Osmosis Practical

This practical is designed to investigate how osmotic activity is related to the surface area to volume ratio of a potato. The surface area to volume ratio is very important in living organisms. For example, Amoebas are uni-cellular organisms with a very large surface area to volume ratio. This is crucial for them, as they absorb all of their oxygen through their membrane. If they were any bigger the cells in the middle would die because the diffusion distance would be too big. They would then need lungs, like mammals have. These have a large surface area to volume ratio so that the oxygen can diffuse into their blood stream and be transported around the organism.

Apart from the diffusion of substances, surface area to volume ratio is also important in heat loss. A small mammal like a mouse has a large surface area to volume ratio, so it will loose a lot of heat through its skin. This is why they have fur coats, to keep the heat from escaping. But a large mammal like an elephant needs a larger surface area to volume ratio to disperse of its body heat, so it has large ears with a huge surface area to volume ratio.

Research:

Osmosis is the movement of water across a semi-permiable membrane from a high concentration of water to a low concentration of water; in cells water would diffuse from a dilute cell solution to a higher concentrated medium. This is particularly important in animal cells, because if an animal cell is placed in a high concentration of water, it will absorb the water until it bursts. But plant cells have a cell wall, which prevents them from bursting. When a plant cell is placed in water, the water will move into the cell by osmosis, the cytoplasm swells and presses against the cell wall. The cell wall resists any further intake of water. When this happens the cell is fully 'turgid'. If the cell is placed in sucrose, the water travels down the osmotic gradient out of the cell. The cell would then shrink, and if this continues the cell membrane will be pulled away from the cell wall, making the cell 'flaccid'.

If a cell is placed in pure water, the water will travel down the osmotic gradient into the cell. Increasing the cell size. But if the incubation medium is more concentrated, for example sucrose solution, the water will travel from inside the cell down the osmotic gradient into the solution, making the cell smaller.

Prediction:

Osmosis works more efficiently through a large surface area to volume ratio. This is because there is a larger surface area for the water to diffuse through. Therefore, from my experiments I would expect to find that the larger the surface area to volume ratio, the greater the difference in size of the potatoes.

Method:

The preliminary experiment that I carried out involved cutting potatoes into cylinders then dividing each cylinder into 3 parts. One 3cm, one 2cm and one 1cm segments. I pre-soaked the potato in water to make it fully turgid, and then weighed each segment and found its surface area to volume ratio. I then incubated them in a 1moler solution of sucrose for 20 minutes. And then measured the amount of water loss by weighing them again after the 20 minutes.

These are the results I gained from this experiment:

Before:

After:

Now I will write a table of percentage decrease in mass for each sample this is the best way to process results because they are all proportional to each other so it is easier to see which sample decreased the most.

Percentage change:

3cm SA: Vol= 1: 0.52cm2

2cm SA: Vol= 1: 0.56cm2

1cm SA: Vol= 1: 2.27cm2

Analysis:

From these results I can see that as the surface area of the chip increased, the percentage change increased as well. This means that ...

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These are the results I gained from this experiment:

Before:

After:

Percentage change:

3cm SA: Vol= 1: 0.52cm2

2cm SA: Vol= 1: 0.56cm2

1cm SA: Vol= 1: 2.27cm2

Analysis:

From these results I can see that as the surface area of the chip increased, the percentage change increased as well. This means that the larger the surface area to volume ratio, the more the chip decreased in mass.

Given the amount of time I had for this practical, it was not feasible to carry on in this manner. There isn't a big enough difference in SA: Vol for each chip, therefore it doesn't give a large enough range for the investigation.

The new investigation involves using cubes instead of cylinders. This gives a greater range of SA: Vol, it also makes it easier to find there surface areas and volumes, using these formulas:

SA = 6x2, V = x3, so SA/V = 6/x.

I will need to calculate a range of different cube sizes to measure the rate of osmotic activity when cubes of different sizes are incubated in a 1moler sucrose solution. I will use cubes of 1cm3, 2cm3, and 3cm3.

