Cambridge O Level Biology · Syllabus 5090 · Transport in Flowering Plants
Transpiration
What is Transpiration?
The loss of water vapour from the leaves of a plant. Water reaching the leaf in the xylem passes into the mesophyll cells, evaporates from their moist surfaces into the air spaces of the leaf, and the water vapour then diffuses out through the stomata down a water-vapour concentration gradient into the atmosphere. Transpiration is a loss of water vapour at the leaf, and is not the same as the transpiration stream, which is the movement of water up the plant that this loss causes.
This definition is part of the Transport in Flowering Plants chapter in Cambridge O Level Biology.
Transpiration 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.
Transpiration is the loss of water vapour from leaves. Water evaporates from the moist surfaces of the mesophyll cells into the air spaces of the leaf, and the water vapour then diffuses out through the stomata, down a water-vapour concentration gradient, into the air.
Common mistakes with Transpiration
- 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.
- 6. “Transpiration is water moving up the stem.” Why wrongThat is the transpiration stream, which transpiration causes. Confusing the cause with its consequence loses the definition mark. WriteTranspiration is the loss of water vapour from leaves.
- 8. “Wind blows the stomata shut, so transpiration stops.” Why wrongOrdinary wind increases transpiration by sweeping away the humid air just outside the leaf. Only extreme conditions, usually combined with water shortage, cause the stomata to close. WriteMoving air removes water vapour from around the leaf, keeping the concentration gradient steep, so transpiration is faster.
- 9. “Light increases transpiration because the lamp heats the plant.” Why wrongIf that is what happened in your experiment, you changed two variables at once and the investigation was invalid. The biological link being tested is stomatal opening. WriteIn the light the stomata open to allow carbon dioxide in for photosynthesis, so more water vapour can diffuse out. Temperature must be controlled separately, with a heat shield or a water bath.
- 10. “Humid air holds more water, so the plant transpires more.” Why wrongThe direction of diffusion depends on the difference between inside and outside. Humid air outside makes that difference smaller, not larger. WriteIn humid air the water-vapour concentration gradient between the leaf air spaces and the atmosphere is smaller, so transpiration is slower.
- 11. “A potometer measures transpiration.” Why wrongIt records the water drawn into the shoot. Water loss is not measured at all; it is inferred. WriteA potometer measures the rate of water uptake, which is used as an estimate of the rate of transpiration.
- 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.
- 16. “All the water a plant absorbs is lost by transpiration.” Why wrongMost of it is, but not all. Some is used as a raw material in photosynthesis, some is retained in new tissue as the plant grows, and some stays in the cells keeping them turgid. WriteMost of the water taken up is lost as water vapour, which is why uptake is a good estimate of transpiration; a small amount is used in photosynthesis, in growth, and in maintaining turgor.
Questions students ask about Transpiration
What is transpiration in simple terms?
Transpiration is the loss of water vapour from the leaves of a plant. Water evaporates from the moist surfaces of the mesophyll cells inside the leaf, and the vapour then diffuses out through the stomata into the air.
What is the difference between transpiration and translocation?
Transpiration is the loss of water vapour from leaves, and it drives the movement of water and mineral ions upward in the xylem. Translocation is the movement of sucrose and amino acids in the phloem, from a source that makes or releases them to a sink that uses or stores them, and it can travel in either direction.
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.
Does a potometer measure transpiration?
No. A potometer measures the rate at which a cut shoot takes up water, which is used as an estimate of the rate of transpiration. The two differ because a small amount of the water absorbed is used as a raw material in photosynthesis, retained in new tissue during growth, and held in cells to maintain turgor.
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.
Does high humidity increase or decrease transpiration?
It decreases it. Humid air already contains a lot of water vapour, so the water-vapour concentration gradient between the air spaces inside the leaf and the atmosphere outside is smaller, and vapour diffuses out of the stomata more slowly.