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How do green plants make their own food, and what do they need to do it?

State the word equation for photosynthesis, describe the raw materials and conditions needed, and explain how light, carbon dioxide and temperature affect the rate

A scaffolded answer to the N(A)-Level Biology outcome on photosynthesis. The word equation, the raw materials and conditions needed, what the products are used for, and the limiting factors that affect the rate, with a worked rate example.

Generated by Claude Opus 4.88 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this dot point is asking
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  3. Examples in context
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What this dot point is asking

This outcome wants the word equation for photosynthesis, the raw materials and conditions a plant needs, what it does with the products, and how light, carbon dioxide and temperature affect the rate. Photosynthesis is how green plants make their own food, so it is the start of nearly every food chain. The most tested idea is limiting factors, the things that can hold back the rate.

The answer

What photosynthesis is

Photosynthesis is the process by which green plants make their own food (glucose) using light energy. It happens in the chloroplasts, which contain the green pigment chlorophyll. Chlorophyll absorbs light energy and uses it to drive the reaction.

The word equation

carbon dioxide+water→light, chlorophyllglucose+oxygen\text{carbon dioxide} + \text{water} \xrightarrow{\text{light, chlorophyll}} \text{glucose} + \text{oxygen}

The raw materials are carbon dioxide (taken in from the air through the leaf) and water (taken up by the roots). Light and chlorophyll are both needed for the reaction. The products are glucose (the food) and oxygen (released as a waste product).

What the products are used for

The plant uses the glucose in several ways: for energy in respiration, to make starch for storage, to make cellulose for cell walls, and to help build other substances needed for growth. The oxygen is released into the air, which is the source of the oxygen that animals breathe.

Limiting factors

A limiting factor is the condition in shortest supply that holds back the rate of photosynthesis. The three main ones are:

  • Light intensity. More light means more energy, so a faster rate, until something else limits it.
  • Carbon dioxide concentration. More carbon dioxide means a faster rate, until something else limits it.
  • Temperature. A higher temperature speeds up the reactions, but only up to a point; too hot and the enzymes denature, so the rate falls.

Examples in context

Example 1. Greenhouses and extra carbon dioxide. Growers sometimes add extra carbon dioxide and lighting to a greenhouse. By removing carbon dioxide and light as limiting factors, they raise the rate of photosynthesis so crops grow faster, showing limiting factors put to practical use.

Example 2. Why plants are the start of food chains. Because plants trap light energy and store it as food, they are producers. Animals cannot photosynthesise, so they must eat plants (or other animals) to get this stored energy, which is why almost every food chain begins with a green plant.

Try this

Q1. State the two raw materials needed for photosynthesis. [2 marks]

  • Cue. Carbon dioxide and water.

Q2. State what is meant by a limiting factor. [1 mark]

  • Cue. The factor in shortest supply that holds back (limits) the rate of photosynthesis.

Q3. Explain why the rate of photosynthesis levels off at high light intensity. [2 marks]

  • Cue. Light is no longer the limiting factor; another factor, such as carbon dioxide concentration or temperature, has become the factor holding back the rate.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original4 marksWrite the word equation for photosynthesis and state what the plant uses the glucose for.
Show worked answer →

Word equation: carbon dioxide + water, in the presence of light and chlorophyll, produce glucose + oxygen.

Uses of glucose: the plant uses glucose for energy (in respiration), to make starch for storage, to make cellulose for cell walls, and to help make other substances the plant needs for growth. Any two correct uses are usually enough.

What markers reward: the correct reactants (carbon dioxide and water) and products (glucose and oxygen), with light and chlorophyll shown as needed, plus at least two sensible uses of glucose. Writing oxygen as a reactant instead of a product is a common error.

Original4 marksA student measures the rate of photosynthesis of pondweed by counting the bubbles of oxygen given off each minute, while increasing the light intensity. Describe and explain the results expected.
Show worked answer →

As the light intensity increases, the number of bubbles per minute increases, because light provides the energy for photosynthesis, so more light means a faster rate.

At high light intensities the rate levels off (the bubble count stops rising). This is because another factor, such as carbon dioxide concentration or temperature, has become the limiting factor; light is no longer the factor holding back the rate.

What markers reward: the rise in rate with light (light supplies energy), and the levelling off explained by a limiting factor taking over. Just saying it goes up forever loses the marks for the plateau.

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