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Chapter 3

BIO204 CH37 - Water and Sugar Transport in Plants - Lecture 3

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University of Toronto Mississauga
Sanja Hinic- Frlog

3 (CH37 - Water and Sugar Transport in Plants) October-05-13 8:04 AM KEY CONCEPT 1 • Water movement along potential energy gradients ○ from areas of high water potential to areas of low water potential • Water’s potential energy in plants is a combination of ○ movement in response to differences in solute concentration ○ the pressure exerted on the water KEY CONCEPT 2 • Plants do not expend energy to replace water lost through transpiration • Instead, water moves from soil and roots to leaves along a water potential gradient ○ Gradient exists because water at air-water surface in leaves is under negative pressure (tension) ○ Negative pressure -> great enough to pull water up from roots through xylem Water loss (Transpiration) is a side effect of photosynthesis • Plant's obtain carbon and release oxygen • Requires opening of stomata • Stomata cannot control loss of water vapor during photosynthetic CO u2take • How do plants replace this lost water? ○ Without replacement, cells will dry out and die ○ Distance between root hairs and leaves  Hybrid poplar 13 - 17 m  Jack pine 16 - 22 m  Douglas fir 60 - 65 m  Redwood 70 - 100 m ○ Movement of water from roots to leaves passive process -> No expenditure of ATP! Water Potential and Water Movement Transpiration - water loss via evaporation from aerial parts of plant - stomata are open - usually during daytime when photosynthesis is occurring - Humidity < 100% -> air surrounding leaves is drier than air inside leaves Water potential (gradient) - potential energy of water in a particular environment compared with potential energy of pure water at atmospheric pressure and room temperature (=0) • Differences in water potential determine the direction that water moves • Water always flows from areas of higher water potential to areas of lower water potential ○ Water potential gradient (Roots to Shoots) - From high (soil & roots) to low (leaves & atm) Solute concentration affects water potential Some general ideas: • Solution - mixture of liquid (often water) and or solutes (ions, proteins, sugars) • Isotonic solution ○ solute concentration = solute concentration cell surrounding • Hypotonic solution (inside the cell) when ○ solute concentration surrounding solute concentration cell More salt = decrease water potential Wall pressure - the force exerted by the wall Turgor pressure - the pressure inside the cell increases until wall pressure is induced Turgid - cells that are firm and experience wall pressure Water potential has two components- Solute potential and pressure potential • Solute potential (Osmotic potential) - tendency of water to move by osmosis ○ Solute potential inside cell = solute potential surrounding solution  No net movement of water ○ Solute potential inside cell < solute potential in surrounding water  Water moves into cell via osmosis • Pressure potential - tendency of water to move in response to pressure ○ Plant cells cannot increase their volume in response to incoming water ○ Cell wall resists expansion How does this apply to plant water potential (Ψ)? • Water potential results from combined effects of pressure potential and solute potential • selectively permeable membranes (e.g. living cell): water moves by osmosis from high solute potential to low solute potential solute potential • no membranes (e.g. Xylem): water moves from high pressure potential to low pressure potential • Water potential results from combined effects of pressure potential and solute potential ○ Water potential (ψ) equals the pressure potential  (ψp) plus the solute potential (s ): ◊ ψ = ψ + ψ p s ○ Water potential is measured in megapascals (MPa) ○ Describes the pressure per unit area Gravity plays a role in comparing large plant's to a small plant' water potential-> not going to discuss in this lecture When cell has reached max amount of water, cell wall prevents further uptake Flaccid - cell has no turgor pressure (pressure potential = 0 ) From cells to entire plants: Water Potentials in Soils, Plants, and the Atmosphere • Similar to single cells, water in tissue, roots and shoot systems, and entire plants have a pressure potential and a solute potential • Air and soil also have water potentials • A water potential gradient exists between the soil, plants, and atmosphere • Plants gain water from soil and loose it to atmosphere When do plants wilt? • Water potentials are dynamic! ○ Water potentials in plant tissues, air, and soil change in response to weather • Transpiration = Loss of water in cells of leaf or stem to the atmosphere ○ When loss exceeds replacement -> plasma membranes
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