Xylem and Phloem

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).

• DEAD when mature — cytoplasm and nucleus break down.
• Form long, HOLLOW tubes (cells join end-to-end, end walls dissolve).
• Walls are LIGNIFIED — impregnated with LIGNIN, a strong waterproof polymer.
• Lignin makes the wall WATERPROOF (water doesn't leak) AND STRONG (resists collapse from suction).

Why dead is best.

• A hollow tube has the lowest possible resistance — water flows freely.
• No cytoplasm or organelles in the way.
• No need to feed the cells (saves the plant energy).

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?

• Bark removal cuts the PHLOEM (which lies just inside the bark) but leaves the xylem deeper inside.
• Water still flows up (xylem intact) → leaves keep photosynthesising.
• But sugars can't get DOWN to the roots (phloem cut) → roots starve, plant dies over weeks/months.

Cambridge tip. Always include "lignified" when describing xylem walls. Cambridge marks the keyword.

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.

• Two main cell types:
  • Sieve tube elements — long, narrow, LIVING cells. End walls have holes (sieve plates) for sap to flow through.
  • Companion cells — alongside each sieve tube. Have nuclei + many mitochondria. Provide energy/management for the sieve tube.
• Together: a continuous tube of living, energy-supported cells.

Direction of flow. TWO-WAY — wherever needed.

Source vs sink.

• Source: where sugars come from. Usually green leaves (just photosynthesised) OR storage organs being broken down.
• Sink: where sugars go. Growing tips, developing fruits, expanding flowers, storage organs being filled.
• A potato can be a sink in summer (filling up) and a source in spring (emptying).

What it carries.

• SUCROSE — main transport sugar (more stable than glucose; doesn't react in transit).
• AMINO ACIDS — going to growing tissues to build proteins.
• Some hormones, mineral ions.

Worked qualitative. Why is sucrose the transport sugar (not glucose)?

• Glucose is reactive (used in respiration, joins to make starch).
• Sucrose is more stable → doesn't react with the cells while travelling.
• At sinks, sucrose is split back into glucose + fructose for use.

Cambridge tip. TRANSLOCATION is the key term. Mark schemes credit "translocation in phloem from source to sink".

Stem (cross-section).

• Vascular bundles arranged in a RING, near the OUTSIDE.
• In each bundle: XYLEM toward the centre, PHLOEM toward the outside.
• Often a layer of CAMBIUM between them — undifferentiated cells that can divide for growth.
• Centre: PITH (storage of starch etc.).

Root (cross-section).

• Xylem is in the CENTRE, often in a CROSS (X) shape (or star).
• Phloem fills in the gaps between the xylem arms.
• This central position helps the root resist BENDING forces (lignified xylem provides strength against pulling).

Leaf (cross-section through a vein).

• Vein contains both xylem and phloem.
• XYLEM on TOP (closer to the upper surface).
• PHLOEM on BOTTOM (closer to the spongy mesophyll).
• Surrounded by sclerenchyma fibres for support.

Why different in each organ?

• Root: needs strength against pull (xylem central).
• Stem: needs strength against bending (vascular bundles in a ring give a flexible, strong design — like steel rods in concrete).
• Leaf: thin, branched veins reach every cell.

Worked qualitative. Why does a celery stalk in dyed water turn coloured?

• Dyed water taken up by xylem (it carries water).
• Visible "lines" of dye in the celery match the position of the vascular bundles.
• Cutting the stalk shows the bundles in a ring.

Cambridge tip. Practise drawing both stem and root cross-sections — Cambridge sometimes asks you to label vascular tissues.