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Transport in Plants
By:
Danny Nemeth
Michelle Drabish
Water and Minerals Move
Upward Through the Xylem
Overview of Water and Mineral
Movement Through Plants.
Local Changes Result in the Long-Distance,
Upward Movement Through Plants.
How can water move upward through trees and other
plants when common sense tells us gravity should "pull"
the water down?
• The answer is in the xylem of the plants.
• Water moves though the spaces between the protoplasts
of cells, through plasmodesmata (the connection between
cells), though the plasma membrane, and through the
interconnected, water-conducting xylem elements that
extend through the plant.
• Water enters roots, rises through xylem, and exits through
stomata and leaves.
What helps water move through xylem?
• Water moves the greatest distance through the xylem of
the plant with the help these factors:
o a small "pushing" pressure from water entering roots
o a larger "pulling" pressure cause by water evaporating
called transpiration
 these are made possible by:
 adhesion: where water molecules stick to the
walls of the tracheid or xylem vessel
 cohesion: water molecules stick to each other
o aquaporins: water channels that enhance osmosis
Water Transport at the Cellular Level
While this chapter focuses on the
movement of water through xylem,
the movement of water at the cellular
level plays a large role in bulk water
transport in the plant as well.
Aquaporins are water-selective
pores in the plasma membrane that
increase the rate of osmosis. They
do not alter the movement of water,
however
Water Potential
• Plant biologists often discuss the
forces that act on water within a
plant in terms of potentials.
• There are two components to water
potential:
o physical forces, such as plant cell
wall or gravity, and
o the concentration of solute in each
solution
Pressure Potential:
the turgor pressure, a
physical pressure
that results as water
enters the cell
vacuoles.
Cell walls
exert pressure in the
opposite direction of
turgor pressure.
Solute Potential:
the smallest amount
of pressure needed
to stop osmosis of
the solution
Using the given
solute potentials, you
can determine the
direction of the water
movement
Water Potential:
the sum of its pressure
potential and solute
potential; it represents the
total potential energy of
water in the plant.
Water will move out of the
cell because the water
potential is negative
Transpiration
• Transpiration from leaves,
which creates a pull on the
water columns, indirectly
plays a role in helping water
enter the root cells.
• Water evaporating from the
leaves through the stomata
causes additional water to
move upward in the xylem
and also to enter the plant
through the roots.
Water and Mineral Absorption
The Pathways of Mineral Transport in Roots:
• Minerals are absorbed at the surface of the root, mainly
by root hairs.
• In passing through the cortex, they must either follow the
cell walls and the spaces between them, or go directly
through the plasma membranes and the protoplasts of the
cells, passing from one cell to the next by way of the
plasmodesmata.
• When they reach the endodermis, their further passage
through the cell walls is blocked by the Casparian
strips, and they must pass through the plasma
membrane and protoplast of the cell before entering the
xylem.
More on Water/Mineral Absorption
• In terms of water
• Under certain circumstances,
potential, active transport
root pressure can be so
increases the solute
strong that water will ooze
potential of the roots.
out of a cut plant stem for
hours or even days!
• The result is movement
of water into the plant
• When root pressure is high,
and up the xylem
it can force water up to the
columns despite the
leaves, where it may be lost
absence of transpiration.
in a liquid form in a process
• This phenomenon is
called guttation.
called root pressure
Water and Mineral Movement
• The tensile strength of a water column varies inversely
with the column diameter.
• More than 90% of water taken in by the roots is lost to
the atmosphere through transpiration.
• Plants have evolved to conserve water loss with
dormancy, thick hard leaves, and trichromes.
• Aerenchyma is a tissue that facilitates gas exchange in
aquatic plants.This is mostly used in flooded areas where
oxygen deprivation is common due to standing water.
Stomata
• Stomata open and close due to
changes in turgor pressure
resulting from the active
uptake of potassium.
• Other factors, such as CO2
concentration,light, and
temperature,can also effect
and control stomatal opening.
Dissolved sugars and hormones are
transported in the phloem
• Translocation- carbohydrates manufactured in
leaves are distributed through the phloem to the rest of
the plant.
o
o
o
provides suitable carbohydrate building blocks for the roots
and other actively growing regions of the plant.
studies have shown, using radioactive carbon dioxide, that
sucrose moves both up and down in the phloem.
Phloem also transports plant hormones and mRNA.
Energy Requirements for Transport
• Mass-flow hypothesis- dissolved carbohydrates flow
from a source and are releases at a sink.
 most widely accepted model of how
carbohydrates in solution move through the
phloem.
• Carbohydrates enter sieve tubes by osmosis, and the
resultant increased turgor pressure drives the water
throughout the sieve-tube system.
o At the sink, carbohydrates are actively removed.
FIN!!!
Thank you for putting up with this!
Hopefully you didn't fall asleep! :)