Experiment to show that the concentrations ofliquids can affect osmosis

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Experiment to show that the concentrations of

liquids can affect osmosis

 Pre-Experiment

Background Information

Osmosis is, by definition, the diffusion of water from a weaker, or more dilute concentration, to a stronger solution (i.e. down a concentration gradient) through a semi-permeable membrane, and it is this pinciple which I want to study. In order to observe any results, and draw a conclusion from them, I need to thoroughly research osmosis, and it’s effect on plant and animal cells. This will allow me to design some preliminary experiments, and finally the main experiment, which may or may not prove that the concentrations of liquids have an affect on osmosis. Osmosis can occur in both plant and animal cells, and allows processes such as transpiration to occur. It also can change the structure of a cell, to a point where the cell can no longer function. Osmosis is the only way that water can diffuse in and out of a cell.

When the solutions on either side of the semi-permeable membrane are equal, or isotonic, the net movement of water is zero. As there is no difference between the two solutions, no water needs to be moved in order to dilute, and osmosis does not occur.

        

In plant cells, osmosis can occur because the cell membrane acts as the semi-permeable membrane, and water from dilute solutions outside the cell enters through the membrane into the strong solution of the vacuole, and keeps the cell turgid. This process allows for the transport of water through a plant, as each successive cell becomes more dilute than its neighbour. Turgidity is achieved when the cell membrane is pushed against the cell wall due to turgor pressure. The cell does not burst, however high the turgor pressure is. This is due to the cell wall, unique to plant cells, which holds the cell together. Turgidity is the reason that plants stay upright, using the cell wall and the interior pressure to hold it’s shape, much like a blown-up balloon. This ability to use a state of turgor to stay upright makes up for a plant not having a skeleton of any kind.

The opposite of turgidity is flaccidity. If a cell contains comparatively little water (and is therefore not turgid), it is deemed to be flaccid, and the cell becomes shrivelled up without any interior turgor pressure to keep it’s shape. Flaccidity is the cause of wilting, for the same reason that turgidity holds a plant upright - when there is too little turgor pressure inside the plant to hold it straight, the plant wilts. When a cell becomes too flaccid, the cell membrane peels away from the cell wall, and the cell collapses in upon itself. This is called plasmolysis.

When the concentrations on either side of the cell membrane reach equilibrium, as I mentioned above, osmosis stops, and there is no diffusion of water.

In animal cells, these two states are also found, although with slightly different effects. An animal cell with a lot of water is turgid. However, if an animal cell becomes too turgid, it bursts. This is because there is no cell wall to hold the cell together and contain the turgor pressure. This is the problem that causes hæmolysis, the bursting of red blood cells because of excess water in the blood stream. The causes of hæmolysis vary from a rejected blood transfusion to poisons, or allergies. When flaccidity occurs in an animal cell, the cell slowly collapses in on itself, shrivelling up and losing its shape.

This background information will allow me to make educated conclusions from my experiments and design an experiment that will efficiently prove or disprove my hypothesis.

Preliminary Experiments

        

In preparation for my main experiment I will complete two preliminary experiments, which will allow me to:

  • Determine the effect that osmosis has on potatoes and raisins, and how noticeable this effect is;
  • Determine the choice of indicator for my main experiment, i.e. what will be experimented with to gain some results;
  • Ensure the validity of my background information; and finally;
  • Examine exactly what happens as a result of water exchange
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Experiment One: Raisins in Water

In the experiment (which was completed at home), I placed 8 raisins into a glass of water for 24 hours, to see whether osmosis would occur between the strong sugar solution inside the raisin, and the water in the glass. At the start of the experiment all of the raisins were shrivelled and sank to the bottom of the glass, as shown on the left side of the diagram.

I left the glass in a cool dark cupboard for 24 hours, and after this retrieved the glass to record results. After 24 ...

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