Photosynthesis

$6\text{CO}_2 + 6\text{H}_2\text{O} \xrightarrow{\text{light}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

• Endothermic reaction (absorbs energy from light)
• Occurs in chloroplasts (which contain chlorophyll)
• Chlorophyll absorbs light energy

The rate of photosynthesis is limited by whichever factor is in shortest supply:

Light Intensity

• More light → faster rate (up to a point)
• Beyond saturation: another factor becomes limiting
• Inverse square law: $\text{light intensity} \propto \frac{1}{d^2}$

CO₂ Concentration

• More CO₂ → faster rate (up to a point)
• Atmospheric CO₂ ≈ 0.04%

Temperature

• Rate increases with temperature (enzymes work faster)
• Above ~45°C: enzymes denature → rate drops rapidly
• Optimum ≈ 25-35°C

Typical graph shapes:

• Light intensity: rate increases then plateaus (limited by CO₂ or temp)
• CO₂ concentration: similar — increases then plateaus
• Temperature: increases to optimum then falls sharply

Using pondweed (Elodea):

1. Place pondweed in water with NaHCO₃ (source of CO₂)
2. Shine lamp at different distances
3. Count bubbles of O₂ per minute (or collect gas)
4. Calculate light intensity from distance ($I \propto 1/d^2$)
5. Plot rate vs light intensity

• $6\text{CO}_2 + 6\text{H}_2\text{O} → \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$
• Occurs in chloroplasts; endothermic
• Limiting factors: light, CO₂, temperature
• Glucose used for: respiration, cellulose, starch, amino acids, lipids
• Light intensity ∝ 1/d²