77 · Transport in Plants

· Transport in Plants — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 8 parts Lesson 1 of 15 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision

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Lesson summary

7 Transport in Plants: Transport Tissues and Water Uptake — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe the structure of xylem and phloem and the pathways by which water crosses the root.

Vascular tissue
Root hair cells
Pathways across the cortex
Endodermis
Mineral ions
Root pressure

Part 1 of 6Why plants need transport systems

Why plants need transport systems

Size

Size plants have many layers of cells and many cells far from the surface, so diffusion alone is too slow to supply them.

Needs

Needs glucose, hormones and mineral ions are required for processes in all parts of the plant.

Movement

Movement water and mineral ions move up from the roots, and sucrose and amino acids (assimilates) move to where they are needed.

Vascular tissue

Vascular tissue a network of vessels runs through the leaves, stem and roots, and xylem and phloem are arranged together in vascular bundles.

Part 2 of 6Xylem and phloem compared

Feature	Xylem	Phloem
Transports	Water and mineral ions	Sucrose and amino acids (assimilates)
Direction	Upwards from roots to leaves	Up and down, from source to sink
Cells	Dead vessels with lignified walls, no end walls and no cytoplasm	Living sieve tube elements with sieve plates and companion cells
Adaptations	Lignin rings or spirals give strength and stop collapse; pits let water move sideways	Companion cells provide ATP for loading

Part 3 of 6Water uptake by roots

1Root hair cells

Root hair cells extensions give a large surface area, and water enters down a water potential gradient from the soil.

2Pathways across the cortex

Pathways across the cortex three routes.
Apoplastthrough cell walls and spaces with no membrane crossing, until blocked by the waterproof Casparian strip in the endodermis.
Symplastthrough cytoplasm and plasmodesmata by osmosis.
Vacuolarthrough vacuoles as well as cytoplasm.

3Endodermis

Endodermis the Casparian strip forces water into the symplast,
- so it crosses a selectively permeable membrane and the plant can control which ions enter.

Part 4 of 6Ions and root pressure

Mineral ions

Mineral ions they are taken up by active transport into root hairs, which lowers their water potential and draws in water.

Root pressure

Root pressure it raises the water pressure in the roots and pushes water into the xylem.

Water potential

Water potential soil water has a higher water potential than root hair cells, which have dissolved ions and sugars.

Part 5 of 6Tissues and evidence

Stem section

Stem section vascular bundles contain xylem on the inside and phloem on the outside, separated by cambium.

Root section

Root section xylem forms a central star or X shape inside the endodermis.

Xylem dye evidence

Xylem dye evidence a cut shoot placed in a dye such as eosin or methylene blue stains the xylem as the dye rises.

Root pressure evidence

Root pressure evidence sap exudes from a cut stump.

Active uptake evidence

Active uptake evidence mineral ion uptake is reduced by respiratory inhibitors.

Part 6 of 6More from the specification

Key terms

Term	Meaning
Xylem	Tissue carrying water and minerals up the plant.
Phloem	Tissue carrying assimilates.
Apoplast	Pathway through cell walls.
Symplast	Pathway through cytoplasm and plasmodesmata.
Casparian strip	Waterproof band in the endodermis.
Root pressure	Pressure that pushes water up the xylem.

Avoid these mistakes

Saying xylem cells are living — mature xylem vessels are dead.

Confusing symplast and apoplast.

Saying water moves into roots by active transport — it moves by osmosis, while ions are actively taken up.

Placing the Casparian strip in the epidermis — it is in the endodermis.

Last updated: 15 October 2026