Photosynthesis
What is Photosynthesis?
Photosynthesis is the process by which green plants, algae and cyanobacteria convert light energy into chemical energy stored in glucose. It is an endothermic process — it takes in energy from the environment.
Word equation: Carbon dioxide + Water → Glucose + Oxygen
Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Where it occurs: In chloroplasts, which contain the green pigment chlorophyll. Chlorophyll absorbs red and blue light (reflects green — why leaves look green).
Two stages (Higher Tier detail):
- Light-dependent reactions — in the thylakoid membranes; light splits water (photolysis) → releases O₂ as a by-product; ATP and NADPH are made
- Light-independent reactions (Calvin cycle) — in the stroma; CO₂ is fixed using ATP and NADPH to make G3P → glucose
Limiting Factors
The rate of photosynthesis is controlled by the factor in shortest supply — the limiting factor. At any moment, only one factor is limiting.
1. Light Intensity
As light intensity increases, the rate of photosynthesis increases — until another factor becomes limiting. Beyond a certain point, increasing light has no further effect.
Inverse-square law: Light intensity is inversely proportional to the square of the distance from the source: $$I propto rac{1}{d^2}$$
Example: If a lamp is moved from 10 cm to 20 cm, the distance doubles → light intensity decreases to 1/4 (not 1/2).
Practical: Count bubbles of O₂ produced per minute from aquatic plants (e.g., Elodea/Cabomba) at different distances from a lamp.
2. Carbon Dioxide Concentration
Increasing CO₂ concentration increases the rate — until light or temperature becomes limiting. At atmospheric CO₂ (~0.04%), CO₂ is often the limiting factor in greenhouses.
3. Temperature
Increasing temperature increases the rate of photosynthesis (up to the optimum, ~25–35 °C for most plants) by increasing enzyme activity. Above the optimum, enzymes denature → rate falls sharply.
Note: Temperature affects the enzyme-controlled light-independent stage; the light-dependent stage is less sensitive to temperature.
Summary — Interpreting Graphs
On a graph of rate of photosynthesis vs light intensity:
- Rising section = light is the limiting factor
- Plateau = light is no longer limiting (CO₂ or temperature is limiting)
- Higher plateau after raising CO₂ = CO₂ was the limiting factor at the plateau
Uses of Glucose Produced by Photosynthesis
Plants use glucose for:
| Use | How |
|---|---|
| Respiration | Releases energy for all metabolic processes |
| Cellulose | Forms cell walls for structural support |
| Starch | Storage carbohydrate (insoluble → doesn't affect osmosis) |
| Sucrose | Transport form in phloem (soluble) |
| Proteins | Glucose + nitrate ions (from roots) → amino acids → proteins |
| Fats/oils | For energy storage in seeds |
Starch test: Iodine solution turns from brown to blue-black in the presence of starch — confirms photosynthesis has occurred.
Greenhouses
Commercial growers manipulate limiting factors to maximise crop yield:
- Increase CO₂: burn natural gas inside greenhouses (also releases heat)
- Artificial lighting: extends growing season and intensity
- Heating: maintains optimum temperature for enzyme activity
- Cost vs benefit: extra CO₂/lighting costs money → economic balance required
AI-generated · claude-opus-4-7 · v3-wjec-combined-science