Photosynthesis and limiting factors
How plants make food, and what restricts the rate.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 6.1.1State the word and balanced symbol equation for photosynthesis and describe the process.
- 6.1.2Investigate and explain the effects of light intensity, carbon dioxide concentration and temperature as limiting factors.
6 minute read
Photosynthesis and limiting factors
Photosynthesis is the process that puts almost all the energy into the living world.
The reaction
Word equation: carbon dioxide + water → glucose + oxygen, in the presence of light and chlorophyll. Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ Chlorophyll, the green pigment in chloroplasts, absorbs light energy and transfers it to chemical energy in glucose. Note that chlorophyll is not a reactant — it is not used up, which is why it does not appear in the equation.
What the plant does with the glucose
It is used for respiration, converted to starch for storage (insoluble, so it does not affect osmosis), converted to cellulose for cell walls, and — combined with nitrate ions absorbed from the soil — used to make amino acids and proteins.
Limiting factors
A limiting factor is the factor in shortest supply, which alone determines the rate: increasing any other factor has no effect until the limiting one is raised.
- Light intensity — rate rises as light increases, then levels off when something else becomes limiting.
- Carbon dioxide concentration — usually the limiting factor for crops on a warm, bright day, which is why glasshouse growers add CO₂.
- Temperature — photosynthesis is enzyme-controlled, so the rate rises to an optimum then falls sharply as enzymes denature.
Reading a graph: while the line is rising, the factor on the x-axis is limiting. Where it plateaus, something else has become the limit.
Think of it like this
Limiting factors work like a production line: if you have one packer, hiring ten more box-makers changes nothing. Only adding packers speeds things up — until packers stop being the bottleneck and something else does.
Worked examples
Method, step by step
A graph shows the rate of photosynthesis rising with light intensity, then levelling off. Explain the shape of the graph.
- 1On the rising part, light intensity is the limiting factor — more light means more energy for the reaction.
- 2At the plateau, further light makes no difference, so light is no longer limiting.
- 3Another factor, such as carbon dioxide concentration or temperature, has become the limiting factor.
Initially light is the limiting factor so rate increases with intensity. At the plateau another factor — carbon dioxide concentration or temperature — has become limiting, so extra light has no effect.
Common misconceptions
- Thinking plants respire only at night. Plants respire continuously, day and night; in daylight photosynthesis simply outpaces it.
- Writing chlorophyll into the equation as a reactant. It is a catalyst-like pigment, written above the arrow at most.
- Assuming a plateau on a light-intensity graph means light is limiting. A plateau means light is *no longer* limiting — something else now is.
In the exam
- When a graph levels off, name a specific alternative limiting factor — usually carbon dioxide concentration or temperature — rather than saying "something else".
- Remember why glucose is stored as starch: it is insoluble, so it has no osmotic effect on the cell.