Transcript Plant Transport – Transpiration and Phloem Movement

Plant Transport – Transpiration and
Phloem Movement
Brown algae – Macrocystis and Laminaria - California
Giant
Sequoia
Water transport in plants
• https://www.youtube.com/watch?v=w6f2BiFiXiM
Plant Transport
Plant transport occurs at three levels:
1. The uptake and loss of water and solutes by
individual cells, such as absorption of water and
minerals from the soil by root cells.
2. Short-distance transport of substances from cell to
cell at the level of tissues and organs, such as moving
sugar from photosynthetic cells of leaf to the phloem
sieve tubes
3. Long distance transport of sap within xylem and
phloem at the level of the entire plant.
Xylem Transport
Water Potential
• The movement of water in and out of plant cells is
driven by water potential. The net uptake or loss of
water by a cell occurs by osmosis, the passive
transport of water across a membrane. Water usually
moves from hypotonic (low solute concentration) to
hypertonic (high solute concentration) – this is what
happens in animal cells. But plants have a rigid cell
wall that provides physical pressure. So in plants the
movement of water depends upon a combination of
solute concentration and physical pressure known as
water potential symbolized by the Greek letter psi Ψ
More Water Potential
• Plant biologists measure water potential in units of pressure
called megapascals MPa – 1 MPa equals 10 atmospheres of
pressure – an atmosphere of pressure is the pressure of a
column of air at sea level
• A car tire is usually inflated to about 0.2 MPa (or two
atmospheres); water pressure in home plumbing is 0.25 MPa
• Adding solutes to water lowers its water potential. This is
because the water molecules form shells around the solute and
have less freedom to move than they do in pure water. Pure
water has a water potential of 0 MPa – so adding solutes
results in a solution with a negative measure of MPa.
Water Potential
• Water potential = physical pressure potential + solute potential
(AKA osmotic potential)
•
Ψ = Ψ p + Ψs
• where Ψ = total water potential; Ψ p = physical pressure
potential; Ψs = solute pressure potential
• If we add physical pressure to a solution – which can occur via
a partially elastic cell wall or by pushing on the solution as by
the plunger of a syringe (compression – a positive pressure) –
we raise its water potential
• Tension is a negative pressure – as if we are pulling the
plunger of a syringe out of the syringe to draw in liquid –
tension lowers water potential
Aquaporins
Aquaporins
• Water can move through transport proteins
known as aquaporins. Aquaporins do not
effect the water potential gradient or direction
of water flow but they effect the rate at which
water diffuses down its water potential
gradient
Vacuolated Plant Cells
Plant cells have three basic compartments.
1. Outside the cell is a thick cell wall that helps maintain the
plant cells shape. It does not regulate the movement of
material in and out of the cell – that is done by the plasma
membrane.
2. The plasma membrane serves as the barrier between the cell
wall and the cytosol – the cytoplasm inside the cell but outside
of the organelles
3. Most mature plant cells have a large vacuole that contains cell
sap. It may occupy 90% of the cell volume. It is surrounded
by a membrane called the tonoplast that regulates traffic
between the cytosol and the cell sap.
Vacuolated Plant Cells
• Most plant cells have openings in the cell walls called
plasmodesmata. The plasmodesmata connect the
cytosol compartments of neighboring cells allowing
easy movement of substances between cells. The
connected cytoplasms of many cells is known as the
symplast.
• The cell walls form a continuum of spaces between
cells. This is known as the apoplast.
Lateral Transport – localized short distance
movement in plant organs
Root with mycorrhizae
Bulk Flow and Xylem Sap
• Once water and minerals reach the xylem via lateral transport
they move up the xylem vessels via bulk flow - the movement
of a fluid driven by pressure.
• Xylem sap flows upward to the veins of leaves due to the
pressure of transpiration – the loss of water vapor from the
leaves and other aerial parts of the plant.
• An early botanical question was whether xylem sap was
pushed up or pulled up the plant.
• Pushing of the xylem sap occurs via root pressure – root cells
expend energy to pump minerals into the xylem. Minerals
accumulate in the xylem sap lowering water potential there.
Thus water flows into the xylem. It can cause guttation,
where water is extruded from pores in leaves.
Guttation
Transpirational Pull
• Xylem sap is pulled up the plant via transpirational pull.
Leaves actually generate the negative pressure necessary to
bring water to them.
• The transpirational pull on xylem sap is transmitted all the way
down the plant – from the leaves, through the stem (shoot) to
the roots. The cohesion of water due to hydrogen bonding
makes it possible to pull a column of sap from above without
the water separating.
• Transpirational pull can only work through an unbroken chain
of water molecules. Cavitation, the formation of a water vapor
pocket in a xylem vessel such as when xylem sap freezes in
winter, breaks the chain.
Water transport potentials
• Dry air can generate a negative water potential of
-100 MPa. Typical water potential of a transpiring
leaf is -1 to -1.5 MPa (negative 10 to 15 atmospheres)
and that potential is transmitted all the way down to
the roots to pull water up
• Note – water will cavitate (develop air bubbles) at
pressures of -0.2 MPa – so how do plants prevent this
from happening? Living phloem and parenchyma
cells near the xylem act as water reservoirs and
actively push water into the xylem to keep interior
xylem pressure around 0 MPa and to repair any
breaks in the water column
Translocation of phloem sap
• The transport of food throughout a plant is known as
translocation.
• Sugar from mesophyll cells in the leaves and other sources
must be loaded before it can be moved. In some species, sugar
moves all the way from mesophyll cells to sieve tube members
via the symplast. In other species, sugars moves by a
combination of symplast and apoplast.
• Often sieve tube members accumulate very high sucrose
concentrations – 2 to 3 times higher than concentrations in the
mesophyll – so phloem requires active transport using proton
pumps.
• At the sink end of a sieve tube, the phloem unloads its sugar.
Phloem unloading is a highly variable process.
Phloem bulk flow
• Phloem moves at up to 1 m/hour – too fast to be by
diffusion. So phloem also moves via bulk flow –
pressure drives it. Pressure flow in phloem comes
about because water flows into the phloem from the
xylem due to the high sugar concentration in the
phloem sap. This sets up a pressure gradient that
drives phloem sap downhill. When the sugar is
unloaded, the water flows out by osmosis and is
recycled via the xylem.
Phloem Movement
• Phloem movement is still not well understood.
We know that is not the rate of photosynthesis
that limits plant growth and crop yields, rather
it is the ability to transport sugars away from
the leaf that limits yield. If we could somehow
increase the rate of sugar movement, we could
probably increase crop yields.