Osmosis In A Potatoe

AT1- Osmosis In Potatoes

Aim: Investigate the movement of osmosis through a selectively permeable membrane, in this case potato.

Introduction: 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 concentrations 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 in 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 stems strong and upright.

Osmosis diagram:

Key:

Semi permeable membrane

Solvent molecule

Water molecule

Preliminary Work: In a sense, preliminary work is as important as the actual experiment. It allows you to make hypotheses before performing actual tests.

For my preliminary work, I used 33mm of potato. It was easy to cut 33mm and get it to weigh around 1g. To begin, we used 15mls of Glucose solution- this was the amount needed to cover one piece of potato. The first thing we needed to observe, was how different concentrations affected the weight and length of the potato. We therefore needed to observe each end of the spectrum. We tried 0.2 molar concentration (the lowest) and 10 molar (the highest.)

We then left each test for 5 minutes, but found after that time no reaction had taken place. It was then we predicted we would have to leave the tests for at least triple that time.

Again, we used the same proportions of potato chip, concentration and amount, but changed the length of time the tests were left for. This time we left them for 15minutes and saw that the results we collected were much easier to compare because there was more of a range.

Hypothesis: I predict, that as concentration increases, the weight and length of the potato chip will decrease. My reasoning behind this, is that the higher the concentration of glucose in a solution, the lower the concentration of water. When the potato chip is put into the solution, it will, by osmosis lose some of its water, and the water will diffuse into the solution of glucose, causing the potato chip to lose water, thus decreasing in weight and length.

However, if a potato chip is placed into a solution of 0 molar concentration, it should gain weight, width and length. This is because the solution has more water potential (its molecules' ability to move) than the potato chip, and so water moves from a region of high concentration (the solution) to a region of low concentration (the potato chip.)

Predicted graphs

I predict my graphs will look like this because, in graph 1, the potato chip used for 0 molar concentration will gain water, as the water moves from a region of high concentration (the solution) to a region of low concentration (the potato chip.) This result will be an example of when the solution is Hypotonic to the potato. However, in the middle of the scale, say 0.2 molar concentration; the graph will go right down, due to the fact that the solution will be isotonic to the potato. In other words, the solution and potato chip are about equal in concentration. The reason for the minus results for 1 molar concentration is that the solution has a lower concentration of water to that of the potato, and so, by osmosis water moves from the potato to the solution, causing the weight to decrease.

Graph 2 shows the average change in length. Basically, the pattern is the same as average weight loss in graph 1. It shows how osmosis is affected by different concentrations.

Method After having done our preliminary work, we knew exactly which measurements we needed to use in order to obtain the correct sort of results. We set up 14 test tubes, because in order to save time, we intended on doing repeats at the same time as the original experiments were going on.

For organisation's sake, we set up all of the test tubes at once, and labelled them individually to avoid confusion later on. Into each test tube, we poured 15mls of Glucose solution, and added one of the potato chips, each weighing around 1 gram, and measuring 33mm by 5mm. In order to make it as much of a fair test as possible, we added the potato chips very quickly, to make sure one chip didn't have a longer amount of time in the solution than another. We then left each one for 30 minutes.

After 30 minutes, we emptied each test tube, leaving only the potato chips inside. Before we measured and weighed each chip, we dabbed them with paper towel. If we had of left excess water on the chips, the weights would not be