The need for transport and transport in plants — Edexcel International GCSE Biology
Why large organisms need transport systems, how xylem and phloem work, and how water is absorbed and lost by plants.
Revision notes
5 short notes, in the order of the specification. Each one in short:
Multicellular organisms need transport systems because their surface area to volume ratio is small and most of their cells are deep inside the body. Diffusion over these long distances is too slow to supply oxygen and food or to remove waste. A unicellular organism has a large surface area to volume ratio, so diffusion alone is enough.
Xylem and phloem are the two transport tissues in flowering plants. Xylem carries water and mineral ions from the roots up to the stem and leaves, in dead, hollow tubes strengthened with lignin. Phloem carries sucrose and amino acids up or down the plant to where they are needed, in living sieve tubes with sieve plates.
Water uptake by root hair cells
Biology onlyRoot hair cells absorb water from the soil by osmosis and mineral ions by active transport. Soil water is more dilute than the cell sap, so water moves into the cell through the partially permeable membrane. The long, thin root hair gives a large surface area, and many mitochondria release energy for the active transport of mineral ions.
Transpiration
Biology onlyTranspiration is the evaporation of water from the surface of a plant, mostly from the leaves. Water evaporates from the cells inside the leaf into the air spaces, then water vapour diffuses out through the stomata. As water leaves, more is pulled up the xylem, forming the transpiration stream, which also carries mineral ions to the leaves.
The rate of transpiration increases with higher temperature, wind speed and light intensity, and decreases with higher humidity. Humid air already holds a lot of water vapour, so the concentration gradient is smaller. Wind carries water vapour away, and stomata open wider in light. A potometer measures the rate by how far an air bubble moves in a set time.
Exam questions
7 exam-style questions (21 marks), each with its mark scheme.
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14 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of structure and functions in living organisms on one page, so you can see where this subtopic fits.
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Method, variables, risks and a results table, with questions to answer.
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Quick check questions
Why can a unicellular organism rely on diffusion?
Show answer
It has a large surface area compared with its volume, so diffusion distances are short.
Which tissue carries water and mineral ions from the roots?
Show answer
Xylem.
Which substances does phloem transport?
Show answer
Sucrose and amino acids.
Biology only Define transpiration.
Show answer
The evaporation of water from the surface of a plant.
Biology only How does an increase in humidity affect the rate of transpiration?
Show answer
It decreases the rate, because the concentration gradient of water vapour is smaller.
Frequently asked questions
Why is diffusion insufficient in multicellular organisms?
Diffusion is insufficient in multicellular organisms because they have a small surface area to volume ratio and most cells are deep inside the body. Diffusion over these long distances is too slow to supply every cell with enough oxygen and food, or to remove waste quickly. So they need transport systems, such as blood, or xylem and phloem.
What is the difference between xylem and phloem?
Xylem carries water and mineral ions upwards from the roots, while phloem carries sucrose and amino acids up or down the plant to where they are needed. Xylem is made of dead, hollow cells with walls strengthened by lignin. Phloem is made of living cells joined end to end, with sieve plates in the end walls.
How are root hair cells adapted to their function?
Biology only Root hair cells have a long, thin projection that gives a large surface area for absorbing water and mineral ions. A thin wall and membrane give a short distance for water. Cell sap more concentrated than soil water keeps water moving in by osmosis, and many mitochondria release energy for active transport of mineral ions.
How does water move from roots to leaves?
Biology only Water enters root hair cells by osmosis, passes across the root into the xylem and moves up the stem to the leaves. In the leaves it evaporates from the cells into the air spaces, and water vapour diffuses out through the stomata. This pulls more water up the xylem, forming the transpiration stream.
How does humidity affect the rate of transpiration?
Biology only An increase in humidity decreases the rate of transpiration. Humid air already holds a lot of water vapour, so the concentration gradient between the inside of the leaf and the air is smaller, and water vapour diffuses out of the stomata more slowly. Higher temperature, wind speed and light intensity all increase the rate.
Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026