Transport in Plants

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Cambridge International AS & A Level Biology (9700)

Transport in Plants

The big idea: Xylem carries water and ions up from the roots, pulled by transpiration through cohesion and adhesion. Phloem carries sucrose and amino acids from sources to sinks by mass flow down a pressure gradient that companion cells build actively.

Summary — What This Chapter Covers

Plan diagrams and the distribution of xylem and phloem in stems, roots and leaves.
Structure and function of xylem vessel elements, sieve tube elements and companion cells.
Mineral ions and organic compounds transported dissolved in water.
Water from soil to xylem: apoplast and symplast pathways, Casparian strip.
Transpiration; cohesion–tension and adhesion in the xylem.
Xerophyte leaf adaptations.
Phloem loading with proton pumps and cotransporters; mass flow from source to sink.

1. Structure of transport tissues

Plan diagrams

Where xylem and phloem are found

A plan diagram shows the outlines of tissues in the correct proportions, with no individual cells drawn. Follow the usual drawing rules: sharp pencil, clear continuous lines, no shading, ruled label lines.

Organ (herbaceous dicot)	Distribution of xylem and phloem
Stem	Vascular bundles arranged in a ring near the outside. In each bundle, xylem is towards the centre and phloem towards the outside, with cambium between them. Central pith, cortex, epidermis. The ring resists bending.
Root	Vascular tissue in the centre (resists pulling forces). Xylem forms an X or star shape with phloem between its arms, surrounded by pericycle and the endodermis. Wide cortex; epidermis with root hairs.
Leaf	Vascular bundles in the midrib and veins. Xylem on the upper side of each bundle, phloem on the lower side. Upper epidermis with cuticle, palisade mesophyll, spongy mesophyll with air spaces, lower epidermis with stomata.

Cells

Xylem vessel elements, sieve tube elements and companion cells

Cell	Structure	How structure fits function
Xylem vessel element	Dead, no cytoplasm or contents; end walls broken down; walls thickened with lignin (rings, spirals or networks); pits in the walls	Empty lumen and no end walls form a continuous tube, so water flows with little resistance. Lignin gives strength so vessels do not collapse under tension, and waterproofs them. Pits let water move sideways between vessels and to other tissues
Phloem sieve tube element	Living; cell surface membrane, thin layer of cytoplasm; no nucleus, few organelles (no ribosomes, no vacuole); end walls form sieve plates with pores	Little cytoplasm gives less resistance to mass flow; sieve pores let sap flow from element to element while the plates strengthen the tube
Companion cell	Nucleus, dense cytoplasm, many mitochondria, ribosomes; linked to its sieve tube element by many plasmodesmata	Carries out metabolism for the sieve element; mitochondria supply ATP for the proton pumps that load sucrose; plasmodesmata let sucrose pass into the sieve tube

Drawing these cells

Xylem: draw the lignin thickening and pits; no contents. Sieve tube: show the sieve plate and a thin peripheral cytoplasm; companion cell: smaller, with a nucleus, next to the sieve tube.

2. Transport mechanisms

Water from soil to xylem

Apoplast and symplast pathways

mineral ions organic compounds

root hair cells

Apoplast pathway

Phloem

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Also in the full note

From leaf to air, and the pull up the xylem
Leaves adapted to reduce water loss
Translocation from source to sink

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Transport in Plants