Transcript Absorption mechanisms, Water Movements and Factors

Awais Ahmad
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Length of roots
= 12 kilometre
Surface area
= 5 sq metres
Length of roots + root hairs =
220
kilometre
Surface area
= 14 sq metres
Degree of soil contact
=
1%
Maximum distance for H2O to move to a root
= 10 millimetres
There are two ways for water to enter into the
plant roots
 Active Transport
 Passive Transport
Active Transport:
Water is absorbed due to activities going on in
roots. Absorption of water occurs with the help
of energy in the form of ATP. Absorption takes
place against concentration gradient - even
when the concentration of cell sap is lower than
that of soil water.
Passive Transport:
Passive absorption is by osmosis. Passive
absorption takes place along the concentration
gradient - when the concentration of cell sap is
higher than that of soil water. Water is absorbed
when transpiration rate is high or soil is dry.
Due to high transpiration rate, water deficit is
created in transpiring cells. Rapid transpiration
removes water and reduces turgor pressure in
living cells of root. The suction force thus
developed is transmitted to root xylem. It pulls
water from surrounding root cells to make up
water deficit.
Diffusion:
Diffusion is the movement of molecules from a
region of high concentration to a region of low
concentration by means of random molecular
motion.
Diffusion requires kinetic energy from the
environment but does not require cellular
energy. Hence diffusion is a form of passive
transport.
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Osmosis:
Omosis is diffusion of water across a
semipermeable membrane. Again Osmosis like
diffusion in general does not require any
cellular energy but just the kinetic energy
related to the heat on either side of the
membrane. Hence its also a passive transport.
 Aquaporins:
Are transport proteins in the cell membrane
that allow the passage of water.
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Most of the water absorbed by plants comes in
through root hairs
-Collectively provide enormous surface area
-Almost always
turgid because
their water
potential is greater
than that of soil
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An expenditure of energy is required for ions to
accumulate in root cells
-Once in the roots, the ions are transported via
the xylem throughout the plant
Surface area for water and mineral absorption is
further increased by mycorrhizal fungi
-Particularly helpful in phosphorus uptake
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Three transport routes exist through cells
-Apoplast route = Movement through the cell
walls and the space between cells
-Symplast route = A cytoplasm continuum
between cells connected by plasmodesmata
-Transmembrane route = Membrane transport
between cells and across the membranes of
vacuoles within cells
-Permits the greatest control
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Cont…
Apoplast route
Symplast route
Transmembrane route
Plasma membrane Cell wall
Plasmodesma Vacuole
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Eventually on their journey inward, molecules
reach the endodermis
-Any further passage through the cell walls is
blocked by the Casparian strips
-Molecules must pass through the cell
membranes and protoplasts of the
endodermal cells to reach the xylem
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display.
H2O and
minerals
H2O and
minerals
Endodermis
apoplastic route
symplastic route
Phloem
Xylem
Casparian strip
Cell membrane
H2O and
minerals
H2O and
minerals
Endodermal cell
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Lateral transport of water in roots
Casparian strip
Endodermal cell
Pathway along
apoplast
Pathway
through
symplast
1 Uptake of soil solution by the
hydrophilic walls of root hairs
provides access to the apoplast.
Water and minerals can then
soak into the cortex along
this matrix of walls.
Casparian strip
2 Minerals and water that cross
the plasma membranes of root
hairs enter the symplast.
3 As soil solution moves along
the apoplast, some water and
minerals are transported into
the protoplasts of cells of the
epidermis and cortex and then
move inward via the symplast.
1
Plasma
membrane
Apoplastic
route
Vessels
(xylem)
2
Symplastic
route
Root
hair
Epidermis
4 Within the transverse and radial walls of each endodermal cell is the
Casparian strip, a belt of waxy material (purple band) that blocks the
passage of water and dissolved minerals. Only minerals already in
the symplast or entering that pathway by crossing the plasma
membrane of an endodermal cell can detour around the Casparian
strip and pass into the vascular cylinder.
Cortex EndodermisVascular cylinder
5 Endodermal cells and also parenchyma cells within the
vascular cylinder discharge water and minerals into th
walls (apoplast). The xylem vessels transport the wate
and minerals upward into the shoot system.
Water first enters the roots and then moves to
the xylem, the innermost vascular tissue.
Water rises through the xylem because of a
combination of factors.
There are to types of movement on the basis of
distance covered.
Short-distance movement:
Movement of water at the cellular level plays a
major role in bulk water transport. Water can
diffuse through cell membranes. (Herbaceous)
 Long-distance movement:
Water movement in most of the vascular plants
from roots to leaves. (Sometimes more than
100m)
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Potentials are a way to represent free energy
Water potential (yw) is used to predict which way
water will move
-Measured in units of pressure called
megapascals (MPa)
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Diffusion of water across a semi-permeable
membrane is termed osmosis
If a plant cell is placed in a solution with high
water potential (low osmotic concentration)
-It will become swollen or turgid
If a plant cell is placed in a solution with low
water potential (high osmotic concentration)
-It will exhibit shrinkage or plasmolysis
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Pressure potential (yp): Turgor pressure against
the cell wall
-As turgor pressure increases, yp increases
Solute potential (ys): Pressure arising from
presence of solute in a solution
-As solute concentration increases, ys
decreases (< 0 MPa)
The total potential energy of water in the cell
yw = yp + ys
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When a cell is placed in pure water, water moves
into the cell because the water potential of the
cell is relatively negative
When a cell is placed in a solution with a
different ys, water moves in the direction that
eventually result in equilibrium
-Both cell and solution have the same yw
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or
display.
