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Detailed notes on Transport in Plants for Cambridge IGCSE Biology, covering key concepts, explanations, examples, and exam-focused revision points.
Plants have TWO transport systems. Xylem moves water UP. Phloem moves sugars EVERYWHERE.
Mapped to the Cambridge IGCSE 0610 syllabus (2026-2028).
Dead, hollow, lignified tubes. Water + minerals up from roots to leaves.
Function. Carry water and dissolved MINERAL IONS from the ROOTS UP to the rest of the plant — particularly the leaves.
Structure of xylem cells (vessels).
Why dead is best.
Direction of flow. ONE-WAY only — UP, from roots to leaves. Driven by transpiration pull (see Transpiration topic).
Worked qualitative. Why does cutting a ring of bark off a tree (girdling) eventually kill it?
Cambridge tip. Always include "lignified" when describing xylem walls. Cambridge marks the keyword.
Living cells. Sucrose + amino acids in either direction. Source to sink.
Function. TRANSLOCATION — moving SUCROSE and AMINO ACIDS from where they're made/stored (a SOURCE) to where they're used (a SINK).
Structure of phloem.
Direction of flow. TWO-WAY — wherever needed.
Source vs sink.
What it carries.
Worked qualitative. Why is sucrose the transport sugar (not glucose)?
Cambridge tip. TRANSLOCATION is the key term. Mark schemes credit "translocation in phloem from source to sink".
Stem: ring near outside. Root: cross in middle. Leaf: vein with xylem on top.
| Feature | Xylem | Phloem |
|---|---|---|
| Carries | Water + mineral ions | Sucrose + amino acids |
| Direction | One-way: UP (roots → leaves) | Both ways: source → sink |
| Cells | Dead, hollow, lignified | Living (sieve tubes + companion cells) |
| End walls | Dissolved (open tube) | Sieve plates (perforated) |
| Energy needed | No (passive, transpiration pull) | Yes (active, needs ATP) |
Stem (cross-section).
Root (cross-section).
Leaf (cross-section through a vein).
Why different in each organ?
Worked qualitative. Why does a celery stalk in dyed water turn coloured?
Cambridge tip. Practise drawing both stem and root cross-sections — Cambridge sometimes asks you to label vascular tissues.
Verbatim phrases and definitions Cambridge mark schemes credit.
Xylem and phloem appear Paper 4 (5-7 marks). Common formats: label a vascular bundle, compare structure/function. Examiner reports flag students saying xylem cells are alive, phloem carries water, or that phloem only flows down.
Sources: Cambridge IGCSE Biology 0610 syllabus 2026-2028 (8.2); 0610/42 May/Jun 2024 — Q7 (xylem structure); 0610 Examiner Reports 2022-2024. Last reviewed 2026-05-07.
Step-by-step solutions to past-paper-style questions on xylem and phloem, written exactly the way a tutor would explain them at the board.
Question
Compare the function of xylem and phloem.
Step-by-step solution
Step 1
Xylem: carries WATER and dissolved MINERAL IONS. Flow is one-way — UP from roots to leaves.
Step 2
Phloem: carries SUCROSE and AMINO ACIDS. Flow can be in either direction (source to sink).
Step 3
Both run together as vascular bundles through roots, stems and leaves.
Answer
Xylem: water and minerals, upwards. Phloem: sucrose and amino acids, source to sink. Both: transport tissues in vascular bundles.
Question
Describe the arrangement of the vascular bundles in a typical young (herbaceous) stem.
Step-by-step solution
Step 1
The vascular bundles are arranged in a ring near the OUTSIDE of the stem.
Step 2
Within each bundle, the XYLEM is towards the inside (centre) and the PHLOEM is towards the outside.
Step 3
The centre of the stem is pith, and the bundles' outer position helps the stem resist bending.
Answer
Vascular bundles form a ring near the edge of the stem; in each bundle the xylem is on the inside and the phloem on the outside.
