Osmosis

Question 1

What is the definition of osmosis?

Answer 1

Osmosis is the net movement of water molecules across a partially permeable membrane, from a region of higher water concentration, to a region of lower water concentration.

Question 2

What is meant by “Water Concentration”?

Answer 2

The amount of water, as compared to other molecules (like sugars or salts) that are dissolved in water. We call these dissolving molecules ‘solutes’.

Note that it’s the proportion of water:solutes that determines concentration.

Question 3

How does osmosis apply to cells?

Answer 3

Water particles will diffuse through the cell’s partially permeable membrane from a region of higher water concentration to a region of lower water concentration.

Question 4

How do you investigate osmosis in a living system?

Answer 4

Potato Cylinders

1. Cut a potato into identical cylinders, and get some beakers with different sugar solutions in them. One should be pure water, another should be a very concentrated sugar solution. Then you can have a few other others with concentrations in between.
2. You measure the length of the cylinders, then leave a few cylinders in each beaker for half an hour or so. Then you take them out and measure their lengths again.
3. If the cylinders have drawn in water by osmosis, they’ll be a bit longer. If water has been drawn out, they’ll have shrunk a bit. Then you can plot a few graphs.

Question 5

How do you investigate osmosis in a non-living system?

Answer 5

Visking Tubing

Visking tubing is a partially permeable membrane.

1. Fix some Visking tubing over the end of a thistle funnel. Then pour some sugar solution down the glass tube into the thistle funnel.
2. Put the thistle funnel into a beaker of pure water - measure where the sugar solution comes up to on the glass tube.
3. Leave the apparatus **overnight*, then measure where the solution is in the glass tube. Water should be drawn through the Visking tubing by osmosis and this will force the solution up the glass tube.