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Esme Nias 6.3.16
Movement in the root
1. Water enters through the root hair cells and then moves across into the xylem tissue
in the centre of the root. Water moves in this direction because the soil water has
higher water potential , than the solution inside the root hair cells.
2. This is because the cell sap has organic and inorganic molecules dissolved in it. The
root hairs provide a large surface area over which water can be absorbed.
3. Minerals are also absorbed but their absorption requires energy in the form of ATP
because they are absorbed by active transport . They have to be pumped against the
4. Water taken up by the root hairs moves across the cortex of the root either via the
cytoplasm of the cells in between the
root hair cell and the xylem (the
symplast pathway ) or through the
cell walls of these cells (the apoplast
pathway ). The root hair cell will have
higher water potential than the cell
next to it. Water moves by osmosis
to where the water potential is
lower. In this way, as water is always
being absorbed by the root hairs,
water will always move towards the
centre of the root.
5. When the water reaches a part of the root called the endodermis , it encounters a
thick, waxy band of suberin in the cell walls. This is the Casparian strip and it is
impenetrable . In order to cross the endodermis, the water that has been moving
through the cell walls must now move into the cytoplasm. It then moves towards the
xylem vessel in the symplast pathway.
6. Once it has moved across the
endodermis, it continues down
the water potential gradient
until it reaches a pit in the
xylem vessel. It enters the
vessel and then moves up
towards the leaves.
Movement in the xylem
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Esme Nias 6.3.16
1. Water evaporates from the mesophyll cells into air spaces in the leaf. If the air
surrounding the leaf has less water vapour than the air in the intercellular spaces,
water vapour will leave the leaf through stomata.
2. This process is called transpiration and will continue as long as the stomata are open
and the air outside is not too humid. On dry, windy days when water vapour is
continually diffusing out and being removed, transpiration will increase in rate.