Transport in Plants
Figure 36.1 The Pathways of Water and Solutes in the Plant
36
Uptake and Movement of Water and Solutes
• For osmosis to occur, a membrane
must be permeable to water but not
to the solutes.
• Plant cells have a rigid cell wall.
• As water enters the cell, the plasma
membrane presses against the cell
wall, restricting expansion.
• The opposing force exerted by the rigid cell wall as water
enters is called the pressure potential, or turgor pressure.
• Water enters a plant cell until the pressure potential exactly
balances the solute potential. The cell is then called turgid.
36
Uptake and Movement of Water and Solutes
• Water always moves
across a semipermeable
membrane toward the
region of more negative
(lower) water potential.
• The structure of many
plants is maintained by the
pressure potential in their
cells. Loss of pressure
potential causes wilting.
36
Uptake and Movement of Water and Solutes
• Mineral ions generally require transport proteins in order
to cross membranes.
• When the concentration of ions is greater in the soil than
in the plant, the plant can take up ions by facilitated
diffusion, a passive process.
• If the concentration of ions is lower in the soil than in the
plant, however, ion uptake requires energy.
36
Uptake and Movement of Water and Solutes
• Where bulk flow of water is occurring, dissolved
minerals are carried along.
• When movement is less, minerals move by
diffusion.
• Minerals must be actively transported across
certain membranes.
• The cells at the surface of the root hairs actively
transport ions.
• Water moves into the cells of the root because the
root cells have more negative water potential than
the soil solution.
36
Transport of Water and Minerals in the Xylem
• In the xylem, water and minerals constitute the
xylem sap.
36
Transport of Water and Minerals in the Xylem
• The transpiration–cohesion–tension mechanism:
• The concentration of water vapor is higher inside the
leaf than outside, so water diffuses out of the leaf
through the stomata. This process is called
transpiration.
• This creates a tension in the mesophyll that draws
water from the xylem of the nearest vein into the
apoplast surrounding the mesophyll cells.
• The removal of water from the veins, in turn,
establishes tension on the entire volume of water in
the xylem, so the column is drawn up from the roots.
Figure 36.8 The Transpiration–Cohesion–Tension Mechanism
36
Transport of Water and Minerals in the Xylem
• Hydrogen bonding between water molecules
results in cohesion, the tendency of water
molecules to stick to one another.
• The narrower the tube, the greater the tension the
water column can stand.
• The water column is also maintained by adhesion
of water molecules to the walls of the tube.
36
Transport of Water and Minerals in the Xylem
• The key elements in water transport in xylem:
 Transpiration
 Tension
 Cohesion
• The transpiration–cohesion–tension mechanism
does not require energy.
• These things makes water flow up the stems to
the leaves.
• At each step, water moves passively toward a
region with a more negative water potential.
36
Transport of Water and Minerals in the Xylem
• Mineral ions in the xylem sap rise passively with
the solution.
• Transpiration also contributes to the plant’s
temperature regulation, cooling plants in hot
environments.
36
Transpiration and the Stomata
• Leaf and stem epidermis has a waxy
cuticle that is impermeable to water, but
also to CO2.
• Stomata, or pores, in the epidermis
allow CO2 to enter by diffusion.
• Guard cells control the opening and
closing of the stomata.
• In most plants, the stomata are open
during the day (when light is intense
enough to maintain photosynthesis)
and closed at night (to prevent water
loss).
36
Translocation of Substances in the Phloem
• Sugars, amino acids, some
minerals, and other solutes are
transported in phloem and move
from sources to sinks.
• A source is an organ such as a
mature leaf or a starch-storing root
that produces more sugars than it
requires.
• A sink is an organ that consumes
sugars, such as a root, flower, or
developing fruit.
Sources
↓
Sinks
36
Translocation of Substances in the Phloem
• Translocation (movement of
organic solutes) stops if the
phloem is killed.
• Sieve tube cells at the source
have a greater sucrose
concentration that surrounding
cells, so water enters by
osmosis. This causes greater
pressure potential at the source,
so that the sap moves by bulk
flow towards the sink.
• At the sink, sucrose is unloaded
by active transport, maintaining
the solute and water potential
gradients.
Figure 36.14 The Pressure Flow Model