BIOLOGY COURSEWORK:
OSMOSIS
AIM:
To investigate and find out the osmotic potential of a potato.
PREDICTION;
I predict that the potato cores in the distilled water and weaker solutions, which contain low concentrations of salty water, will expand when left in the solution for a certain amount of time. I say this because the molecules in the weaker solution will diffuse to an area of higher solute concentration, the potato (which will be done by osmosis).
I also predict that the potato cores in the 1.0 molar concentration of brine (highest concentration) will shrink this is because the molecules in the potato will diffuse in to the brine. This will also be done by osmosis, is the movement of water molecules from an area of weak solution to an area of strong solution through a semi-permeable membrane (cell membrane).
Therefore the potato cores once left in the higher concentrated solutions will shrink. And the potato cores in the weaker solutions will expand.
THEORY:
Osmosis is defined as ‘the movement of water molecules from an area of high water concentration to an area of low water concentration, across a semi-permeable membrane’ (Collins, 1999).
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. sugar) 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).
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.
