Investigating Osmotic Effects on Plant Cells

Investigating Osmotic Effects on Plant Cells
Introduction ==== Water is not only the medium in which metabolic reactions occur but it provides skeletal support and thus is vital for plants. Animal cells are not used to demonstrate osmotic effect in this study because it would raise moral and ethical issues in regards to animal rights and the use of animals in scientific experimentation. Also animal cells don?t have cell walls so if they were in dilute solutions the cell membrane would expand and eventually split. This type of change due to osmosis would be difficult to measure. A plant cell (fig 1) maintains its shape due to the cell wall being rigid and the cell wall pushing outwards onto it. Water will enter a cell by osmosis. Osmosis is the movement of water molecules from a dilute solution to a concentrated solution through a selectively permeable membrane (fig 2). When a plant cell gains water the vacuole expands causing the cell to stretch and become larger. The cell is turgid (fig 3). This occurs when the cell is surrounded by a more dilute solution. If a plant cell is put in a more concentrated solution then what is in, its vacuole becomes flaccid. The cell is plasmalysed. This is because water travels from a dilute solution to a concentrated solution by osmosis.The vacuole then shrinks causing the pressure inside the cell to reduce and the cell gets shorter (fig 4). Aim I am going to investigate what happens to plant cells when placed in different concentrations of sucrose. During my preliminary studies and trial experiments (see appendix) I studied rhubarb cells in sucrose & in water and I then examined them with a microscope. The information I gained from these trial experiments led me to use potatoes as my source of plant cells (see study p â??â??â??â?? Appendix). Hypothesis 1. I predict that potato cells in dilute solution will become turgid. 2. In these conditions each cell will become moderately larger causing the potato cylinder as a whole to expand. For hypothesis 1 & 2 the changes will happen due to osmosis. This is because there is a more dilute solution outside the potato cylinder than in the potato cells? vacuoles. This causes water to move from a dilute solution to a concentrated solution through a selectively permeable membrane 3. I predict that potato in concentrated solution will become flaccid. 4. This will cause each cell to shrink causing the entire potato cylinder to decrease in size. For predictions 3 & 4 the changes will occur due to osmosis. This is because there is a more concentrated solution outside the potato cylinder than in the potato cells? vacuole. Water moves from inside the cells to outside of them from a dilute solution to a concentrated solution through a selectively permeable membrane. 5. I think that when there is an equal concentration of solution inside and outside of the cell then the cell?s size will remain constant. Variables ===== 1. I am going to use the same variety of potatoes for all parts of the experiment to keep cell contents similar so changes are not due to genetic variation. 2. The size of the potato cylinder will be kept the same by using the same borer to cut it to the same diameter. I will measure all the cylinders using the same ruler so changes are not due to differences in original length. 3. I will keep all the potato cylinders at the same temperature by keeping all the experiments in the same place because molecular movements by osmosis speed up with a high temperature. 4. The concentration of sucrose is a variable that will be changed. 5. The volume of solution will be the same in each experiment so that there is no variation in the results due to solution. Plan ==== This plan was devised following my pilot study and trial experiments detailed in the appendix. Each preliminary experiment I carried out led me to develop this final method, to the choice of apparatus and how the variables could be controlled. The preliminary studies helped inform the plan. Apparatus ===== Â? Permanent Marker ? to label solutions = Boiling tubes ? to hold the solution and potato cylinders. = Cork borer ? to give each potato cylinder an even diameter. = Tile ? to prevent cork borer from damaging my hand or the work surface. = Scalpel ? to cut the potato to an appropriate size. = Ruler ? to measure precise lengths of potato cylinders. = Solutions ? solutions of sucrose and water in varying concentrations. = Bungs ? to cover the boiling tube to prevent the evaporation of water. This keeps the overall concentration of water the same throughout the experiment. = Measuring Cylinder ? to measure the volume of the solutions accurately. Safety == = Use cork borer downward onto a tile to prevent it damaging your hand. = Don?t spill