B8.2 The Rate of Photosynthesis

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: limiting factors: light intensity, carbon dioxide, temperature and chlorophyll, how they interact, and the inverse square law. 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 and explain why each one matters
• Explain the shape of graphs of rate against light intensity, carbon dioxide concentration and temperature
• Decide which factor is limiting at different points on a graph with two or three factors (Higher tier)
• Use the inverse square law to compare light intensities at different distances (Higher tier)

Key words

• Rate: How fast a reaction happens: amount of product made (or reactant used) per unit time.
• Limiting factor: Anything in short supply that stops the rate from going any higher.
• Deficiency: A lack or shortage, e.g. magnesium deficiency.
• 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).

Limiting factors

Photosynthesis needs light, carbon dioxide, a suitable temperature and chlorophyll. Whichever is in shortest supply holds the rate back: it is the limiting factor. Think of a bike workshop that has plenty of frames but runs out of wheels: more frames will not make more bikes.

• Light intensity; Why it affects the rate Transfers the energy for the reaction; Often limiting… at dawn and dusk, 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 Molecules move faster and collide more; enzymes control the reaction; Often limiting… on cold mornings, in winter
• Amount of chlorophyll; Why it affects the rate Absorbs the light; Often limiting… in plants short of magnesium, or diseased leaves

Roots absorb magnesium ions from the soil by active transport. Magnesium is needed to make chlorophyll, so a plant with a magnesium deficiency has yellow leaves and photosynthesises slowly.

Light intensity and carbon dioxide

At low light intensity, more light means more energy for the reaction, so the rate rises steadily. Then the line levels off (a plateau): increasing the light no longer helps because another factor, often CO₂ or temperature, is limiting. A graph of rate against CO₂ concentration has the same shape.

Exam tip: on the flat part of the graph the plant is still photosynthesising, just not any faster. Say “the rate stays constant”, not “photosynthesis stops”.

Temperature

At low temperatures the molecules have less kinetic energy, so they collide less often and the enzyme-controlled reactions are slow. Warming increases the rate up to an optimum. Above it the enzymes start to denature: the active site changes shape and the rate falls steeply.

Worked example: percentage change

Pondweed gives off 14 bubbles per minute at 12 °C and 35 bubbles per minute at 24 °C. Calculate the percentage increase.
(35 − 14) ÷ 14 × 100 = 21 ÷ 14 × 100 = 150%.

Answer: 150% increase

Factors interacting (Higher tier)

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 10 °C line, what is limiting (a) at low light and (b) on the flat part?
(a) The rate rises when light increases, so light intensity is limiting.
(b) More light does not help, but a warmer temperature does, so temperature is limiting.
On the flat part of the 25 °C line, CO₂ concentration is probably limiting.

Answer: (a) light intensity (b) temperature

The inverse square law (Higher tier)

Light spreads out as it travels from a lamp, so light intensity falls quickly with distance:

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

Worked example: comparing two distances

A lamp is moved from 10 cm to 25 cm from a plant. By what factor does the light intensity change?
Distance is multiplied by 25 ÷ 10 = 2.5.
Light intensity is multiplied by 1 / 2.5² = 1 / 6.25 = 0.16.

Answer: The light intensity falls to 0.16 (about one sixth) of its value

Worked example: relative light intensity

Work out 1/d² for a lamp 15 cm (0.15 m) away.
1 / 0.15² = 1 / 0.0225 = 44.4 (arbitrary units).

Answer: 44.4 arbitrary units

Common mistakes

• Saying the rate “stops” on the flat part of a graph. It stays constant.
• Naming the x-axis factor as limiting on the plateau. A different factor limits there.
• Saying high temperatures kill enzymes. They are denatured.
• Halving the intensity when the distance doubles. It falls to one quarter.

Revision questions

Try these, then check your answers below each one.

Question 1. Name the mineral ion that plants absorb from the soil to make chlorophyll.

Answer: magnesium

Working: Magnesium ions are needed to make chlorophyll. Without enough, less light is absorbed and the rate of photosynthesis falls.

Question 2. The graph shows how the rate of photosynthesis changes with light intensity. At which point or points is light intensity the limiting factor? (Choose from: A only, C only, Both A and C, Neither A nor C.)

Answer: A only

Working: At A the line is still rising, so more light increases the rate: light is limiting. At C the line is flat, so another factor (such as CO₂ or temperature) is limiting.

Question 3. (Higher tier) The graph shows the rate of photosynthesis at two carbon dioxide concentrations, both at 25 °C. What is the limiting factor at point X? (Choose from: Light intensity, Carbon dioxide concentration, Temperature, Oxygen concentration.)

Answer: Carbon dioxide concentration

Working: At X the 0.04% line is flat, so more light does not help. The 0.12% line is higher at the same light intensity, so raising CO₂ increases the rate: CO₂ is limiting.

Related lessons

• Previous lesson: B8.1 Photosynthesis
• Next lesson: B8.3 How Plants Use Glucose
• In this chapter: B8.4 Making the Most of Photosynthesis
• In this chapter: B8 RP Required Practical: Rate of Photosynthesis

Continue in the full course

This lesson is section B8.2 (AGCSC0802) of our free GCSE Combined Science 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 Combined Science course.