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What is photosynthesis, and what factors limit its rate?

Describe photosynthesis as an endothermic reaction, explain the effect of temperature, light intensity and carbon dioxide as limiting factors, and use the inverse square law for light intensity.

A focused answer to Edexcel GCSE Biology 6.1 to 6.6, covering photosynthesis as an endothermic reaction, temperature, light intensity and carbon dioxide as limiting factors, the light-intensity core practical, and the inverse square law.

Generated by Claude Opus 4.810 min answer

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  1. What this dot point is asking
  2. The photosynthesis reaction
  3. Limiting factors
  4. The core practical: light intensity and photosynthesis
  5. The inverse square law
  6. Try this

What this dot point is asking

Edexcel statements 6.1 to 6.6 want you to describe photosynthesis as an endothermic reaction, explain how temperature, light intensity and carbon dioxide act as limiting factors, carry out the core practical on light intensity (6.5), and use the inverse square law linking light intensity to distance (6.6 is Higher tier).

The photosynthesis reaction

carbon dioxide+water→lightglucose+oxygen\text{carbon dioxide} + \text{water} \xrightarrow{\text{light}} \text{glucose} + \text{oxygen}

The symbol equation:

6CO2+6H2O→C6H12O6+6O26CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2

Photosynthetic organisms are the main producers of food and biomass, forming the base of food chains. The glucose they make is used for respiration, stored as starch, and used to build cellulose, proteins (with nitrate) and lipids.

Limiting factors

These factors also interact: on a bright warm day, carbon dioxide is often the limiting factor, which is why growers sometimes add carbon dioxide and warmth in greenhouses to increase yield.

The core practical: light intensity and photosynthesis

In the core practical (6.5) you place pondweed (such as Elodea) in water and count the bubbles of oxygen it releases (or measure the volume of gas) at different distances from a lamp. As the lamp moves closer, light intensity increases and the rate increases, until another factor (carbon dioxide or temperature) becomes limiting and the rate levels off. To keep it fair, hold the temperature constant (a heat shield or water bath stops the lamp warming the water) and use the same piece of pondweed.

The inverse square law

Light intensity is inversely proportional to the square of the distance (dd) from the light source. This means small changes in distance have a large effect: doubling the distance cuts the intensity to a quarter.

Try this

Q1. Name the three main limiting factors of photosynthesis. [3 marks]

  • Cue. Light intensity, carbon dioxide concentration and temperature.

Q2. A lamp is moved from 30 cm30\ cm to 60 cm60\ cm from a plant. State what happens to the light intensity. [1 mark]

  • Cue. The distance doubles, so the light intensity falls to one quarter (inverse square law).

Exam-style practice questions

Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Edexcel 20194 marksA student measures the rate of photosynthesis of pondweed at increasing light intensities. Explain the shape of the graph, which rises and then levels off.
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A 4-mark explain question rewards the limiting-factor reasoning for both parts of the curve.

  1. At low light intensity, light is the limiting factor, so as light intensity increases the rate of photosynthesis increases (more light means more energy for the reaction).
  2. The graph then levels off because another factor has become limiting, such as carbon dioxide concentration or temperature.
  3. At this point, increasing the light intensity no longer increases the rate, because the other factor is now in short supply.

Markers reward light as the limiting factor on the rising part, and a different factor (carbon dioxide or temperature) limiting on the flat part. Saying the plant gets tired or has enough light scores little.

Edexcel 20213 marksA lamp is moved from 10 cm to 20 cm from a plant. Using the inverse square law, explain what happens to the light intensity reaching the plant.
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A 3-mark question rewards applying the inverse square relationship.

Light intensity is inversely proportional to the square of the distance: intensity∝1d2\text{intensity} \propto \frac{1}{d^{2}}.

Doubling the distance from 10 cm10\ cm to 20 cm20\ cm multiplies d2d^{2} by 22=42^{2} = 4, so the light intensity falls to one quarter (14\frac{1}{4}) of its original value.

Markers reward the inverse square relationship and the correct factor (a quarter, because the distance doubled). Saying the intensity simply halves ignores the squaring and loses marks.

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