Limiting Factors
Higher Tier Only
Interaction of two factors
- More than one limiting factor can have an effect on the rate of photosynthesis
- Graphs may show the effect of two factors interacting:
The rate of photosynthesis increases with increasing light intensity, temperature and carbon dioxide
- At both temperatures tested the rate of photosynthesis is limited by low light intensity - as demonstrated by the lines showing the same rate
- As the light intensity increases the rate of photosynthesis at 15℃ is lower than 25℃
- Both lines level off, this shows that light intensity is no longer the limiting factor
Interaction of three factors
- Graphs may show the interactions between three different factors, the graph below shows the relationship between temperature, carbon dioxide as light intensity is increased:
The rate of photosynthesis increases with increasing light intensity, temperature and carbon dioxide
- All three experiments level-off when light intensity is no longer the limiting factor
- Experiment 1 (red line) has the highest temperature and concentration of carbon dioxide so the rate of photosynthesis is much higher
- In experiment 2 (blue line), the concentration of carbon dioxide is the limiting factor
- The results of this experiment demonstrate that the rate of photosynthesis is controlled by carbon dioxide levels
The inverse square law
- The inverse square law shows the relationship between light intensity and distance.
- As the distance from the lamp increases the light intensity decreases
- Light intensity and distance (from the light source) are inversely proportional to each other
- This means that as the distance doubles the intensity of the light will be four times less
- This is called the inverse square law and shown by the equation below:
Worked example
Calculate the light intensity when the distance of the plant is 30 cm from the lamp
Step 1: Use the equation
Light intensity = 1/d2
Step 2: Substitute the values you know
Light intensity = 1/302
Step 3: Calculate the light intensity
Light intensity = 0.001 a.u. (arbitrary units)
