How Photosynthesis Works

Explain how plants use light energy to make glucose, and what limits the rate of photosynthesis.

About 3 min read
  • Write the equations for photosynthesis
  • Describe how plants use glucose
  • Explain the effect of limiting factors
Before you start

You know that chloroplasts contain chlorophyll, and that plants take in water through their roots.

How plants make food

Photosynthesis is the process by which plants make glucose. Chlorophyll in chloroplasts absorbs light energy. This energy is used to convert carbon dioxide and water into glucose and oxygen. Oxygen leaves the leaf through tiny pores called stomata.

Definition: photosynthesis

The process in which plants use light energy to make glucose from carbon dioxide and water.

Maths
carbon dioxide + water → glucose + oxygen
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Limiting factors

The rate of photosynthesis is limited by light intensity, carbon dioxide concentration and temperature. If one factor is in short supply, it holds the rate back even when the others are plentiful. Very high temperatures reduce the rate because enzymes are damaged.

Definition: limiting factor

The factor in shortest supply that restricts the rate of a process.

Read a limiting factor graph

The rate rises as light increases, then levels off. Explain the flat part of the graph.

More light no longer increases the rate, so light is not the limiting factor.

Something else, such as carbon dioxide or temperature, is now limiting.

Answer: Another factor has become limiting.

Energy is absorbed

Light energy is transferred to chemical energy in glucose. Plants do not make energy from nothing.

Exam focus
Say it like this“Enzymes are denatured at very high temperatures, so the rate falls.”
Not like this“The plant gets too hot and dies.” — Refer to enzymes when explaining temperature.
Common mistakeWriting oxygen as a reactant, or saying that plants respire only at night.

Practice

Name the gas released in photosynthesis.
Oxygen.
Where in the cell does photosynthesis happen?
In the chloroplasts.
Give two ways a plant uses glucose.
For respiration, and to make starch, cellulose, proteins or fats.
Going further
A farmer heats a greenhouse with a paraffin burner. Explain two ways this may increase growth.
The burner raises the temperature, so enzymes work faster, and it releases carbon dioxide, which increases a raw material for photosynthesis.

Quick check

Photosynthesis quick check

Which are the raw materials for photosynthesis?

Plants absorb these from the air and soil, then rearrange them into glucose.

What does chlorophyll do?

This green pigment traps the energy needed for the reaction in chloroplasts.

Which is a limiting factor of photosynthesis?

Too little of it holds the rate back, even with enough carbon dioxide.
Key takeaways
  • Plants use light energy to make glucose and oxygen.
  • Photosynthesis happens in chloroplasts.
  • Light, carbon dioxide and temperature can limit the rate.
  • Very high temperatures denature enzymes.
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