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Learn4BI1Paper 2B · Paper 2 (all content)Structure and functions in living organismsPhotosynthesis
Structure and functions in living organisms · Subtopic 5 of 21

Photosynthesis

In photosynthesis, plants use light energy, absorbed by chlorophyll in chloroplasts, to make glucose from carbon dioxide and water, releasing oxygen: 6CO2+6H2O→C6H12O6+6O2\mathrm{6CO_2+6H_2O\rightarrow C_6H_{12}O_6+6O_2}. Glucose is used for respiration or converted to starch (storage), cellulose (cell walls), sucrose (transport) and, with nitrate ions, amino acids. The rate is controlled by limiting factors: light intensity, carbon dioxide concentration and temperature; whichever is in shortest supply limits the rate.

Cambridge 5090 Paper 1Cambridge 5090 Paper 2Cambridge 0610 Papers 2, 4Cambridge 0610 Papers 1, 3Edexcel 4BI1 Paper 1BEdexcel 4BI1 Paper 2B≈ 45 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Write the word and balanced equations for photosynthesis.
  • Describe uses of glucose in plants.
  • Explain limiting factors and interpret graphs.
  • Describe experiments showing the need for light, CO₂ and chlorophyll.
Pause and think

Greenhouse growers pump extra carbon dioxide into their greenhouses in winter. Why?

Section 2 of 14

Core idea

Start hereMaking food from air

A tree's wood comes mostly from carbon dioxide pulled from the air. Using sunlight, plants turn a gas and water into sugar.

The big idea

Chlorophyll absorbs light energy, which is used to make glucose from carbon dioxide and water, releasing oxygen. The rate is set by whichever factor is in shortest supply.

1. The equation

6CO2+6H2O→chlorophylllightC6H12O6+6O2\mathrm{6CO_2+6H_2O\xrightarrow[\text{chlorophyll}]{\text{light}}C_6H_{12}O_6+6O_2}
where
\mathrm{CO_2}
from the air, through stomata
\mathrm{H_2O}
from the soil, through xylem

2. Limiting factors

Worked exampleReading the graph

At 0.04% CO₂ the rate levels off at light intensity 40; at 0.1% CO₂ it continues to rise until 70.

Goal
What limits the rate at intensity 50 with 0.04% CO₂?
Strategy
Rate is flat there, so not light.
  1. flat at 0.04%\text{flat at }0.04\%
    More light doesn't help.
  2. higher at 0.1%\text{higher at }0.1\%
    Adding CO₂ raises the rate.
AnswerCarbon dioxide concentration.
✓ CheckTemperature might also limit but CO₂ is shown to matter ✓.
Where glucose goes

One product, many uses.

Exam tipStarch test Boil in water, warm in ethanol in a water bath (no flame), rinse, add iodine. Blue-black shows starch, evidence that photosynthesis occurred.
Pause and think

At night, does a plant take in or give out CO₂?

Summary

  • CO₂ + water → glucose + O₂.
  • Glucose has many uses.
  • Light, CO₂, temperature limit rate.
Key words
Chlorophyll
Green pigment that absorbs light energy.
Limiting factor
The factor in shortest supply that limits the rate.
Destarch
Keep a plant in the dark so it uses up its starch.
HistoryVan Helmont's willow. In the 1640s Jan van Helmont grew a willow tree in a weighed pot of soil. After five years the tree had gained 74 kg, but the soil had lost only 57 g. The mass came from air and water.
Fun factEvery breath. About half of the oxygen you breathe was made by tiny ocean phytoplankton, not trees.
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