8.2 The Rate of Photosynthesis

A free GCSE Biology lesson for AQA (8461) from OBA School: limiting factors: light intensity, carbon dioxide, temperature and chlorophyll, and how they interact. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Name the four factors that can limit the rate of photosynthesis
• Explain the shape of graphs of rate against light intensity, carbon dioxide concentration and temperature
• Identify the limiting factor at different points on a graph (Higher tier)
• Use the inverse square law to compare light intensities (Higher tier)

Key words

• Rate: How fast something happens: amount of product made (or reactant used) per unit time.
• Limiting factor: The factor in shortest supply, which stops the rate going any higher.
• Light intensity: How much light energy reaches a surface. It falls as a lamp is moved further away.
• Optimum: The temperature at which the enzymes work fastest.
• Denatured: An enzyme whose active site has changed shape, so it no longer works.
• Inverse square law: Light intensity is proportional to 1 / distance² (Higher tier).

What is a limiting factor?

Photosynthesis needs light, carbon dioxide, a suitable temperature and chlorophyll. If any one of these is in short supply, it holds the rate back. That factor is the limiting factor.

Think of a sandwich shop: it can only make sandwiches as fast as its slowest supply. Doubling the bread does nothing if it has run out of filling.

• Light intensity; Why it affects the rate Supplies the energy for the reaction; Often limiting… at night, in shade, in winter
• CO₂ concentration; Why it affects the rate A reactant; air is only about 0.04% CO₂; Often limiting… on bright, warm days
• Temperature; Why it affects the rate Photosynthesis is controlled by enzymes; Often limiting… in early morning, in winter
• Amount of chlorophyll; Why it affects the rate Absorbs the light; Often limiting… in diseased or nutrient-poor leaves

A plant short of magnesium ions cannot make enough chlorophyll and its leaves turn yellow (chlorosis). Some diseases also destroy chlorophyll.

Light intensity and carbon dioxide

As light intensity rises, the rate rises in proportion, because more energy is available. Then the line levels off: now something else (often CO₂ or temperature) is limiting. Carbon dioxide gives a graph of the same shape.

Chlorophyll absorbs mainly red and blue light and reflects green light, which is why leaves look green. Under green light alone, the rate is very low.

Temperature

Temperature affects the enzymes that control photosynthesis. As it warms up, molecules move faster and collide more often, so the rate rises. Above the optimum (often around 25–35 °C for UK plants) the enzymes start to denature: the active site changes shape and the rate falls quickly.

Exam tip: say enzymes are denatured, not “killed”. Enzymes are proteins, not living things.

Factors interacting (Higher tier)

In real life the factors work together. To find the limiting factor at a point on a graph, ask: “If I increase this factor, does the rate go up?”

Worked example: reading the graph

On the 0.04% CO₂ line, what is limiting (a) at low light intensity and (b) on the flat part?
(a) The rate rises when light increases, so light intensity is limiting.
(b) More light no longer helps, but raising CO₂ to 0.10% does, so carbon dioxide is limiting.

Answer: (a) light intensity (b) carbon dioxide concentration

The inverse square law (Higher tier)

Light spreads out as it travels. Light intensity is inversely proportional to the square of the distance from the lamp:

Inverse square law: light intensity ∝ 1 / d² Double the distance → one quarter of the intensity.

Worked example: comparing two distances

A lamp is moved from 15 cm to 45 cm from a plant. By what factor does the light intensity change?
Distance is multiplied by 45 ÷ 15 = 3.
Intensity is multiplied by 1 / 3² = 1/9.

Answer: The light intensity falls to 1/9 of its value

Worked example: relative light intensity

Work out the relative light intensity 1/d² for a lamp 20 cm (0.20 m) away.
1 / 0.20² = 1 / 0.04 = 25 (arbitrary units).

Answer: 25 arbitrary units

Common mistakes

• Saying the rate “stops” on the flat part of a graph. It is still photosynthesising, just not any faster.
• Naming the factor on the x-axis as limiting on the flat part. On the plateau, a different factor is limiting.
• Saying high temperatures kill enzymes. They are denatured.
• Halving the intensity when the distance doubles. Intensity falls to one quarter, because of the square.

Revision questions

Try these, then check your answers below each one.

Question 1. Which of these is not a limiting factor for photosynthesis? (Choose from: Light intensity, Carbon dioxide concentration, Oxygen concentration, Temperature.)

Answer: Oxygen concentration

Working: Light, CO₂, temperature and the amount of chlorophyll can limit the rate. Oxygen is a product.

Question 2. A graph of rate against light intensity rises and then levels off. What is true on the flat part? (Choose from: Another factor, such as CO₂ or temperature, is limiting, Photosynthesis has stopped, The enzymes are denatured, Light intensity is the limiting factor.)

Answer: Another factor, such as CO₂ or temperature, is limiting

Working: On the plateau more light has no effect, so a different factor is limiting. Photosynthesis is still happening.

Question 3. Above the optimum temperature, the rate of photosynthesis falls because the enzymes are ____.

Answer: denatured

Working: High temperatures denature the enzymes that control photosynthesis.

Related lessons

• Previous lesson: 8.1 Photosynthesis
• Next lesson: 8.3 How Plants Use Glucose
• In this chapter: 8.4 Making the Most of Photosynthesis
• In this chapter: 8.5 Required Practicals: Photosynthesis

Continue in the full course

This lesson is section 8.2 (AGCBH0802) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.