Limiting factors and light intensity
Limiting factors
The rate of photosynthesis can be affected by:
- Temperature
- Carbon dioxide concentration
- (e.g. reduced by disease or a lack of nutrients such as magnesium)
A is whichever factor is in shortest supply and is stopping photosynthesis from happening any faster. Increasing the limiting factor increases the rate; increasing a factor that isn't limiting has no effect.
Key terms
Reading limiting factor graphs
: as either increases, the rate of photosynthesis rises at first. The graph then levels off (plateaus), because some other factor has become limiting.
Temperature: the rate rises as temperature increases, up to an . Above this, the rate falls sharply, because the enzymes that control photosynthesis are .
Higher tier: in the real world, these factors interact — e.g. raising light intensity only helps if CO₂ and temperature aren't already limiting. Farmers use greenhouses to control all three, balancing the extra cost against the extra yield.
Key terms
The inverse square law (Higher tier)
Light intensity is inversely proportional to the square of the distance from the light source:
So if you double the distance from the lamp, the light intensity falls to one quarter. If you halve the distance, the light intensity becomes four times greater.
Example: moving a lamp from 10 cm to 30 cm (3× further) reduces light intensity to 1/9 of its original value.
Key terms
Required practical: light intensity and pondweed
- Place pondweed in a boiling tube of water with some added, so carbon dioxide is not limiting.
- Place a lamp at a measured distance (e.g. 10 cm) from the tube.
- Leave the pondweed to adjust for a few minutes, then count the oxygen bubbles released in one minute (or collect the gas in a measuring cylinder or gas syringe).
- Repeat at different distances, e.g. 20, 30 and 40 cm.
: keep the temperature constant (use an LED bulb, or place a beaker of water between lamp and tube to absorb heat), the CO₂ concentration, and the same piece of pondweed.
The closer the lamp, the more bubbles — until another factor becomes limiting.