osmosis

Osmosis Coursework


Planning

Aim

The aim of our experiment is to find out how the strength of a sugar solution affects potato cells through osmosis.

Method

1. Cut out potato chips, making sure every piece is skinless, the same length (40mm) and the same diameter (6mm). The length was measured with a ruler and a bore was used to ensure the diameter was consistent.

2. Mix up distilled water and 1.0M sugar solutions, to make mixtures with known molar contents using measuring cylinders.

Volume 1.0M Sugar solution Volume distilled water Molar Content
50.0ml 0.0ml 1.0
37.5ml 12.5ml 0.75
25.0ml 25.0ml 0.5
12.5ml 37.5ml 0.25
0.0ml 50.0ml 0.0

3. Put a single potato chip in with 50 ml of each solution.

4. Leave for sufficient time for osmosis to happen. How long is needed will be determined in a preliminary investigation.

5. After leaving measure lengths of the potato chips to see if this has changed.

6. Calculate percentage length change.

To make this a fair test it is important to ensure that the chips are all identical in size and shape and that the same amount of solution is going is used in each tube. The length of time the chips are in the solution might be an important factor so in comparing results we will only compare tests where the time is the same. Another thing that might vary is the individual potato, so for each set of tests the same potato was used for each solution strength.

The size of the potato chips was chosen to be 4cm long and 6mm diameter because they are completely submerged by 50 ml of fluid in a test tube and will fit in the diameter of the test tubes.

Apparatus

• Multiple test tubes
• Test tube rack
• Scalpel
• Accurate ruler (to mm)
• 3mm radius bore
• Distilled water
• 1.0M Sugar Solution
• Beaker
• Measuring Cylinder

Prediction

Each cell of the potato is surrounded by a semi permeable cell membrane. Semi permeable means that water molecules can travel through the membrane while sugar molecules cannot because they are too large. This will mean that over a period of time water will diffuse through the membrane from a weaker solution to stronger solution. Where the cells are in a weaker solution than that of the cells’ cytoplasm I would expect the cells to loose water by osmosis and the cytoplasm to shrink. Where the concentrations are the other way round I would expect the cells to take on water through osmosis and the cytoplasm to expand.

When we measure the length of the chips this is really a way of measuring the size of all the cells in one go.

Not all cells will be in the solution as they might be in the centre of the potato chip. I don’t think this will affect the experiment because I would expect water to pass from cell to cell in the chip through osmosis.

In the experiment we used sugar in the solution. Sugar is a large molecule in comparison with water and I would expect that it would not be able to diffuse through the semi permeable cell membrane. This means water is able to travel in and out of the potato’s cells while sugar cannot. A strong sugar solution has a lower concentration of water than a weak sugar solution so the water will diffuse from the weak solution to the stronger solution. Eventually all the solution strengths will equalise so that everywhere has an equal strength of sugar solution. When everywhere has the same sugar strength this process stops. This means that given sufficient time the growth there is only a certain amount the chips can grow or shrink.

I predict that if the concent...