Revision notes · Bioenergetics
Photosynthesis
Photosynthetic reaction4.4.1.1
Definition: Photosynthesis is an endothermic reaction in which energy is transferred from the environment to the chloroplasts by light.
Plants make their own food using light energy absorbed by chlorophyll.
- •The word equation for photosynthesis is carbon dioxide + water → glucose + oxygen.
- •The balanced chemical equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
- •Photosynthesis occurs inside chloroplasts, which contain the pigment chlorophyll that absorbs light.
- •It is an endothermic reaction because energy is taken in from the surroundings.
⚠️ Common mistake: Thinking photosynthesis is exothermic instead of endothermic.
🧠 Remember: Remember COW produces GO: Carbon dioxide + Water → Glucose + Oxygen.
Rate of photosynthesis4.4.1.2
Definition: A limiting factor is an environmental condition that restricts the rate of photosynthesis when it is in short supply.
The rate of photosynthesis is limited by light intensity, carbon dioxide concentration, temperature, and amount of chlorophyll.
- •Increasing light intensity or carbon dioxide concentration increases the rate of photosynthesis until a different factor becomes limiting.
- •If temperature is too high, the enzymes controlling photosynthesis denature and the rate drops to zero.
- •Light intensity follows the inverse square law: light intensity is proportional to 1 / distance².
- •Commercial growers control temperature, light, and carbon dioxide levels in greenhouses to maximise crop yield.
⚠️ Common mistake: Saying that high temperatures 'kill' or 'destroy' enzymes rather than stating that enzymes denature.
🧠 Remember: Double the distance from a light source and light intensity quarters (inverse square law).
Uses of glucose from photosynthesis4.4.1.3
Definition: Cellulose is a tough carbohydrate made from glucose that strengthens plant cell walls.
Glucose produced during photosynthesis is used immediately or converted into other essential plant substances.
| Use | Product / purpose |
|---|---|
| Respiration | Releases energy for growth and biological processes |
| Storage | Converted to insoluble starch (in roots, stems, leaves) |
| Protein synthesis | Combined with nitrate ions from the soil to make amino acids |
| Structure & storage | Converted to cellulose (cell walls) or lipids (fats/oils in seeds) |
⚠️ Common mistake: Thinking plants store energy as glucose instead of insoluble starch, which does not affect osmosis in plant cells.
🧠 Remember: Remember SCARL for glucose uses: Starch, Cellulose, Amino acids, Respiration, Lipids.
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