Pressure Potential p
Turgor pressure p=-0.5MPa
Wall
pressure
–
Cell wall
+
Cell
membrane
Pure water
a.
Solute Potential s
s=-0.2MPa
s=–0.7MPa
Sucrose
molecules
b.
Water Potential
water
movement
 = s + p
cell = –0.7 MPa + 0.5 MPa = –0.2 MPa
solution = –0.2 MPa (solution has no pressure potential)
c.
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Cell Initially Introduced into Solution
Solution
ys = –0.7 MPa
yp = 0 MPa
ysolution = –0.7 MPa
Cell wall
Cell membrane
Cell
ys = –0.2 MPa
yp = 0.5
MPa
ycell = 0.3 MPa
a.
b.
Cell at Equilibrium Is Plasmolyzed
ycell = ysolution = 0.7 MPa
Cell
yp = 0
–0.7 MPa =
ys + 0 MPa
ys = –0.7 MPa
Cell membrane
Cell wall
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Aquaporins are water channels that exist in
vacuole and cell membranes
-They speed up
osmosis, without
changing the
direction of water
movement
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Water potential regulates movement of water
through the whole plant as well
-Water moves from the soil into the roots only
if the soil’s water potential is greater
-It then moves along gradients of
successively more negative water
potentials in the stems, leaves and air
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Evaporation of water in a leaf creates negative
pressure or tension in the xylem
-This “negative water potential” literally pulls
water up the stem from the roots
The driving force for transpiration is the gradient
in vapor pressure
-From 100% relative humidity inside the leaf, to
much less than 100% outside the stomata
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There are three main forces for water
movement trough xylem
 Transpiration Pull
 Cohesion Adhesion Forces
(Cohesive Tension Theory)
 Root Pressure
Transpiration Pull:
It is the pulling force responsible for lifting the
water column. As water is lost in form of water
vapour to atmosphere from the mesophyll cells
by transpiration, a negative hydrostatic
pressure is created in the mesophyll cells which
in turn draw water from veins of the leaves.
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The negative tension is then gradually
transmitted downwards via xylem tissues of
the leaf, stem and finally to the roots. As a
result there is a continuous upward
movement of water column in the plant.
 Thus the transpiration pull acts as pull from
above on the-whole of water column of the
plant which pushes the water column of
xylem vessels of roots lowers leaves i.e. in an
upward direction. This is how ascent of sap is
affected in plants.
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Cohesion Adhesion Forces:
(Cohesive Tension Theory)
 The water molecules in the chain are held
together by hydrogen bonds which exist between
neighboring water molecules. (cohesion)
 The chain of molecules is prevented from being
pulled down because each water molecule in the
chain is attracted to the walls of the xylem by
hydropyllic attraction between water and the
cellulose in the cell walls. (Adhesion)
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Hence the water column which is held
together by cohesion and prevented from
lowering by adhesion is pulled up by the
tension generated from above by
transpiration.
It is valuable both for herbaceous grasses as
well as vascular plant.
Root Pressure:
Root pressure is caused by active transport of
mineral nutrient ions into the root xylem. Without
transpiration to carry the ions up the stem, they
accumulate in the root xylem and lower the water
potential. Water then diffuses from the soil into the
root xylem due to osmosis. Root pressure is caused
by this accumulation of water in the xylem pushing
on the rigid cells. Root pressure provides a force,
which pushes water up the stem, but it is not
enough to account for the movement of water to
leaves at the top of the tallest trees.
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or
display.
Water exits plant
through stomata.
Air
Smooth Rippled
surface surface
H2 O
The water film that
coats mesophyll
cell walls evaporates.
Rippled cell surfaces
result in higher rate of
transpiration than
smooth cell surfaces.
Water moves up plant
through xylem.
Plant
Adhesion due to polarity
of water molecules
Cohesion by
hydrogen bonding
between water
molecules
Water
enters
plant
through
roots.
Soil
H2 O
Soil
Soil
Cytosol
0 –0.5–1.0
–100
y w Water potential (MPa)
Proton pumps
contribute to the H+
y w gradient that
Mineral
determines the
ions
directional flow
of water.
Proton
pump
Water
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1.
Physical Factors:
a) Soil Factors
Soil water contents
Soil Temperature
Soil Aeration
Flooding
Texture and Structure
Speed of Water Movement
Effective Root Zone
b) Atmospheric Factors:
Temperature
Relative Humidity
VDP
Wind Speed
Stress (chemical)
2. Biological Factors
Plant Class (Herbaceous or Vascular )
Root Length
Root System
Plant Health
Biological Stresses
Tolerance to Stresses (Salinity)
Genetic Makeup
Growth Rate
Growth Hormones