Find out if osmosis occurs in a potato, and how it affects the potato in different molar solutions of sucrose and water.

Sam Dixon 10E

GCSE Biology Course Work

Planning experimental procedures

The Potato Investigation, Osmosis

Aim: My main aim in this experiment is to find out if osmosis occurs in a potato, and how it affects the potato in different molar solutions of sucrose and water.

Prediction: Osmosis is the movement from one side to the other of a semi-permeable membrane, when on one side of the membrane there is a solution of both large and small molecules, but on the other side there are fewer or no large molecules.

So I predict that when the potato is submerged in dilute concentration of sucrose solution, the mass, as well as the length of the potato will increase.

The semi-permeable membrane allows small water molecules to pass through it, but not the large sugar ones. There is a diffusion of small water molecules from side where there they are in high concentration to the side where they are in lower concentration.

So I predict that when the potato is in dilute sucrose solution below 0.5 molar concentration, the small water molecules will be in higher concentration than the ones inside the potato, so they will diffuse through the semi-permeable membrane, to the side where there is a lower concentration, therefore, increasing the mass and length of the piece of potato.

So, when in a higher concentration of sucrose solution, there will be a lower concentration of small water molecules outside the potato, so the water molecules in higher concentration inside the potato will diffuse out of the potato through the semi-permeable membrane, thus decreasing the mass and length of the piece of potato.

I can also include in my predicting that there will be a point at which osmosis doesn’t occur, when the concentration of water molecules inside the potato is the same as outside the potato so we can expect to see no change in mass or length.

Method (Simple, safe procedure):

With a cork borer, produce 18 cylindrical pieces

of potato and cut all of them to exactly 30mm long.

Collect 18 boiling tubes and set 3 up for each

concentration of Sucrose solution.

Weigh the pieces of potato, this will give a

starting point for us to calculate the mass increase.

Place the pieces of potato in their solutions and

leave for 30 minutes, then remove and dry them

before weighing them again and recording the results.

After replacing the potato pieces, leave them again for a further 60 minutes and then repeat the above procedure recording the results of mass and length increase.

We will repeat the experiment 3 times so that we can make sure that all our results are similar, and so that we will be able to take the average of ...

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starting point for us to calculate the mass increase.

Place the pieces of potato in their solutions and

leave for 30 minutes, then remove and dry them

before weighing them again and recording the results.

After replacing the potato pieces, leave them again for a further 60 minutes and then repeat the above procedure recording the results of mass and length increase.

We will repeat the experiment 3 times so that we can make sure that all our results are similar, and so that we will be able to take the average of the 3 experiments. This makes sure that we get accurate results.

Apparatus (appropriate equipment):

· Stop Clock – to time our experiment
· Cutting tile – to cut the potato on
· Knife – to cut the potato
· 25cm Measuring cylinder – to measure the solutions
· Distilled water – part of the experiment
· sucrose – part of the experiment
· Potatoes – part of the experiment
· Tissue paper – to dry the potatoes after the osmosis takes place
· Balance – to weigh the potatoes
· Cork borer – to cut out potato cylinders

Fair Test: Throughout the procedure, there are key factors to vary and control in order to produce accurate results and precise, reliable evidence.

The ways in which I will do this are as follows;

Making a suitable number and range of observations- Getting a wide range of results by varying the concentration of the sucrose solution will create the basis for which we can investigate osmosis. I will use concentrations starting with distilled water, then 0.2, 0.4, 0.6, 0.8, and 1 molar.

For very accurate results, I will take each reading in the solution 3 times, which is why 3 test tubes are needed for each concentration. This will allow me to calculate an average, eliminating any inaccurate results. I will also use the same ruler when measuring the pieces of potato to ensure no mistakes occur when recording results.

I will be using the same volume of sucrose solution for each concentration so that this is another variable, which cannot hinder my results.

When using the balance, we will make sure that the balance is reading zero before we put the 3 potato pieces on it. This is so that we don’t get a false reading. And after the experiment, we will it in the same way, taking the reading to the nearest 2 decimal places.

Provided this is a successful fair test, my prediction should prove correct.

Preliminary Work:

My hypothesis for my prediction came from preliminary work carried out before this investigation, on a similar subject.

This experiment proved that osmosis occurs through a semi-permeable membrane, in this case a visking tube.

