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.
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