Osmosis
 Osmosis drives the
absorption of water and
minerals from the soil by
the root tips.
 Osmosis=movement of
water from higher
concentration to lower
concentration across a
semi-permeable
membrane.
 Water then moves deeper
into the roots until it
reaches the endodermis.
Microscopic root hairs absorb
water and nutrients by
osmosis.
Water Movement Across the
Casparian Strip
 Once inside the root itself,
water and nutrients must
get past the Casparian
strip—they can only travel
through the selective
endodermal cells that
choose which nutrients
and minerals they let
through.
 If they pass the “test”, they
move into the vascular
cylinder.
The Endodermis is shown in bright red in
the root cross section above.
Water and nutrients must move into the
vascular cylinder across the Casparian
Strip next to the Endodermis.
Osmosis—How does it drive water
movement?
 Osmosis is the driving
force for moving water
from the soil into xylem
cells.
 How does the plant keep
the concentration
gradient going that is
necessary for osmosis to
occur?
What maintains the concentration
gradient of water in the root?
 Two Contributing Factors:
 1)Water is constantly
moving away from the root
tips creating more space
for water to enter
 2)There is a higher mineral
concentration inside the
vascular cylinder, which
drives water into the xylem
by a force known as root
pressure.
Capillary Action
 Capillary action is the
force of adhesion
between water and a
passageway that puls
water up along the sides.
 Along with osmosis, this
mechanism is a minor
contributor to the
movement of water up
the xylem .
Cohesion-Tension Theory and
Transpiration
 This process is the major
mover of water in the
xylem.
 Transpiration creates a
negative pressure in the
leaves and xylem tissues
due to water evaporation
from the leaves.
 Transpiration is
evaporation of water from
the surface of leaves.
Water Molecules & Hydrogen
Bonds
 Water molecules are
attracted to other water
molecules. Why?
 The molecular attraction
of water to water is called
cohesion.
 This attraction
essentially creates a
single united column of
water that runs the
length of the plant
How does Cohesion pull water
molecules up the plant?
 Imagine that you tie a
bunch of soda cans to a
rope. If you are standing
in a tree, and pull up on
the cans at the top of the
rope, the cans at the
bottom will follow—not
really because they are
loyal to the other cans,
but because they are
connected (bonded).
Transpiration
 The movement of water
through the xylem is due
mostly to cohesion
between the water
molecules.
 When water evaporates off
the surface of the leaf, the
water is pulled up through
the xylem toward the
leaves.
 http://www.phschool.com/
science/biology_place/lab
bench/lab9/transpull.html
Regulating Stomate Activity
 Stomata are the
structures that regulate
the flow of O2, CO2, and
H2O in/out of the leaf
 When closed,
photosynthesis is halted.
Why?
 When open, mesophyll
cells have access to water
and CO2.
Opening and Closing Stomata
 With every reward, there
is a risk.
 When the stomata are
open, the plant could dry
out as a result of
excessive transpiration.
 The process of opening
and closing the stomata,
then, must be very
carefully controlled.
Guard cells such as these shown above
control the opening and closing of the
stomata.
Guard Cells
 Guard cells surround and tightly
regulate the actions of the stomata.
 When water flows into neighboring
guard cells (leading to an increase of
turgor pressure), a structural change
occurs that causes the opening of
the stomata.
 When water flows out, (a decrease in
turgor pressure), the stomata will
close.
http://www.phschool.com/science/b
iology_place/labbench/lab9/stoma
mov.html
Transpiration Lab—AP Lab 9
 http://www.phschool.co
m/science/biology_place
/labbench/lab9/design.h
tml
Carbohydrate Transport Through
Phloem
 The transport of
carbohydrates through
the phloem is called
translocation.
 After they are produced,
they are moved into the
phloem (the sugar
superhighway), near the
site of their creation.
 They will be distributed
throughout the plant.
Translocation Through Phloem
 The movement of sugar
into the phloem creates a
driving force because it
establishes a concentration
gradient.
 Because there are more
sugars in the phloem, the
gradient causes water to
move into the phloem by
osmosis. This increases the
pressure inside these cells.
Pressure Inside Phloem cells
 The pressure of these cells
is what drives the
movement of sugars and
water through the phloem.
 As the sugars arrive at
various destination sites,
the sugar is consumed by
plant cells, causing a
reversal in the driving force
for water.
 This pushes water out of
the phloem. As water exits
the phloem, the increased
pressure disappears.
Translocation occurs in a series of
steps:
 1. Sugars enter sieve-tube members by active transport.
(now there is a greater concentration of sugars inside
these cells)
 2. Water enters sieve-tube members by diffusion.
Why?
 3. Pressure in sieve-tube members at the source
(leaves)moves water and sugars to sieve-tube members
at the sink (where the sugar will be used)through
sieve tubes. The water and sugars move by bulk flow.
 4. Pressure is reduced in sieve-tube members at the
sink as sugars are removed and used. Water then
leaves also.