The transport of xylem sap is the most rapid movement

The transport of xylem sap is the most rapid movement of materials in plants.
(b) How does water move to the top of plants? It is either pushed up from the bottom of the plant
or pulled up by the top of the plant. Although plants use both mechanisms, current evidence
indicates that most water is transported through xylem by being pulled to the top of the plant.
B. Water movement can be explained by a difference in water potential
(a) To understand how water moves, it is helpful to introduce water potential , which is defined
as the free energy of water. Water potential is represented by the Greek letter psi and is
important in plant physiology because it is a measure of a cell's ability to absorb water by
osmosis. Recall osmosis is a special kind of diffusion that involves the net movement of water
through a selectively permeable membrane from a region of higher concentration to one of lower
concentration. Water potential also provides a measure of water's tendency to evaporate from
cells.
(b) The water potential of pure water is set to 0 megapascals (MPa) because it cannot be directly
measured. Botanists can measure differences in the free energy of water molecules in different
situations though. When solutes dissolve in water, the free energy of water decreases. Solutes
induce hydration , in which water molecules surround ions and polar molecules, keeping them in
solution by preventing them from coming together. The association of water molecules with
hydrated molecules and ions reduces the motion of water molecules, thus decreasing their free

energy. Thus, dissolved solutes lower the water potential to a negative number. Water moves
from a region of higher (less negative) water potential to a region of lower (more negative) water
potential.
(c) The water potential of the soil varies, depending on how much water it contains. When soil is
moister, its water potential is higher, although it still has a negative value because dissolved
minerals are present in dilute concentrations.
(d) The water potential in root cells is also negative because of the presence of dissolved solutes.
Roots contain more dissolved materials than soil water does, unless the soil is extremely dry.
This means that under normal conditions, the water potential of the root is more negative than
that of the soil. Thus, water moves by osmosis from the soil into the root.
C. According to the Tension-Cohesion model, water is pulled up a stem
(a) In 1986, Irish botanist Henry Dixon proposed the tension cohesion model in order to explain
the ascent of water against the force of gravity. According to the tension-cohesion model , also
known as the transpiration-cohesion model , water is pulled up the plant as a result of a tension
produced at the top of the plant. This tension, which resembles that produced when drinking a
liquid through a straw, is caused by the evaporative pull of transpiration. Transpiration is the
evaporation of water vapor from plants.