Question
Explain how the structure of a xylem vessel is suited to its function of carrying water.
Step-by-step solution
Step 1
Xylem cells are dead and hollow (no cytoplasm or nucleus), so water can flow freely through them.
Step 2
The end walls have broken down, forming a continuous tube from root to leaf.
Step 3
The walls are thickened with lignin, which is strong, so the vessels resist the inward pull of the water and do not collapse, and the plant is supported.
Step 4
Lignin is also waterproof, so water does not leak out as it travels up.
Answer
Dead, hollow, open-ended tubes allow free water flow; lignified walls make them strong (resist collapse, give support) and waterproof.
Question
Describe the structure of phloem and how it differs from xylem.
Step-by-step solution
Step 1
Phloem is made of living cells, unlike the dead xylem.
Step 2
The main cells are sieve tubes, joined end to end; their end walls have small pores (sieve plates) that let the sugary sap pass through.
Step 3
Each sieve tube has little cytoplasm and no nucleus, so it is supported by a companion cell alongside it, which provides the energy and control the sieve tube needs.
Step 4
Because the cells are living and the flow follows need, phloem can carry sucrose and amino acids in either direction.
Answer
Phloem is living tissue of sieve tubes (with sieve plates) and companion cells, carrying sucrose and amino acids in either direction — unlike the dead, one-way xylem.
Examiner tip
Key contrast: phloem is LIVING (sieve tubes + companion cells); xylem is DEAD and lignified.
Question
Describe how you could use a stem (such as celery) and a coloured dye to show that water is carried up the stem in the xylem.
Step-by-step solution
Step 1
Stand a cut leafy stem in water containing a red (or blue) dye and leave it for a few hours in a warm, light place.
Step 2
The dye is carried up the stem with the water as the plant transpires.
Step 3
Cut thin cross-sections of the stem and look at where the dye has collected.
Step 4
The dye is found only in the xylem (the inner part of the vascular bundles), showing that water travels up the stem in the xylem.
Answer
Stand a cut stem in dyed water; after a few hours, cross-sections show the dye only in the xylem, proving water is carried up in the xylem.
Examiner tip
Keep the plant warm/in light so it transpires and draws the dye up. The conclusion comes from where the dye is — the xylem.
Question
In a 'ringing' experiment, a complete ring of bark (containing the phloem) is removed from a woody stem. After several weeks a swelling forms just above the ring. Explain what this shows.
Step-by-step solution
Step 1
In a woody stem, the phloem is in the bark (outer tissue) and the xylem is in the wood (inside).
Step 2
Removing a ring of bark removes the phloem but leaves the xylem intact.
Step 3
Sugars made in the leaves are carried down the phloem but cannot pass the gap, so they accumulate above the ring, causing the swelling.
Step 4
The leaves do not wilt at first, because the xylem (in the wood) still carries water up. This shows that sugars are transported in the phloem and water in the xylem.
Answer
Removing the phloem stops sugars moving down, so they build up above the ring (the swelling). Water still rises in the intact xylem. This shows phloem carries sugars and xylem carries water.
Examiner tip
The swelling above the cut and the fact the plant does not immediately wilt together prove phloem (sugars) and xylem (water) have separate roles.
High-scoring sample answers for xylem and phloem on the Cambridge IGCSE 0610 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.
State the function of the xylem. (1 mark)
Model answer
To transport water and dissolved mineral ions from the roots up to the leaves (and to give support).
Why this scores
One mark for 'transports water (and minerals)'. Direction (up/roots to leaves) strengthens it.
State two differences between xylem and phloem. (2 marks)
Model answer
Xylem is made of dead cells, whereas phloem is made of living cells. Xylem carries water and minerals (only upwards), whereas phloem carries sucrose and amino acids (in either direction).
Why this scores
Two genuine contrasts, each comparing both tissues. Dead vs living, and what is transported, are the safest.
