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Investigating Osmosis.

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Investigating Osmosis Introduction Knowing that osmosis (a diffusion of water) will occur across a semi-permeable membrane whenever there is a difference between the water concentrations on the two sides of the membrane, and knowing that when this happens to cells they will either become turgid if water flows into them, or plasmolysed if water flows out of them, and thus change their volume, we want to test the hypothesis that: If the concentration of a solution into which a cylinder of potato is placed is greater than a certain level the cylinder will contract, and if the concentration is less than that level it will expand. We have studied turgidity and plasmolysis in a textbook (Key Science-Biology, pages 143-144) and in a preliminary experiment, where we first added 2% sucrose solution to rhubarb epidermal cells, and saw them become plasmolysed, and then added water, and saw them become turgid. However, we did not use different solution concentrations, and did not measure the amount of contraction or expansion that took place. From our results in the main experiment, we should be able to work out not only the amount of contraction or expansion caused by each strength of solution, but also the concentration of the sap inside the cells. Apparatus * For the experiment we will require: * Either cylinders of potato with a diameter of 6.5mm and a height of 5mm, or a potato, a borer with a diameter of 6.5mm and a scalpel. (To allow us to make our own). * Solutions of varying strengths (of sucrose and NaCl), or a solution of a known strength and distilled water. (To allow us to make our own). * Pins (To ensure that cylinders remain separate while in the solutions.) * Test-tubes * Callipers (To measure cylinder height and diameter.) Diagram Method We take a cylinder of potato, with a diameter of 6.5mm, from the potato, and cut it into separate cylinders each with a height of 5mm. ...read more.


the cell will lose water by osmosis. Again, water crosses the cell membrane in both directions, but this time more water leaves the cell than enters it. Therefore the cell will shrink. Firstly what happens to plant cells: Plant cells always have a strong cell wall surrounding them. When the take up water by osmosis they start to swell, but the cell wall prevents them from bursting. Plant cells become "turgid" when they are put in dilute solutions. Turgid means swollen and hard. The pressure inside the cell rises, eventually the internal pressure of the cell is so high that no more water can enter the cell. This liquid or hydrostatic pressure works against osmosis. Turgidity is very important to plants because this is what make the green parts of the plant "stand up" into the sunlight. When plant cells are placed in concentrated sugar solutions they lose water by osmosis and they become "flaccid"; this is the exact opposite of "turgid". If you put plant cells into concentrated sugar solutions and look at them under a microscope you would see that the contents of the cells have shrunk and pulled away from the cell wall: they are said to be plasmolysed. When plant cells are placed in a solution which has exactly the same osmotic strength as the cells they are in a state between turgidity and flaccidity. We call this incipient plasmolysis. "Incipient" means "about to be". When I forget to water the potted plants in my study you will see their leaves droop. Although their cells are not plasmolsysed, they are not turgid and so they do not hold the leaves up into the sunlight. And now for the animal cells: When animal cells are placed in sugar solutions things may be rather different because animal cells do not have cell walls. In very dilute solutions, animal cells swell up and burst: they do not become turgid because there is no cell wall to support the cell membrane. ...read more.


So we see that the cell membrane is a very intricate and important component of the cell. Dynamic Equilibrium Introduction: The concept of equilibrium is a very important one to scientists in all fields. Static equilibrium refers to a condition in which the parts of a system have stopped moving, and is rare in nature. Dynamic equilibrium refers to a condition in which the parts of a system are in continuous motion, but they move in opposing directions at equal rates so that the system as a whole does not change Water molecules move randomly. When water is enclosed by a membrane, living or artificial, some of the moving water molecules will hit the membrane, exerting pressure on it. This pressure is known as water potential. As the number of water molecules increases, the number of collisions between the molecules and the surrounding membrane increases. This causes the pressure on the surrounding membrane to increase so the water potential will increase. Water potential is represented by the symbol ? (Greek letter, "psi"). It is measured in units of pressure, usually kilopascals (kPa). Pure water has a water potential of zero. A solution will have a lower concentration of water molecules so it will have a negative water potential. Passive Transport Passive transport is the diffusion of substances across a membrane. As we stated above, this is a spontaneous process and cellular energy is not expended. Molecules will move from where the substance is more concentrated to where it is less concentrated. Although the process is spontaneous, the rate of diffusion for different substances is affected by membrane permeability. Since membranes are selectively permeable, different molecules will have different rates of diffusion. For instance, water diffuses freely across membranes, an obvious benefit for cells since water is crucial to many cellular processes Osmosis is a special case of passive transport. Water will diffuse from a hypotonic solution to a hypertonic one. Generally speaking, the direction of water flow is determined by the solute concentration and not by the "nature" of the solute molecules themselves. ...read more.

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