Most natural (and some Man-made) membranes can be thought of as having extremely tiny pores or holes of various sizes; such partially permeable membranes allow some substances to pass but not others, depending on the relative particle sizes.

This effect is responsible for the process which biologists call OSMOSIS.


Osmosis is a special form of diffusion: the MOVEMENT OF WATER from a dilute solution to a more concentrated one through a PARTIALLY PERMEABLE MEMBRANE.

This type of membrane (also called semi-permeable) allows only water, but not other (dissolved) substances to pass through.


All cells are surrounded by a cell membrane, which is partially permeable, and the cytoplasm contains dissolved substances (solutes), e.g. sugars, salts, proteins. Whether or not water enters or leaves the cell depends on the relative concentrations of the cytoplasm and the fluid surrounding them.

It is important to remember that a concentrated solution has a lower concentration of water than a dilute one, or water itself.


Add a third slide to your powerpoint presentation on Elodea & Plasmolysis. Draw two basic cell membranes and add water and solutes to your illustration. Draw in arrows in the boxes to show the movement of water in the case of these two cells placed into two different liquids similar to the illustration above.