \xrightarrow{\text{light}} C_6H_{12}O_6 + 6O_2$$
Carbon dioxide + water → glucose + oxygen (using light energy absorbed by chlorophyll).
Photosynthesis occurs in the chloroplasts of plant cells and algae.
Limiting factors
A limiting factor is whatever factor is present in least sufficient quantity:
| Factor | Effect of increasing |
|---|---|
| Light intensity | Increases rate (up to a plateau) |
| CO₂ concentration | Increases rate (up to a plateau) |
| Temperature | Increases rate up to optimum (enzyme-controlled); denatures enzymes above optimum |
| Water | Rarely limiting in normal conditions |
Inverse square law for light: $$\text{Light intensity} \propto \frac{1}{\text{distance}^2}$$
If distance doubles, light intensity quarters; rate of photosynthesis decreases.
Uses of glucose by plants
Plants use glucose from photosynthesis for:
- Respiration: release energy for all cellular processes.
- Cellulose synthesis: for cell wall construction.
- Starch: stored as an insoluble energy reserve.
- Proteins: combined with nitrate ions from the soil.
- Fats and oils: stored in seeds.
Required practical
Light intensity experiment: place aquatic plant (e.g. Elodea) in water at different distances from a lamp; count bubbles of O₂ per minute. Control: CO₂ concentration (add NaHCO₃ to water), temperature.
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