Photosynthesis - AQA
Plants make their own food using photosynthesis. The food that plants produce is important, not only for the plants themselves, but for the other organisms that feed on the plants.
Water, light, carbon dioxide, temperature and chlorophyll
Water
In the UK, water is often the main limiting factor for photosynthesis.
You won't see graphs for its effects, as water is important in many other areas of a plant's life, and not just photosynthesis. Most important is its role as a solvent for all the chemical reactions in cells.
Light
Light intensity affects the rate of photosynthesis. The light intensity fluctuates during the day, and will also be affected by the weather. The rate of photosynthesis will change with the time of day.
The rate of photosynthesis will also change during the year in countries like the UK.
Carbon dioxide
Levels of carbon dioxide in the atmosphere are rising because of greenhouse gas emissions. They are currently at around 0.04 per cent. This concentration is still very low in terms of being the optimum for photosynthesis.
Carbon dioxide concentration is therefore an important limiting factor for photosynthesis.
Temperature
Plants can photosynthesise over a wide range of temperatures from 0°C to around 50°C.
The optimum temperature for most plants is 15°C to around 40°C.
Temperature affects the rate of photosynthesis in crop plants and affects where certain crops can be grown.
The table is an example of the optimum and maximum temperatures for the growth of some crops:
| Crop | Optimum | Maximum | Minimum |
|---|---|---|---|
| Maize | 22 - 25°C | 34°C | 20°C |
| Potato | 15 - 20°C | 34°C | 12°C |
| Rice | 30 - 33°C | 40°C | 18°C |
| Soya beans | 25 - 28°C | 40°C | 10°C |
| Wheat | 20 - 25°C | 38°C | 5°C |
Most scientists think that with rising temperatures, crop production will be negatively affected by climate change.
Chlorophyll
The position of the compensation point is different from plants grown in brighter conditions.
For shade-adapted plants, the compensation point is lower – their rate of photosynthesis will exceed the rate of respiration at lower light intensities than the plants adapted to sun.