6.2 - Rate of Photosynthesis

Limiting factors in photosynthesis

• The rate of photosynthesis isn't fixed - it rises and falls with the conditions around the plant.
• A limiting factor is the factor in shortest supply at that moment, so it's the one holding the rate back.
• Increasing the limiting factor raises the rate; increasing any other factor makes no difference until the limiting one is raised.
• Which factor is limiting depends on the surroundings:
  • At night - Light is limiting
  • A cold winter day - Temperature is often limiting
  • Warm and bright - Carbon dioxide is usually limiting

The three environmental factors

1. Light intensity - Provides the energy for photosynthesis. Raising it from low increases the rate steadily, but only up to a point. Beyond that, more light makes no difference because temperature or carbon dioxide has become limiting instead. The rate-against-light-intensity graph climbs, then levels off at a plateau.
2. Carbon dioxide concentration - A raw material of photosynthesis. Increasing it raises the rate up to a point, then the graph flattens - showing light or temperature is now limiting. Same rise-then-plateau shape as the light graph.
3. Temperature - Affects the enzymes that control photosynthesis:
   • In the cold, enzymes work slowly, so the rate is slow - this is usually why temperature is limiting.
   • As temperature rises, the rate increases up to an optimum at around 25°C.
   • Above the optimum the rate falls sharply, and by around 45°C the enzymes are denatured (their shape is destroyed) and the rate drops to zero.
   • The rate-against-temperature graph therefore rises to a peak then drops steeply - unlike the light and carbon dioxide graphs, which plateau.

The amount of chlorophyll can also limit the rate

• Chlorophyll is the pigment that absorbs the light, so less chlorophyll means a reduced rate.
• Things that can damage chloroplasts or stop the plant making enough chlorophyll include:
  • Disease - e.g. infection with tobacco mosaic virus
  • Environmental stress - e.g. a shortage of nutrients
• With less chlorophyll, the plant can't absorb as much light, slowing photosynthesis.