Water transport in plantsAQA A-Level Biology: Revision notes

Xylem is the tissue that transports water and dissolved mineral ions from the roots up the stem to the leaves. Mature xylem vessels are made of dead cells joined end to end, with no end walls, so they form a continuous hollow tube with no cytoplasm to slow the flow. Their walls are thickened with lignin, which makes them strong and waterproof, so they do not collapse inwards when the water inside is under tension. Small gaps called pits let water move sideways between vessels and into surrounding cells.

Transpiration is the loss of water vapour from the leaves, mainly through the stomata. Water evaporates from the wet surfaces of the mesophyll cell walls into the air spaces of the leaf, then diffuses out of the stomata down a water vapour gradient. This lowers the water potential of the mesophyll cells, so water moves into them from the xylem by osmosis, through the cell walls.

The loss of water from the top of the xylem creates tension (negative pressure) which pulls on the water column below.

The theory explains how water rises, sometimes over 100 m, without being pumped.

1. Water evaporates from the leaf, creating tension in the xylem.
2. Water molecules are polar and stick to each other by hydrogen bonds (cohesion), so the water moves up as a continuous column without breaking.
3. Water molecules are also attracted to the walls of the xylem (adhesion), which helps to support the column.
4. The pull draws water into the roots from the soil to replace what is lost.

The process is passive: no energy is needed from the xylem cells, because the energy comes from the Sun, which drives evaporation.

Observations that support the theory:

• When a xylem vessel is cut in a transpiring plant, air is drawn in rather than water flowing out, showing that the water is under tension.
• Trunk diameter decreases during the day, when transpiration is fastest and tension is greatest, and recovers at night.
• Water movement is faster when transpiration is faster (for example in warm, dry, windy conditions).

A potometer measures water uptake by a cut shoot as the distance moved by an air bubble. Volume taken up = πr² × distance moved, and rate = volume ÷ time.

Worked example: r = 0.5 mm, bubble moves 30 mm in 5 minutes. Volume = π × 0.5² × 30 = 23.6 mm³, so rate = 4.7 mm³ min⁻¹.

A strong correlation between the rate of transpiration and the rate of water movement in the xylem suggests that they are related. To show that transpiration causes the water movement, scientists look for a mechanism, show that changing transpiration changes water movement, and check that other variables are controlled (for example light and temperature, which may affect both). Several independent lines of evidence that agree make a causal link more convincing.