- OSMOSIS -
Osmosis is diffusion of water. It is the overall movement of water from a dilute solution (high concentration of water) to a more concentrated solution through a partially permeable membrane (only allows certain substances to diffuse through it).
A potato contains plant cells, and the reason I shall be using plant cells in this Biology Investigation is because they have a membrane and a cell wall, unlike animal cells that only have a membrane (as shown on the picture on the next page). If too much water was to move into an animal cell, it would burst. When water moves into a plant cell, the cell wall will prevent the cell from bursting; it will only swell, but shrink if water has moved from the cell to a solution with low water concentration.
In this Biology Investigation, I will be looking at osmosis in a plant cell (potato). I will observe the change in mass of pieces of potato before putting them in salt solutions/distilled water and after they are taken out. If the potato's mass decreases, that means that water has moved from the potato (high concentration of water) into the beaker filled will salt solution (low concentration of water). If the potato increases in mass, this means that water has moved from a high concentration of water (in beaker) into a low concentration of water (potato). Water always moves from a high to low concentration of water. If the mass of the potato remains the same, the water concentration in the beaker and potato must be equal.
I will be using equipment, such as:
§ a stopwatch,
§ digital scales and
§ a measuring jug
that give accurate results. I will also be using a knife and tile for cutting the potato into pieces. I shall be using a stopwatch because it is more reliable than a clock. The digital scales and measuring jug are more accurate - digital scales will give me the exact mass of a piece of potato in grams (with 2 decimal places), and using a measuring jug gives me the exact amount of solution, rather than estimating the approximate amount of solution I need then pouring it into the beaker. I will also pay attention to safety; I will be carefull while cutting the potato, and I will use a tile to cut on, rather than cutting/scratching the surface I am working on.
My preliminary work to check the method gave some results. I placed a piece of potato into 500ml of distilled water, 1%, 3% and 5% salt solution (range of solutions) and left them for 30 minutes.
1% salt soln
2% salt soln
3% salt soln
The results from my preliminary work follow a pattern, because as the concentration of salt solution increases, the change in mass is going from positive to negative , which means that starting from distilled water, the mass of the potato is decreasing - water is moving from a high concentration of water (potato) to a low concentraion of water (beaker with salt soln).
Because my preliminary work has given appropriate results, I will only change the time the pieces of potato are kept in the salt soln/distilled H2O from 30 minutes to 40 minutes. I will keep the amount of salt soln/distilled H2O the same (500ml). I will place pieces of potato into distilled water and different concentrations of salt solutions. I will measure the mass of each piece of potato and record the mass when taken out of DH2O/salt soln. I will be using 500ml of distilled water and 500ml of 1%, 2%, 3%, 4% and 5% concentation of salt solutions. The pieces of potato shall be left in distilled H2O and the salt soln for 40 minutes. The test will be repeated 3 times.
As you increase the salt concentration, the water concentration decreases (there will be a lower concentration of H2O), so I would predict that the potato will lose more mass in the higher concentration of salt solution, because in osmosis water moves from a high concentration of water (in potato) to a low concentration of water (salt solution).
The line on my Average Change in Mass Graph is going from positive to negative. There is a pattern; you can see as the concentration of salt solution increases (over x-axis), the average change in mass results are decreasing (going from positive to negative - over y-axis), which shows as the concentration of salt solution increases, the mass of the potato decreases because water is moving from a dilute solution (potato) to more concentrated solution (salt solution). However, there is an outlier (4% salt soln, circled in graph). The case could have been that the 3% and 4% salt soulution god mixed up/switched around, because lookingat the Change in Mass Table, the potatos in the 3% salt soln have decreased in mass somewhat more than the potatos in the 4% salt soln, even though the potatos have been in the solutions for the same amount of time (40mins). The potatos in the 4% salt solution should have decreased more in mass than the potatos in the 3% salt solution, because more water from the potatos is moving to the more concentrated solution (salt solution) in the beaker. There is a lower concentration of H2O in the 4% salt soln than the 3% salt soln, which is why more water from the potato should move to the salt soln, causing a lower mass of the potato in the 4% salt soln, reason being that water always moves from a high concentration of water (from potato) to a low concentration of water (salt solution).
The method I used was reliable because the results have given a pattern you can see in the Average Change in Mass Graph (as concentraion of salt soln increases, the mass of the potato decreases). The change in mass results for each solution were similar and reliable as I ended up with a pattern. There was an outlier (circled in the graph) for the 4% salt soln. I suggested that the 4% salt soln could have got mixed up with the 3% salt soln as the potatos in the 3% salt soln have decreased more in mass than the potatos in the 4% salt soln. My results support my conclusion: by increasing the concentration of salt solutions, the mass of the potatos decreases. My method and results allow my conclusion the be valid because they have given appropriate results. My choice of equipment allowed my results and conclusion the be valid, because I was using equipment, eg. stopwatch, measuring jug and digital scales, that give accurate results. I could have used a potato cutter instead of a knife, so that each potato would have the same size, making my results more accurate. I could extend the experiment by using plant cells from a different vegetable or a plant, eg. carrot or stem of a flower, and leave them for a longer amount of time in water (50/60 minutes) in more concentrated salt solutions (6%, 7%, 8%, 9% and 10%) so they would decrease in mass much more.