Xylem — transport and support
Xylem is made of dead, hollow, tube-shaped cells with lignin-thickened walls, joined end to end into a continuous, unobstructed pipe from root to leaf. It has two named functions in B8.1: transport of water and mineral ions from the roots to the stem and leaves, and support — the lignin thickening gives mechanical strength that keeps the plant upright. "State the functions of xylem" is normally 2 marks, one for each; "carries water" alone scores half.
Phloem — sucrose and amino acids
Phloem is made of living cells — sieve tube elements with companion cells — joined end to end into tubes; they stay alive because moving substances through phloem needs energy. B8.1 names one function: transport of sucrose and amino acids, for example from the leaves to growing regions, roots or storage organs. This movement is called translocation. Glucose made in photosynthesis is converted to sucrose before it is loaded into the phloem.
Vascular bundles — arrangement by organ
Xylem and phloem occur together in a vascular bundle, whose shape and position differ by organ in a non-woody dicotyledonous plant. Stem: several separate bundles in a ring near the outer edge, xylem towards the centre of each bundle, phloem outside. Root: one central bundle, xylem forming a star/cross shape, phloem between the arms. Leaf: one small oval bundle in the midrib or vein, xylem on the upper (adaxial) side, phloem on the lower (abaxial) side.
Drawn from real examiner reports.
Phloem carries sucrose, not water
The most repeated B8.1 error is naming the wrong substances for phloem. Candidates wrote that phloem carries water, nutrients (unqualified) or glucose — none of these score. Phloem transports sucrose and amino acids; water and mineral ions are xylem's cargo. Unqualified "food", "sugar" or "minerals" are not credited for either tissue.
Flagged Jun 2022 P32 Q10d, P42 Q7d; Jun 2023 P43 Q4c
Translocation ≠ moving glucose
Candidates who correctly recall translocation as movement in phloem then often name glucose as the substance moved. Glucose is the sugar made by photosynthesis in the leaf, but it is converted to sucrose before being loaded into the sieve tubes; amino acids are the second substance moved. "Translocation moves food around the plant" is too vague to score.
Flagged Jun 2022 P42 Q7d
Root star ≠ stem ring
A root has one central, star/cross-shaped block of xylem; a stem has several separate bundles arranged in a ring near its edge. Mixing these two patterns is the commonest position-identification error — always check whether the vascular tissue is single-and-central (root) or multiple-and-ringed (stem) before naming any tissue.
In a stem bundle, xylem is the inner tissue
Within each stem vascular bundle the xylem lies towards the centre and the phloem towards the outside. Reversing them is a standard diagram error — as is drawing the bundles scattered randomly, fused into one solid ring, or placed at the centre of the section (which is the root pattern, not the stem pattern).
In a leaf, xylem is on the upper side
In a leaf vein or midrib the xylem is on the upper (adaxial) side, nearer the palisade mesophyll, and the phloem on the lower (abaxial) side, nearer the spongy mesophyll. Swapping them loses the mark, and so does giving no orientation at all — the leaf pattern must be described against the upper and lower surfaces.
Support is xylem's job, not phloem's
Support belongs to xylem alone, and comes from the lignin thickening in its cell walls. Phloem cells are living and thin-walled, so they cannot provide comparable mechanical strength. "Phloem helps support the stem" is wrong: phloem's only named function in B8.1 is transport of sucrose and amino acids.
Xylem is dead; phloem is living
Xylem cells are dead, hollow and lignified — being dead and empty is exactly what makes them an unobstructed pipe, and the lignin is what gives support. Phloem cells stay living because energy is needed to move sucrose and amino acids into and out of the sieve tubes. Do not describe phloem as dead, hollow tubes.
Read the arrangement before naming the organ
Read the whole pattern of vascular tissue first. Separate bundles in a ring near the edge -> stem. A single central star/cross block -> root. One oval bundle in a flattened blade with upper and lower surfaces -> leaf. Then apply that organ's xylem/phloem rule.
Use cell appearance as a tie-breaker
If the arrangement is ambiguous, look at the individual cells. Xylem cells are large, empty/hollow and thick- (often darker-) walled, because they are dead and lignified. Phloem cells are smaller and thinner-walled, and some show visible cell contents because they are living.
One rule links all three organs
In every organ, xylem sits on the structurally more "central" or "upper" side of the vascular bundle and phloem on the more "outer" or "lower" side. The root's central star, the stem's xylem-inside bundles and the leaf's upper xylem are three versions of a single underlying rule.
Let the mark count set your answer
A plural "functions" for xylem signals two marks: water and mineral-ion transport, plus support. Phloem has one function, so a 2-mark phloem question is asking for both named substances. Match the number of scoring points to the marks available before you write.
Cambridge 0654 spec reference: Section B8 "Transport in plants", sub-topic B8.1 (Core only in this leaf — no Supplement). This leaf covers (1) stating the functions of xylem and phloem, and (2) identifying the position of xylem and phloem in transverse sections of roots, stems and leaves of non-woody dicotyledonous plants. The mechanism of water uptake (root hair cells), transpiration pull and the detail of translocation sources/sinks are covered separately in B8.2-B8.4.
Xylem — dead, hollow, tube-shaped cells with lignified (thickened) walls, joined end to end into continuous pipes. Functions:
Phloem — living cells (sieve tube elements, with companion cells) joined end to end into tubes. Function:
| Tissue | Cell state | Wall | Transports | Also provides |
|---|---|---|---|---|
| Xylem | Dead | Lignified (thick) | Water, mineral ions | Support |
| Phloem | Living | Thin | Sucrose, amino acids | — |
Position in transverse sections of a non-woody dicotyledonous plant — xylem and phloem always occur together as part of a vascular bundle, but the bundle's shape and position differ by organ:
| Organ | Arrangement of vascular tissue | Xylem position | Phloem position |
|---|---|---|---|
| Stem | Several separate bundles in a ring, near the outer edge | Towards the centre (inside) of each bundle | Towards the outside of each bundle |
| Root | One central bundle | Forms a central star/cross shape | Fills the tissue between the arms of the xylem star |
| Leaf | One small oval bundle in the midrib/vein | On the upper (adaxial) side, nearer the palisade mesophyll | On the lower (abaxial) side, nearer the spongy mesophyll |
State the function of phloem tissue.
State one function of xylem and one function of phloem. (2 marks)
