My aim in this experiment is to find out how osmosis affects potato chips' mass when placed in different concentrations of glucose and water.

An Investigation into Osmosis

Aim:

My aim in this experiment is to find out how osmosis affects potato chips’ mass when placed in different concentrations of glucose and water.

Prediction:

I will now predict how I think the outcome of the experiment will be. I believe that some of the potatoes will shrink and lose weight, some will swell and gain weight and some will stay the same. I think that when there is a high concentration of glucose, osmosis will occur through the potato and it will shrink as water molecules escape into the solution. When there is a very low concentration of glucose I believe the potato will swell as it gains water molecules due to osmosis trying to equal out the balance of water. I find it unlikely that any of the potatoes will stay exactly the same weight but some that are in the middle concentrations like 0.4 and 0.6 may change only slightly.

However, there will be a point where the concentrations of water inside and outside the potato cells are equal (isotonic). At this point there will be no change in the length, volume and mass of the potato, as the net movement of water will be zero, no osmosis has occurred.

Using this information a graph and prediction can be made.

At point A the graph suggests that no osmosis has occurred, suggesting that the concentration of water inside the cell is equal to the solution outside.

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 concentrations), 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. This backs up my prediction on the previous pages.

References

(Collins, 1999 'The Biological Dictionary)

Background Knowledge:

Osmosis is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration. Osmosis is basically diffusion but it only occurs with water molecules whereas diffusion can occur in liquids or gasses.

Explanation

Semi-permeable membranes are very thin layers of material (cell membranes are semi-permeable) which allow some things to pass through them but prevent others.

Cell membranes will allow small molecules like oxygen, water, carbon dioxide, ammonia, glucose and amino-acids to pass through. They will not allow larger molecules like sucrose, starch or protein to pass through.

A region of high concentration of water is either a very dilute solution of something like glucose or just pure water.

A region of low concentration of water is a concentrated solution of something like glucose.

How osmosis works

When you put an animal or plant cell into a liquid containing water, if the liquid surrounding the cell has a higher water concentration than the cell itself (a very dilute solution), the cell will gain water by osmosis. Water molecules are free to pass across the cell membrane in both directions, but more water will come into the cell than will leave because there was less water in the cell to begin with and osmosis is trying to balance the level of water concentration out. This is likely to cause the cell to swell up.

If the liquid is exactly the same water concentration as the cell (isotonic) there will be no overall movement of water across the cell membrane. Water crosses the cell membrane in both directions, but the amount going in is the same as the amount going out, so there is no overall movement of water. The cell will stay the same size.

If the liquid has a lower concentration of water than the cell (a very concentrated solution) the cell will lose water by osmosis. Again, water crosses the cell membrane in both directions, but this time more water leaves the cell than enters it to try and make the balance of water equal. This would cause the cell to shrink.

The Consequences of Osmosis

Plant cells always have a strong cell wall surrounding them. When they take up water by osmosis they start to swell, but the cell wall prevents them from bursting. Plant ...

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The Consequences of Osmosis

Plant cells always have a strong cell wall surrounding them. When they take up water by osmosis they start to swell, but the cell wall prevents them from bursting. Plant cells become "turgid" when they are put in dilute solutions (solutions with a high water concentration). Turgid means swollen and hard. The pressure inside the cell rises; eventually the internal pressure of the cell is so high that no more water can enter the cell. This water pressure works against osmosis. Turgidity is very important to plants because it’s what makes the green parts of the plant "stand up" into the sunlight.

When plant cells are placed in concentrated sugar solutions (solutions with low water concentration) they become "flaccid". This is the exact opposite of "turgid". If you put plant cells into concentrated sugar solutions and look at them under a microscope you can see that the contents of the cells have shrunk and pulled away from the cell wall. They are said to be plasmolysed.

When plant cells are placed in a solution, which has exactly the same water concentration as the cells themselves, they are in a state between being turgid and being flaccid. This is called incipient plasmolysis. "Incipient" means, "about to be". When cells are not turgid they do not hold their leaves up into the sunlight.

When animal cells are placed in sugar solutions things can be different because animal cells do not have cell walls. In very dilute solutions, animal cells swell up and burst. They do not become turgid because there is no cell wall to stop the cell membrane from breaking. In concentrated solutions, water is sucked out of the cell by osmosis and the cell shrinks.