Precautions:

To make this experiment fair, I have to take some basic precautions:

Make sure that the dilution of the medium around the potato is always constant. If this changes, then it will affect the rate of osmotic activity. Making it an unfair test.
The ratio of potato to the volume of medium also has to be constant. If there were a higher ratio of medium in one beaker, then osmosis would take place faster in that potato. Also, as the experiment progresses the solution becomes more dilute with the water lost from the potato. If there were a large volume of potato then the solution would become more dilute than a beaker with a small volume of potato in it. This would affect the rate of osmosis.
The potato skin must be removed because it is waterproof. And if it were left on, no water would escape from the potato, so it must be removed.
All of the potato samples must be from the same potato because different potatoes may have different concentrations in their cells. And one potato may be void, if it is dead, for example.
I have to do 2 or 3 repeats of the same experiment, to get an average result and to discard any anomalous ones.
I will have to put one cube of each size in each beaker so the ratio of medium to potato stays constant.
I have to make sure that I blot each cube dry before and after the experiment to remove any surface solution, so the calculation of its mass will be fair.
The time aloud for incubation must be kept constant as well because if one is left for longer than any others, it would loose more water. If it were left for too long, the potato would eventually equilibrate with the water voiding the results.
I will also have to work very quickly to avoid any water evaporation.

I have decided to incubate the cubes for 15 minutes, as this should be long enough to see a significant difference in the potatoes.

Method:

In my first experiment I had weighed two cubes of 3cm3,two of 2cm3 and two 1cm3 then placed one of each in one beaker, and the others in another beaker each with 200cm3 of sucrose in them. I then left them to incubate for 15minutes. Then weighed them again after.

Because I'm using the same size cubes for each experiment. Their surface areas and volumes won't change. So I will write them all at the beginning:

1cm3: SA = 6cm2

Vol = 1cm3

SA: Vol = 6cm2

2cm3: SA = 24cm2

Vol = 8cm3

SA: Vol = 3cm2

3cm3: SA = 54cm2

Vol = 27cm3

SA: Vol = 2cm2

Results: Experiment 1

Before:

After:

Percentage Change:

I then repeated this experiment two more times but with an extra beaker, and 3 more potato samples. I have kept the results separate because I performed the experiments on different days, so there may be slight variations to the results.

Here are those results:

Experiment 2

Before:

After:

Percentage Change:

Experiment 3

Before:

After:

Percentage Change:

Analysis:

As you can see from the results. As the size of cube decreased, the percentage decrease in mass increased. So the larger the surface area: vol the more water was lost through osmosis. Because the graphs are linear the percentage change in mass is directly proportional to the SA: Vol. As the SA: Vol increases, the percentage decrease also increases, proportionally.

As you can see from the graphs, the error bars show that the variation between samples can be quite large, but sometimes there is hardly any difference at all. This is why some of the points are slightly off line. Experiment 3 seemed to have the most anomalous results. The 2cm3 experiment had the largest difference in results. It has a large error bar and the line of best fit doesn't pass through it. There are quite a few reasons why this could have happened.

The other two experiments have a good set of results. And as you can see from the graph of all the experiments, they average out to a near perfect straight line.

Conclusion:

Even though some of the results are slightly out, as a whole they show that my prediction was correct. As the surface area to volume ratio increased, the more water was lost via osmosis. The graphs show that the SA: Vol. is proportional to the percentage decrease in water. So the more the SA: Vol. increases, the Average percentage decrease in water also increases proportionally.

There were some anomalous results, most of them were because the amount of water lost was smaller than in the other experiments. This could have happened for a number of reasons. Water is lost from the potato cells into the sucrose solution because the sucrose is less dilute than the potato cell. The water travels down the osmotic gradient into the sucrose until the concentration of the cell is equal to the concentration of the sucrose solution. The cells on the outside of the cube lose their water first, until there is none left. The cell next to it then starts to diffuse its water to the outside cell, the water then diffuses out of the potato, and so on. Because I had a limited incubation time not all of the cells lost water, this amount would differ between different potatoes, making the percentage decrease vary.

