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How do plants make their own food from light, and how is the leaf adapted for the job?

Describe photosynthesis as the process that makes glucose using light energy, state its equation and limiting factors, and relate leaf structure to its function

A focused answer to the O-Level Combined Science outcome on photosynthesis. The word and balanced equations, the role of chlorophyll, limiting factors of light, carbon dioxide and temperature, and how the leaf is adapted.

Generated by Claude Opus 4.89 min answer

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

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  3. Examples in context
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What this dot point is asking

SEAB wants you to describe photosynthesis as the process by which plants make glucose using light energy, to state its equation, to explain the limiting factors that control its rate, and to relate the structure of a leaf to its function. The equation, the limiting-factor graphs and the leaf adaptations are all common exam content.

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 the green pigment chlorophyll. It takes place mainly in the chloroplasts of leaf cells. Oxygen is released as a by-product.

The equation

The word equation is:

carbon dioxide+water→glucose+oxygen\text{carbon dioxide} + \text{water} \rightarrow \text{glucose} + \text{oxygen}

In symbols (with light and chlorophyll): 6CO2+6H2O→C6H12O6+6O26\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2.

What the plant does with the glucose

The glucose made is used to:

  • respire (release energy),
  • make starch for storage,
  • make cellulose for cell walls,
  • make proteins (using nitrate ions from the soil) and other substances.

Limiting factors

The rate of photosynthesis is controlled by whichever factor is in shortest supply, the limiting factor. The three main ones are:

  • light intensity: more light gives a faster rate, until another factor limits,
  • carbon dioxide concentration: more carbon dioxide gives a faster rate, until another factor limits,
  • temperature: a higher temperature speeds the rate (the enzymes work faster), but too high a temperature denatures the enzymes and the rate falls.

On a graph, the rate rises as a limiting factor is increased, then levels off when a different factor becomes limiting.

Leaf adaptations

A leaf is well adapted to photosynthesise:

  • broad and flat to give a large surface area for catching light,
  • thin so gases diffuse in and out quickly,
  • many chloroplasts in the upper cells where most light reaches,
  • stomata (pores, mostly on the underside) to let carbon dioxide in and oxygen out,
  • a network of veins to supply water and carry away the glucose.

Examples in context

Example 1. Why crops grow faster in summer. Longer days and warmer temperatures in summer increase light and temperature, so photosynthesis is faster and crops grow more quickly. Farmers plan planting around these natural changes in the limiting factors.

Example 2. Greenhouses and controlled growing. Commercial greenhouses raise light, temperature and carbon dioxide together to push photosynthesis as fast as possible, because increasing only one factor soon hits a new limit. This is a direct application of limiting-factor theory to food production.

Try this

Q1. State the raw materials and the products of photosynthesis. [2 marks]

  • Cue. Raw materials: carbon dioxide and water; products: glucose and oxygen.

Q2. Name three factors that can limit the rate of photosynthesis. [2 marks]

  • Cue. Light intensity, carbon dioxide concentration, and temperature.

Q3. Explain why a leaf is broad and thin. [2 marks]

  • Cue. Broad gives a large surface area to catch light; thin means a short distance for gases to diffuse in and out.

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, name the source of energy and the green pigment that absorbs it, and state what the plant does with the glucose it makes.
Show worked answer →

Word equation: carbon dioxide + water gives glucose + oxygen (in the presence of light and chlorophyll).

The source of energy is light (sunlight). The green pigment that absorbs the light energy is chlorophyll.

The plant uses the glucose for respiration (to release energy), to make starch for storage, to make cellulose for cell walls, and to make other substances such as proteins (with mineral ions).

Markers reward the correct word equation, light as the energy source and chlorophyll as the pigment, and at least one correct use of the glucose (respiration, storage as starch, cellulose, or proteins).

Original4 marksA scientist measures the rate of photosynthesis of a plant as the light intensity is increased. The rate rises at first then levels off. Explain the shape of this graph in terms of limiting factors.
Show worked answer →

At low light intensity, light is the limiting factor: as the light intensity increases, the rate of photosynthesis increases because more light energy is available.

At high light intensity, the rate levels off (plateaus). Light is no longer limiting; instead some other factor, such as the carbon dioxide concentration or the temperature, has become the limiting factor and is now holding the rate back.

Markers reward light being the limiting factor at low intensity (so increasing light increases the rate), and another factor (carbon dioxide or temperature) becoming limiting where the graph levels off.

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