Cambridge O Level Biology · Syllabus 5090 · Transport in Flowering Plants
Transpiration Pull
What is Transpiration Pull?
The tension set up in the water column of the xylem when water is lost from the leaves. Evaporation from the mesophyll cell surfaces, followed by diffusion of water vapour out of the stomata, lowers the water potential of the leaf cells, so water is drawn out of the xylem into them by osmosis. This puts the water in the xylem under tension, and because water molecules are attracted to one another the column does not break, so the whole column is drawn upwards from the roots. No living process and no pumping is involved.
This definition is part of the Transport in Flowering Plants chapter in Cambridge O Level Biology.
Transpiration Pull in context
A flowering plant runs two separate transport systems in the same vascular bundles. Xylem carries water and dissolved mineral ions in one direction only, from the roots up to the leaves, through dead hollow vessels with lignified walls. Phloem is living tissue that carries sucrose and amino acids from a source that makes or releases them to a sink that uses or stores them, so its direction depends on where the source and the sink happen to be. The engine of the xylem stream is not a pump: water evaporates from moist mesophyll surfaces inside the leaf and diffuses out through the stomata, and the tension this creates — the transpiration pull — draws a continuous column of water up the plant, held together by the attraction between water molecules.
Common mistakes with Transpiration Pull
- 5. “Xylem vessels pump the water up.” Why wrongA mature xylem vessel is dead and empty. It has no membrane, no cytoplasm and no mitochondria, so it cannot do anything that requires energy. WriteWater is drawn up the xylem by the transpiration pull, a tension created when water is lost from the leaves.
- 13. “It does not matter whether the shoot is cut in air or under water.” Why wrongCutting in air lets air into the open xylem vessels. The water column is broken, the transpiration pull cannot be transmitted past the air lock, and uptake falls or stops. WriteThe shoot is cut under water so that air does not enter the xylem and break the continuous water column.
Questions students ask about Transpiration Pull
How does water get to the top of a tall plant without a pump?
Evaporation and diffusion of water vapour from the leaves lower the water potential of the leaf cells, so water is drawn out of the xylem into them by osmosis. This puts the water in the xylem under tension — the transpiration pull. Because water molecules are attracted to one another, the column does not break, so the tension is transmitted the whole way down and the entire column is pulled upward.
Why must a potometer shoot be cut under water?
To stop air entering the xylem vessels. An air bubble inside the xylem would break the continuous column of water, so the transpiration pull could not be transmitted and water uptake would slow or stop, giving a falsely low reading.