Plant transport

Plants have two transport tissues: xylem (one-way, water + minerals up) and phloem (two-way, sugars from source to sink).

Xylem

• Made of dead cells stacked end-to-end with no end walls — forms a continuous tube.
• Walls strengthened with lignin (a tough waterproof polymer).
• Carries water and dissolved minerals UP from roots to leaves.
• Lignin also gives the plant structural support.

Phloem

• Made of living sieve tube cells with perforated end walls (sieve plates), supported by companion cells.
• Carries dissolved sucrose / amino acids from sources (leaves) to sinks (growing tips, fruits, roots).
• Movement is two-way depending on where sugars are needed — this is called translocation.

Transpiration

The flow of water through a plant.

1. Water absorbed by root hair cells (osmosis).
2. Water moves across the root and up the xylem.
3. Water evaporates from leaf mesophyll cells.
4. Water vapour diffuses out through the stomata.

The transpiration stream is driven by evaporation from the leaves — it pulls water up.

Factors affecting transpiration rate

Factor	Effect on rate	Reason
Light intensity	Increases	Stomata open in light for photosynthesis
Temperature	Increases	Faster evaporation, faster diffusion
Wind / air movement	Increases	Removes humid layer near stomata, maintains gradient
Humidity	Decreases	Smaller water-vapour gradient between leaf and air

Stomata and guard cells

Stomata are pores on the underside of leaves, each controlled by two guard cells.

• When light + water present, guard cells become turgid and curve apart — stoma OPENS (allows CO2 in for photosynthesis, water and O2 out).
• In darkness or water shortage, guard cells become flaccid and stoma CLOSES (reduces water loss).

WJEC exam tip

When asked to compare xylem and phloem, structure your answer in pairs: "xylem cells are dead, phloem cells are alive"; "xylem moves water UP only, phloem moves sucrose in BOTH directions". Examiners reward parallel comparison statements.