Osmosis Osmosis is the process where water moves across a selectively permeable cell membrane by diffusion. Water moves from the solution with more water to the solution with less water. The process of osmosis must be tightly controlled by cells, otherwise they will die. For example, if you place a red blood cell in pure (distilled) water, it will quickly take up water until it bursts. That is why plasma, the liquid portion of our blood is made of water with proteins and salts dissolved in it, preventing the unnecessary gain of water by our blood cells. In plants, osmosis is just as important. Plants with too little water will wilt. This happens when water moves out of the cells by osmosis. Without this water there is little pressure inside the cells and the plant can no longer support itself against the pull of gravity. However, after watering the plant, the cells become re-inflated with water and the plant stands upright. The effect of water loss on plant cells is shown in the diagram below. The term hypertonic describes a solution which has more solute (salt) and less solvent (water) than the cell. In this case the cell will lose water. The term hypotonic describes a solution which has less solute (salt) and more solvent (water) than the cell. In this case the cell will gain water. The term isotonic describes a solution which has an equal concentration of solute (salt) and an equal amount of water as the cell. In this case the water moves equally into and out of the ell. Figure 1 A. Plant cell placed in pure water. This cell will become inflated because the water outside the cell is at a higher concentration than the water inside the cell. As water moves in by osmosis the vacuole fills up and presses out against the cell wall. B. Plant cell in salt solution equal to the concentration inside the cell. This cell has no overall gain or loss of water because whatever moves out will be replaced by water moving in. C. Plant cell placed in a salt solution greater than the concentration inside the cell. This cell will lose water as the water moves by diffusion from higher to lower concentration. The following experiment was done to observe the effects of osmosis on plant cells. You will use the weight of potato to see how much water moves in and out of cells in different salt solutions. Materials: cylinders of potato (using cork borer) knife cork borer; one big one, and one small one 4 beakers Deionized water salt solutions (1%, 5%, 10%) digital balance Glass rod Procedure: 1. Label 4 beakers (use small—50ml—plastic beakers) with the following: 0 % salt (deionized water), 1% salt, 5% salt and 10% salt. (Label beakers using masking tape – do NOT write on the beakers) 2. Put the correct solution in each beaker. Use enough of the appropriate solution to fill about half of the beaker. 3. Using a cork borer, take four cores from a potato and use a knife to cut four potato cylinders about 3 cms. long. Caution: be very careful with the knife. Always cut away from yourself. Make sure there is no peel left on the cylinder. 4. Place the potato cylinders on weighing dishes and place one dish in front of each beaker. When you are ready, remove one cylinder at a time and weigh it. You should weigh them to the nearest 0.1 g. Record the initial weight in Table. Note: Use a weighing dish and tare the balance. Never place specimens directly on the balance pan. 4. Immediately return the cylinder to the weighing dish in front of the correct beaker. Cut into two longitudinal pieces and transfer them to the first beaker and note the time. 5. Repeat steps # 3 and 4 for the remaining cylinders. Incubate the potatoes in various solutions for 30 min. 6. Stir the potato cylinders occasionally with a glass rod. 7. After 30 minutes, remove two pieces of each cylinder from its beaker. Briefly blot them with a paper towel to remove excess water. Do not leave the cylinders sitting on paper towels. Use the weighing boat and weigh both pieces of each cylinder and record the weight in the Table. (Don’t forget to tare the balance before each use) Repeat this process until all potatoes have been weighed in the same order in which they were initially placed in each solution. 8. Calculate the total weight change by subtracting the initial weight from the 30 minute final weight. Record this in Table. 9. Calculate the percent change for each cylinder. Percent change is calculated as follows: Final Weight at 30 minutes - Initial weight X 100 Initial weight For example, If the initial weight was 3 grams and the weight at 30 minutes was 5 grams, then 5g-3g = 2g 2g/3g = .666 x 100 = 67% Remember: Positive numbers indicate weight gains while negative numbers indicate weight losses. 9. Record your data in a table. 10. Display your data in a line graph using the graph paper provided. Graphs should be clearly labelled (include title, axis labels (with units), legend/key, clear unit measurements on each axis. Data Table: Change in Weight Based on Concentration of Solution Solution Initial Wt. Final Wt. (g) = at 30 min (g)= (I) Total Wt. % Weight Change*= Change= (F-I) (F-I)/I X 100 (F) Deionized 5 7 2 40% 1% salt 5 6 1 20% 2% salt 5 4 -1 -20% 3% salt 5 3 -2 -40% Water 0 % salt * Total Weight Change = Final Weight at 30 minutes – Initial weight (positive numbers indicate weight gain while negative numbers indicate weight loss.) Write down the following: Hypothesis: Potato cores placed in salt solutions of higher concentrations will lose more weight than those in little to no salt solutions. Prediction: The potato core placed in a higher salt solution will lose more weight due to Osmosis and water leaving the cell. The potato core that is placed in salt solutions with less or no salt will gain weight. Variables Dependent Variable Independent Variable Controlled Variables The dependent variable is the weight changes that are affected by the independent variable change (salt level changes) The independent variable is the change in salt levels throughout the experiment. 1. Weight of potatoes 2. Time potatoes are in beakers 3. Size of beakers used Effect of varying salt solutions on weight of potato cylinders Answer the following questions based on the graph: Which solution is most likely isotonic? The 1% salt solution is the solution that is most likely isotonic. Which solution is most hypertonic? The 0% solution is the most hypertonic because of the drastic weight gain. Which solution is most hypotonic? The most hypotonic solution is the 3% salt solution because of the drastic weight loss.
