Paramecium is a single-celled organism that lives in pond water.

If too much water went into a Paramecium cell, it would eventually burst.

Inside the Paramecium cell is a structure called a contractile vacuole. This collects the excess water that enters the cell, and then empties this water out.

Paramecium

1. Would you expect there to be a higher concentration of dissolved molecules in the pond water or in a cell?

2. Use your answer to 1 to explain why you would expect water to enter a Paramecium cell.

Look at the Paramecium video clip. Watch the contractile vacuole filling and emptying. Ask if you’re not sure which part it is.

Time how long it takes for the contractile vacuole to fill completely.

3. If you put a Paramecium into distilled water, would you expect the contractile vacuole to fill more quickly or more slowly? Explain your answer.

4. Some single-celled organisms very similar to Paramecium live in the sea. They do not have contractile vacuoles. What does this suggest about the concentration of the solution inside their cells compared with the concentration of the sea water? Explain your answer.

5. Diagram A shows a magnified human red blood cell. Estimate the diameter of this cell. Give your answer in micrometres (µm). Note that 1 mm = 1000 µm.

6. Diagram B shows a magnified human red blood cell which has been in a salt solution.

a Describe the changes to this cell.

b Use your knowledge of osmosis to explain why it has changed shape.

7. A student put some red blood cells in distilled water for one hour. When he looked at a drop under a microscope, he saw

 a pale yellowish-red liquid

 no cells

Use your knowledge of osmosis to explain what might have happened to the membranes of these cells.