Explain how the structure of a xylem vessel is adapted to transport water. (3 marks)
Model answer
A xylem vessel is made of dead, hollow cells with their end walls broken down, forming a continuous tube through which water can flow freely. Its walls are strengthened with lignin, which is strong so the vessel does not collapse under the pull of water, and waterproof so water does not leak out.
Why this scores
Three marks: dead/hollow/continuous tube for free flow; lignified walls for strength/no collapse; waterproof so no leakage.
Compare the structure and function of xylem and phloem. Give two points about each. (4 marks)
Model answer
Xylem is made of dead, hollow, lignified cells, and it carries water and mineral ions upwards from the roots to the leaves. Phloem is made of living cells (sieve tubes with companion cells), and it carries sucrose and amino acids in either direction, from sources to sinks. So xylem is dead and one-way for water, while phloem is living and two-way for sugars.
Why this scores
Four marks: xylem dead/lignified; xylem carries water up; phloem living/sieve tubes; phloem carries sugars (either direction). Comparison of both tissues required.
Describe how you could show that water travels up a stem in the xylem. (5 marks)
Model answer
Take a leafy stem, such as celery, and stand the cut end in a beaker of water that contains a coloured dye (for example red or blue). Leave it in a warm, well-lit place for a few hours so that the plant transpires and draws water (and the dye) up the stem. Then cut several thin cross-sections of the stem and examine them. The dye will be seen only in the xylem — the inner tissue of the vascular bundles. Because the dye marks the path the water took, this shows that water is transported up the stem in the xylem.
Why this scores
Five marks: cut stem in dyed water; warm/light to encourage transpiration; leave for time; cut cross-sections; dye only in the xylem → conclusion.
Describe a ringing experiment and explain how it shows that sugars are transported in the phloem. (6 marks)
Model answer
In a ringing experiment, a complete ring of bark is carefully cut and removed from around a woody stem. Because the phloem is in the bark and the xylem is in the wood deeper inside, this removes the phloem but leaves the xylem intact. The plant is then left for several weeks. Sugars made in the leaves are normally carried down the stem in the phloem, but they cannot pass the gap where the phloem has been removed, so they accumulate just above the ring, producing a swelling. The tissues below the ring receive no sugar and may begin to die. Meanwhile the leaves do not wilt immediately, because the xylem in the wood is still intact and continues to carry water up. The swelling of sugars above the cut, together with the continued supply of water, shows that sugars are transported in the phloem while water is transported in the xylem.
Why this scores
Up to 6 marks: remove a ring of bark/phloem; xylem left intact; sugars cannot pass; accumulate above the ring (swelling); region below deprived/dies; leaves still get water via xylem → conclusion that phloem carries sugars.
Definitions to memorise and the exact keywords mark schemes credit for xylem and phloem answers — sharpened from recent examiner reports for the 2026 0610 sitting.
Tissue of dead, hollow, lignified cells that transports water and dissolved mineral ions from the roots to the leaves.
Tissue of living sieve tubes and companion cells that transports sucrose and amino acids around the plant.
The movement of sucrose and amino acids in the phloem from sources (e.g. leaves) to sinks (e.g. growing tips, roots, fruits).
Strong, waterproof material in xylem cell walls. Provides support and prevents water loss.
A group of xylem and phloem running together as a transport unit through stems, roots and leaves.
The traps other students keep falling into on xylem and phloem questions — taken from recent Cambridge IGCSE 0610 examiner reports and mark schemes — and how to avoid them.
Why it happens
Plant cells are usually alive.
How to avoid it
Mature xylem cells are DEAD — empty hollow tubes, much better for free water flow.
Why it happens
Both are vessels in the plant.
How to avoid it
PHLOEM carries SUCROSE and amino acids. XYLEM carries water (and minerals).
Why it happens
Sugars made in leaves often go down to the roots.
How to avoid it
Phloem flow is from SOURCE to SINK and can go up or down. In spring, sugars travel UP from storage organs to new shoots.
The things students keep getting wrong in this sub-topic, answered.