The aim of this experiment is to investigate different concentrations of sucrose solutions and eventually determine the molarity of the cells inside the potato.

The cell membrane in cells is semi-permeable and the vacuole contains a sugar/salt solution. So when a cell is placed in distilled water (high water concentration) water will move across the semi-permeable membrane into the cell (lower water concentration) by osmosis, making the cell swell. This cell is now referred to as turgid. I predict that the cells will increase in length volume and mass because of the extra water.

Over 48 hours I think the colour of the potato cylinder will get lighter and fade in the distilled water and possibly 0.25M solution. E.g. if the potato cylinder was at first a golden yellow colour I think that after 24 hours it would change to a pale white colour. This is because the cell in the potato has become turgid, and water has entered the cell by osmosis. I also predict the other cylinders will be dark in colour, and bendy and brittle in texture. This is because the cell has become flaccid. A flaccid cell is the complete opposite of a turgid cell.   

 

Below is a table of results showing the weight and length of the cylinders before and after they were put in various solutions

 

Experiment 1

Experiment 2

Experiment 3

Averages

After these results were obtained, the potato cylinders were dried carefully with a paper towel and then immediately weighed on electronic scales. The new mass of each cylinder can be compared with its mass reading before being placed in the solution in the test tube. Also the cylinders will be measured to note its new length. This new length can then be compared with its original length of four centimeters and thus with this information compiled together, I will be able to analyze what has happened with the help of what I have learnt before on the topic of osmosis.

From this table and the graph overleaf, it is obvious that with a less concentrated solution, the potato cylinder gains length. For example, when the potato cylinders are placed in distilled water, from four centimetres, they increase in length to 4.42 centimetres on average, an increase of 42 millimetres. This relies on the fact of osmosis; if a cell is placed in a less concentrated solution, then water must move into the cell so that the concentration gradient is eliminated. Since the potato cells inside the cylinder have a mixture of salts and sugars with water, it forms a weak solution. Even though it is weak, it is still more concentrated than the distilled water. In order for equalization to occur, some of the distilled water must move across the cell membrane of the potato cell and enter it. In this case, the cells will become bloated or turgid; they become firm and full. Due to this, the potato cylinder will increase in length as the potato cells have become firm and turgid; they occupy more space and so the cylinder increases in length.

It is seen that the potato cylinder increases in length the most when placed in distilled water because the concentration gradient is greater; the difference in concentration between the external and internal solution is greater and so more water enters the potato cells filling its vacuole more rapidly. When the potato cylinders are placed in a 0.25M sucrose solution, the potato cylinders increase in length but by a smaller margin. This must mean that there is a smaller concentration gradient; the concentration difference between the external solution and the solution inside the cell is smaller even though the solution in the cell is more concentrated than the 0.25M sucrose solution. This means that less water must enter the cell in order to cause equalization of the external and internal solutions.

With the concentration is of 0.5M sucrose solution, the potato cylinders no longer increase in length but in fact become shorter. This means that the potato cell is less concentrated than its external solution; therefore in order for equalization to occur, water must move out of the cells and they become flaccid. In this case, the cells become shrivelled up and so the potato cylinder becomes smaller in size, and they decrease in length.

Since there is a sudden change between these concentrations of 0.25M and 0.5M, that is, at 0.25M the potato cylinders gained length while at 0.5M the cylinders decreased in length, it shows that obviously the isotonic solution exists between these two values; the isotonic solution is where the external solution is equal to the internal solution and is where there is no net flow of water molecules. The isotonic solution must exist between 0.25M and 0.5M sucrose solutions as at one point there will be no increase in length of the potato cylinders. The point marked X on the graph shows the molarity of the cells inside the potato, 0.43M.

Also from the graph, it is seen that with higher concentrations of sucrose solution such as those of 0.75M and 1M, there is a greater change in length as there is now a greater concentration and more water must move out of the potato cylinders and so more cells become even more flaccid. With higher concentrations of sucrose solution, the potato cylinders become shorter and shorter.

I have now completed my experiment and have determined the average concentration of the potato cell used in my experiment to be 0.43M. I believe this value to be reliable as my method itself is both practical and suitable to determine the concentration of the solution. It is a reliable method as I have taken into account the original masses and lengths of the potato cylinders and compared them with their new masses and lengths appropriately. It also allows the potato cylinders to be submerged fully in their respective solutions for up to 24 hours so that enough time is given for the process of osmosis to occur. Nevertheless, there are several improvements that could be suggested in order to make the method even more reliable.

The first way to improve my experiment would be to keep the temperature constant. If I were to repeat this experiment again, I would keep the test tubes containing the cylinders inside a stored area, where the temperature is kept constant always; this will prevent the chance of any change in the rate of osmosis and thus will prevent any inaccuracies in the mass and length readings. In order to prevent the cylinders from being irregular, I will use only those potato cylinders that are almost a perfect cylinder. In this case, the readings in mass and length would be more accurate. Another common error to overcome is that when using the measuring jar to confirm 30mL of solution to be used, I will make sure to take this reading at eye level so that I am actually using an exact volume of 30mL exactly. Also an error can be avoided when cutting the potato cylinders to an exact four centimeters; its length should be noted at eye level to confirm the length of the cylinder is four centimeters. And finally, in order to prevent the concentration of the potato cell from altering, I will use the same potato.