Xylem — water and mineral ions, plus support
Xylem carries water and mineral ions in one direction only: upward from the roots, through the stem, to the leaves, pulled by the loss of water vapour from the leaves. Its second function is support — thick lignified walls keep the tissue rigid and the plant upright. Mineral ions such as nitrate, magnesium and potassium dissolve in soil water and enter through root hair cells. Xylem never carries sucrose or amino acids.
Phloem — sucrose and amino acids by translocation
Phloem transports sucrose and amino acids from sources to sinks, and the process has a name: translocation. A source is where the substance is made or released (a photosynthesising leaf); a sink is where it is used or stored (roots, growing tips, storage organs). Unlike xylem, the flow is bidirectional — down to the roots and up to the shoot tips. Glucose made in photosynthesis is converted to sucrose before it is loaded into phloem.
(Extended) Xylem vessel structure suits transport
Xylem vessels are dead at maturity, and each structural feature earns a functional mark. Lignified walls are rigid and waterproof, so the vessel resists collapse under the pull of transpiration and supports the plant. No cell contents leaves a hollow lumen through which water and ions flow unobstructed. No end walls joins neighbouring cells into one continuous tube, so water travels the length of the vessel without resistance.
Drawn from real examiner reports.
Sucrose, not glucose, in phloem
Photosynthesis makes glucose in mesophyll cells, but the plant converts it to sucrose before loading it into phloem. The 0610 mark scheme specifies sucrose — glucose, and the vague word "sugar", are not accepted. Write: "phloem transports sucrose and amino acids from the leaves to the rest of the plant".
Glucose given instead of sucrose: s23 P23 Q16 · w22 P43 Q4c · s22 P31
Xylem vessels are dead
A developing xylem cell deposits lignin in its walls, loses its cytoplasm, nucleus and vacuole, and loses its end walls — leaving a dead, hollow tube. Calling xylem "living cells containing cytoplasm" loses the mark and wrecks any structure-function answer, because the hollow lumen exists precisely because the contents broke down.
Xylem described as alive: w22 P32 Q7d — the most penalised Extended error here
Root: xylem is the central star
In a root cross-section, xylem sits at the centre as a star with several arms; phloem lies between the arms, in the surrounding tissue. Two wrong answers recur: placing phloem at the centre, and drawing xylem and phloem as concentric rings — that ring pattern belongs to the stem, not the root. Memory aid: the star is xylem, phloem fills the gaps.
Root/leaf sections confused: s23 P11/13 Q17 · w22 P12/13 Q7 · s22 P12/22 Q15/Q17
Stem and leaf: xylem on the inside
In a stem, xylem and phloem are grouped into vascular bundles arranged in a ring near the outside, with xylem on the inner side of each bundle and phloem on the outer side. A leaf vein repeats the pattern: xylem on the upper side, phloem on the lower side. Learn each organ separately — the three arrangements are not the same.
Root/leaf sections confused: s23 P11/13 Q17 · w22 P12/13 Q7 · s22 P12/22 Q15/Q17
Water reaches xylem, never phloem
The uptake pathway is root hair cell → cortex cells → xylem at the centre of the root, then up the stem to the leaf. Candidates who route water into phloem first, or who say phloem carries water to the leaves, lose the mark. Phloem is not part of the water pathway at all.
Water said to enter phloem before xylem: s22 P13/23 Q16/Q18
Mineral ions come from the soil
Nitrate, magnesium and potassium ions are dissolved in soil water and absorbed through root hair cells, then carried upward in the xylem. Answers saying that mineral ions come from the air, or are made in the leaves, are wrong — the only thing the leaf takes from the air is carbon dioxide.
Mineral ions stated to come from leaves or air: consistent across all three sittings
Name the process: translocation
The movement of sucrose and amino acids in phloem has a specific name — translocation. Writing "transport" or "movement" instead is commonly not credited when the question asks you to name the process. Describe the direction too: from source to sink, not simply "downwards".
Translocation omitted as a named process across all three sittings
Feature → structure → benefit
For "relate structure to function", give three linked steps per feature: name it ("no end walls"), state the structural result ("cells form one continuous hollow tube"), then the benefit ("water flows without resistance"). Repeat for lignified walls and no contents.
Core answers: name, cargo, direction
For a function question, give the tissue name, then its cargo, then its direction. Xylem: water and mineral ions, upward from roots to leaves. Phloem: sucrose and amino acids, from source to sink. Three elements, three marks — most lost marks are a missing direction.
Red dye stains the xylem
A cut stem stood in red dye (eosin) stains the xylem, because dye dissolved in water travels in the xylem. In a stem section the stained tissue lies on the inner side of each vascular bundle — use the stain to identify xylem rather than guessing.
Check the organ before you label
Before labelling a cross-section, decide whether it shows a root, a stem or a leaf — the arrangements differ. A central star means root; a ring of separate bundles means stem; a vein between mesophyll layers means leaf. Labelling a root with the stem pattern is a standard trap.
Xylem: vascular tissue that transports water and mineral ions from the roots upward to the leaves, and also provides structural support to the plant. The flow in xylem is unidirectional (upward only). Xylem vessels are dead at maturity.
Phloem: vascular tissue that transports sucrose and amino acids from leaves (sources) to all other parts of the plant (sinks) — a process called translocation. The flow in phloem is bidirectional (up or down, depending on where the source and sink are).
Translocation: the movement of sucrose and amino acids in the phloem from a source (where they are made or released) to a sink (where they are used or stored).
Mineral ions: dissolved ions (e.g. nitrate, magnesium ions, potassium ions) absorbed from the soil through root hair cells and carried upward in the xylem. They do NOT come from the air or from leaves.
State TWO functions of xylem in a plant.
(a) State TWO functions of xylem.
(b) State the function of phloem and name TWO substances transported in it.
(c) State the name of the process by which substances move in phloem.
