osmosis plant physiology 2009 uni. two ways to move water bulk flow osmosis both move water from...
TRANSCRIPT
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
START LATER
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
START LATER
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
STARTLATER
air
cell wall
fresh waterfresh 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
START LATER
air
typical cell