Light & the Rate of Photosynthesis (Edexcel GCSE Biology: Combined Science)

Higher tier only

• The intensity of the light available to a plant determines the amount of energy that it has to carry out photosynthesis
  • This means that more light a plant receives, the faster the rate of photosynthesis
  • In other words, the rate of photosynthesis is directly

    proportional to light intensity

• However, the intensity of the light available to a plant depends on how far the light source is from the plant
  • The further away a plant is from a light source, the lower the rate of photosynthesis
  • In other words, the rate of photosynthesis is inversely
    proportional to the distance from the light source

Light intensity & distance

• A light meter can be used to measure light intensity
• Light intensity outside (natural light) changes gradually throughout the day as the sun rises and sets
• Artificial light sources (e.g. bulbs) can also vary in light intensity when they are at different distances from a plant
• As a light source gets further away from a plant (i.e. as the distance from a light source increases), the light intensity decreases
  • Also, therefore, as the distance decreases, the light intensity increases

• This means that light intensity is inversely proportional to the distance between the plant and the light source
• Light intensity actually decreases in proportion to the square of the distance - this is known as the inverse square law
  • Therefore, the inverse square law is as follows:

light intensity ∝ 1 ÷ distance²

• For example, if the distance between a plant and a light source was doubled, the light intensity would be four times smaller