Chapter 28.128.4Transport in Plants
28.1a Short Distance Transport mechanisms move molecules across plant
Two mechanisms for transporting molecules across a plasma membrane is passive transport and
Passive transport moves substances with their concentration gradient which means that the cells
do not need to expend energy.
Passive includes both transport of simple diffusion (nonpolar molecules and small polar molecules)
and facilitated diffusion (transport of polar and charged molecules that move across the membrane
via transport proteins).
Active transport moves ions and large molecules across membranes via transport proteins, since
solutes are being moved against their concentration gradient, cells must expend energy.
The energy for active transport can be provided either by ATP hydrolysis or by harnessing the
energy in concentration gradient.
Cells gain and lose water by osmosis, the passive transport of water across a selectively
permeable membrane either by simple diffusion or by facilitated diffusion through waterconducing
channel proteins called aquaporin’s.
Whether water will move into or out of a given cell is determined by water potential
28.1b The relationship between Osmosis and Water Potential
Water potential is measured in megapascals(MPa)
Factors that determine water potential are the presence of solutes and physical pressure.
The effect of dissolved solutes on water potential is called solute potential.
When solutes are added to water, they disrupt some of the hydrogen bonding between water
molecules and the polar water molecules interact with the solutes, forming a hydration bonding
between water molecules, and the polar water molecules interact with the solutes, forming a
hydration shell that surrounds the solute molecules.
Water potential is lower in a solution with more solutes than in pure water while solution containing
solutes will have values less than zero.
The relationship between water potential and solute potential is vital to understanding transport in
plant because water movies by osmosis.
Pressure can also change how water moves.
If we exert a positive pressure on the water, giving it more energy than the water in the other side of
utube, causing water to flow to that side. If we pull on the plunger, reducing its energy, pulling water from the other side of the utube to the
side under tension.
28.1c Osmosis in Plant cells creates Turgor Pressure, which is necessary for plant support
most of the volume of a mature plant cell is occupied by a large central vacuole, which is
surrounded by a vascular membrane(tonoplast).
Cells that enter a plant cell are transported from the cytoplasm into the central vacuole through ion
channels in the tonoplast.
The pressure of waterfilled vacuole and cytoplasm against the wall keeps the cell firm known as
As long as the water potential of soil is higher than that of the root epidermal cells, water will follow
the water potential gradient and flow into root cells, making them turgid.
If soul around the roots dries out, the soul water potential becomes more negative than that of root
epidermal cells and water no longer flows into the roots from soil.
When turgor pressure in the cells of leaves and stems drops to very low levels, it is called wilting.
28.2a Water travels across the root to the root xylem by two pathways
Minerals can either travel by two paths: across the root to the xylem, either through interconnected
cytoplasm of living cells or through cell walls and intercellular spaces. Living cells make up the symplast (inner side of the plasma membrane) and are interconnected by
plasmodesmata, allow water to flow from the cytoplasm of one cell to the next via the symplastic
The continuous network of cell walls and spaces between cell makes up the nonliving areas of the
root or the apoplast.
Water moves through the apoplastic pathway as it flows through these nonliving spaces without
crossing plasma membranes.
When water enters a root some diffuses into the symplast but most enters into the apopplast.
Apoplastic water travels inward until it encounters the endodermis, the innermost layer of the
Endodermal cells are tightly packed.
Each endodermal cell also has a ribbonlike Casparian strip in its radical and transverse walls
The Casparian strip is impregnated with suberin, a waxy substance that is impermeable to water
and blocks the apoplastic movement of water at the endodermis.
Once the water reaches the casparian strip, they are forced to detour from the apoplast, moving
across the plasma membrane of endodermal cells and into the symplastic pathway.
Finally, the water then pass through plasmodesmata in the outer layer of stele.
The casparian strips allow the endodermis to control which substances enter and leave a plant’s
28.2b Roots take up ions by Active Transport
Ions can enter the symplast immediately and travel to the xylem via the symplastic pathway or they
can move inward following the apoplastic pathway until they reach the casparian strip if the
Once the ion reaches the stele, it diffuses from cell to cell until it is loaded into the xylem.
28.3 LongDistance Transport of Water and Minerals in the xylem.
How does the solution of water and minerals called xylem sap move up?
Inside a plant’s tubelike vascular tissue, a large amounts of water travel by bull flow the mass
movement of molecules in response to a difference in pressure between two locations
The driving force for the upward movement of xylem sap is the evaporation of water from leaves
and other above ground parts of land plants.
Transpiration is able to pull xylem sap upward through the plant body because of the cohesion of
water molecules and the tension created by the evaporation of water from plant surfaces.
23.a The properties of water play a key role in its transport
Two properties of water which are important in water movement in the xylem:
Water molecules are strong cohesive: they tend to form hydrogen bonds with one another.
Water molecules are adhesive: they form hydrogen bonds with molecules of other substances,
including carbohydrates in plant cell walls.
Both these properties pull water molecules into small spaces.
Henry Dixon of xylem transport is now called the cohesiontension theory of water transport. According to the theory, water transport begins as water evaporates from the walls of mesophyll
cells inside leaves and the intercellular spaces.
When a water molecule moves out of a leaf vein into the mesophyll, its hydrogen bonds with the
next molecule in line stretch but don’t break.
The stretching creates tension a negative pressure gradientin the column,
Adhesion of the water column adds to the tension.
Under tension, the entire column of water pulls upward.
As the soil dries, the remaining water molecules are held tightly; this tension pulls the water