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Topic 9 Plant
Biology
9.1 Transport in the Xylem
Nature of Science
Use models as
representations of the real
world: mechanisms
involved in water transport
in the xylem can be
investigated using
apparatus and materials
that show similarities in
structure to plant tissue.
Transpiration

Transpiration is the inevitable
consequence of gas exchange in the
leaf.
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Primary organ of photosynthesis in plants =
Leaves
Stomata = pores in the underside of leaves for
gas exchange
If stomata allow carbon dioxide to be
absorbed, they will usually also allow water
vapor to escape.
Transpiration = the loss of water vapor from the
leaves and stems of plants
Plants can minimize water losses through
stomata using guard cells.
Guard cells = cells that open and close
stomata
Xylem structure helps withstand low pressure
• The cohesive property of water and the structure of the xylem vessels
allow transport under tension.
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Xylem = long continuous tubes (vascular tissue in a plant) that helps them transport
water.
The structure of xylem vessels allows them to transport water inside plants very
efficiently.
Xylem vessels are long, continuous tubes.
Xylem walls are thickened, and the thickenings are enhanced with a polymer
called lignin, which strengthens the walls, so that they can withstand very low
pressures without collapsing.
Xylem vessels are formed from files of cells, arranged end-to-end. When mature,
xylem cells are non-living, so the flow of water must be a passive process.
Water molecules are POLAR and the partial negative charge on the oxygen atom
in one water molecule attracts the hydrogen atom in a neighboring water
molecule (COHESION).
Water is also attracted to hydrophilic parts of the cell walls of xylem (ADHESION).
As a result of the connections between the molecules, water can be pulled up
from the xylem in a continuous stream.
Tension in leaf cell walls maintains the
transpiration stream
 The adhesive property of water and evaporation generate tension forces in leaf cell
walls.
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When water evaporates from the surface of the wall in a leaf, adhesion
causes water to be drawn through the cell wall from the nearest
available supply to replace the water lost by evaporation. The nearest
available supply is the xylem vessels in the veins of the leaf.
Even if the pressure in the xylem is low, the force of adhesion between
water and cell walls in the leaf is strong enough to suck water out of the
xylem, further reducing its pressure.
The low pressure generates a pulling force that is transmitted through the
water in the xylem vessels down the stem and to the ends of the xylem in
the roots = TRANSPIRATION-PULL
Passive process for the plant. All the energy needed comes from thermal
energy (heat) that causes transpiration.
The pulling of water upwards in xylem vessels depends on the cohesion
that exists between water molecules.
Active Transport of Minerals in the roots
 Active uptake of mineral ions in the roots causes absorption of water by
osmosis.
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Water is absorbed into root cells by osmosis.
Happens because the solute concentration inside the root cells is greater than that in the
water in the soil.
Most of the solutes in both the root cells and the soil are mineral ions.
Concentration of mineral ions in the root can be 100 times greater than those in the soil.
These concentration gradients are established by active transport, using protein pumps in
the plasma membrane of root cells. There are separate pumps for each type of ion the
plant requires.
Mineral ions can only be absorbed by active transport if they make contact with an
appropriate pump protein. This can occur by diffusion, or by mass flow when water
carrying the ions drains through the soil.
Some ions move through the soil very slowly because ions bind to the surface of soil
particles. To overcome this problem, certain plants have developed a relationship with
fungus. The fungus grows on the surface of the roots and sometimes even into the cells of
the root. The thread-like hyphae of the fungus grow out into the soil and absorb mineral
ions such as phosphate form the surface of soil particles. These ions are supplied to the
roots, allowing the plant to grow successfully in mineral-deficient soils. (mutualistic
relationship)