IGCSE Biology / Transport in plants
The transpiration stream revision notes
Edexcel International GCSE Biology (4BI1), chapter 11: Transport in plants, specification point 2.56B. Free lesson.
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Transpiration is water evaporating from leaves and then diffusing out through the stomata. This loss of water pulls a continuous column up through the xylem, helping the plant transport water and minerals.
How water leaves the leaf
The transpiration stream is a continuous column of water moving through the plant. It starts at the top because water is lost from the leaves. Water evaporates from the wet cell walls of the spongy mesophyll into the leaf’s air spaces. It then diffuses out through the open stomata, because the air outside the leaf is drier.
How water is pulled up the plant
As water leaves the leaf, mesophyll cells draw in more water from neighbouring cells by osmosis. This draws water from the xylem in the leaf vein. Water molecules attract each other, a property called cohesion, so pulling at the top pulls the whole water column up the xylem. More water enters the root hair cells by osmosis to replace the water that has moved up. The column is under tension as it is pulled upwards.
The route through the plant
Water follows one continuous route from the root to the stomata. It enters through a root hair cell, crosses the root from cell to cell, then travels up the xylem and into a leaf vein before reaching the mesophyll. From the mesophyll, water evaporates into the air spaces and diffuses out through the stomata.
Why the stream is pulled, not pushed
Roots do not push water up the stem. A plant has no pump. The Sun provides the energy for evaporation at the top of the plant, which creates tension in the xylem. Cohesion between water molecules allows that pull to move the whole column upwards.
What transpiration is useful for
Transpiration is not only water loss. As water evaporates, it cools the leaf. The moving water also transports dissolved mineral ions and helps keep cells turgid, which supports the plant and helps it stay upright.