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To determine the water potential of a potato tuber cell using varying salt solution.

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Aim To determine the water potential of a potato tuber cell using varying salt solution. Introduction Osmosis is the diffusion of water molecules from a region where it has higher water potential to a region where it has lower water potential through a partially permeable membrane1. As osmosis is a type of diffusion the same things that affect diffusion have an effect on osmosis some of theses things are: * The concentration gradient - the more the difference in molecules on one side of the membrane compared to the other, the greater the number of molecules passing through the membrane and therefore the faster the rate of diffusion2. * The surface area - the larger the area the quicker the rate of diffusion * The size of the diffusing particles - the smaller the particle the quicker the rate and polar molecules diffuse faster than non-polar ones3. * The temperature - the higher the temperature the more kinetic energy the particles have and so the faster they move. From the diagram4 we can see the process of osmosis in a simple expression. On the right side there is pure water, which has the maximum water potential of 0. Water potential is the pressure created by water. As you can see from the diagram the pure water is pushing its way through the semi permeable membrane at a high pressure. This is its water potential. Water potential is measured in kilopascals (kPa) and is represented by the letter ?. Prediction I predict that the lower the concentration of salt solution the higher the water potential will be and therefore the higher the % change in mass will be, I predict this because osmosis is the diffusion of water molecules from a region where it has higher water potential to a region where it has lower water potential through a partially permeable membrane5. Therefore the distilled water will have the highest water potential and the 1.0 mole of salt solution will have the lowest water potential. ...read more.


* Record these results also on the results table. * The experiment is now complete. Tidy the area and make sure everything is put away properly Variables Variable Why it is a variable How it will be controlled The concentration on salt solution The concentration of the salt solution will change how much water potential there is and therefore effect how much water enter or leaves the cell. This is the independent variable that will be changed by dilutions. Therefore it does not need to be kept controlled. Temperature Higher temperatures would make osmosis occur more quickly as the particles gain kinetic energy and therefore diffusion takes place more quickly. There fore the rate of the osmosis increases. At low temperature the molecules do not have a lot of kinetic energy and therefore the rate of osmosis would take longer. If the potatoes are kept at different temperatures the rate of osmosis in each potato cell will be different and therefore some cells will gain an extreme amount of weight in a little amount of time and vice versa. The experiment will be conducted at room temperature and at the same time, which means there will be a constant temperature and if it the temperature does change then all will be affected not distorting the results drastically. The surface area of the potato The bigger the surface area the more molecules that can pass through into the cell via osmosis. If the potatoes are not cut at the same length their surface areas will change. This means a potato cell with a large surface area will have a quicker rate of diffusion. This means the amount of mass gained will be distorted like the effect of temperature. To keep this constant the potato will be cut at in 2cm length pieces insuring all 12 are cut at exactly the same length The amount of salt solution The amount of salt solution will affect how much weight the potato cell gains. ...read more.


There were some limitations such as time and the range of equipment available such as buffer solution, but overall the method fulfilled its purpose to help me find out the water potential of the potato pieces. Reliability I think my experiment was reliable to an extent as I did conclude a result that satisfied my prediction. I think my experiment was reliable because most of my equipment was consistent as I didn't have any problems with any of the apparatus, also the fact that the experiment was repeated and similar results were achieved both times showed it was a relatively steady experiment. From the results I can see that only one of the point is an anomalous point this could have been caused from the fact that the experiment didn't have a very good range of concentrations. I think I should have used a wider range of concentrations to see what would happen after 1Mole as the solution that was given was of 2Mole. Also as my constant error table shows, my I believe my most significant error was that of the 2Mole salt solution that was provided having said this my results are similar to others so the experiment must have been reliable. To gain an even further understanding into water potential I could do experiments that change water potential like the affect of temperature on osmosis and the effect of different types of potatoes. I think this would give more accurate and reliable evidence to the conclusion of my experiment. 1 Essential AS Biology- Glenn and Susan Toole chp.4.3 pg 60 2 Biology 1- Mary Jones chp.4.1 pg 55 3 Essential AS Biology- Glenn and Susan Toole chp.4.2 pg 59 4 http://www.dr-evans.com/advancedbiology/osmosis.html 5 Essential AS Biology- Glenn and Susan Toole chp.4.3 pg 60 6 http://homepages.nildram.co.uk/~gaz/waterpot.htm 7 http://homepages.nildram.co.uk/~gaz/pressurepot.htm 8 http://www.biologymad.com/master.html? 9 Essential AS Biology- Glenn and Susan Toole chp.4.4 pg 62 10 http://www.purchon.com/biology/osmosis.htm 11 http://www.purchon.com/biology/osmosis.htm 12 Essential AS Biology- Glenn and Susan Toole chp.4.4 pg 63 ?? ?? ?? ?? Sneha Patel Page 1 5/6/2007 ...read more.

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An excellent account with a very high level of detail and correct use of A level biological terminology throughout.

Marked by teacher Adam Roberts 10/05/2013

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