To find out the internal concentration of a potato cell.

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Osmosis in Potato Cells

Coursework Plan

Aim: To find out the internal concentration of a potato cell.

Hypothesis:

        Osmosis is defined as the diffusion of water across a partially permeable

membrane “from the dilute to the concentrated solution” (Mackean GCSE Biology

textbook).

        There are more water molecules in a dilute solution than in a concentrated

solution so the diffusion gradient favours water passing across the membrane from the

dilute to the concentrated solution.  A concentrated solution e.g. sugar solution has a

lower concentration of water molecules because the glucose molecules attract some of the

water molecules when they dissolve in the water, preventing them from moving freely.  

Therefore there are more free water molecules in the dilute solution which can move

through to the concentrated solution by osmosis.  Sugar molecules are larger because the

water molecules cluster around them, so they move more slowly and some may pass

through the membrane pores slowly in the other direction.  The diagram below shows

osmosis across a partially permeable membrane:

           

(diagram from Mackean GCSE Biology textbook)

        The water molecules actually pass both ways through the membrane but because

there are more on one side than the other, there is a steady net movement of water

molecules flowing from the weaker solution to the stronger solution by osmosis.  The

more concentrated sugar solution fills up with water molecules, which act in an attempt to

dilute it, until there is an equal concentration either side of the membrane.

        In plant cells the cell wall is freely permeable to water and dissolved substances,

but the cell membrane is partially permeable.  If the plant cell is surrounded by a solution

more dilute than its contents, which has more free water molecules, then water passes into

the vacuole from the surroundings by osmosis.  This causes the vacuole to expand,

pressing the cytoplasm against the cell wall.  The plant cell swells up and becomes turgid,

so it has gained in mass. If the plant cell is surrounded by a more concentrated solution

than its contents, water will pass out of the cell so it will lose mass and become flaccid.

Preliminary Experiment

        An experiment was conducted to test the effects of differing concentrations on

osmosis in visking tubing.  Six pieces of visking tubing were filled with 0.6M sugar

solution and tied at both ends with cotton.  They were dried to remove excess liquid and

the mass of each was taken.  Each piece of visking tubing was submerged in a different

external sugar solution of 0.0M (distilled water), 0.2M, 0.4M, 0.6M, 0.8M and 1.0M.  

After 30 minutes they were removed, dried and the mass was re-recorded.  The

percentage change in mass was calculated to obtain a true comparison of change in mass,

since the visking tubing bags started at different masses.  

        It was discovered that for the 0.0M, 0.2M and 0.4M solutions, water passed

through the membrane into the visking tubing by osmosis because the sugar solution

inside the visking tubing was more concentrated, so the mass increased.  The mass did

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not change for the 0.6M external sugar solution because the internal and external

concentrations were equal. For the 0.8M and 1.0M solutions, the mass of the visking

tubing decreased because the external sugar solution was greater and water molecules had

passed out of the visking tubing by osmosis.  In conclusion, there was a greater change in

mass when the difference between the external and internal concentrations was higher

because there were more free water molecules in the less concentrated solution that were

able to pass though the membrane by osmosis.

Graph of Results

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