Animals which live on dry land and the sea (because of its saltiness) must conserve water so they don’t become dehydrated, but animals, which live in freshwater, have the opposite problem. They have to get rid of extra water as fast as it gets into their bodies by osmosis so they don’t become too saturated with it.

Planning:

From my background research I have discovered that there are 3 basic consequences of osmosis – flaccidity, turgidity and no change at all – and various gradients between these. From this knowledge and because I want to do as accurate an experiment as possible I know it is important for me to vary my sugar concentrations so as to get a good, even scope of results so my conclusions will be as accurate as possible. To do this I will vary the glucose content from 0 to 1 moles (M) making 6 concentrations at equal points in the scale.

This would mean taking concentrations of 0M, 0.2M, 0.4M, 0.6M, 0.8M and 1.0M.

Concentration is measured in Moles. It is vital for me to work out the amount of sugar in grams I will need to get the correct molarity for each concentration. The formula for glucose is:-

C6H12O6H2O

By reading my background knowledge in relation to potato chips I can fully understand how osmosis will work in this case. Now knowing that osmosis will occur across a semi-permeable membrane whenever there is a difference between the water concentrations on either side of the membrane, and knowing that when this happens to the cells they will either become turgid if

water flows into them, or plasmolysed if water flows out of them, and change in mass, I can plan a suitable approach to testing the theory with potato chips.

Method:

Prepare the 6 different molar concentrations – 0, 0.2, 0.4, 0.6, 0.8, and 1 – in 6 clean beakers using a certain amount of glucose to get the correct concentrations and making sure each one is properly stirred.

Fill 6 test tubes with 25mls of each concentration using the pipette and place in a test tube holder.

Cut 6 potato chips with size of 4*1*1cm with a sharp knife on a cutting board.

Weigh each potato chip using an electric mass balance, record results.

Place potato chips in test tubes, make note of which potato went to which concentration.

After 24 hours remove potatoes from test tubes. Wipe them on kitchen roll to remove any excess moisture.

Weigh each potato on an electric mass balance and record results.

Complete a results table showing the change in weight of the potato chips.

Apparatus:

6 test tubes
Test tube holder
6 beakers
Pipette
Tile
Knife
Potato
Water
Sugary Substance (glucose)
Electric mass balance
Kitchen roll
Measuring cylinder

Diagram:

I have drawn a diagram to show what the experiment will look like. I have used the best equipment available to me to carry out this experiment thoroughly.

Fair Test:

It is very important to make the experiment a fair test. If it isn’t then the results I discover could be incorrect. To make sure it is a fair test I should take special care to make sure I weigh all the potatoes carefully and record their weights correctly. Also, to pay extra attention when weighing the water and glucose solutions out by using a very precise pipette. I will not be able to make all these things exact but I can try and make my measurements as accurate as possible. I should also make sure I use very sensitive scales, which clearly say zero before I place the potato on so as not to get a false reading. Another very important thing to make the experiment fair is to make sure all the potatoes are fully submerged in the solutions so the surface area being affected by osmosis on each potato is equal. Also, it is important to make sure all the potatoes get an equal amount of time in their solutions, but obviously their times will differ slightly but I will try to make the differences as little as possible. I will also try to make the molar concentrations of the solutions as exact as possible to prevent the test from being unfair.

Safety:

No matter how harmless the experiment, safety is always a very important issue. It is always necessary to take the relevant safety precautions. In this experiment it is important to be careful when cutting the potatoes because the knife we will use will need to be very sharp. Also it is important not to let the solutions into our mouths because no matter how clean the test tubes are there could still be residue left over from previous experiments, which could be harmful to our bodies.

Results:

Table showing the change in weight of potato chips put in different concentrations of sugar and water

% = difference ^ time * 100

Graph to show the percentage of weight difference in the potato chips

Analysis:

I have discovered that as I predicted, some of the potatoes lost mass and some gained in mass. My results show that my predictions are mostly correct. The weight of the potato is decreased slightly in the cases of the higher glucose concentrations and increased in the cases of the lower concentrations. I also had one rogue answer and if I had more time I would redo the experiment for the 0.4 concentration to try and come up with a more accurate result. From my graph I can see that the most likely result for the 0.4 concentration would be about a 0% change which would mean that the chip already had the same concentration as the solution, 0.4 (isotonic). Another piece of evidence to support this theory is that my result for the end weight for the potato chip could have been 4.42 instead of 4.52. If it were 4.42 it would match the start weight proving that there had been no weight change. It is most likely that I accidentally wrote down a 5 instead of a 4 at some point during the investigation.

