Investigation Into How Osmosis In Potatoes Is Affected By Solution Concentration.

GCSE Science

Emily Grant

2003

Investigation Into How Osmosis In Potatoes Is Affected By Solution Concentration

PLAN

Aim

To investigate how the concentration of a salt and water solution affects the rate of osmosis in a potato.

Osmosis

Osmosis is the passage of water from an area of high water concentration to an area of lower water concentration, through a partially permeable membrane (a membrane which allows small molecules like oxygen, carbon dioxide, water, ammonia, glucose etc to pass through, but does not allow larger molecules such as sucrose, starch, protein etc). Cell membranes are partially permeable, which allows water to pass from one cell to another within the organism, and also, as I will be exploring in this investigation, from outside to inside, or vice versa.

Osmosis occurs when the area of liquid outside the cell contains more water particles than the area of liquid inside the cell. In the solution, the particles are moving all the time. In a solution in which the concentration of water is high, there are more water particles per cm3 than in a dilute solution, and therefore a higher rate of movement, causing more particles to come into contact with the membrane. In a more dilute solution in which there are less water particles per cm3, less movement occurs and therefore less particles will pass through the membrane. Also, in the solution with low water concentration, there are more particles of sugar or in this case salt, which form a weak attraction with the water particles, inhibiting movement. Therefore, there will be more water moving from the area of high water concentration.

This diagram shows how osmosis occurs.

The diagram shows an area of high water concentration (on the left) and one of low water concentration. The dashed line at the centre represents a partially permeable membrane, while the large red circles symbolize the salt particles and the smaller blue ones, the water. The arrows show that there are more water particles moving towards and through the membrane from left to right (high water concentration to low water concentration) than from right to left (low water concentration to high water concentration).

Osmosis in plants causes the plant cells to become turgid. A plant cell has a strong cellulose wall outside the cell membrane—this is fully permeable. In the centre of the cell, there is the vacuole which contains the solution of salts, sucrose etc. The thin layer of cytoplasm surrounding the vacuole is what acts as the partially permeable membrane. When water enters the cell by osmosis, the cell swells up; however, the cellulose wall prevents it from bursting. The wall stretches, but does not break, and eventually the pressure inside the cell is so high that no more water can enter the cell and the cell reaches a point where it cannot stretch anymore; we say it is turgid. (Turgidity is very important in plants, because it allows the appropriate parts of the plant to ‘stand up’ in order to get enough sunlight).

However, if you put a plant cell into a solution of less water concentration than that inside the vacuole, water is drawn out of the vacuole. This is known as reverse osmosis. When this occurs, water is drawn out of the vacuole and the cell shrinks. It loses its turgor and becomes flaccid. If the external solution is strong enough, the cytoplasm eventually pulls away from the cellulose wall in what we call plasmolysis; I do not expect this to occur in this investigation, as the solutions I will be using will not be strong enough for this to happen and I will not be leaving it for long enough.

When plant cells are placed into a solution which has exactly the same osmotic strength as the cells they are in a state between turgidity and flaccidity; this we call incipient plasmolysis, meaning that plasmolysis is about to occur.

Prediction

In this investigation, I want to find out how the concentration of the salt solution outside the potato affects the amount of osmosis taking place in a certain amount of time (the rate). I think that my results will show that the lower the concentration of the salt solution—meaning, a solution with higher amounts of water in it—the higher the rate of osmosis. I think this will occur because a higher water concentration has a higher number of water particles in it, thus there will be more movement of water particles and therefore a faster rate of osmosis.

I will expect the potato that had been in the strongest solution to have experienced a slight loss in mass and length, and become flaccid (feeling soft and ‘flabby’ to the touch). I expect this to happen because I expect reverse osmosis to be occuring in this situation, assuming that the solution is of lower water concentration than that inside the cell. However, I am making these statements without knowing the concentration of the solution inside the cell; if the solution outside the cell is of sufficient water concentration to still allow osmosis to occur, I would expect to ...