Osmosis and Potatoes!

What is Osmosis ? Osmosis is the movement of water molecules, across a partially permeable membrane, from a region of high concentration of water to a low concentration of water molecules. Text book definition

Osmosis has the following uses: What is Osmosis? Osmosis has the following uses: It allows plants to take in water through their roots. It is used in kidney dialysis machines to filter the blood.

What is a partially permeable membrane? Remember!! A membrane controls what goes in and out of the cell. Permeable means it allows substances to go through it. A partially permeable membrane is a membrane with holes in it small enough for only WATER to go through it. Big molecules like glucose can not fit through See next slide…

A partially permeable membrane Water Sugar Small molecules, such as water can pass through it. Large molecules like sugar can not pass through the membrane Diagram shows the net flow of water

Osmosis - what is actually happening ? Water Sugar The water molecules pass both ways through the membrane The overall flow (net flow) of water molecules is from a region with lots of water molecules to region of less water molecules. Lots of water molecules Few water molecules The water moves into the sugar - rich region to fill it up with water. The water moves as if to even up the concentration either side of the membrane.

Summary OSMOSIS is the movement of water molecules across a partially permeable membrane. The membrane only allows small molecules, e.g. water to pass through it. Large molecules e.g. glucose can NOT pass through the membrane. Water molecules move into the region where there are less water molecules The water acts to even up the concentration of water either side of the membrane.

Osmosis and Potato Investigation Using the above facts you should now be able to understand what happens in the potato investigation W What happens here? Pure Water Potato In pure water the potato tubes swell, because water enters their cells by osmosis. Remember, potatoes are rich in starch – which is a complex sugar.

What happens here? Sugar Solution Potato In rich sugar solution the potato tubes shrink, because water leaves the potato by osmosis.

Hope you have got the idea now... If you would like to go over some of the points again , choose one of the options below. BACK TO START What is Osmosis? What is a partially permeable membrane? A partially permeable membrane Osmosis – what is actually happening? Summary Potato tubes in pure water Potato tubes in sugar – rich water