Transcript Plant Structure and Function Ch. 35

Transport in Plants
AP Biology
Ch. 36
Ms. Haut
Physical forces drive the transport of
materials in plants over a range of distances
• Transport in vascular plants occurs on three scales:
– Transport of water and solutes by individual cells, such as
root hairs
– Short-distance transport of substances from cell to cell at the
levels of tissues and organs
– Long-distance transport within xylem
and phloem at the level of the whole plant
CO2
O2
Light
H2O
• A variety of physical processes are
involved in the different types of
transport
Sugar
O2
H2O
Minerals
CO2
Transport at Cellular Level
Relies on selective permeability of
membranes
• Transport proteins
– Facilitated diffusion
– Selective channels (K+ channels)
• Aquaporins—water-specific protein
channels that facilitate water diffusion
across plasma membrane
Transport at Cellular Level
• Proton pumps
– create a hydrogen ion gradient that is a form of
potential energy
– contribute to a voltage known as a membrane
potential
CYTOPLASM
EXTRACELLULAR FLUID
ATP
Proton pump
generates membrane potential
and
gradient.
Transport at Cellular Level
• Plant cells use energy stored in the proton gradient
and membrane potential to drive the transport of
many different solutes
CYTOPLASM
EXTRACELLULAR FLUID
Cations (
, for
example) are
driven into the cell
by the membrane
potential.
Transport protein
Membrane potential and cation uptake
Transport at Cellular Level
• In the mechanism called cotransport, a transport
protein couples the passage of one solute to the
passage of another
Cell accumulates
anions (
,
for example) by
coupling their
transport to; the
inward diffusion
of
through a
cotransporter.
Cotransport of anions
Effects of Differences in Water
Potential
• To survive, plants must balance water
uptake and loss
• Osmosis determines the net uptake or water
loss by a cell is affected by solute
concentration and pressure
Effects of Differences in Water
Potential
• Water potential is a measurement that combines
the effects of solute concentration and pressure
• Water potential determines the direction of
movement of water
• Water flows from regions of higher water potential
to regions of lower water potential
How Solutes and Pressure Affect
Water Potential
• Both pressure and solute concentration
affect water potential
– The addition of solutes reduces water potential
• The solute potential of a solution is
proportional to the number of dissolved
molecules
• Pressure potential is the physical pressure
on a solution
 = P + S
Differences
in Water
Potential
Drive Water
Transport in
Plant Cells
 = P + S
Three Major Compartments of
Vacuolated Plant Cells
• Transport is also regulated by
the compartmental structure
of plant cells
• The plasma membrane
directly controls the traffic of
molecules into and out of the
protoplast
• The plasma membrane is a
barrier between two major
Cell compartments
compartments, the cell wall
and the cytosol
Cell wall
Cytosol
Key
Vacuole
Symplast
Apoplast
Plasmodesma
Vacuolar membrane
(tonoplast)
Plasma membrane
• The third major compartment in most
mature plant cells is the vacuole, a large
organelle that occupies as much as 90% or
more of the protoplast’s volume
• The vacuolar membrane regulates transport
between the cytosol and the vacuole
ell compartments
Cell wall
Cytosol
Vacuole
Plasmodesma
Key
Symplast
Apoplast
Vacuolar membrane
(tonoplast)
Plasma membrane
• In most plant tissues, the cell walls and
cytosol are continuous from cell to cell
• The cytoplasmic continuum is called the
symplast
• The apoplast is the continuum of cell walls
and extracellular spaces
Key
Symplast
Apoplast
Transmembrane route
Apoplast
Symplast
Symplastic route
Apoplastic route
Transport routes between cells
Lateral Transport of Minerals and Water
Casparian strip—waxy material (suberin)
that creates selectivity (only minerals
already in symplast can enter stele)
The Roles of Root Hairs,
Mycorrhizae, and Cortical Cells
• Much of the absorption of water and
minerals occurs near root tips, where the
