Photosynthesis — the equation, limiting factors and what plants do with glucose
Photosynthesis is the process by which plants and algae make their own glucose using light, water and carbon dioxide. Without it, almost no life on Earth could exist.
The equation
Word equation:
carbon dioxide + water → glucose + oxygen (light absorbed by chlorophyll)
Balanced symbol equation:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
This is an endothermic reaction — energy is taken in from light.
Where it happens
In the chloroplasts of leaf cells (mainly the palisade mesophyll). Chlorophyll absorbs light energy — most strongly red and blue, reflecting green (which is why most leaves look green).
Limiting factors
A factor is limiting if increasing it would speed up photosynthesis (the others being held constant). The four key factors:
- Light intensity — more light → faster reaction (up to a point).
- Carbon dioxide concentration — more CO₂ → faster reaction (up to a point).
- Temperature — increases rate up to ~30–35 °C; above ~45 °C enzymes denature and rate falls sharply.
- Chlorophyll amount — fewer chloroplasts (e.g. due to disease) lower the maximum rate.
A typical graph plots rate against the factor:
- Initial straight line — the factor is limiting.
- Plateau — another factor has become limiting.
The inverse-square law (HT)
Light intensity falls with the square of the distance from the source: I ∝ 1/d². Doubling the distance from a lamp to a plant cuts light intensity to a quarter (not a half).
In a practical, you can measure rate of photosynthesis by counting bubbles of O₂ from pondweed (or measuring volume in a gas syringe), at various distances from a lamp.
Commercial use of limiting factors
Greenhouse growers manipulate all four factors to maximise growth (within economic limits):
- Add artificial lighting at night
- Burn propane heaters — supplies heat AND extra CO₂
- Maintain optimum temperature (~25 °C)
There is a cost-benefit balance — at some point, raising one factor more is not financially worthwhile.
What plants use glucose for
Plants don't store free glucose; they convert it for various uses:
- Respiration — releases energy for growth and other processes.
- Convert to starch for storage (compact, insoluble — doesn't affect water potential).
- Convert to cellulose to build cell walls.
- Combine with mineral ions (e.g. nitrate) to form amino acids → proteins.
- Convert to lipids and oils — e.g. for seed energy stores.
⚠Common mistakes— Common mistakes / exam traps
- "Plants make energy" — they don't; they trap light energy as chemical energy in glucose.
- "Plants only photosynthesise during the day" — true, but they also respire continuously (day AND night). At night, there's a net release of CO₂; during bright sunlight, a net uptake.
- Confusing limiting factor graphs — when you've increased one factor and the rate is no longer rising, something else has become limiting.
- "Doubling distance halves light intensity" — wrong; inverse-square law means it quarters intensity.
Links
Connects to B4.2 (respiration), B2.3 (leaf as an organ for photosynthesis), and B7.3 (carbon cycle).
AI-generated · claude-opus-4-7 · v3-deep-biology