What limits the rate of photosynthesis?

The factor in shortest supply limits the rate, and growers optimise all of them together. Three factors affect the rate: temperature, light intensity and CO2 concentration. The rate plateaus once another factor becomes the limiting factor. High temperature denatures enzymes.

How do we measure photosynthesis rate?

Count oxygen bubbles from pondweed at different lamp distances, controlling temperature and CO2. Keep temperature and CO2 constant. A gas syringe measures the volume of gas more precisely than counting bubbles.

Photosynthesis Notes

Edexcel GCSE Biology: Revision notes

Key facts

Producers (plants and algae) turn light energy into chemical energy in glucose.
$\ce{6CO2 + 6H2O -> C6H12O6 + 6O2}$ , and it is endothermic.
The limiting factor is whichever of temperature, light intensity or $\ce{CO2}$ is scarcest.
Greenhouses optimise all the factors together.
Light intensity is proportional to $\dfrac{1}{\text{distance}^2}$ .

Carbon dioxide + waterglucose + oxygen

The photosynthesis reaction

Producers use light absorbed by chlorophyll to turn carbon dioxide and water into glucose and oxygen.

Plants and algae are the main producers. They convert light energy into chemical energy in glucose, forming the base of biomass in ecosystems. Photosynthesis is endothermic, using light absorbed by chlorophyll.

Labelled diagram of a chloroplast — Chlorophyll in the chloroplast absorbs the light used for photosynthesis.

$\ce{6CO2 + 6H2O}$ $\ce{C6H12O6 + 6O2}$

What absorbs the light energy in photosynthesis?

Limiting factors

The factor in shortest supply limits the rate, and growers optimise all of them together.

Three factors affect the rate: temperature, light intensity and $\ce{CO2}$ concentration. The rate plateaus once another factor becomes the limiting factor. High temperature denatures enzymes. Improving one factor alone is wasted once another limits, so greenhouses control temperature, light and $\ce{CO2}$ together.

Illustrative rate against light intensity: the rate plateaus when another factor limits it; more CO₂ raises the plateau.

Rate of photosynthesislimited by the scarcest factor

Temperature

High values denature enzymes

Light intensity

Rate rises with intensity

$\ce{CO2}$ concentration

Rate rises with concentration

A greenhouse grower adds more light but the rate stops rising. Why?

Core practical: light intensity

Count oxygen bubbles from pondweed at different lamp distances, controlling temperature and $\ce{CO2}$ .

Keep temperature and $\ce{CO2}$ constant. A gas syringe measures the volume of gas more precisely than counting bubbles.

1
Set up
place pondweed a set distance from a lamp
2
Control
keep temperature and $\ce{CO2}$ constant
3
Measure
count oxygen bubbles (or use a gas syringe)
4
Repeat
change the lamp distance and measure again

Pondweed practical

Why use a gas syringe instead of counting bubbles?

Inverse square law

Light intensity is proportional to 1 over distance squared, so doubling the distance gives a quarter.

At low levels, rate is proportional to light intensity. Light intensity follows the inverse square law: it is proportional to $\dfrac{1}{\text{distance}^2}$ .

Relative intensity

\frac{1}{d^2}

, with distance

d

in units of 20 cm

Worked example

The lamp moves from 20 cm to 40 cm. What happens to the light intensity?

1
Intensity at 20 cm is proportional to $\dfrac{1}{20^2} = \dfrac{1}{400}$ .
2
Intensity at 40 cm is proportional to $\dfrac{1}{40^2} = \dfrac{1}{1600}$ .
3
Ratio: $\dfrac{1}{1600} \div \dfrac{1}{400} = \dfrac{1}{4}$ .

What happens to light intensity when the distance doubles?

Try an exam question

A student investigates how light intensity affects the rate of photosynthesis in pondweed. Describe the method and name two variables to control.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.