Over time, the mass of the boiling tube decreased, this was due to the diffusion of highly concentrated water molecules from outside the visking tubing, moving through its semi-permeable membrane, to the inside where there was a lower concentration of water molecules as the tubing contained sucrose solution.

Safety:

This is an important aspect in our experiment. So we should take this into consideration, no matter what.
We will be using a very sharp knife, which could injure someone if it’s not handled properly. And we should also be careful that the solutions don’t get into our bodies internally, because we are not fully aware of the damage it could do to us.

Obtaining evidence:

After the experiment, I had to dry the potatoes first, so that the water outside the tissue of the potatoes wouldn’t alter the weight of what it is supposed to be. I quickly took all of them out, and put them onto a piece of tissue paper, into their own groups.
When obtaining my results, I reset the balance, so that it would read zero so as to obtain the most accurate of results.

Analysis:

On the graph shown above, I have made the ‘different solutions’ as my independent variable, since it won’t be changing on any event. And I have made the ‘percentage’ as my dependent variable, because it doesn’t change in any particular order or a pattern. I have decided to make it a bar graph, because the independent variables aren’t exactly in the same category. For example, I would have used a line graph if the independent variable in my experiment was time, which is changed deliberately, but this is different subjects of matter. And also, the results were very different, as well as the columns and there wouldn’t be any advantages of using a line graph to view the results. I have taken the average result of the 6 different solutions and put it on the graph to be more accurate.

The results show that, in accordance with our hypothesis, the cylinders will expand when external solution concentration is low (high water concentration), and contract in strong solutions (low water concentration). This is due to osmosis, where water passes from weak solutions to strong solutions across a semi-permeable membrane. The graphs of % change against solution strength show that the results tend to form a curve, crossing the x axis (where there is no change in length), at approximately 0.2 molar concentration.

Conclusion:

In this experiment, I believe that I have collected enough data to support my hypothesis. This investigation was, I think, successful. Successful meaning my results proved my predictions.
The potato cells, working to stay alive, took in, or gave out the water depending on the concentration in the potato, and the concentration of the solution it was surrounded in.
The results were fine and by looking at the weights measured before the experiment, you can see that there is no reading, which seems to be out of the line. As the weights before the experiment range between 1.76g and 1.9g, this tells us that the potato pieces were cut well, and I believe accurate enough. There were, I believe, no out of line results after the experiment, as the weights were very similar in their own category. And this tells us that my experiment was successful.
The graphs and the results show that:
. Osmosis actually took place in the experiment.
. As the molars increased, the weight decreased until it was losing weight.
There doesn’t seem to be any results, which undermine my predictions, and our group didn’t find any bad results, which means we didn’t have to repeat any of our experiments more than three times.
Having the results, which backs up my hypothesis, proves that this investigation was fully accomplished.

In the table above, the percentage shows a steady decrease, telling us that the percentage of the difference decreases as the water concentration decreases.
This experiment helped me find out that osmosis occurs between 2 liquid substances with a semi-permeable membrane, and that the higher the water concentration is, the larger increase in grams. This means that the percentage will also be higher, increasing with the grams.

Evaluation:

The experiment was very successful in my opinion. I obtained a large quantity of very accurate results from which I was able to create informative graphs. I think I took easily enough results for the amount of concentrations that I was using, and the time that I used for the experiment to last was enough to allow sufficient osmosis to occur. However if I was to repeat the experiment I might well increase the time of the result to allow more osmosis to happen and possibly find out the saturation point of the chips. The range of concentrations was adequate but I would possibly create more concentrations if I repeated the experiment so that I would have more varied results.

The cutting of the potatoes was the most difficult part of the experiment as although I was recording my results by mass, it could well have affected the surface area and so the overall rate of osmosis. If I were to repeat the experiment I would have possibly found a machine to cut the potato, as it would ensure that all potatoes would be the same weight and dimensions. As well as the potato I could have found a more accurate way to measure out the solutions and to determine the molar concentrations. This would ensure that I have an accurate amount of fluid in each test tube. I could also weigh each chip on a more accurate scale.

There were not any out of the ordinary results, but some were not as close to the line as others. I may have caused this. When the potato chips were removed from the test tubes and dried I may well have dried some potatoes more thoroughly than others and so some would have more excess water, which would add to the mass. If the experiment was repeated I could find another way to dry the potatoes that would ensure that all were dried in the same way for the same time. However with all this said I think that the experiment was truly successful and I was very pleased with the complete comparison of my results with my initial prediction.