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The Pathway of Water through
a Plant
OCR Unit 1 Module 2
Root Hair Cell
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1. Minerals ions either move down their diffusion gradient
into the root hair cell or are actively transported across
the membrane into the root hair cell using energy from
ATP.
2. This lowers the water potential in the root hair cell so
water will flow into it from the soil.
3. Water including dissolved mineral ions will move
through the cortex cells down the diffusion gradient until
it reaches the xylem. The pathway taken through the
cortex cells could be the APOPLASTIC, the
SYMPLASTIC or the VACUOLAR pathway.
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Pathways
A)
B)
C)
The apoplast pathway. The cell walls
are made of cellulose strands. Water
can travel in between these strands
and through spaces between cells.
The water does not pass through any
membranes so the mineral ions are
carried with the water.
The symplast pathway. Water enters
the cytoplasm through the plasma
membrane of the root hair cell. It can
then travel from cell to cell through
plasmodesmata. These are gaps in
the cell wall that allow cytoplasm from
adjacent cells to connect.
The vacuolar pathway. Similar to the
symplast pathway but the water can
enter the vacuoles as well.
Casparian Strip
• The endodermis is a layer of cells surrounding the xylem in the root. It is
sometimes called the starch sheath as it contains granules of starch (a sign
that energy is being used).
• The endodermal cells also have a Casparian Strip. This is a band of
waterproof suberin in their cell wall. This blocks the apoplast pathway
between the cortex and the xylem.
Functions
• The strip ensures water and
dissolved ions (e.g. nitrate ions)
have to pass into the cytoplasm
through cell membranes.
• There are transporter proteins in
the cell membranes that actively
transport nitrate ions from the
cortex into the xylem.
•This lowers the water potential in
the xylem and so water enters in
by osmosis.
• In this way the plant has control
over substances entering the
xylem.
• Once in the xylem the water can move by root
pressure, capillary action or by the transpiration pull.
• Water molecules are attracted to each other by forces
of COHESION. These forces hold the water molecules
together in one continuous column.
• As water is lost from the top of the column, the whole
column is pulled up as one chain – the transpiration
stream.
• The pull can create tension in the column of water.
The xylem vessels need to be reinforced with lignin to
stop them collapsing under the tension. The
COHESION- TENSION theory relies on an unbroken
column of water. If one xylem vessel gets damaged the
column can be maintained through another vessel via
the pits.
• Most of the water exits the plant through stomata (tiny
pores in the epidermis)
• Water evaporates from the cells immediately above
the guard cells and diffuses out of the leaf.
• This lowers the water potential in those cells, and so
water enters them by osmosis from surrounding cells
and so on, until eventually water leaves the xylem and
enters the innermost leaf cells.