At point B (high water concentration), there is no indication that the cell is increasing further in size. This is because the cell is fully turgid and no more water can enter.
At point C (low water concentration), there is no indication that the cell is decreasing further in size. This is because the cell is fully plasmolysed and no more water can leave the cell.
Plan
- I intend to fully immerse 3 cm lengths, with a diameter of 1 cm², of raw potato chips in 15cm³ of sucrose solutions of varying concentration.
- I will measure the mass of the raw potato chip before and after the immersion. I will time it for 10 minutes from when I drop the raw potato chip in to the boiling tube containing the sucrose solution.
- I am going to repeat the experiment three times as I intend to record the average mass in grams, to 2 decimal places i.e. 4.00g, and not take just one result for each solution.
- By repeating the experiment and taking the average results, the more accurate my results will be.
Introduction
The movement of Osmosis through a selectively permeable membrane,
of raw potato chips.
Before starting the investigation, I decided to do some research about Osmosis.
Osmosis is the movement of water through a semi permeable membrane, separating solutions of different concentrations. The water passes from a region of high concentration to a region of low concentration, until the two concentrations are equal in concentration of water.
Many cell membranes behave as semi permeable membranes, and Osmosis is a vital mechanism in the transport of fluids in living organisms, for example, in the transport of water from the soil to the roots of plants.
If a cell is in contact with a solution of lower water concentration than its own contents, then water leaves the cell by Osmosis, through the cell membrane. Water is lost first from the cytoplasm, then the vacuole through the tonoplast. The living contents of the cell contracts and eventually pulls away from the cell wall and shrinks, this is known as Plasmolysis.
If you put a plant cell in water, water enters by Osmosis then swells up. However the cell will not burst. This is due to the fact that the cell walls are made from cellulose, which is extremely strong. Eventually, the cell stops swelling, and when this point is reached, we say the cell is turgid. This is important, because it makes plant stem strong and upright.
Preliminary Work
In a sense, preliminary work is as important as the actual experiment. It allows you to make prediction before performing actual tests.
For my preliminary work, I used 3cm (30mm) and a diameter of 1cm of raw potato. It was easy to cut 3 cm of raw potato and get it around 1g. To begin we used 15mls of sucrose solution – this was the amount needed to cover one piece of raw potato. The first thing we needed to observe was how different concentration affected the weight and length of the raw potato chip. We therefore needed to observe each end of the spectrum. We tried 0 molar concentration (the lowest) and 1 molar (the highest used).
We then left each test for 5 minutes, but found after that time, hardly any or no reaction had taken place. It was then we gave our opinion that we should leave the experiment going for at least double that time.
Again we used the same dimension of raw potato chip, concentration and amount, but changed the length of time the tests were left out for. This time we left them for 10 minutes and saw that the results we collected were much easier to compare because there was more of a range.
Safety
Safety is a very important part of all experiment done in the laboratory. The things that you will need to do to keep your experiment a safe one for yourself and other students around you are as follows:
- OBEY ALL LABORATORY RULES
- Take care in using glassware since it is sharp when broken and can cut skin.
- Take care when returning all glassware and equipment at the end of the experiment.
- Wash all your equipment after your experiment.
- Don’t leave any thing lying around (e.g. Boiling tube Bench on the table, it might roll off).
- Make sure all the boiling tubes are in the rack.
- Listen to all instructions that are given.
- Use the knife carefully to cut the pieces of raw potato to accurate size. (Do not mess around with the knife, as it is very dangerous).
Fair test
To ensure that the experiment is done fairly I will consider the following factors:
- Make sure the raw potato pieces are all of equal lengths (3 cm) and diameter (1 cm).
- All three experiments are done together so all are done fairly under the same conditions.
- Use the same type of raw potato (different types of potatoes may give different results.
- Use an accurate volume of sucrose solution.
- Keep the volume of the sucrose solution the same (if different volumes are used then it will effect the osmosis).
- Use the same equipment all the way through the experiment.
- Use accurate weighing scale (make sure it reads 0.00gm prior to weighing each time), and write the results to 2 decimal places.
- Weigh consecutively so there is less chance of mistakes.
