Osmosis Essay, Research Paper

Title: Osmosis in Potato Cells

I am going to investigate: the diffusion of water by osmosis in plant cells, when changing the concentrations of sucrose in water, and measuring the change of mass of pieces of potato left in the solution for twelve hours.

I predict that

I predict that the stronger the solution of sucrose the more mass the pieces of potato will loose. This is because the solution will be more concentrated than the solution in the potato. So the water in the potato will come out through the partially permeable membrane of the potato cells into the more concentrated sucrose solution. This means that the mass of the potato will decrease because the water from the solution will have moved out of the potato into the sucrose solution. When the concentration of the sucrose solution is weak (weaker than the solution inside the potato) the potato will gain mass. This is because the solution in the potato is more concentrated than the sucrose solution. So the water in the sucrose solution will soak into the potato by passing through the partially permeable membranes of the potato cells because the potato is more concentrated than the sucrose solution. This means that the potato will gain mass because the potato now has more water in it. This is all because in osmosis the partially permeable membrane in plant cells will allow the water in the less concentrated side to seep through to the more concentrated side.

Method

I started off my experiment by using a knife and chopping board to cut up five small pieces of potato 5cm, by 1cm, by 1cm. I weighed all the pieces of potato before the experiment and made sure the pieces all weighed within 5 grams. I then put the pieces of potato separately in petri-dishes. In the first petri-dish I used 20 ml of water, in the second one I put 16ml of water and 4ml of sucrose. I then carried on changing the solution 4 ml at a time. Here are the concentrations that I used:

Amount of sucrose (ml) Amount of water (ml)

0 20

4 16

8 12

12 8

16 4

20 0

In each petri-dish I put 1 piece of potato, which was labelled, weighed and the put lid on and then left for 12 hours. Then after 12 hours I weighed the potato pieces making sure there was no excess solution on them and repeated the experiment another 2 times. This allowed me to work out averages so that my results were very accurate.

Conclusion

My graph shows that the stronger the solution of sucrose the more mass the pieces of potato lost. This is because the sucrose solution is more concentrated than the solution in the potato. So the water in the potato came out through the partially permeable membrane of the potato cells into the more concentrated solution. This means that the mass of the potato decrease because the water from the solution moved out of the potato into the sucrose solution. When the concentration of the sucrose solution was weak (weaker than the solution inside the potato) the potato gained mass. This was because the solution in the potato was more concentrated than the sucrose solution. So the water in the sucrose solution will soak into the potato by passing through the partially permeable membranes of the potato cells because the potato is more concentrated than the sucrose solution. This means that the potato gained mass because the potato now has more water in it. This was all because in osmosis the partially permeable membrane in plant cells will allow the water in the less concentrated side to seep through to the more concentrated side.

From my results I found that the average solution of a potato is 43 % sucrose, 57% water. This means when the sucrose solution was 43% sucrose there was no movement of water so there wasn t any mass change because the two concentrations were the same. My graph shows this quite clearly.

I also have one or two anomalous results showing on my graph. There are a few that are just of the best-fit line, which is probably because the potatoes were different so they have different concentrations or the sizes of the potato pieces may be different. At the bottom of my graph where the concentration is the strongest there is one result that doesn t fit the best-fit line at all well. This may be because the potato can not shrink up any more because all the water has already been taken out of it therefore no extra mass can be lost. This means that at the bottom of the graph the best-fit line may start to curve off because the pieces of potato can not loose any more mass.

Evaluation

I am pleased with my results being accurate as they all shown the same thing (that in osmosis a partially permeable membrane in a plant cell will allow water in a less concentrated solution to seep through to the more concentrated solution). I found a few difficulties that may have varied my results a little. One difficulty was cutting the pieces of potato into the exact same sizes. This would make a difference because the surface area will be different so one piece of potato may have more or less sucrose solution exposed to its cells than other pieces. Also some of the potato pieces may have had more solution covering them than others. So some potato pieces may have had more or less sucrose solution exposed to the potato s cells than others. Twelve hours may not have been long enough for the potato pieces to be in the solution. If I had done the experiment again I would have done some preliminary work to test whether or not 12 hours is long enough time. One last thing is that different potatoes have different concentrations so the results may vary for different potatoes. If I did the experiment again I would have a vaster range of concentrations to give me a wider range of results.