When you submerge a plant cell in water, water flows through the cell wall and cell surface area into the vacuole from the outside. As a result, the cell swells up, but does not burst. This is because the cell wall stops the cell expanding too much.

Introduction

When you submerge a plant cell in water, water flows through the cell wall and cell surface area into the vacuole from the outside. As a result, the cell swells up, but does not burst. This is because the cell wall stops the cell expanding too much. When this point is reached we say that the cell is fully turgid. This type of diffusion is called osmosis, and takes place through a partially permeable membrane. The water moves from a dilute solution to a concentrated solution. This stops when the levels of water in each solution are equal. The experiment we are about to do will prove that osmosis takes place, and that it takes place in a living cell. We will then attempt to find out the percentage of sugar within a potato.

Preliminary Experiments

Viskin tubing is a special kind of membrane, as it has millions of tiny holes in it. The experiment that we are going to do will show how a special kind of diffusion, called osmosis occurs through a membrane that lets small molecules through but not large molecules. This type of membrane is known as a semi permeable membrane, or a partially permeable membrane.

The following experiment will show that osmosis takes place.

Plan

I shall follow the following plan to complete the experiment:

Tie one end of a viskin tube to make a bag.
Fill the bag with 20% sucrose solution.
Push a capillary tube into the end of the bag, and tie the bag to the bottom of it.
Clamp the capillary tube to a stand and lower the bag into a beaker of water.
Mark the level of the sucrose solution in the capillary tube. Re-measure this level every five minutes for half an hour.

The level or water in the capillary tube will increase. This is because water moves from a dilute solution to a concentrated solution. There is a larger amount of water in the beaker, than in the viskin tube so water moves from the beaker in to the viskin tube via the partially permeable membrane. The movement of the water will stop when the amount of water in both the beaker and viskin tube is equal.

The following diagram shows how water molecules ...