Transport in Plants
Senior 2 · Biology
Transport in Plants
Lesson 3 of 5
20 min
Let's begin
Some trees are more than 100 m tall, yet a plant has no pump. If you suck water up a drinking straw, you can lift it only as high as the pull of your mouth allows. So what lifts water to the top leaves of a tall eucalyptus every sunny afternoon? Scientists have tested three ideas: root pressure, capillarity and transpiration pull.
The flow of water from the soil, through the plant and out into the air is called the transpiration stream: soil → root hairs → root cortex → xylem of the root → xylem of the stem → xylem in the leaf veins → mesophyll cells → evaporation into the air spaces → out through the stomata as water vapour.
Root cells actively pump mineral ions into the xylem. Water follows by osmosis, building up a pressure that pushes water a short way up the stem.
Evidence: when a plant stem is cut near the ground, sap oozes from the stump for some time. On cool, humid mornings, grasses and some other plants push out droplets of water at the tips and edges of their leaves; this is called guttation.
Limit: root pressure can raise water only a few metres, and it is absent in many plants when they are transpiring fast. It cannot explain tall trees.
Worked example
Three clean glass tubes, 2 mm, 1 mm and 0.5 mm wide, stand in a dish of water. The water rises highest in the 0.5 mm tube. Explanation: water molecules are attracted to the glass (adhesion) and to each other (cohesion), so water creeps up the walls. The narrower the tube, the higher it rises.
Xylem vessels are very narrow, so capillarity helps a little, but it lifts water only a few centimetres.
This pull comes from evaporation, powered by the sun's heat, so the plant spends no energy of its own on it. At night, when the stomata close, transpiration slows and the columns are simply held in place by cohesion and adhesion.
Worked example
Florists recut the stems of roses under water before putting them in a vase. Why? Cutting a stem in air lets air bubbles enter the xylem. A bubble breaks the water column, so cohesion can no longer pull water up and the flower wilts. Cutting under water keeps the columns unbroken.
| Mechanism | What drives it | How far it lifts water |
|---|---|---|
| Root pressure | osmosis after active pumping of ions into root xylem (push) | a few metres at most |
| Capillarity | adhesion and cohesion in narrow tubes | a few centimetres |
| Transpiration pull | evaporation from leaves, with cohesion and adhesion (pull) | to the top of the tallest trees |
Tip
Evaporation pulls, cohesion holds the chain together, adhesion grips the walls
If asked which force is most important in a tall tree, answer "transpiration pull", and support it: root pressure and capillarity can each lift water only a short distance.
Define the transpiration stream.
Solution: The continuous flow of water from the soil into the roots, up the xylem to the leaves, and out through the stomata as water vapour.
What is guttation, and which mechanism does it provide evidence for?
Solution: Guttation is the loss of liquid water droplets from leaf edges, usually on humid mornings when transpiration is slow. It is evidence for root pressure pushing water up from below.
Explain the part played by cohesion in moving water up a tree.
Solution: Water molecules attract one another, so the water in each xylem vessel forms an unbroken column. When water evaporates from the leaves, the pull is passed down the whole column, lifting water from the roots.
Why does water rise more slowly in the xylem on a cool, cloudy, humid day?
Solution: Less water evaporates from the leaves, so transpiration pull is weaker and water is drawn up more slowly.
Remember
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