Transcript 36plant transport

Chap 36 Transport in Plants
• In most plant tissues, two of the three cellular
compartments are continuous from cell to cell.
– Plasmodesmata connect the cytosolic compartments
of neighboring cells.
– This cytoplasmic continuum, the symplast, forms a
continuous pathway
for transport.
– The walls of
adjacent plant cells
are also in contact,
forming a second
continuous
compartment, the
apoplast.
Fig. 36.6b
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
The Casparian Strip
blocks the sides of the
endodermis,
preventing water from
moving all the way in
using the apoplast
Water entering the stele through both the apoplast and symplast
must pass through a membrane before entering the stele
symplast=root hair, apoplast=endodermis
thus the plant has control over the intake of water
What are the
connections
between plant
cells called?
• Most plants from partnerships with symbiotic
fungi for absorbing water and minerals from
soil.
• “Infected” roots form mycorrhizae, symbiotic
structures consisting of the plant’s roots united with
the fungal hyphae.
• Hyphae absorb water and selected minerals,
transferring much of these to the host plants.
• The mycorrhizae create an
enormous surface area for
absorption and can even
enable older regions of the
roots to supply water and
minerals to the plant. Fig. 36.8
In order to move material into the cell a
diffusion gradient must be established
One way is to form an electrostatic gradient
(using a proton pump) which will attract
cations. Most dissolved minerals are in the
form of cations: Ca2+, K+, Mg2+, Fe+
The Proton Pump also allows for the Co-Transport
of anions and neutral solutes such as nitrates and
sugars.
Most nutrients in the soil are available at neutral pH in
areas of acid rain - certain nutrients such as nitrates
wash out of the soil and become unavailable to the
plant
1
2
Pure water is O
megapascals
Solutes lower the potential
Water potential is the
combined effects of
solute concentration
and pressure
Water always
moves from
higher to lower
water potential
One way to
increase or
decrease a
solution’s water
potential is to
change the
pressure
Adding sugar
lowers the
environment’s
water potential
Distilled water
will always have
a higher water
potential than
the cell
The cell at
equilibrium
The pressure in
the cell
increases to .7
since the solute
concentration
cannot reach 0
• When psip and psis are equal in magnitude (but
opposite in sign), psi = 0, and the cell reaches a
dynamic equilibrium with the environment,
with no further net movement of water in or
out.
• A walled cell with a greater solute
concentration than its surroundings will be
turgid or firm.
– Healthy plants are turgid
most of the time as
turgor contributes to
support in nonwoody
parts of the plant. Fig. 36.5
5. Vacuolated plant cells have three
major compartments
• While the thick cell wall helps maintain cell
shape, it is the cell membrane, and not the cell
wall, that regulates the traffic of material into and
out of the protoplast.
– This membrane is a barrier
between two major
compartments: the wall
and the cytosol.
– Most mature plant have
a third major compartment,
the vacuole.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 36.6a
Root Pressure
is due to the
active transport
of nutrients into
the stele which causes
water to follow
Guttation
occurs when
the root
pressure
pushes the sap
out of the
leaves
Root Pressure
is not sufficient
to explain the
movement of
water up tall
trees
Page 38
Water is adhesive
to the charges
found on the
surface of glass
and this causes
capillary action.
Why does the
water move up
less in the larger
glass tube and
not at all in the
plastic tube?
Capillary action
accounts for a
little movement
of water up the
xylem.
Transpiration and Tension
create a pull of water up
the xylem
As water
evaporates the
meniscus
becomes more
curved - creating
more tension on
the water film
Adding pressure to
counteract the
upward flow of water
can be use to
measure the leaf’s
water potential
Besides Tension and
Transpiration
Cohesion between water
molecules help to bring
other water molecules up as one leaves it pulls the
next one up- creating
tension
Adhesion of water to the
walls of the xylem help to
pull water upwards
(capillary action)
Adhesion and Cohesion
are due to the hydrogen
bonds that form from the
polar water molecules
TATC is only upwards
in xylem
If this was Ca,
which only
moves upwards
in the xylem, it
would be stuck
here and not be
able to move out
to new leaves
P can move upwards in
the xylem and back out
and on to new leaves
through the phloem
Guard cell mediate the photosynthesistranspiration compromise
• A leaf may transpire more than its weight in
water each day.
– To keep the leaf from wilting, flows in xylem vessels
may reach 75 cm/min.