solutions onto the floor, as they will make it slippery. = Be careful with glassware. Method == I picked a number 7-cork borer and pushed it through my potatoes until I had enough cylinders for my experiment. I did this on a tile to prevent harm to myself or the bench. I then cut each of the cylinders to precisely 30mm long. I placed 3 of these cylinders in each boiling tube. I used 3 cylinders per tube to average out errors and so increase the reliability of the results. I used 40cm³ of solution for each test, I measured this in a measuring cylinder at the bottom of the meniscus. I chose to use sugar solutions of 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, & 1.0 molarities. I then placed a bung in each boiling tube to reduce evaporation and maintain the concentration of the solution. I left the potato cylinders in the solution for 48hrs and then re-measured them. The potato cylinders felt turgid at: 0.0, 0.1, 0.2 & 0.3 molarities. In 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, & 1.0 molarities the potato cylinders felt flaccid, however only some of the 0.4 cylinders felt flaccid. Concentration of solution (molarity) Length (mm) Start Length (mm) end Change in length (mm) % change in length Average % change in length Observation / feel 0.0 (1) 30 (1) 33 (1) 3 (1) 10 10 Turgid (2) 30 (2) 32 (2) 2 (2) 6.7 Turgid (3) 30 (3) 34 (3) 4 (3) 13.3 Turgid 0.1 (1) 30 (1) 32 (1) 2 (1) 6.7 5.6 Turgid (2) 30 (2) 32 (2) 2 (2) 6.7 Turgid (3) 30 (3) 31 (3) 1 (3) 3.3 Turgid 0.2 (1) 30 (1) 32 (1) 2 (1) 6.7 4.4 Turgid (2) 30 (2) 31 (2) 1 (2) 3.3 Turgid (3) 30 (3) 31 (3) 1 (3) 3.3 Turgid 0.3 (1) 30 (1) 31 (1) 1 (1) 3.3 1.1 Turgid (2) 30 (2) 30 (2) 0 (2) 0 Turgid (3) 30 (3) 30 (3) 0 (3) 0 Turgid 0.4 (1) 30 (1) 29 (1) -1 (1) -3.3 -2.2 Flaccid (2) 30 (2) 30 (2) 0 (2) 0 Turgid (3) 30 (3) 29 (3) -1 (3) -3.3 Flaccid 0.5 (1) 30 (1) 28 (1) -2 (1) -6.7 -5.6 Flaccid (2) 30 (2) 28 (2) -2 (2) -6.7 Flaccid (3) 30 (3) 29 (3) -1 (3) -3.3 Flaccid 0.6 (1) 30 (1) 27 (1) -3 (1) -10 -8.9 Flaccid (2) 30 (2) 27 (2) -3 (2) -10 Flaccid (3) 30 (3) 28 (3) -2 (3) -6.7 Flaccid 0.7 (1) 30 (1) 27 (1) -3 (1) -10 -12.2 Flaccid (2) 30 (2) 26 (2) -4 (2) -13.3 Flaccid (3) 30 (3) 27 (3) -4 (3) -13.3 Flaccid 0.8 (1) 30 (1) 26 (1) -4 (1) -13.3 -13.3 Flaccid (2) 30 (2) 25 (2) -5 (2) -16.7 Flaccid (3) 30 (3) 27 (3) -3 (3) -10 Flaccid 0.9 (1) 30 (1) 25 (1) -5 (1) -16.7 -17.8 Flaccid (2) 30 (2) 24 (2) -6 (2) -20 Flaccid (3) 30 (3) 25 (3) -5 (3) -16.7 Flaccid 1.0 (1) 30 (1) 24 (1) -6 (1) -20 -17.77 Flaccid (2) 30 (2) 24 (2) -6 (2) -20 Flaccid (3) 30 (3) 26 (3) -4 (3) -13.3 Flaccid Conclusion The results I gained (as in the preliminary experiments) are due to osmosis. The potato cells in distilled water (0.0 molarity) to 0.3 molarity increased in size and became turgid due to there being a greater concentration of water outside the cell than inside the cell. Thus, water moved into the cell through a selectively permeable membrane from a dilute solution to a concentrated solution by osmosis. This causes each cell?s vacuole to push out against the cell wall causing the potato to become turgid. The potato cells in a solution between 0.5 and 1.0 molarities decreased in size and became flaccid due to there being a smaller concentration of water outside the cell than inside the cell. Thus, water left the cell through a selectively permeable membrane from a concentrated solution to a dilute one. This causes each cell?s vacuole to shrink causing the cell membrane to become withdrawn from the cell wall and the potato to become flaccid. This shows that the concentration of sugar inside the potato cells lies somewhere between 0.3 and 0.4 molarities. If you follow the line of best fit on the graph it suggests that average percentage change would be 0 at 0.31 molarity. The graph also shows that the data forms a strong negative correlation. This shows overall that the stronger the concentration of sugar the more cell will decrease in size. The correlation also shows that the potato cell?s decrease in size in a regular manner, making it possible to predict the results from other molarities. Evaluation == My final experiment allowed me to gather exactly the results I wanted. It produced a large range of data and due to using 3 potato cylinders per solution the anomalous results were obvious and thus didn?t hinder my experiment. The experiment proved all of my first four hypotheses to be perfectly correct showing that my overall theories of osmosis were correct. My fifth hypothesis was null hypothesis because no solution had an average percentage change of 0. However, from my graph you can see that the concentration of 0.31 is likely to give an average percentage change of 0. Thus this hypothesis is partially proved.

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