To determine concentration of cell sap in potato tuber cells.

Apparatus-        6 strips of potato tuber of equal shape and mass

6 beakers containing different concentrations of distilled water and sucrose solution from 0M to 1M in 0.2M intervals. Each should contain 300ml

                Measuring cylinder

                Balance

                Filter Paper

                Scalpel

                Tile

                Safety Glasses

                

Diagram-

Prediction- I predict that the concentration of cell sap lies between 0M and 1M. I know this from my preliminary work. I also predict that the mass of the potato tubers will increase when I put them in distilled water- they will become turgid, and the mass will decrease when I put them in 1M sucrose solution- they will become plasmolysed. This is because osmosis will occur when the water molecules move from the regions of low concentration of water molecules through the partially permeable cell membranes to the regions of high concentration of water molecules.

Method-

~ Take six ready cut strips of potato tuber, weigh them and record their masses

~ Place each one in a test tube containing distilled water/sucrose solution and leave for 24 hours

~ Take them out and blot them on filter paper

~ Weigh them and record their masses

~ Record which potato tuber chip has changed the least in mass

~ Repeat experiment to improve reliability in case the first set of results were incorrect

I will control the mass and shape of the potato tubers and the amount of solution in the test tube by keeping them the same throughout the experiment. This is because we do not know whether the surface area could affect the rate of osmosis and I need to make this experiment a fair test. I will vary the concentration of the solution so as to work out what concentration cell sap has. If I do not leave the potato for long enough I will limit the process of osmosis so I will leave it to react for 24 hours. I will also make it a fair test by blotting the tubers on filter paper before I weigh them to remove any excess water. It is important to keep it a fair test so that my results are correct and also accurate.

Risk Assessment-

I am using scalpels in this experiment, which are potentially dangerous. If I use them safely I am unlikely to be harmed. When I am using a scalpel I should work carefully and wear safety goggles.

Results-

Analysis-

This graph shows that the greater the concentration of the solution in M, the less the increase in mass. The mass of the potato tubers increased when I put them in distilled water- they became turgid (held as much water as they could hold) and the mass decreased when I put them in 1m sucrose solution- they became plasmolysed. This is because osmosis occurred when the water molecules moved from the regions of low concentration of water molecules through the partially permeable cell membranes to the regions of high concentration of water molecules. This is because molecules in a solution move in constant random motion because they have kinetic energy and the water molecules pass through the pores of the partially permeable cell membranes in the potato tuber cells but the sucrose molecules are to big to get through since they contain more atoms (C12H22O11 as compared to H2O). Since they are in constant random motion the water molecules eventually moved across the membrane to the area with less water molecules in (down the concentration gradient of concentration of water molecules) until the two sides were in equilibrium: there was no net movement of water molecules.

My aim for this experiment was to determine the concentration of cell sap in potato tuber cells. My graph passes through the point on the x-axis of 0.328M, which tells me that is the point where no net osmosis occurs: the iso-osmotic point. Therefore that is the concentration of cell sap in the vacuoles of the potato tuber cells.  My results support my prediction in general because the scientific ideas in the prediction and the results are the same. I predicted that the concentration of cell sap was between 0M and 1M and my results proved it to be at 0.328 so my prediction was right but my results showed the correct value to be nearer the bottom end of my predicted scale.

        At the bottom of my graph it becomes less and less steep. This means that as the concentration of the solution gets bigger and the percentage increase gets smaller the amount of mass that is lost is getting smaller and smaller. I think that if I extended my experiment so that I measured using solutions more concentrated than 1M I would see that the line of my graph would level off and there would come a point when the potato chip does not increase any more in mass. This would be because there must be a point when a potato chip cannot lose any more water: there is only a finite amount of water inside a potato tuber cell.  

Evaluation- When I was doing this experiment I didn’t have any major problems, however I found it hard to accurately measure the amount of solution used in a measuring cylinder. If I repeated the experiment I would use a measuring pipette.

To make the results reliable I used a good range of concentration solutions and kept other variables the same. I knew these results were reliable because they fitted in with my prediction and previous scientific knowledge, the line of best fit was the shape I expected it to be and I took all appropriate measures to keep it reliable. To increase my confidence in the reliability of this experiment I could do it with more different solution concentrations and therefore make my line of best fit better.

        My results were very close to my line of best fit. There were no very odd results in my experiment. My measurement for the concentration of solution at 0.8M was slightly above my line of best fit. This could have been because of me not blotting the potato tuber enough. Since blotting the potato tubers after I took them out of the solutions is not accurately measured and done by a human not a machine this is an easy place to lose accuracy. If I repeated this experiment this is an area I could take more care in. I tried to take accurate readings by blotting the potato tubers on a piece of filter paper before I weighed them and using a balance that went into one decimal place. To improve accuracy I could use a balance that weighed more accurately, going into more decimal places. I made sure it was a fair test by keeping all variables the same (temperature, amount of solution, mass and shape of potato tuber,) apart from the one I was altering (concentration of solution).

        I have quite a lot of confidence in these results because I have repeated my experiment a few times and every time I have repeated it I have come up with the same results.

        Looking at this topic I could also investigate plasmolysis more fully. I could investigate the maximum decrease in mass a potato can have through osmosis. To do this I could use solution concentrations of 1M, 1.1M, 1.2M, 1.3M, 1.4M, 1.5M, 1.6M, 1.7M, 1.8M 1.9M and 2M. I could plot this on my graph I have done for my cell sap experiment and see at what point the graph levels off and not longer curves round. This would indicate no further decrease in mass. I will leave them for 24 hours because that is a sensible amount of time to leave a potato tuber chip for if I want osmosis to occur.

I could also find out the concentration of cell sap in onion cells to see if it is the same as potato cells. To do this I would use exactly the same method as I used for the potato cells:

 ~ Cut 6 pieces of onion, weigh them and record their masses

~ Place each one in a test tube containing distilled water/sucrose solution and leave for 24 hours

~ Take them out and blot them on filter paper

~ Weigh them and record their masses

~ Record which piece of onion has changed the least in mass

~ Repeat experiment to improve reliability in case the first set of results were incorrect

I will use exactly the same solution concentrations as I did in the potato experiment going from 0M distilled water to 1M sucrose solution in 0.1M intervals. I will plot it on the same graph as the potato cell sap concentration graph so that I can easily compare the two cell sap concentrations. The concentration of solution at which the line of best fit will cross the x-axis, where the onion is no longer losing or gaining any mass (iso-osmotic point) will be the concentration of cell sap in onion cells.