• Guard cells, by
controlling the size
of stomata, help balance
the plant’s need to
conserve water with
its requirements for
Fig. 36.12
photosynthesis.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
When the guard cells
expand the microfibrils
cause an increase in
length
The inner cell
walls are also
thicker
Active transport of H+
out of the cell drives K+
into the guard cell and
tonoplast - causes an
influx of water and
increased turgor
pressure
H+
Stomatal Opening
• Usually open during the day closed at night
• Blue light receptors in GC triggers opening
• ATP powers proton pump, increases K+
intake
• Low CO2 levels in air spaces
• Circadian Rhythms
Stomatal Closing
• Night
• water deficiency
• abscisic acid released by mesophyll in
response to low water
• high temp - increases respiration and CO2
Xerophyte Leaf Adaptations
•
•
•
•
Small, thick leaves w/ reduced surface area
thick cuticle
stomata mainly on lower leaf
stoma located in depressions, some lined
with hairs
• shed their leaves during the dry season
• CAM plants take in their CO2 at night and
store it as crassulacean acid
• In some xerophytes, the stomata are
concentrated on the lower (shady) leaf surface.
– They are often located in depressions (“crypts”) that
shelter the pores from the dry wind.
– Trichomes (“hairs”) also help minimize
transpiration by breaking up the flow of air, keeping
humidity higher in the crypt than in the surrounding
atmosphere.
Fig. 36.15
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• An elegant adaptation to arid habitats is found
in ice plants and other succulent species of the
family Crassulaceae and in representatives of
many other families.
– These assimilate CO2 by an alternative
photosynthetic pathway, crassulacean acid
metabolism (CAM).
– Mesophyll cells in CAM plants store CO2 in
organic acids during the night and release the CO2
from these organic acid during the day.
• This CO2 is used to synthesize sugars by the conventional
(C3) photosynthetic pathway, but the stomata can remain
closed during the day when transpiration is most severe.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Pearson Transpiration Lab
• Turn in both Lab quizzes tomorrow.
7. Bulk flow functions in longdistance transport
• Diffusion in a solution is fairly efficient for
transport over distances of cellular dimensions
(less than 100 microns).
• However, diffusion is much too slow for longdistance transport within a plant - for example,
the movement of water and minerals from roots
to leaves.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Water and solutes move through xylem vessels
and sieve tubes by bulk flow, the movement of
a fluid driven by pressure.
– In phloem, for example, hydrostatic pressure
generated at one end of a sieve tube forces sap to
the opposite end of the tube.
– In xylem, it is actually tension (negative pressure)
that drives long-distance transport.
• Transpiration, the evaporation of water from a leaf,
reduces pressure in the leaf xylem.
• This creates a tension that pulls xylem sap upward from
the roots.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• The phloem transports the organic products of
photosynthesis throughout the plant via a process
called translocation.
– In angiosperms, the specialized cells of the phloem
that function in translocation are the sieve-tube
members.
• These are arranged end to end to form long sieve tubes with
porous cross-walls between cells along the tube.
• Phloem sap is an aqueous solution in which
sugar, primarily the disaccharide sucrose in most
plants, is the most prevalent solute.
– It may also contain minerals, amino acids, and
hormones.
1. Phloem translocates its sap from
sugar sources to sugar sinks
• In contrast to the unidirectional flow of xylem sap
from roots to leaves, the direction that phloem
sap travels is variable.
• In general, sieve tubes carry food from a sugar
source to a sugar sink.
– A sugar source is a plant organ (especially mature
leaves) in which sugar is being produced by either
photosynthesis or the breakdown of starch.
– A sugar sink is an organ (such as growing roots,
shoots, or fruit) that is a net consumer or storer of
sugar.
Pressure Flow Hypothesis explains bulk
flow of materials through the phloem
The ingrowths of the transfer cells may
increase the surface area for the
transfer of solutes from the apoplast to
the symplast
Chemiosmosis helps
in the cotransport of
sugar into the cell
Bidirectional
Unidirectional
A
B
C
• The pressure flow model explains why phloem
sap always flows from sugar source to sugar
sink, regardless of their locations in the plant.
• Researchers have devised several experiments to test
this model, including an innovative experiment that
exploits natural phloem probes: aphids that feed on
phloem sap.
• The closer the aphid’s stylet is to a sugar source, the
faster the sap will flow out and the greater its sugar
concentration.
Fig. 36.18