Photosynthesis (B4.1)

The equation

Word: carbon dioxide + water → glucose + oxygen
Symbol: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (requires light energy and chlorophyll)

Photosynthesis is endothermic — it takes in energy from light.
It takes place in chloroplasts, mainly in palisade mesophyll cells.

Uses of glucose by plants

1. Respiration — releases energy for the plant
2. Stored as starch — tested with iodine (blue-black if positive)
3. Stored as fats/oils — especially in seeds
4. Cellulose for cell walls
5. Combined with nitrate ions → amino acids → proteins

Limiting factors

A limiting factor is whatever is in shortest supply at a given moment. Increasing it raises the rate; once another factor is limiting, the rate plateaus.

The three main limiting factors:

1. Light intensity
2. Carbon dioxide concentration
3. Temperature (enzyme-controlled)

Inverse-square law for light:

light intensity ∝ 1 / d²

Double the distance → intensity falls to ¼. Triple → falls to ⅑.

Temperature:

• 10–30°C: rate increases (more KE, more enzyme–substrate collisions)

• 40°C: enzymes denature → active site changes shape → rate drops sharply

Required practical: investigating rate of photosynthesis

Set-up: pondweed (Cabomba/Elodea) in sodium hydrogen carbonate solution (CO₂ source); lamp at variable distances; count O₂ bubbles/min or measure gas volume.

Control variables: same pondweed length, constant temperature (water bath), constant NaHCO₃ concentration.

Common exam errors

1. Saying "more light always gives more photosynthesis" — rate plateaus when another factor limits.
2. Forgetting the plateau on a graph requires stating which factor is now limiting.
3. Saying enzymes "slow down" above 40°C — they denature (irreversible change in shape).
4. Forgetting doubling distance quarters intensity (not halves).