BIOSC 0100 Lecture Notes - Lecture 8: Water Potential, Transpiration, Phloem

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6 Feb 2017

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Chapter 38
-Transpiration is water loss via evaporation from the aerial parts of a plant
-Transpiration occurs when: stomata are open, air surrounding leaves is drier than the air inside leaves
Water Potential:
-The potential energy that water has in a particular environment compared to the potential energy of
pure water at room temperature and atmospheric pressure
-Ψ for pure water = 0
-Water always flows from areas of high water potential energy to areas of lower water potential
-Two factors that affect water potential: solute potential and pressure potential
Solute Potential:
-The potential of water to move by osmosis
-Presence of solutes decreases the solute potential below that of water
-As solute concentration increases, solute potential becomes lower
-Pure water has no solutes
-Will always be negative because they are measured relative to pure water
Pressure Potential:
-Tendency of water to move in response to pressure
-In the absence of pressure, water moves from areas of high solute potential to low solute potential
-The potential energy of water in a particular location is the sum of the pressure potential and the solute
potential that it experiences
Water Potential Gradient
-Exists between soil, plants and the atmosphere
-Water moves along a steep water-pressure gradient from root to shoot in vascular tissue called xylem
-The other type of vascular tissue in plants is phloem
-Phloem conducts sugars and other substances in two directions: from roots to shoots and from shoots
to roots
-Movement of sugars through a plant from sources to sinks
-Source: a tissue where sugar enters the phloem
-Sink: a tissue where sugar exits the phloem
-Sugar (sucrose) concentrations are high in sources and low in sinks
-Made of two specialized cell types: sieve tube elements and companion cells
-Sieve-tube elements are connected to one another, end to end, by perforated structures called sieve
-The pores in these plates create a direct connection between the cytoplasm of adjacent cells
Pressure-Flow Model:
-Explains how sugars are moved through the phloem
-Phloem sap moves down a water-potential gradient created by changes in pressure potential
-Differences in turgor pressure in the phloem near source tissues and the phloem near sink tissues
generate the forms
-Generating the pressure differences requires ATP
-High turgor pressure in the phloem at the source is created by movement of sugars into phloem
(phloem loading)
-There is a one-way flow of sucrose and a continuous loop of water movement, as water is supplied to
and from the xylem
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