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How do green plants make their own food from light?

State the word equation for photosynthesis and explain the conditions needed and the limiting factors

A focused answer to the O-Level Biology outcome on photosynthesis. The word equation, the raw materials and conditions, the role of chlorophyll, and how light, carbon dioxide and temperature act as limiting factors.

Generated by Claude Opus 4.88 min answer

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

SEAB wants you to state the word equation for photosynthesis, list the raw materials and conditions a plant needs to carry it out, explain the role of chlorophyll and light, and explain how light intensity, carbon dioxide concentration and temperature can each limit the rate. The idea of a limiting factor is a frequent exam target.

The answer

What photosynthesis is

Photosynthesis is the process by which green plants make their own food (glucose) from carbon dioxide and water, using light energy absorbed by chlorophyll. It is the way energy from the Sun enters living things.

The word equation

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

In words: carbon dioxide plus water, in the presence of light and chlorophyll, produce glucose and oxygen. The oxygen is released as a by-product.

Raw materials and conditions

Photosynthesis needs:

  • Carbon dioxide, which enters the leaf from the air through the stomata.
  • Water, absorbed by the roots and carried up to the leaves.
  • Light energy, usually from the Sun.
  • Chlorophyll, the green pigment in chloroplasts that absorbs light energy.
  • A suitable temperature, because the reactions are controlled by enzymes.

What happens to the products

The glucose made is used for respiration (to release energy), converted to starch for storage, or used to make other substances such as cellulose for cell walls. The oxygen is released into the air through the stomata.

Limiting factors

The rate of photosynthesis is set by whichever needed factor is in shortest supply, called the limiting factor. The three main limiting factors are:

  • Light intensity: in dim light, more light speeds photosynthesis; in bright light, light is no longer limiting.
  • Carbon dioxide concentration: more carbon dioxide speeds the rate until it is no longer the factor in short supply.
  • Temperature: a higher temperature speeds the rate up to the optimum, because enzymes work faster; too high a temperature denatures the enzymes.

On a graph, the rate rises as the limiting factor is increased, then levels off once a different factor takes over as the limiting one.

Examples in context

Example 1. Greenhouses. Growers raise crop yields by removing limiting factors: extra lighting, burning fuel to add carbon dioxide, and heaters to keep a warm temperature. Each change lifts the rate of photosynthesis until the next factor becomes limiting.

Example 2. A pond plant in sunlight. Bubbles of oxygen rise faster from pondweed in bright light than in dim light, showing light intensity affecting the rate of photosynthesis. Counting bubbles is a classic way to measure the rate.

Try this

Q1. Write the word equation for photosynthesis. [2 marks]

  • Cue. Carbon dioxide + water, in the presence of light and chlorophyll, give glucose + oxygen.

Q2. State three conditions needed for photosynthesis. [3 marks]

  • Cue. Any three of: carbon dioxide, water, light energy, chlorophyll, a suitable temperature.

Q3. Explain what is meant by a limiting factor in photosynthesis. [2 marks]

  • Cue. The factor that is in shortest supply and so holds back the rate; increasing it raises the rate until another factor becomes limiting.

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.

Original6 marks(a) Write the word equation for photosynthesis. (b) State what is needed for photosynthesis to take place. (c) A graph shows the rate of photosynthesis rising as light intensity increases, then levelling off. Explain why the rate levels off.
Show worked answer →

(a) carbon dioxide + water, in the presence of light and chlorophyll, gives glucose + oxygen.

(b) Photosynthesis needs carbon dioxide, water, light energy, chlorophyll, and a suitable temperature.

(c) At low light intensity, light is the limiting factor, so increasing it raises the rate. As light intensity keeps rising, the rate levels off because some other factor, such as the carbon dioxide concentration or the temperature, has become the limiting factor; light is no longer what is holding the rate back.

Markers reward the correct word equation, the full list of requirements including chlorophyll, and the explanation that the rate levels off because another factor has become limiting.

Original4 marksA leaf is tested for starch after being left in sunlight. Explain why the presence of starch is taken as evidence that photosynthesis has occurred, and why the plant is kept in darkness before the experiment.
Show worked answer →

Photosynthesis makes glucose, which the plant quickly converts to starch for storage. So if starch is present in the leaf, glucose must have been made, which is evidence that photosynthesis has taken place.

The plant is kept in darkness beforehand (destarched) so that any starch already in the leaves is used up or removed. This means any starch found after the experiment must have been made during the experiment, in the light, making the test a fair one.

Markers reward glucose being converted to starch (so starch shows photosynthesis), and destarching in the dark so that new starch can be attributed to the experiment.

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