Movement of water up stems

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  • Created on: 07-05-14 10:10
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  • Movement of water up stems
    • The main force that pulls water up the stem of a plant is the evaporation of water from leaves, a process called transpiration
    • Movement of water through the stomata
      • The humidity of the atmosphere is usually less than that of the air spaces next to the stomata.
      • Provided the stomata are open, water vapour molecules diffuse out of the air spaces into the surrounding air
      • Water lost from the air spaces is replaced by water evapourating from the cell walls of the surrounding mesophyll cells.
    • Movement of water across the cells of a leaf
      • Water is lost from mesophyll cells by evaporation from their surfaces to the air spaces of the leaf.
      • This is replaced by water reaching the mesophyll cells from the xylem by either the apoplastic or symplastic pathways.
      • In the case of the symplastic pathway, the water movement occurs because
        • Mesophyll cells lose water to the air spaces
          • These cells now have a lower water potential and so water enters by osmosis from the neighbouring cells
            • The loss of water from the neighbouring cells lowers their water potential. They in turn take in water from neighbours by osmosis
    • Movement of water up the stem in the xylem
      • The two main factors that are responsible for the movement of water up the xylem, from the roots to the leaves, are cohesion tension and root pressure
      • Cohesion tension operates as follows
        • Water evapourates from the leaves as a result of transpiration
          • Water molecules from hydrogen bonds between one another and hence tend to stick together. This is known as cohesion
            • Water forms a continuous, unbroken pathway across the mesophyll cells and down the xylem
              • As water evapourates from the mesophyll cells in the leaf into the air spaces beneath the stomata, more molecules of water are drawn up behind it as a result of this cohesion


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