What does Edexcel GCSE Biology Topic 6 cover?

Topic 6 covers photosynthesis as an endothermic reaction, temperature, light intensity and carbon dioxide as limiting factors, the inverse square law, how root hair cells absorb water and ions, the structure and function of xylem and phloem, the leaf, transpiration and translocation, the effect of environmental factors on water uptake, and (Biology only) plant adaptations to extreme environments and plant hormones.

What are the limiting factors of photosynthesis?

The three main limiting factors are light intensity, carbon dioxide concentration and temperature. The factor in shortest supply caps the rate; increasing it raises the rate until another factor becomes limiting. Very high temperatures denature the enzymes, so the rate then falls.

How does the inverse square law apply to light intensity?

Light intensity is inversely proportional to the square of the distance from the source, so intensity is proportional to 1 over the distance squared. Doubling the distance cuts the intensity to one quarter, and halving the distance multiplies it by four, which strongly affects the rate of photosynthesis.

What is the difference between xylem and phloem?

Xylem transports water and dissolved mineral ions from the roots up to the leaves in one direction, through dead, hollow cells strengthened with lignin. Phloem transports dissolved sucrose made in the leaves to the rest of the plant in both directions (translocation), through living cells with sieve plates.

How do plant hormones control growth?

Auxin, made at the tips of shoots and roots, controls cell elongation. In phototropism, auxin gathers on the shaded side of a shoot so it grows more and bends towards light. In gravitropism, auxin makes shoots grow up and roots grow down. Auxins, gibberellins and ethene also have commercial uses such as weedkillers, ending dormancy and ripening fruit.

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Edexcel GCSE Biology Topic 6 Plant structures and their functions: a complete overview of photosynthesis, transport, transpiration and plant hormones

A deep-dive Edexcel GCSE Biology guide to Topic 6 Plant structures and their functions. Covers photosynthesis and limiting factors, the inverse square law, root hair cells, xylem and phloem, the leaf, transpiration and translocation, and (Biology only) plant adaptations and plant hormones, with the core practical and exam patterns Edexcel repeats.

Generated by Claude Opus 4.814 min readTopic 6Updated 2026-06-02

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What Topic 6 actually demands
Photosynthesis and limiting factors
Transport in plants
Transpiration, translocation and plant hormones
How Topic 6 is examined
Check your knowledge

What Topic 6 actually demands

Plant structures and their functions is the first Paper 2 topic. The examiners test photosynthesis and its limiting factors (with the inverse square law), the structure-to-function logic of root hair cells, xylem, phloem and the leaf, the factors affecting transpiration, and (Biology only) plant hormones and adaptations. This guide ties together the four dot-point pages, each with its own practice questions.

Photosynthesis and limiting factors

Photosynthesis is an endothermic reaction using light energy to turn carbon dioxide and water into glucose and oxygen:

6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2

Its rate is set by limiting factors: low light intensity, low carbon dioxide or low temperature cap the rate, while very high temperature denatures the enzymes. The core practical uses pondweed at different distances from a lamp, and light intensity follows the inverse square law, proportional to $\frac{1}{d^{2}}$ .

Transport in plants

Root hair cells absorb water by osmosis and ions by active transport, using a large surface area and many mitochondria. Xylem carries water and minerals up through dead, lignified cells; phloem carries sucrose in both directions (translocation) through living cells with sieve plates. The leaf (Biology only) is broad, thin and full of chloroplasts, with stomata for gas exchange.

Transpiration, translocation and plant hormones

Transpiration is water loss through the stomata, pulling a stream of water up the xylem; its rate rises with temperature, light, air movement and low humidity, measured with a potometer. Translocation moves sucrose in the phloem. (Biology only) Auxin controls phototropism (bending towards light) and gravitropism, and plant hormones (auxins, gibberellins, ethene) have commercial uses.

How Topic 6 is examined

Limiting-factor graphs. Explaining the rising and flat parts of a photosynthesis rate graph.
Calculations. The inverse square law and transpiration rates from a potometer.
Structure to function. Root hair cells, xylem, phloem and the leaf.
Application. Predicting the effect of changing conditions on transpiration, and explaining tropisms with auxin.

Check your knowledge

A mix of recall and calculation questions covering Topic 6. Attempt them under timed conditions, then check against the solutions.

Name the three main limiting factors of photosynthesis. (3 marks)
A lamp is moved from $20\ cm$ to $40\ cm$ from a plant. State what happens to the light intensity. (1 mark)
Explain how a root hair cell is adapted to absorb water and ions. (3 marks)
State which tissue carries water up the plant and which carries sucrose around it. (2 marks)
State two environmental factors that increase the rate of transpiration. (2 marks)
Name the cells that open and close the stomata. (1 mark)
Explain how auxin causes a shoot to bend towards light. (3 marks)
Give one commercial use each of auxins and ethene. (2 marks)

Sources & how we know this

Pearson Edexcel GCSE (9-1) Biology (1BI0) specification — Pearson (2016)