SKILL AREA P: PLANNING
The aim of this investigation is to see the effect of varying concentrations of glucose solution on the amount of osmotic activity between the solution and a potato chip. An investigation into Plasmolysis in onion cells was undertaken prior to this experiment, in order to gain some knowledge of osmosis to enable a hypothesis to be formed.
Preliminary Work and Scientific Knowledge
Osmosis is defined as the net movement of water molecules from a region in which they are highly concentrated to a region in which they are less concentrated. This movement must take place across a partially permeable membrane, like a cell wall, which allows small molecules like water through but does not let larger molecules through. The molecules continue to diffuse until the area in which they are found has an even distribution of molecules all round, inside and outside of the cell. Naturally, an environment that is identical inside and out of the cell is the best condition to survive (i.e. the ‘perfect’ environment).
In the case of the onion cells, when placed with a drop of water, after 15 minutes, the cell had a turgid, or swollen, appearance under the microscope and felt fairly strong and sturdy, as the water was diffused through osmosis into the cell. This is because pure water has a 100% concentration of water, and the onion cell has much less, so the water diffused from a higher concentration to a lower concentration. With the 1% glucose solution, the cell had a flaccid appearance and the onion felt particularly floppy, because the cell membrane is pulled into the cell when the vacuole shrinks. This is what Plasmolysis is, the shrinkage of cell cytoplasm, with the membrane being pulled in. This happens because the water inside the vacuole moves outside the cell as the 1% glucose solution is a stronger solution than the solution in the vacuole, resulting in there being a higher concentration of water inside than out.
To ensure that this investigation is a fair test, certain aspects of the experiment will have to be kept the same while one key variable is changed. To gain a result related to the aim of the experiment, the strength of the glucose solution will have to be changed. This will give a set of results from which a conclusion can be drawn, but if a non-variable is changed, the experiment would not be fair. For example if the potato chips are all different lengths, the longer ones will have more surface area enabling more osmosis to occur which could affect the result.
For this investigation, in order to have the same temperature for each experiment, they will all be done at room temperature. The volume of the glucose solution in each boiling tube must be the same and must also completely cover the chip, and to ensure this, a measuring cylinder will be used and 25cm3 of each solution will be used. The mass and length of the potato are dependent variables and so will be measured throughout the experiment. The mass will be measured in grams, using the same set of scales each time. Measuring the mass will allow us to see if osmosis has taken place because if it has then water will have either entered or left the potato. The length will be measured in millimetres using a ruler. The duration of the experiment must also be the same and so each solution test will be done at the same time and left for 15 minutes.
To ensure reliability, the experiment will be repeated in that each solution will have two potato sticks in it. Also, the exact same length of the potato chip, in millimetres, on all four sides cannot be guaranteed, and as a change of only a few millimetres is expected, a corner of the side to be measured will be cut out to make sure the same side is measured at the end of the experiment.
Slab of potato object...