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

Seawater pure water Seawater pure water Example I of simple osmosis 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 air STARTLATER air

fresh water salty water fresh water salty water Example II of simple osmosis 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 air STARTLATER air

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

Seawater typical cell Seawater typical cell Example I of cell osmosis 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 air START LATER air cell wall

fresh water typical cell Example II of cell osmosis 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) air STARTLATER air typical cell