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

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