Movement of water

how water moves through roots, then up the stem and transpiration

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  • Created by: malu
  • Created on: 27-06-13 19:55
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Plants have a special exchange surface in the soil called root hairs to absorb and
transport water.
Root hairs
are the
exchange surfaces in plants that are responsible for the
absorption of water and mineral ions. Plants constantly lose water by
the process of transpiration.
Each root hair is a long, thin extension of a root epidermal cell. They
are very efficient surfaces for exchange of water and mineral ions because:-
They provide a large surface area as they are very long extensions
They have thin surface layer (cell membrane and cell wall)
across which materials can move easily.
Root hairs arise from the epidermal cells a little way behind the tips of young
roots. They grow into the spaces around soil particles. In damp conditions they
are surrounded with small quantities of mineral ions, therefore they
have a very HIGH WATER POTENTIAL (more negative). The root
hairs and other cells in the plant have mineral ions, sugars and amino acids
dissolved in it so it has A LOW WATER POTENTIAL THAN SOIL.
Therefore water moves into cell by osmosis from the soil solution
After being absorbed into the root hair cell, water continues its journey across
the roots in two ways:

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As water is drawn into epidermal cell, it pulls more water along behind it due
to the cohesive properties of the water molecules. That creates a
Cohesion is the mutual attraction of molecules for one another. It is the
ability of water molecules to stick to one another.
This takes place across THE CYTOPLASM OF THE CELLS OF THE
CORTEX as a result of osmosis.…read more

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Casparian strip
Endodermal cells control substances passing from root to plant. Each endodermal
cell has A WATERPROOF LAYER (casparian strip). They are impermeable to
water and mineral ions and it blocks apoplast pathway. All substances must pass
through cell surface membrane of endodermal cells.
Passage of water into the xylem
When water reaches the endodermis by the apoplastic pathway, the waterproof
band that makes up the casparian strip in the endodermal cells prevents it
progressing further along the cell wall.…read more

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Mineral salts are actively transported to the xylem and this
requires energy. It takes place along carrier proteins in the cell surface
membrane. When mineral ions are actively transported into the xylem there is a
low water potential in the xylem, this creates a concentration gradient and water
moves into xylem by osmosis. This creates a force that helps to move water up the
plant. This force is called ROOT PRESSURE.…read more

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Mesophyll cells lose water to the air spaces
These cells now have a low water potential
than its neighbour and water moves
in by osmosis
The loss of water from neighbours lowers its
water potential and they in turn take in
water from their neighbours
In this way, a water potential gradient is
established that pulls water from the xylem, across
the leaf mesophyll and finally out into the
atmosphere.…read more

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When transpiration
rate of low (during night), there is less tension in the xylem and so the diameter
of the tree trunk increases
If a xylem vessel is broken and air enters it, the tree can no
longer draw up water because the continuous column of water is broken
and so the water molecules can longer stick together.
When xylem is broken water does not leak out because the pressure
outside will be higher than inside xylem, so pressure pushes water down.…read more


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