I have drawn a line of best fit onto the graph and by looking at this line you can see that I have a smooth curve of results.

Conclusion:

All in all, I think this investigation was quite successful. By successful I mean that my results basically confirmed my predictions. The potato cells took in, or gave out, water depending on the concentration of the tissue and the concentration of the solution surrounding it to try and equal out the level of water. I can now confirm that osmosis took place.

The original weights of the potatoes range from between 3.82 to 6.4, this tells us that the potatoes were reasonably well cut and in the same kind of area.

There was only one result, which was out of line. I can tell that one of my results was anomalous because when I plotted it on the graph it looked completely out of line with the rest.

The graph and results show that osmosis did occur and as the molars increased the percentage of the weight difference decreased.

This experiment helped me to discover that osmosis occurs between two liquids through a partially permeable membrane and how the concentration of a solution can affect the way that osmosis occurs.

Evaluation:

I followed the plan correctly and believe that I gained accurate and sufficient enough results to conclude my experiment. I do think that my method was the best way of carrying out the experiment but improvements to the method could probably have been made. My final results were quite reliable due to the precautions I took to make it a fair test.

I believe that my one anomalous result was due to misread scales or by accidentally writing down the wrong number when we meant another. This is why if I were to do the experiment again I would concentrate much more when taking down the results and take much more care to be specific and precise about the numbers.

But I also believe that because all my other results look perfectly accurate on the graph my results are still just as sufficient in supporting my conclusion.

Another thing that I believe we could have done to make the experiment better and achieve more accurate results would be to do one test at a time of each concentration, thus reducing the time difference when moving the potatoes from the test tube to the balance. Whilst we are drying the potato chip on the tissue the others are still in the test tubes, which makes us hurry the drying procedure. If we only had to worry about one potato chip at a time we could make sure the test was much fairer by drying each one properly.

Another thing that could improve the experiment would be to get more people to carry out the experiment with me. That way we could each be responsible for different tasks giving us the opportunity to do our delegated task at a much better standard with much more dedication and the ability to be more precise.

A major thing I could have done to improve the experiment would have been to extend the range of which I’d tested. My results show a steady decrease (discounting the anomalous result) but I do not know whether this trend would have continued if I’d carried on testing higher molar concentrations or stopped or changed somehow at some point. I could have also tested more concentrations within the range. From a range of 0 to 1 moles I tested 6 different concentrations but I could have got much more accurate results had I tested 10 concentrations instead.

If I had more time to prove my estimation about the potato chips having about a 0.4 molar concentration themselves I could repeat the procedure a few more times to get really accurate results. But apart from my one anomalous result I am very pleased with my results and with what I have concluded.

Other factors that could have altered my results are listed and explained below:

Temperature: Osmosis is merely a process where the water molecules from the dilute solution diffuse past a semi permeable membrane to a concentrated solution. By increasing the temperature, this rate of diffusion can be increased and thus, the process of osmosis is faster. This is because, by increasing the temperature, we increase the kinetic energy of the water molecules and thus they diffuse across the semi permeable membrane at a greater speed.

Concentration Gradient: The concentration gradient is actually the difference in concentration between the two solutions, separated by the semi permeable membrane. If the concentrated solution is altered so that it has a greater concentration, it will now require more water movement from the dilute solution. Thus, more water molecules must diffuse across the semi permeable membrane. As a result, the process of osmosis is increased as the water molecules diffuse across the membrane faster in order for equalization to occur. When the concentration difference is greater, we say the concentration gradient is steeper.

Size of the Molecules: When a molecule is heavier, then it will obviously take a greater deal of time to diffuse across the semi permeable membrane as the molecules speed is decreased. This is also why the solute molecules from a hyper tonic solution do not move into a solution. In cells, the cell membrane is semi permeable. However, in experiments, the membrane may longer act as a sieve and the solute molecules can diffuse across the membrane. However, since the solute molecules attract water molecules, which form clusters around them, this causes the solute molecules to become heavier. Thus, they diffuse more slowly than the free water molecules in the solution.

Permeability of the Membrane: In osmosis, the water molecules will move across the semi permeable membrane. This membrane in cells is generally the same size. However, plant cells have the presence of the cell wall, which contributes to the thickness of the cell membrane. Therefore, since the membrane is thicker, the rate of diffusion is greater as the water molecules must move over a greater distance. As a result, the rate of osmosis is decreased.