Transpiration pull

Where it happens

Transpiration pull acts throughout the xylem of a plant, a continuous system of dead, hollow vessels running from the roots, up the stem, and into the veins of every leaf.

Inputs and outputs

• Input: liquid water in the xylem of the root, delivered by osmosis from root hair cells.
• Input: heat energy from the surroundings, which supplies the latent heat needed for evaporation in the leaf.
• Input: dissolved mineral ions carried along with the water in the xylem sap.
• Output: water vapour lost through the stomata to the atmosphere.
• Output: a continuous upward flow of water and mineral ions to the mesophyll cells for photosynthesis, turgor and cooling.

The steps

1. Inside the leaf, water evaporates from the moist walls of mesophyll cells into the air spaces between them.
2. Water vapour then diffuses out of the leaf through open stomata, a process called transpiration.
3. This loss of water lowers the water potential of the mesophyll cells, so water moves into them by osmosis from the xylem in the leaf veins.
4. Removing water from the xylem in the leaf creates a tension (pulling force) that is transmitted down the continuous column of water in the xylem.
5. Cohesion between water molecules, caused by hydrogen bonding, keeps this column unbroken, so it is pulled upward from the roots to replace the water lost, the transpiration pull, or transpiration stream.
6. Adhesion between water molecules and the lignified xylem walls supports the column and stops it slipping back, while the narrow bore of the vessels helps the water rise by capillary action.
7. Water drawn out of the root xylem lowers the water potential of the surrounding root cells, so more water enters from the soil by osmosis and the stream is maintained as long as the stomata stay open.

Why it matters and how it is controlled

Transpiration pull is the main force that moves water and dissolved mineral ions from the roots to every part of the plant, including the leaves where they are needed for photosynthesis. It works passively, without the plant using any of its own energy, because it relies entirely on evaporation and the cohesive properties of water.

The rate of the pull is set by the rate of transpiration, which the plant controls through its stomata. Guard cells open the stomata in light, when photosynthesis needs carbon dioxide, and close them in darkness or when the plant is short of water.

Closing the stomata cuts evaporation, so the tension in the xylem falls and the stream slows.

External factors change the rate in predictable ways. Higher temperature, lower humidity and moving air all steepen the water vapour gradient between the air spaces in the leaf and the atmosphere, so evaporation and the pull increase.

High humidity or still air reduces the gradient and slows the stream. Light acts indirectly by opening the stomata.

Transpiration pull also matters for the plant's survival on hot days: the evaporation that drives it removes heat from the leaf, keeping leaf temperature within the range at which enzymes work.

How it is examined

You may be asked to explain the cohesion-tension theory in sequence, to link transpiration to water uptake by roots, or to explain why cutting a stem underwater and then in air affects the continuity of the water column.

A frequent structured question describes a potometer experiment and asks you to explain why the bubble moves faster when a fan is switched on or the humidity falls. The expected answer links the environmental change to the water vapour gradient, the rate of evaporation, and the tension transmitted down the xylem.

Another style shows a wilted plant with a cut stem and asks why an air bubble stops water rising. The marks go to stating that the air breaks the continuous column, so cohesion can no longer transmit the tension from leaf to root.

Common misconceptions

Worked exam-style question

Question. A leafy shoot is set up in a potometer. The air bubble moves 12 mm in 5 minutes in still air.

A fan is then directed at the shoot and the bubble moves 30 mm in the next 5 minutes. (a) State what the movement of the bubble measures.

(b) Explain why the bubble moves faster when the fan is on. (c) Describe how the loss of water from the leaf causes water to rise in the xylem.

Model answer. (a) The rate of water uptake by the shoot, which is taken as the rate of transpiration. (b) Moving air removes the layer of humid air around the leaf, so the water vapour concentration gradient between the air spaces in the leaf and the atmosphere is steeper.

Water vapour diffuses out of the stomata faster, so transpiration increases and more water is drawn into the shoot. (c) Water evaporates from the mesophyll cell walls, lowering the water potential of these cells.

Water moves into them from the leaf xylem by osmosis. This creates a tension in the xylem which is transmitted down the water column.

Cohesion between water molecules, due to hydrogen bonds, keeps the column continuous, so the whole column is pulled upwards from the roots.

Source:SRC-DSKP-EN

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