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The Pathway of Water
through a Plant
Objectives
To be able to describe with the
aid of diagrams the pathway by
which water is transported
from the root cortex to the air
surrounding the leaves, with
reference to the Casparian
strip, apoplast pathway,
symplast pathway, xylem and
stomata.
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
The minerals lower
the water
potential in the
cytoplasm of the
root hair cell so
water crosses the
cell membrane by
osmosis moving
down the water
potential gradient
3
Mineral ions are actively pumped
into the xylem and water follows by
osmosis
Water with
dissolved mineral
ions (such as
nitrate) moves
through the
cortex down the
water potential
gradient until it
reaches the xylem.
Transporter proteins actively pump nitrate
ions into the xylem, reducing the water potential
of the xylem so water moves from the cortex
through the endodermis by osmosis.
The pathway taken by the
water through the
cortex cells could be the
APOPLASTIC, the
SYMPLASTIC or the
VACUOLAR pathway.
A) The apoplast pathway –
Water travels in between the
cellulose strands of the cell wall
and through spaces between
cells. The water does not pass
through any membranes so the
mineral ions are carried with the
water.
B) The symplast pathway Water enters the cytoplasm
through the cell membrane of
the root hair cell and then
travels from cell to cell through
plasmodesmata.
C) The vacuolar pathway Similar to the symplast pathway
but the water can enter the
vacuoles as well.
The Casparian
Strip
• The endodermis (or starch sheath because
starch granules are present) is a layer of cells
surrounding the xylem in the root
• The endodermal cells have a band of waterproof
suberin in their cell wall forming the Casparian
Strip
• This blocks the apoplast pathway between the
cortex and the xylem forcing water into the
symplast pathway just before the endodermis
The Casparian strip ensures the plant controls
substances entering the xylem.
Movement of Water up the Stem
Three processes enable water to
move up the stem:
1. Root Pressure
2. Capillary Action
3. Transpiration Pull
Root Pressure
• Active transport of minerals from
cortex to xylem forces water into the
root
Capillary Action
• Water molecules are charged so are
attracted to the sides of the xylem
vessels by ADHESION. This pulls the
water up the sides of the narrow vessels.
Transpiration Stream
• Water molecules are attracted to
each other by forces of COHESION
which hold the water molecules
together in a continuous column.
• As water is lost from the top of the
column (evaporation from leaves), the
whole column is pulled up – the
transpiration stream.
• It creates tension in the lignified
xylem vessels
•This is the COHESION- TENSION
theory of water movement and it
relies on an unbroken column of
water. If a xylem vessel is damaged
the column can be maintained
through another vessel via the pits.
• Most water exits the plant through
stomata (pores in the epidermis)
• It evaporates from the cells
immediately below 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 water leaves
the xylem and enters the leaf cells.