Investigation of Water Potential in a Root Vegetable
Osmosis is the passage of water from a region of high water potential to a region of low water potential, down a water potential gradient through a semi-permeable membrane. If a plant cell, or an item with similar properties is put in water, three different things can happen: • If the surrounding area has a higher water potential, the cell will increase in mass through osmosis. The cell will become turgid. • If the surrounding area has a lower water potential, the cell will decrease in mass through osmosis. The cell will become flaccid. (If you look at the cells under a microscope, you will see the plasma membrane + cell contents have pulled away from the cell wall: They have been plasmolysed) • If the medium is exactly the same water potential as the cell there will be no net movement. (Water crosses the cell membrane but the amount going in is the same as the amount coming out, so there is no net movement, When the cell is in this state between turgidity and flaccidity, It’s called Incipient Plasmolysis. Osmosis is a form of passive transport, passive transport doesn’t use energy, the molecules go down a concentration gradient. Active transport is the opposite it uses ATPase for energy, and molecules move against a concentration gradient.
When the potato cylinder is placed in a high water potential sucrose solution I think it will increase in mass, and length. This is because the solution has a higher water potential than the cells in the cylinder, water molecules will travel from a high water potential (The solution) to a low water potential (The potato cylinder) through a semi-permeable membrane by osmosis. More molecules will enter the cell then leave it, so the potato cylinder will swell, as the cells become turgid. The contents of the cell in this state of turgidity push against the cell wall, which causes pressure to it, and the cells expand, meaning the potato cylinder as a whole would increase in mass, and length. I predict that any solution under 0.4M will have this effect. When the potato is placed in a low water potential sucrose solution I think it will decrease in mass, and length. This is because the solution has a lower water potential than the potato cylinder, so this time the opposite effect occurs. Water molecules travel from a high water potential, inside the potato cylinder, to a low water potential, the solution, through a semi permeable membrane by osmosis. More molecules leave the cell then enter it, so the potato cells will shrink, and become flaccid. The contents of the cell pull away from the cell wall. The cell wall pushes on the contents and the cell shrinks and loses its shape. This is called Plasmolysis. The result is that the potato cylinder as a whole loses weight, and decreases in length. I think any solution over 0.5M will have this effect. I think somewhere between 0.4M and 0.5M, there will be an equilibrium between the solution and the potato. This is when the water potential of the solution and the water potential of the potato cells are equal. There is no net movement of water between the two, and there should not be any change in the mass or length of the potato.
As a class we conducted an experiment as a preliminary study for the osmosis experiment. The aim of this experiment was to work out the water potential onion tissue by calculating the strength of sucrose solution at which 50% of the cells are plasmolysed. From this we could find out where the point of incipient plasmolysis was. After creating a graph, and discussing anomalous results, we agreed that the point of incipient plasmolysis was around 3.4-3.6M. Any molarity over this amount more cells were plasmolysed, and under more cells were turgid. This backs up my prediction about roughly where I think there will be an equilibrium. The results...
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