5.4 Osmosis• The diffusion of water across a membrane

– Depends on solute concentration inside and outside of cell

– Water moves across a membrane until the concentration of solute is equal on both sides of the membrane

– Type of passive transport

Selectivelypermeablemembrane

Solutemolecule

Lowerconcentration

of solute

H2O

Solute molecule withcluster of water molecules

Net flow of water

Watermolecule

Equalconcentration

of solute

Higherconcentration

of solute

5.5 Water Balance: Tonicity• Tonicity:

o the ability of a solution to cause a cell to gain or lose water

o dependent on the concentration of a solute on each side of the membrane

5.5 Water Balance: Tonicity– Isotonic indicates that the concentration of a solute is

the same on both sides

– Hypertonic indicates that the concentration of solute is higher outside the cell

– Hypotonic indicates a higher concentration of solute inside the cell

Osmosis: Isotonic Solutions

Osmosis: Hypertonic Solutions

Osmosis: Hypertonic Solutions

Osmosis: Hypotonic Solutions

Osmosis: Hypotonic Solutions

Isotonic solution

(B) Lysed (C) Shriveled

(D) Flaccid (E) Turgid (F) Shriveled

Hypertonic solutionHypotonic solution

Plantcell

Animalcell

(A) NormalPlasma

membrane

(plasmolyzed)

5.5 Water Balance: Osmoregulation• Osmoregulation: ability to maintain water balance within cells

– prevents excessive uptake or excessive loss of water

– Plant, prokaryotic, and fungal cells have different issues with osmoregulation because of their cell walls

– Organisms that live in freshwater are constantly pumping water out of their cells because they live in a hypotonic environment.