Investigating osmosis - coursework for biology includes prediction evaluation and results

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Prediction

Osmosis is defined as ¡§the movement of water molecules across a partially permeable membrane from a region of high water concentration to a region of low water concentration¡¨.

In a high concentration of water the amount of solute (e.g. sugar) is low. This could be called a weak or dilute solution. In a low concentration of water the amount of solute (e.g. sucrose) is high. This could be called a strong or concentrated solution. When two such solutions are divided by a semi-permeable membrane the water will move from the area of high concentration to the area of low concentration, until both sides are equal (have reached equilibrium).

Knowing that osmosis 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. This can be seen in living cells. The cell membrane in cells is semi-permeable and the vacuole contains a sugar/salt solution. So when a cell is placed in distilled water (high water concentration) water will move across the semi-permeable membrane into the cell (lower water concentration) by osmosis, making the cell swell. This cell is now referred to as turgid. If done with potato cells the cells would increase in length volume and mass because of the extra water. If these potato cells were placed in a solution with a low water concentration, then the opposite would happen. Water would move out of the cell into the solution. In extreme cases the cell membrane breaks away from the cell wall and the cell is referred to as plasmolysed. The potato cells will have decreased in length, volume and mass.

The greater the concentration of water in the external solution the greater the amount of water that enters the cell by osmosis. The smaller the concentration of water in the external solution the greater the amount of water that leaves the cell.

However, there will be a point where the concentrations of water inside and outside the potato cells are equal (isotonic). At this point there will be no change in the length, volume and mass of the potato, as the net movement of water will be zero, no osmosis has occurred.

Using this information a graph and prediction can be made:

At point A the graph suggests that no osmosis has occurred, suggesting that the concentration of water inside the cell is equal to the solution outside.

At point B (high water concentration), there is no indication that the cell is increasing further in size. This is because the cell is fully turgid and no more water can enter.

At point C (low water concentrations), there is no indication that the cell is decreasing further in size. This is because the cell is fully plasmolysed and no more water can leave the cell.

Apparatus

For the experiment we will require:

"h Cylinders of potato with a diameter of 6.5mm

"h a borer with a diameter of 6.5mm

"h Sucrose solution of varying strengths (ranging from 0M to 1M)

"h A burette to measure out the sucrose solution.

"h Six jar specimen bottles

"h Razor blade

"h Callipers (To measure cylinder height.)

Method

We take a cylinder of potato from the potato, and cut it into separate cylinder. We then put three measured cylinders into six jar specimen bottles and added 20cm3 of sucrose solution into each jar (jar one was 0M-distilled water ranging to jar six-1M sucrose solution and leave for 23 hours.

We assume that this means that the pressure and temperature in each case is the same, as these are factors which could affect osmosis, and we know that the...
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