Movement of water up stems
- Created by: Laura
- 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
- These cells now have a lower water potential and so water enters by osmosis from the neighbouring cells
- Mesophyll cells lose water to the air spaces
- 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
- Water forms a continuous, unbroken pathway across the mesophyll cells and down the xylem
- Water molecules from hydrogen bonds between one another and hence tend to stick together. This is known as cohesion
- Water evapourates from the leaves as a result of transpiration
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