Transpiration is part of the water cycle. It is the loss of water vapour from parts of a plant. The process explains how water is absorbed by a plant’s root hair cells down a concentration gradient. This causes the solution inside root hair cells to become more dilute, meaning that there is now a concentration gradient between the dilute root hair cell and other root hair cells. This allows water to move from the dilute root hair cell to other root hair cells through the process of osmosis. This continues until water has reached the xylem vessels within the root. Water is then carried up the stem against gravity. In daylight, water is continuingly being drawn up the xylem vessels to replace the water already drawn up the stem. Water molecules tend to stick together, which causes them to travel through the xylem vessels after each other. The water then reaches the leaves and is released as water vapour into the air by stomata (pores) in a plant’s leaves. The above diagram illustrates the transpiration stream.
As you may remember from an earlier topic, stomata are mostly found on the surface under the leaf. They are tiny pores whose main function is the exchange of gases into and out of the plant by the process of diffusion. More stomata are found on the surface under the leaf because otherwise the plant would lose too much water. If stomata were on the top of leaves, they would be exposed to direct sunlight, causing an increase in the rate of evaporation from them. Stomata have evolved like this to help reduce the loss of water. The below diagram illustrates the exchange of gases through stomata:
The stomata have the ability to open and close. Guard cells surround each stoma and are the only cells in the lower epidermis to contain chloroplasts. In daylight, the guard cells use energy to accumulate solutes in their vacuoles. This causes them to draw in water through the process of osmosis, which causes them to become turgid. As the guard cells swell, their shape also changes and this results in them bending outwards, opening the stoma. At night, the guard cells become flaccid as they lose water and this results in the stoma closing. Stomata do not need to be open in the dark; this is because without sunlight, there is no need for the loss of water vapour to help cool the leaves.
Water is an essential component of photosynthesis but leaves do not have the ability to photosynthesise when it is dark; another reason why stomata do not need to be open.
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