Osmosis Investigation

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Biology Coursework                                                                    By Rachael Cathrew

Prediction

        I predict that in a high salt solution the potato core will loss mass because water will leave the cells because of osmosis and the net movement through the partially permeable membrane will be from the potatoes to the water. Also if in pure water the potato cores will swell and gain mass because water will enter the cells by osmosis and the net movement through the partially permeable membrane will be from the water to the potatoes.

        At some point the concentration of the water and the concentration of the potato cores will be the same and there will be a movement of particles through the partially permeable membrane but it will have no net movement e.g. the mass will remain unchanged.

Scientific Reason for Prediction

        Osmosis is the movement of water molecules across a partially permeable membrane from a region of high water concentration to a region of low water concentration.

First the definition of osmosis:

Partially permeable membranes are very thin layers of material (cell membranes are partially permeable) which allow some things to pass through them but prevent other things from passing through.

        Cell membranes will allow small molecules like Oxygen, water, Carbon Dioxide, Ammonia, Glucose, amino-acids, etc. to pass through. Cell membranes will not allow larger molecules like Sucrose, Starch, protein, etc. to pass through.

A region of high concentration of water is either a very dilute solution of something like sucrose or pure water. In each case there is a lot of water: there is a high concentration of water.

Now to explain osmosis:

        When you put a potato core into a liquid containing water one of three things will happen.

        If the liquid surrounding the potato core has a higher water concentration than the cells in the core (a very dilute solution) the cell will gain water by osmosis.

        Water molecules are free to pass across the cell membrane in both directions, but more water will come into the potato core than will leave. The net (overall) result is that water enters the potato core. The potato core is likely to swell up and increase in mass.

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        If the liquid is exactly the same water concentration as the cells in the potato core there will be no net movement of water across the cell membrane and no change in mass.

        Water crosses the cell membrane in both directions, but the amount going in is the same as the amount going out, so there is no overall movement of water. The potato core will stay the same size and mass.

        If the liquid has a lower concentration of water than the potato core cells (a very concentrated solution) the cells will lose water by osmosis.

        Again, water crosses the cell ...

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