Download 3 - Bio5090

Lesson 3
Learning Objectives
1.Relate the structure and functions of root hairs to their surface area and to water
and ion uptake.
2.Explain the movement of water from the soil to the root hairs.
3.Explain the mechanism of root pressure.
4.Explain the mechanism of capillary action.
5.State that root pressure and capillary action are ways to draw water up the stem
6.Define transpiration
7.Explain the mechanism of transpiration producing a pull from above.
Outline

Transport of water
– Soil to root hairs (osmosis)
– Up the stem (root pressure, capillary action, transpiration pull)
Transport of water
• From soil into root
hairs
– Osmosis
• Up the stem
– Root pressure
– Capillary action
– Transpiration pull
Transport of water into the root hairs
cytoplasm
soil particles
vacuole
2
1
2
The thin film of liquid
surrounding each soil
particle is a dilute
solution of mineral salts.
cell surface
membrane of
root hair cell
nucleus
cell wall
film of liquid
(dilute
solution of
mineral salts)
1
Each root hair is a fine tubular outgrowth of
an epidermal cell. It grows between the soil
particles, coming into close contact with the
water surrounding them.
3
4
The sap in the root hair
cell is a relatively
concentrated solution of
sugars and various salts.
Thus, the sap has a
lower water potential
than the soil solution.
These two solutions are
separated by the
partially permeable cell
surface membrane of the
root hair cell. Water
enters the root hair by
osmosis.
a section of root
showing the path of
water through it
5
C
water entering
the root hair
4
B
A
3
root hair
xylem
piliferous layer
phloem
The entry of water dilutes the sap. The sap of the root
hair cell now has a higher water potential than that of
the next cell (cell B). Hence, water passes by osmosis
from the root hair cell into the inner cell.
cortex
5
Similarly, water passes from cell B into the
next cell (cell C) of the cortex. This process
continues until the water enters the xylem
vessels and moves up the plant.
As water enters the cells, it can take one of two
routes depending upon whether or not it has
entered the cytoplasm of the cell.
These routes are called the
– Apoplastic
• Water is transported between cells up to the
endodermis.
– Symplastic
• Water is transported through the cell membrane
and cytoplasm.
• It takes longer to transport and requires ATP.
• Water can pass through the endodermis.
Transport of mineral salts into root hairs
• Diffusion
– When the concentration of mineral salts in the soil
solution is higher than that in the root hair cell.
• Active transport
– When the concentration of ions in the soil solution is
lower than that in the root hair cell.
– The energy comes from respiration in the root hair
cells.
Transport of water up the stem
• Root pressure
• Capillary action
• Transpiration pull
Root Pressure
• By using active transport, ions in the living cells
around the xylem vessels in the root are pumped into
the vessels.
• Water potential in the xylem vessels is lowered.
• Water passes from the living cells into the xylem
vessels by osmosis.
• Accumulation of water in the xylem vessels create a
pressure called root pressure.
• Root pressure pushes the water up the stem.
Capillary Action
Water also moves up inside fine capillary
tube by capillary action
• Water molecules attract other
water molecules by the force of
cohesion.
• Water sticks to the inner surface of
the xylem vessels by adhesion.
• Both cause water to move up
xylem stem without breaking.
• This mechanism does not account
for the greater distance that water
can travel in trees.
Transpiration Pull
Transpiration is the loss of water vapour from stomata
of the leaves of the plant.
• When water vapour is lost from the leaves,
there is a decrease in water concentration in
the leaves.
• This creates a suction force that pull water
up from the stems.
• This is called transpirational pull.
• Transpiration stream is the stream of water
going up the stem.
Learning Objectives
1.Relate the structure and functions of root hairs to their
surface area and to water and ion uptake.
2.Explain the movement of water from the soil to the root hairs.
3.Explain the mechanism of root pressure.
4.Explain the mechanism of capillary action.
5.State that root pressure and capillary action are ways to
draw water up the stem.
6.Define transpiration
7.Explain the mechanism of transpiration producing a pull from
above.