Photosynthesis — converting light energy into chemical energy
Photosynthesis is the process by which plants, algae and some bacteria convert light energy into chemical energy (glucose). It is the foundation of almost all food chains.
The photosynthesis equation
Word equation: Carbon dioxide + Water → Glucose + Oxygen (using light energy)
Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (light energy; chlorophyll)
This is an endothermic reaction — energy is taken IN from light. The reaction occurs in chloroplasts, specifically using the green pigment chlorophyll.
Where does it happen?
Chloroplasts are found mainly in palisade mesophyll cells in the upper part of the leaf. The leaf is adapted for photosynthesis:
| Adaptation | Reason |
|---|---|
| Broad, flat surface | Large surface area to absorb light |
| Thin | Short diffusion pathway for CO₂ to reach cells |
| Transparent cuticle/epidermis | Light passes through to palisade cells |
| Stomata (underside) | Allow CO₂ in and O₂ out; controlled by guard cells |
| Vascular bundles (veins) | Deliver water via xylem; remove glucose via phloem |
| Air spaces in spongy mesophyll | Allow gas exchange between cells |
Factors affecting the rate of photosynthesis
The rate of photosynthesis is limited by whichever factor is in shortest supply — this is the limiting factor.
1. Light intensity More light → more energy for the reaction → faster rate (up to a point). If the graph levels off despite increasing light, another factor is limiting.
2. Carbon dioxide concentration More CO₂ → more raw material → faster rate (up to a point).
3. Temperature Higher temperature → enzymes work faster → increased rate (up to the enzyme's optimum temperature). Above optimum, enzymes denature → rate drops sharply.
Typical graph shapes for limiting factors:
- Light and CO₂: rate increases linearly, then plateaus (limited by another factor).
- Temperature: bell-shaped curve — increases to optimum (~30-40°C for most plants), then falls steeply.
Uses of glucose produced by photosynthesis
Plants do not just store glucose — they convert it to many things:
| Product | How formed | Use |
|---|---|---|
| Starch | Glucose polymerised | Long-term energy storage (insoluble, won't affect osmosis) |
| Cellulose | Glucose polymerised | Cell walls (structural support) |
| Sucrose | Glucose + fructose | Transport in phloem to non-photosynthetic parts |
| Lipids (fats/oils) | Glucose converted | Energy storage in seeds |
| Amino acids | Glucose + nitrate ions | Protein synthesis (for growth and enzymes) |
| Respiration (ATP) | Glucose oxidised | Energy for all cell processes |
Measuring the rate of photosynthesis — the pondweed experiment
Aquatic plants (e.g., Elodea/pondweed) release oxygen bubbles that can be counted or collected.
- Count bubbles per minute at different light intensities (by varying the distance from a lamp).
- Collect oxygen in an inverted syringe and measure volume over time.
- Key control: use sodium hydrogen carbonate solution to keep CO₂ concentration constant.
AI-generated · claude-opus-4-7 · v3-ccea-combined-science