Diffusion of Diffusion of waterwater across a across a membranemembrane

Moves fromMoves from HIGH water HIGH water potentialpotential (low (low solute) tosolute) to LOW LOW water potentialwater potential (high solute)(high solute)

High H2O potentialLow solute concentration

Low H2O potentialHigh solute concentration

Water Channels Protein pores used during

OSMOSISWATERMOLECULES

10% NaCL90% H2O

CELLCELL

10% NaCL90% H2O

ENVIRONMENTENVIRONMENT

NO NET MOVEMENTNO NET MOVEMENT

What is the direction of water movement?The cell is at equilibrium!

CELLCELL 20% NaCL80% H2O

10% NaCL90% H2O

What is the direction of water movement?

CELLCELL5% NaCL95% H2O

15% NaCL85% H2O

ENVIRONMEENVIRONMENTNT

What is the direction of water movement?

Isotonic Solution Hypotonic Solution Hypertonic Solution

NO NET MOVEMENT OF H2O (equal

amounts entering & leaving)

CYTOLYSIS

PLASMOLYSIS

Cytolysis Plasmolysis

Isotonic Hypotonic

HypertonHypertonicic

Because the membrane allows only molecules of a certain size to diffuse through it, it is called selectively permeable.

The cell membrane functions as a selectively permeable membrane.

The cell sap and cytoplasm function as fairly concentrated solutions.

There is a greater concentration of free water molecules outside the cell than inside

so water diffuses into the cell by osmosis and the cell swells up.

If osmosis continued the animal cell would burst. This would be bad news for animals. Consequently there are processes in the animal’s

body which control osmosis. Mainly, this is done by keeping the concentration

of body fluids outside the cell the same as it is inside.

In a plant cell, the cell membrane acts as a selectively permeable membrane.

The cell wall is freely permeable to water. The vacuole contains a solution of salts and

sugars. If there is water outside the cell, it will diffuse by

osmosis into the vacuole. The vacuole will expand, pushing the cytoplasm

outwards against the cell wall.

The cellulose cell wall cannot stretch, so the vacuole cannot continue to expand.

But the pressure of the vacuole against the cell wall makes the cell very firm.

A plant cell in this condition is said to be turgid. If all the cells in the plant are turgid, the plant

will be firm and upright and the leaves would be expanded.

cell wall vacuole

cytoplasm and cell membrane

The cell absorbs waterby osmosis .... but the cell wall stops the

cell expanding any more