• Experiment showing osmosis

• What is osmosis?

• What is water potential?

• Water potential of sucrose solution

• Water movement across a differentially permeable membrane

• Osmosis and plant cells

• Osmosis and red blood cells

initial mark

initial mark

final mark

Experiment showing osmosis

30 minutes later

What is osmosis?• The net movement of water molecules

across a differentially permeable membrane from a less concentrated solution to a more concentrated solution

• The net movement of water molecules across a differentially permeable membrane from a region of higher water potential to a region of lower water potential

What is water potential?• The probability of water molecules moving through a membrane from each side of the differentially permeable membrane

• Value as the concentration of solutes (i.e. as the concentration of water molecules )

• Distilled water: water potential = 0greatest water potential

• Solutions: water potential < 0 (-ve value)

0

Water potential of sucrose solution

0

+

-

Water potential (arbitrary unit)

Concentration of sucrose solution (M)

net movement of water moleculeswater molecule sucrose moleculelow water potential high water potential

Water movement across a differentially permeable membrane

Osmosis and plant cells

• distilled water

in distilled water for 30 minutes

When young plant cells are immersed in

they become turgid, bigger and heavier

in concentrated sucrose solution for 30 m inutes

water out

water out

water out

Osmosis and plant cells

• concentrated sucrose solutionWhen young plant cells are immersed in

they become soft, smaller and lighter

in distilled water for 30 m inutes

cell bursts, cell content is released

Osmosis and red blood cells

• distilled water

When red blood cells are immersed in

they burst and release red pigments

Osmosis and red blood cellsWhen red blood cells are immersed in

in concentrated sucrose solution for 30 minutes

water out

water out

water out

• concentrated sucrose solutionthey shrink

Summary• Osmosis: the net movement of water

molecules across a differentially permeable membrane down a water potential gradient

• Effect of osmosisWhen put in a solution with water potential

> cell = cell < cell

Plant cells turgid no change flaccid

Animal cells swell,may burst

no change shrink