Another reason for the anomalous results could be that the potatoes started off with different amounts of water. Because I didn't have time to pre-soak the potatoes in water, each potato would have a different amount of water in their cells. The potatoes with a large quantity of water would be able to lose more, whilst those with less water in their cells wouldn't be able to lose as much.

Because the anomalous results would effect the averages quite significantly, I decided to leave them out. Most of the percentage differences are one or two grams apart, and some of the anomalous results are five grams different so they would change the averages by a noticeable amount, effecting the graphs. This is why I chose to leave them out of the averages.

As you can see from the graph of all the experiments, they average out to a straight line. This shows that as the surface area: volume ratio increases, the percentage decrease in mass also increases proportionally. Also the gradient of the graph shows that when the surface area: volume is doubled, so is the percentage decrease in mass. The graph doesn't show this perfectly, but it is very close.

Evaluation:

This experiment was quite reliable but there are quite a lot of aspects that could go wrong with this experiment:

Because I am cutting each cube from the potatoes by hand, they won't all be the same size, this is why I recorded the mass instead of relying on the sizes. You can see this variation in the sizes of the cubes by looking at the masses in my results tables. For instance, in experiment 1 the masses of the two 3cm3 cubes were 31.35g and 24.15g, this is a large difference showing that the cutting of the potato could not have been very accurate. Although the preliminary experiment did not offer enough variation, it was a lot easier to acquire the correct sized chips than with the cubes. Also the cubes were cut from different parts of the potato, and this could effect the rate of osmosis, depending on how much water is contained in each part of the potato.

When I put the potatoes in the sucrose solution, because they float, only five sides of the potato are in the liquid. So only five sides of the potato will loose water. To try and minimise this, I could stir the potatoes to submerge it entirely. Another thing that stirring could prevent is the uneven dilution of the sucrose. If the solution were left standing then the sucrose around the potato would become very diluted. By stirring, the sucrose would be evenly diluted so osmosis would occur faster.

Because I had to cut the potatoes to make them the correct size I damaged the outside cells. The smaller cubes would have a greater proportion of damaged cells so they would have a disadvantage to the larger cubes. There would be fewer cells to lose water in the smaller cubes so it would effect the rate of osmotic activity. Unfortunately there is nothing I can do to prevent this from happening.

Because the experiments were done on different days the conditions would be slightly different for every experiment. If the temperature changes then it would effect the rate of osmotic activity. Also I had to use different potatoes for the different experiments. This could effect the results because I only had 50 minutes to do the experiment. If I had longer I could pre-soak the potatoes to ensure that they were all fully turgid. Because I didn't have time to do that, the different potatoes would have different dilutions in their cells, effecting the rate of osmotic activity.

By taking into account some of these circumstances I could have minimised the amount of anomalous results.

If I were to do this experiment again I would use a different shaped piece of potato. With the cubes there was not a very large range of sizes that I could use. Because I only used three different sizes, it is very difficult to come to a firm conclusion, whilst if I had more results I could draw much more accurate conclusions. Therefore I would use a different shape, a cuboid for instance, this would give me a greater range of sizes without having to use a very large potato.

An experiment I could perform to extend the enquiry would be to keep the size of the cube constant and to change the concentration of the sucrose. By doing this I could discover the concentration of the sucrose inside the potato cell. I would do this by varying the concentration of the sucrose solution. I would weigh the potatoes before and after the experiment and see how their masses changed. When I use a concentrated sucrose solution I would expect the potato to loose water, and when I use a dilute solution I would expect them to gain mass. Somewhere in the middle the mass of the potato would not change. This would be the same concentration as the sucrose inside the potato cells.

This way you can investigate whether the size of the potato or the difference in concentration has a larger effect on the rate of osmotic activity.