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Osmosis Plant Physiology 2009 UNI Two ways to move water • Bulk flow • Osmosis • Both move water from high energy to low • Differences – Source of energy difference – What gets moved – Structures Bulk flow • • • • Source of energy difference: Δ pressure Requires open “pipes” Everything in the fluid moves at once Advantages – Move lots of material – Move long distances quickly • Disadvantage – No chance to select what gets moved Examples of bulk flow Xylem sap is being pulled up. Phloem sap is being pushed down. Heart contraction pushes blood out into vessels. Osmosis • Source of energy difference: – Solutes control energy (potential) of water – Pressure also controls energy (potential) of water • Requires selective membrane between compartments – Lets water through – Does not let solutes through • Examples: water movement between adjacent cells – Restoration of turgor to wilted leaves – Rehydration of cells of marathon runner • Advantage – Lets organism move (just) the water • Disadvantages – Slow – Small volumes moved – Short distances Lowering water energy with solutes • Start with pure water – Maximum water energy (potential) • Effect of solutes on water energy – Solutes partly tie up water – Solutes reduce water energy – Can water energy by adding solutes • Solute effect on water energy – 0 if no solutes – Negative if solutes present – More solutes? More negative Example I of simple osmosis air pure water START air pure water LATER Seawater Seawater • Water permeable bag allows water to move but not salts • Water moved – From high energy (pure water) – To low energy – Continues until no water left in bag • No pressure differences between bag and surroundings Example II of simple osmosis air air salty water START salty water LATER fresh water fresh water • Water permeable bag allows water to move but not salts • Water moved – From high water energy (pure water) – To low water energy (water tied up by solutes) – Continues until bag breaks • No pressure differences between bag and surroundings Osmotic movement of water • Continues until – energy of water = in both compartments • then molality (not molarity) will be the same in both – or a pressure difference develops • doesn’t happen with flexible bag, animal cells • does happen with stiff cell walls (coming up) • No energy difference = no net movement Changing water energy with pressure • Start with pure water – Maximum water energy (potential) • Effect of pressure – Can by increasing pressure – Can by lowering pressure • Pressure can be +, 0, • Pressure-induced differences add to solute-induced differences Example I of cell osmosis air typical cell air typical cell LATER START Seawater cell wall Seawater • Water permeable bag allows water to move but not salts • Water moves – From high energy (pure water) to low energy – Until concentration of solutes (water energy) = inside and out • No pressure differences between bag and surroundings • Cell with membrane shrinks away from wall=plasmolysis Example II of cell osmosis air air typical cell START typical cell LATER fresh water fresh water • Water permeable bag allows water to move but not salts • Water moved – From high energy (pure water) to low energy – Continues until pressure effect on water energy = solute effect • Big pressure differences between bag and surroundings • Cell volume hardly changes (pressure does)
