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Investigation into osmosis in plant tissue

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Marek Zaremba-Pike 11J 10.10.2002 Biology Coursework My aim is to investigate osmosis in plant tissue. Osmosis is the movement of water from an area of low solute concentration to an area of high solute concentration through a selectively permeable membrane. In this experiment the plant cells will be potato chips and the solution will be sucrose, later sucrose diluted with water. Osmosis is a passive process that will occur across a selectively permeable membrane whenever there is a difference between the water concentrations on the two sides of a selectively permeable membrane. Osmosis is really a special kind of diffusion, because water moves down the concentration gradient. Unlike diffusion, by definition Osmosis only refers to the movement of water across a selectively permeable membrane. I predict the higher the concentration above the isotonic point the shorter the chip, and the lower the concentration below the isotonic point of the solute the longer the chip. I also predict that double the length solute concentration, i.e. double the concentration of sucrose, will result in double the length change. A high concentration of water is called a weak or dilute solution, and it is said to be hypotonic to the stronger solution. A low concentration of water is called a strong or concentrated solution, and it is said to be hypertonic to the weaker solution. When two such solutions are divided by a selectively permeable membrane the water will move from the area of high concentration to the area of low concentration, until both sides have reached equilibrium (become isotonic), are equally concentrated. Water moves from the hypotonic solution into the hypertonic. Solution A is hypertonic and solution B is hypotonic. Water moves from A to B until the concentration is equal, i.e. isotonic. Selectively permeable membrane (only allows water through) One important example of Osmosis is water entering the roots of plants. If plants cells are placed in different strength solutions they will swell up (gain turgor) ...read more.


Repeat this twice, each after a minute's interval. I obtained the following results:- Concentration (molars) Initial Length (mm) Volume of Sucrose (ml) Change in Length (mm) % change 0.9 50 18.0 -4 8 0.6 50 12.0 -3 6 0.3 50 6.0 0 0 0.0 50 0.0 +2 6 It all seems to be reasonable and it places the isotonic point roughly around 0.3 moles. Method I will use three chips for each concentration and eight different concentrations. This is because this will ensure greater accuracy; more results ensure greater accuracy as long as they are not averaged. When you average results you hide anomalous results and the line of best fit is less likely to be accurate. I will use concentrations of 1.0 moles, 0.8 moles, 0.6 moles, 0.4 moles, 0.3 moles 0.2 moles, 0.1 moles and 0.0 moles of sucrose. I decided to use these concentrations so I could gain a wide range and a focus around the isotonic point, which I already know from the preliminary to be around 0.3 moles. To work out how much sucrose to use and how much water I simply use 1.0 molar sucrose, and multiply the desired amount of moles of sucrose I want by 20, the rest of the 20ml should be water: e.g. for 0.9 moles multiply 0.8 x 20 = 16. So I measure 18.0ml sucrose in the measuring cylinder and pour it into the test tube. Then I pour another 4ml of water in and I have a concentration of 0.8 moles. 1 Assemble the apparatus. Make sure the test tubes and measuring cylinder is clean. 2 Measure room temperature. 3 Measure out the desired amount of sucrose accurate to 0.1ml and pour into test tube. Place test tube on test tube rack and fill test tube with desired amount of water. Repeat with all eight desired concentrations. Mark each concentration next to test tube. ...read more.


2 Measure out the desired amount of sucrose accurate to 0.01ml and pour into test tube. Place test tube on test tube rack inside water bath and fill test tube with desired amount of water. Repeat with all eight desired concentrations. Mark each concentration next to tube. 3 Cut 48 potato chips to 50mm long, using the tile as chopping board. Then weigh each individual chip cutting each down till they are all exactly the same weight, to between the nearest 0.01g and 0.05g. 4 Place six chips in two of the sixteen test tubes, three chips in each test tube, preferably the one with the highest concentration. Start the stop watch. 5 After 1 minute place another six chips in the next two test tubes, the one with the next lowest concentration. Repeat this after 2, 3, 4, 5, 6 and 7 minutes. 6 After 40 minutes after start of experiment remove the first six chips from the two test tubes (the highest concentration) and weigh each individually to the previous scale used. Record the results. 7 After 41 minutes since experiment began remove the next highest concentration's chips and weigh. Repeat this twice, each after a minute's interval. 8 Clean apparatus, and tidy away. The use of a water bath also allows use to use temperature as a variable, instead of concentration. Most reactions give double the rate of reaction when the temperature is raised by 10�C. I would suggest this is what would happen here, the reactions would speed up and the chip would lose and gain more water. It would allow us to find us the fully turgid point and when the chip becomes plasmolysed. To carry out such an experiment follow the method for e mass except instead of changing the concentrations change the preheated temperature. It would mean that each different experiment (different temperature) would have to be carried out separately at a different time. At higher temperature the test tube would also need a bung. 1 ...read more.

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