epidermis is permeable to water and root
hairs are located
• Root hairs account for much of the surface
area of roots
Mycorrhizae
2.5 mm
• Most plants form mutually
beneficial relationships
with fungi, which
facilitate absorption of
water and minerals from
the soil
• Roots and fungi form
mycorrhizae, symbiotic
structures consisting of
plant roots united with
fungal hyphae
Pushing Xylem Sap: Root
Pressure
• At night, when transpiration is very low, root
cells continue pumping mineral ions into the
xylem of the vascular cylinder, lowering the
water potential
• Water flows in from the root cortex, generating
root pressure
• Root pressure sometimes results in
guttation, the exudation of water droplets on
tips of grass blades or the leaf margins of
some small, herbaceous eudicots
Transportation of Xylem Sap (Water):
Transpiration-Cohesion Theory
•Water evaporates from leaves
through stomata—creates a low
pressure at top of water column
•Water replaced by water from
xylem—water in areas of high
pressure move to areas of low
pressure
Strong cohesion of water with the
pressure difference helps to pull the
entire water column up from roots
to rest of plant
Transpirational Pull
• Water is pulled upward by
negative pressure in the
xylem
• Water vapor in the airspaces
of a leaf diffuses down its
water potential gradient and
exits the leaf via stomata
• Transpiration produces
negative pressure (tension)
in the leaf, which exerts a
pulling force on water in the
xylem, pulling water into the
leaf
Cohesion and Adhesion in the
Ascent of Xylem Sap
Xylem
sap

Outside air 
= –100.0 MPa
Mesophyll
cells
Stoma
Leaf  (air spaces)
= –7.0 MPa
Transpiration
Leaf  (cell walls)
= –1.0 MPa
Trunk xylem 
= –0.8 Mpa
Water potential gradient
• The transpirational
pull on xylem sap is
transmitted all the way
from the leaves to the
root tips and even into
the soil solution
• Transpirational pull is
facilitated by cohesion
and adhesion
Xylem
cells
Water
molecule
Atmosphere
Adhesion Cell
wall
Cohesion,
Cohesion and by
adhesion in
hydrogen
the xylem
bonding
Water
molecule
Root
hair
Root xylem 
= –0.6 MPa
Soil 
= –0.3 MPa
Soil
particle
Water
Water uptake
from soil
K+
H2 O
K+
Guard
cell
H2 O
K+
K+
•Opening and closing is regulated by turgor pressure
•Stoma of most plants open during the day and closed during the night
During the day, K+ is pumped into the guard cells
H2O flows into cells by osmosis
Turgor pressure increases and guard cells expand, opening the pore
K+
H2 O
K+
K+
H2 O
K+
At night K+ pumped out of cells
H2O flows out of cells by osmosis
Turgor pressure decreases and guard cells shrink, closing the pore
Organic nutrients are translocated
through the phloem
• Translocation is the transport of organic
nutrients in a plant
Movement from Sugar Sources to
Sugar Sinks
• Phloem sap is an aqueous solution that is mostly
sucrose
• It travels from a sugar source to a sugar sink
• A sugar source is an organ that is a net producer
of sugar, such as mature leaves
• A sugar sink is an organ that is a net consumer or
storer of sugar, such as a tuber or bulb
Translocation
Sieve tube
(phloem)
Vessel
(xylem)
H2O
Source cell
(leaf)
Sucrose
flow
sure
Pres
Transpiration stream
H2O
Sink cell
(storage
root)
Sucrose
H2O
• Sugar must be loaded
into sieve-tube
members before being
exposed to sinks
• In many plant species,
sugar moves by
symplastic and
apoplastic pathways
Transportation of Food: Pressure-flow Hypothesis
At the source end of the sieve tube:
•Sugars are made in photosynthetic cells and pumped by active
transport into sieve tubes
•Concentration of dissolved substances increases in the sieve tube
and water flows in by osmosis
•Pressure builds up at the source end of the sieve tube
Water flows in
Transportation of Food: Pressure-flow Hypothesis
At the sink end of the sieve tube:
•Sugars are pumped out
•Water leaves the sieve tube by osmosis
•Pressure drops at the sink end of the sieve tube
•Difference in pressure causes sugars to move from source to sink
Water flows in
Water flows out