1.4.3 Limiting Factors - Photosynthesis

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 you decrease 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

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)