The passage of water through a plant

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  • Created on: 28-03-11 17:30
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AQA Biology for AS ­ Bill Indge ­ Adaptation for mass transport ­ Page 166-169
3.2.7 Factors such as size and metabolic rate affect the requirements of organisms and this gives
rise to adaptations such as specialised exchange surfaces and mass transport systems.
The passage of water through a plant
Specification Points
The structure of a dicotyledonous root in relation to the pathway of water from root hairs through
the cortex and endodermis to the xylem.
Apoplastic and symplastic pathways.
Transpiration and the effects of light, temperature, humidity and air movement.
The roles of root pressure and cohesion-tension in moving water through the xylem.
How does water get to the top of a tree?
The tree has a massive network of roots that penetrate deep into the soil and absorb water from the
soil.
The larger roots anchor the tree into the soil; they have a waterproof coating and do not absorb water.
They branch many times, ending in young, very fine roots.
The tip at the end of each section has no waterproof coating and its job is to absorb water.
The epidermis, cells in its outer layer, have extensions that penetrate between the soil particles ­ these
extensions are the root hairs and they greatly increase the surface area through which water can enter.
Water enters root hairs by osmosis.
The root hairs accumulate mineral ions by absorbing them from the soil by active transport.
Once inside the root, the water crosses to the xylem tissue in the centre.
*The xylem is the mass transport system made up of continuous tubes that distributes
water to other parts of the plant.*
Water crosses the cortex by two different pathways.
The sympathetic pathway ­ water passes through the cells of the cortex, moving from cell to cell
through the membranes that separate them.
This is down a concentration gradient; however it is also a slow route, because the cytoplasm and
membranes restrict the rate at which the water can move.
Apoplastic pathway ­ there is much less resistance in the cellulose walls where water can move quite
freely between the fibres.
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AQA Biology for AS ­ Bill Indge ­ Adaptation for mass transport ­ Page 166-169
Before water can get into the xylem it has to pass through a single layer of cells called the endodermis.
Cannot go via fast route through the cell walls due to a waterproof barrier ­ the casparian strip.
So, water is forced to pass through the membranes and cytoplasm of an endodermis cell before it can
reach the xylem.…read more

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AQA Biology for AS ­ Bill Indge ­ Adaptation for mass transport ­ Page 166-169
This pulling force is so great that the column of water is actually being stretched. The lignin in the walls is
strong enough to stop the vessels collapsing, just as the cartilages in the trachea and bronchi prevent
them from collapsing as we expand the chest to breathe in.…read more

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AQA Biology for AS ­ Bill Indge ­ Adaptation for mass transport ­ Page 166-169
o Often and in many plants the stomata close well before darkness falls and so bright sunlight
does not necessarily mean a high rate of transpiration.…read more

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