transpiration
Transpiration is the evaporation of water from cells and tissues. In the plants studied this is the loss of water from the leaves. The water enters the stem of the leaf from the potometers tube via osmosis. Because the water potential in the stem is lower then that in the tube the water molecules enter the stem down a water potential gradient. Once in the stem, in the xylem cells this occurs again as the next cell along has a lower water potential from the one before it. The theory of how the water moves up the stem and out the leaves is called cohesion tension theory. As the water molecules move from cell to cell, they pull other water molecules with them. This pulling affect, known as the transpiration pull is possible due to cohesive forces between the molecules. The pull creates tension in the xylem cells, which pulls the water through the stem. The water potential in the atmosphere is usually lower then that in the leaf.
There are also many factors which can affect the rate of transpiration:Humidity - The lower the water potential is in the atmosphere, the greater the loss of water from the leaf as the water potential gradient between the leaf and the air is greater, the converse also applies. Light - Stomata open as light intensity increases. Rolled leaves - On the holly leaf the leaves are slightly rolled, trapping moisture within the leaf and hence reduces the water potential between the leaf and the air. As this water evaporates into the substomatal space from the wall of 1 cell, it creates a tension, which pulls water from the cell walls of other cells. If water is lost from the cell through the stomata, the water potential of this cell is lowered. Also with less stomata there is less places for the water to diffuse from, thus decreasing the transpiration rate for the plant. Depression/reduced number of stomata - This lengthens the diffusion path by trapping sill moist air above the stomata so reduces transpiration rate. Wind speed - If there is no wind speed then the water molecules leaving the leaf will remain near the stomata. The pull is transmitted through the plant by cohesive forces between each of the water molecules. Water will now flow into this cell from the cells near the xylem vessels via osmosis. If the stomata are open then the rate of evaporation from the leaf will increase and thus so will rate of transpiration. As wind speed increases the water molecules are swept away more quickly keeping the water potential gradient constant. This is replaced by more water evaporating from the surrounding the substomatal space. The water is taken fro these cells in 2 ways:Apoplast method- Water travels along the cell wall along spaces in-between cellulose fibres.
Common topics in this essay:
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Light Stomata,
water potential,
water molecules,
rate transpiration,
water potential gradient,
via osmosis,
wind speed,
potential gradient,
water potential atmosphere,
lower water potential,
increase rate transpiration,
move cell,
pulls water,
increase rate,
stem water,
stem water potential,
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Acceptance Essays
