PhotosynthesisEdexcel GCSE Biology: Revision notes
Section 1
Producers and Biomass
Plants/algae are main producers, converting light energy into chemical energy (glucose), forming the base of biomass in ecosystems.
Section 2
The Photosynthesis Reaction
Endothermic: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, using light absorbed by chlorophyll.
Photosynthesis is endothermic; respiration is exothermic.
Section 3
Limiting Factors
Temperature, light intensity, CO₂ concentration. Whichever is scarcest limits rate; rate plateaus once another factor becomes limiting; high temperature denatures enzymes.
A plateau on a graph shows a different factor has become limiting.
Section 4
Interaction of Limiting Factors
Growers optimise temperature, light and CO₂ together (e.g. greenhouses) since improving one factor alone is wasted once another becomes limiting.
Paraffin heaters raise both temperature and CO₂ together.
Section 5
Core Practical: Light Intensity and Photosynthesis Rate
Count oxygen bubbles from pondweed at varying lamp distances, keeping temperature/CO₂ constant. Gas syringe more precise than bubble counting.
Section 6
Inverse Square Law
Rate ∝ light intensity (at low levels). Light intensity ∝ 1/distance². Doubling distance reduces intensity to a quarter.
20cm→40cm: intensity ratio = (1/1600)/(1/400) = 1/4.
Must Know
- Photosynthesis endothermic: 6CO2+6H2O→C6H12O6+6O2.
- Limiting factors: temp, light, CO2 — whichever scarcest limits rate.
- Rate ∝ light intensity; intensity ∝ 1/distance².
- Practical: pondweed bubbles/gas volume, control temp & CO2.
That's the notes covered.
Carry on to the next subtopic.