Transport mechanisms

Biology AS · Transport in plants. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

Test this topic free All Biology AS topics

The idea

Water crosses the root in two ways. The apoplast pathway is through the cell walls. The symplast pathway is through the cytoplasm, from cell to cell by plasmodesmata. At the endodermis the Casparian strip blocks the cell walls, so all water must pass through cytoplasm before it reaches the xylem.

In the leaf, water evaporates from the wet walls of mesophyll cells and the vapour diffuses out through the stomata. This is transpiration. It pulls the water column up the xylem under tension.

In the phloem, sucrose and amino acids move from sources to sinks by mass flow. Loading at the source raises the hydrostatic pressure there, and the sap flows towards the lower pressure at the sink.

Rules to remember

Casparian strip: a band of waterproof suberin in the walls of endodermis cells.
Cohesion: water molecules hold together by hydrogen bonds, so the column is pulled up unbroken.
Adhesion: water molecules are attracted to the cellulose in the xylem walls.
Loading: companion cells pump H⁺ out using ATP; H⁺ flows back in through cotransporter proteins, bringing sucrose with it.
Sucrose lowers the water potential in the sieve tube, water enters by osmosis and the hydrostatic pressure rises.
Source: where assimilates are loaded (such as a mature leaf). Sink: where they are removed (such as roots, seeds, growing buds).
Xerophyte leaves: a thick cuticle reduces water loss through the surface; sunken stomata, hairs and rolled leaves trap humid air and lower the water potential gradient.

Common mistake

Writing that water evaporates through the stomata. Evaporation happens at the mesophyll cell walls inside the leaf; water vapour then diffuses out through the stomata.

Worked example

A complete ring of bark, including the phloem, is removed from a tree trunk in summer. The xylem is not damaged. After some weeks the trunk is swollen just above the ring. Explain why.

The xylem is intact, so water still reaches the leaves and they keep making sucrose.
Sucrose moves down the phloem from the leaves (source) towards the roots (sink).
The phloem is cut at the ring, so sucrose collects just above it.
The extra sucrose lowers the water potential, water enters by osmosis and the tissue swells.

Answer: Assimilates moving down the phloem are stopped at the ring and build up above it.

Ask about this

Type what you are stuck on. The helper finds the explanation, worked examples and the notes to read, free and instant.

Read the full notes

Full-course notes

Keep going

← Structure of transport tissues The circulatory system →Take a Biology AS test