- Do all experiments in the same order (following the method).
- Wipe the excess sucrose solution off the potato, after the raw potato chip has been brought out of the boiling tube using a paper towel.
- Make sure the raw potato chip is kept in the boiling tube for the same amount of time.
Safe test
To ensure the experiment is done safely I will consider the following:
- Follow all laboratory rules.
- Use the knife carefully to cut the pieces of raw potato to accurate size.
Apparatus required to carry out the experiment
The apparatus used during the experiment was as follows:
- Sucrose solutions. (Three lots of sucrose solutions of 0.0, 0.2, 0.4, 0.6, 0.8, 1 molar)
- Raw potato chips (length of 3cm, diameter of 1cm).
- Chip machine.
- Top pan balance.
- Boiling tubes (×3).
- Ruler.
- Test tube rack.
- Measuring cylinder (25ml³).
- Knife.
10. Labels.
11. Paper towels.
12. Tile.
13. Stop watch.
14. Table for result.
Method
Make sure you understand everything before starting the experiment - read through the method a couple of times.
I am going to go through the experiment step by step.
After having done the preliminary work, I knew exactly which measurements I needed to use in order to obtain the correct sort of results.
Diagram 1.1
- Collect all equipment.
- Place three boiling tubes in a test tube rack (Set it out like in diagram 1.1).
- Measure the sucrose that you are using e.g. 0.8 molar, with the measuring cylinder.
- Pour in 15ml of the sucrose solution (that you are using e.g. 0.8 molar) in all three boiling tubes.
- Label the boiling tubes with numbers 1, 2 and 3.
- Cut the raw potato into equal diameter of chip using the chip machine (1cm²).
- Place three pieces of raw potato chips on a tile.
- Cut them to 3cm in length, using a knife and ruler. Do not use any chip which still has the skin on.
- Place one chip next to the first boiling tube, another chip to the second boiling tube and another chip to the third boiling tube.
10. Take care not to mix the raw potato chips around.
11. Make a table where you can record your result (see
Diagram 1.2)
- Make sure you know how to use the top pan balance (make sure that no one is leaning on the bench where the top pan balance is because it will effect the results).
- Measure the mass of the pieces of raw potato chips one by one and record it on the table. Reset the stopwatch so it shows 0:000. Place the first raw potato chip inside the first boiling tube when the raw potato hits the surface of the sugar solution, immediately start the stopwatch.
- To make the time efficient after 1-minute place the second chip in the second test tube
- After another minute place the third raw potato chip inside the third boiling tube (make sure that you don’t put it one second before or one second after).
- Make sure your eyes do not move from the stopwatch and slip the time.
- When the time approaches 10 minutes (about 15 seconds to go) take the first boiling tube and start pouring the sugar solution out, leaving the raw potato chip inside.
20. Quickly put the raw potato piece on a paper towel and tap
the potato with it so that any excessive sugar solution can
be removed. (As this might interfere with the result).
- Quickly measure the mass of the raw potato chip on the top pan balance.
21. Record the mass on the results table.
22. When 11 minutes approaches (about 15 seconds to go) on
the stopwatch (this means that it has been 10 minutes for
the second chip because I started 1 minute after the first
one), repeat the same procedure as in step 20,21 and 22.
- When 12 minutes approaches (about 15 seconds to go) on the stopwatch (this means that it has been 10 minutes for the third chip because I started 2 minute after the first one) repeat the same persuader as in step 20,21and22.
- Throw the potato chips away; wash the boiling tubes that you have used.
- Do whole of the experiment again starting from step 13 to step 25 for the next set of concentration (so you have 3 sets for 0.0, 0.2, 0.4, 0.6, 0.8 and 1 molar).
- After you have recoded the results of the mass before and after for all of them, work out the change and then work out the average.
- Record the results on a graph.
My Results
To give me the average mass of the raw potato chip I deducted the result from the third column with the second column.
My results show me that as the concentration of sucrose increased the mass of the of the raw potato piece decreased. As you can see when the concentration level is at its highest (1 molar) the mass has averagely decreased by 0.196 (a decrease in mass by %). I will produce a graph from this result.
Analysing of results
