Pearson Edexcel · GCSE · Biology
Course version: 1BI0 · Version from Sept 2016
Explain how the rate of photosynthesis is directly proportional to light…
Full specification wording
Topic 6 – Plant structures and their functions: 6.6 Explain how the rate of photosynthesis is directly proportional to light intensity and inversely proportional to the distance from a light source, including the use of the inverse square law calculation
Light intensity falls as you move further from a lamp.
Checked against the Pearson Edexcel 1BI0 specification (opens in a new tab) on .
- Point 5 of 15
Study notes
From the checked teaching notes for this specification point.
The idea
Light intensity falls as you move further from a lamp. The rate of photosynthesis is directly proportional to light intensity, so doubling light intensity doubles the rate (provided light is the limiting factor). Light intensity is inversely proportional to the square of the distance from the source: intensity ∝ 1 ÷ distance². So doubling the distance reduces intensity to one quarter, and the rate falls to a quarter. You use the inverse square law to calculate relative intensity at different distances, for example comparing 1 ÷ 0.5² = 4 with 1 ÷ 1² = 1, showing the nearer position gives four times the intensity. Keep units consistent and square the distance before dividing.
What earns the marks
- Rate of photosynthesis is directly proportional to light intensity when light is the limiting factor, so doubling intensity doubles the rate.
- Light intensity is inversely proportional to the square of the distance from the source: intensity ∝ 1 ÷ distance².
- Doubling the distance reduces light intensity to one quarter, so the rate falls to one quarter.
- The inverse square law calculation squares the distance before dividing, for example relative intensity = 1 ÷ d².
- Direct proportionality means a straight-line graph through the origin of rate against light intensity.
Common mistakes and how to put them right
Treating intensity as inversely proportional to distance rather than to distance squared.
Put it right by squaring the distance before dividing.
Forgetting to square the distance in the calculation.
Put it right by applying intensity ∝ 1 ÷ d².
Assuming rate keeps rising with light intensity even when another factor such as carbon dioxide is limiting.
Put it right by noting light only limits the rate up to a point.
Exam tips
- Write the relationship as intensity ∝ 1 ÷ d² and substitute the distance before calculating.
- When comparing two distances, work out the ratio of the squared distances to find the change in intensity.
- State that direct proportionality gives a straight line through the origin on a rate-versus-intensity graph.
Try the lesson
The start of the Topic 6 – Plant structures and their functions: 6.6 Explain how the rate of photosynthesis is directly proportional to light intensity and inversely proportional to the distance from a light source, including the use of the inverse square law calculation lesson, made from checked teaching notes for this specification point.
Have a go first
As light intensity increases, the rate of photosynthesis rises in direct proportion (while light is the limiting factor). If light intensity doubles, what happens to the rate of photosynthesis?
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