Finding the concentration of a Potato cell.

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Aim

The aim is to find the concentration (mole/dm3 (M)) of solute in a potato cell by using the process of osmosis and different concentrations of sucrose solution.

Background information

Osmosis is diffusion of water across a partially permeable membrane. It moves from a solution with less solute concentration (high water potential) to a solution with more solute concentration (low water potential). The one with a high water concentration is called a hypotonic solution and the low water concentration is called hypertonic solution, but these only depend on what type of concentration is on the other side of the partially permeable membrane. When more water passes through to one side of the membrane it is called net movement.

(Toole + Toole "Essential AS level Biology")

This is an example of the net movement through a partially permeable membrane. The right side is the more dilute solution, which makes it the hypotonic solution. The left side is the less dilute solution, which makes it the hypertonic solution. Net movement occurs through the more dilute to the less dilute so it goes from left to right though the partially permeable membrane. As you can see, the dotted line going horizontally through the middle of the two levels of solution shows the level of the solutions at the beginning when the two sides where given equal amounts of its solution. The horizontal solid lines show the level of the two sides after it was left for 2 hours. The right side has more solution in it because of the greater net movement of water going to the right from the left side by the process of osmosis.

(Marveen "GCSE Biology")

This diagram is of two liquids separated by a partially permeable membrane. The left side has large molecules of solute that has dissolved (hydrated), therefore attracting other water molecules to it and reducing the water potential; it is called the hypertonic solution. On the right side there is less solute to attract the water molecules to it and so it has a higher water potential with more free water molecules; it is called a hypotonic solution. The net movement only starts when the solute attracts the water to it and dissolves into it because this lessens the amount of free water molecules in the solution and so movement of water can occur. The amount of water molecules moving into the right solution is less than the movement to the left so the most significant net movement is to the left. Levels of the water in the two sides increase or decrease respectively when the process starts. When both solutions are equal in concentration, however, the two solutions are called isotonic and no net movement occurs.

This theory of osmosis is what we have to use to find the concentration of solute in the potato. I can use this because cells become turgid if water flows into them or flaccid if water flows out of them, and so I can measure the change in their mass as the water moves though the cell's partially permeable membrane. This is because of the stretchable cell wall on the plant cell. I can find the solute concentration of the potato when the two sides (potato and sucrose solution) are isotonic (both solutions have the same concentrations) and there is no net movement. To get the most accurate result we have to test various concentrations of the solution and plot a graph to see were the mass on the potato has not changed. This requires us to only change the variable of the concentration of the sucrose solution (independent variable) and for everything else I have to keep it constant. The various concentrations I have chosen are 0M to 1M with some in between (generally in 0.1 increments). This is to make a fairer and more accurate line of best fit on the final results graph, therefore making the final concentration of a potato cell much more reliable. Repeating the experiment and following a fair test can help get reliable results.

Prediction

I think that as the concentration of the sucrose...
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