The need for transport and transport in plants — Edexcel International GCSE Science (Double Award)
Why large organisms need transport systems, and how xylem carries water and mineral ions and phloem carries sucrose and amino acids.
Revision notes
2 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.
Exam questions
3 exam-style questions (8 marks), each with its mark scheme.
Answer the questionsFlashcards
6 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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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.
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.
Written and checked against the Edexcel IGCSE Science Double Award (4SD0) specification · Updated October 2026