Download Transport in Plants – Chapter 38

Plant Transport
• How does water get from
the roots of a tree to its
top?
• Plants lack the muscle
tissue and circulatory
system found in animals,
but still have to pump
fluid throughout the
plant’s body
Plant Transport
• Water first enters the roots and then
moves to the xylem, the innermost
vascular tissue
• Plants need water
– As a starting product for photosynthesis
– As a solvent to dissolve chemicals
– For support
– To ‘pay’ for water lost by transpiration
Plant Transport
• Heartwood is the
xylem that has died;
much darker
• Sapwood is the
younger, outermost
wood that has not yet
become heartwood;
conducts water from
the roots to the leaves,
and to store
Plant Transport
• Water movement (transport) occurs at
three levels:
– Cellular
– Lateral transport (short-distance)
– Whole plant (long-distance)
Plant Transport: Cellular level
• Diffusion – movement from an area of high
concentration to an area of lower
concentration
• Plays a major role in bulk water transport,
but over short distances
• Although water diffuses through cell
membranes, ions and organic compounds
rely on membrane-bound (protein)
transporters
Plant Transport: Cellular level
• Some protein transporters form channels
that allow molecules to diffuse (passive
transport)
• Others require energy to move minerals
and other nutrients against a concentration
gradient (active transport)
Plant Transport: Cellular level
• Proton pumps hydrolyze ATP and use the
release energy to pump hydrogen ions
(H+) out of the cell; makes a proton
gradient that is higher in H+ outside of the
cell ( membrane potential)
• Makes the inside of a cell more negative
than the outside, driving the transfer of
positive ions (e.g., K+ ions)
Plant transport: Cellular level
• Osmosis – passive transport
– Transport of water (and its solutes) across a
semi-permeable membrane
• Unlike animal cells, plants have cell walls
and this affects osmosis
• Water potential, Ψ (Greek letter psi), is used
to predict which way water will move
• Water will move from solution with
higher Ψ to a solution with lower Ψ
Water Potential, Ψ
•
•
•
•
Water moves from higher Ψ to lower Ψ
The addition of solutes lowers Ψ
Increasing pressure raises Ψ
In essence, Ψ measures the ability of soil
water to move (into or out of the plant)
• Low osmotic concentration = high Ψ
• High osmotic concentration = low Ψ
Water Potential, Ψ
• If a single plant cell is placed into water,
then the concentration of solutes inside
the cell is greater than that of the external
solution, and water will move into the cell
by the process of osmosis
• The cell expands and presses against the
cell wall, a condition known as turgid
(swollen), due to the cell’s increased
turgor pressure
Water Potential, Ψ
• Ψ is measured in units of pressure
• Pure water at standard temperature and
pressure has a Ψ of zero
• The addition of solutes to water lowers its
Ψ (makes it more negative), just as an
increase in pressure makes it more
positive
• Water will move from higher Ψ to lower Ψ
Water Potential, Ψ
• Water will spontaneously flow
from a high potential to a low
potential, like a ball rolling
down a hill
• Ψ are usually negative
• Ψ is measured as pressure
potential, ΨP and solute
potential, ΨS
• The total potential energy of
water in the cell = Ψ+P + ΨS
http://www.steve.gb.com/science/water_potential.html
• Pressure potential, refers to the turgor
pressure resulting from pressure against
the cell wall
• Water pressure also arises from an
uneven distribution of a solute on either
side of a membrane, which results in
osmosis
• Solute potential, ΨS describes the smallest
amount of pressure needed to stop
osmosis
• Water flows from a solution with the less
negative ΨS to the more negative ΨS
A watered plant regains its turgor
Water Potential, Ψ
• Water potential of the
soil is negative, but not
as negative as the cell,
due to the high content
of solutes
• Water moves from high
(less negative) to low
(more negative) water
potential
http://www.steve.gb.com/science/water_potential.html
Plant Transport: Long distance
• Evaporation of water in
a leaf creates a
negative pressure
(negative water
potential) in the xylem,
which literally pulls
water up the stem from
the roots
http://www.steve.gb.com/science/water_potential.html
Plant Transport: Long distance
• Water moves from the soil
into the roots only if the
soil’s water potential is
greater
• It then moves along
gradients of successively
more negative water
potentials in the stems,
leaves and air
Water and Mineral Absorption
• Most of the water absorbed the plant
comes in through root hairs – extensions
of root epidermal cells
• Root hairs are almost always turgid,
because their water potential is greater
than that of the surrounding soil
• Collectively, have enormous surface area
• And don’t forget the mycorrhizae…
Root tips
Water and Mineral Absorption
• Minerals are absorbed at the root hair
• Minerals may either follow the cell walls or
spaces in between them, or go directly
through the plasma membrane of the cells
• They will, however, eventually reach the
endodermis, where their entry is blocked
by casparian strips – a waxy material
that surrounds endodermal cells, before
reaching the xylem
Water and Mineral Absorption
• Apoplast route – movement through cell
walls and the spaces between cells
• Symplast route – cytoplasm continuum
between cells
• Transmembrane route – membrane
transport b/w cells and across membranes
of vacuoles within the cells; provides the
greatest control over which substances
enter and leave
Casparian strip
• Transport into the endodermis is selective
• Passage through the cell walls blocked by
casparian strips
• Substances must enter the cells of the
endodermis in order to pass into xylem
– Allows selectivity
Root
hair
Xylem
• Xylem sap brings minerals to leaves and
water to replace what is lost by
transpiration
• Moves at rates of 15 meters/hour; travels
vertically up distances of 100 meters in the
tallest trees
• At night, when transpiration is low or
absent, root pressure caused by the
accumulation of ions in the roots, causes
more water to enter the root hair cells by
osmosis
Xylem
• Under certain circumstances, root
pressure is so strong that water will ooze
out of a cut plant stem for hours or even
days
– When root pressure is very high, it can force
water up to the leaves, where it may be lost
(guttation)
– Guttation produces what is more commonly
called dew on leaves
Phloem
• Carbohydrates manufactured in leaves
and other green parts are distributed
through the phloem to the rest of the plant
– Translocation
– Phloem sap consists primarily of sucrose
(30%), as well as hormones, amino acids, and
minerals
– Phloem sap travels from sugar sources to
sugar sinks (non-green parts, growing shoots
and roots, and fruits)
Maple syrup is sap!
• Sap in maple trees remains frozen
during the winter
• Begins to flow again when
weather warms, and is triggered
by cold nights and warmer days
• A hole is tapped into the tree
allowing sap to drain
• Sugar maples have the greatest
amount of sugar in the sap;
produce 20 gallons of sap (=2
quarts of syrup)
Maple syrup is sap!
• The sap that
produces maple
syrup flows through
the sapwood; the
living portion of the
xylem
Just in case you’re not starving
yet…