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Steps of transpiration??????????//

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Why bother with a post grad? Are they even worth it? Have your say! 26-10-2016
    • Thread Starter

    Please can someone clearly state the steps of transpiration, including cohesion-tension, positive and negative pressure, capillary action and adhesion forces.
    Thank you

    Transpiration : The evaporation of water form the surface of a leaf.
    The transpiration stream goes as follows:

    1.Water enters the roots of the plant by osmosis and is transported up in the xylem until it reaches the leaves.

    2.Here it moves by osmosis across membranes and by diffusion in the apoplast pathway from the xylem through the cells of the leaf where it evaporates from the freely permeable cellulose cell walls of the mesophyll cells in the leaves into the air spaces .

    3.The water vapour then moves into external air through the stomata along a diffusion gradient.
    This is the transpiration stream.

    Now i will explain the transpiration pull:

    1.Water molecules evaporate from the surface of mesophyll cells into the air spaces in the leaf and move out of the stomata into the surrounding air by diffusion down a concentration gradient.

    2.The loss of water lowers the water potential of the cell , so water moves into the cell from an adjacent cell by osmosis along both the symplast and apoplast routes.

    3.Water molecules form hydrogen bonds with the carbohydrates in the walls of the narrow xylem walls - This is known as adhesion. Water molecules also form hydrogen bonds with eachother and tend to stick to eachother - this is known as cohesion.

    4.The combined effects of adhesion and cohesion result in water exhibiting capillary action. This is the process by which water can rise up a narrow tube against the force of gravity.

    5.Water is drawn up the xylem in a continuous stream to replace the water lost by evaporation. This is known as the transpiration pull.

    6.The transpiration pull results in a tension in the xylem, which in turn helps to move water across the roots from the soil.

    The way in which i have explained of water moving from the soil in a continuous stream up the xylem and across the leaf is known as cohesion-tension theory.

    (Original post by ilovefruit)
    Please can someone clearly state the steps of transpiration, including cohesion-tension, positive and negative pressure, capillary action and adhesion forces.
    Thank you
    Transpiration: loss of water vapour from aerial parts (upper parts) of a leaf by the process of evaporation.


    1) Water enters the leaf by the process of osmosis (movement of water particles from an area of higher concentration to an area of lower concentration down water potential gradient) to the spongy mesophyll via apoplast pathway and plasmodesmata.

    2) Water evaporates in the spongy mesophyll to form water vapour

    3) Water vapour leaves out of the leaf through stomata by diffusion. This relies on water potential difference, meaning that there is higher concentration of water molecules inside the cell than outside (plasmolysis) in order for this process to occur.

    Capillary action, transpirational pull and root pressure:

    Transpirational pull - Attracted together by hydrogen bonding from cohesion, where they are pulled to the column of water. Tension is created during this process called cohesion-tension theory. Lignification is used for mechanical strength and preventing the xylem from collapsing.

    Root pressure - A high pressure is created by osmosis in the medulla which forces towards the xylem.

    Capillary action - They have narrow tube and attracted together by adhesion which forces water to be forced towards the side of a xylem.
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