Potato coursework

Clair Kidd-Stanton, 2107 Page The George Ward School

How does the concentration of a solution affect the osmosis rate of a potato

Aim:

The aim of my experiment is to see if the concentration of sugar solution I use will affect the rate of osmosis in potato cells. I will do this by recording data collected on the potatoes placed in different concentrations of sucrose.

Research:

Osmosis is a special kind of diffusion that only concerns the movement of water.

If two solutions are separated by a semi-permeable membrane, which only allows certain sized molecules through there will be a net movement of the water molecules, from the less concentrated solution (the one with more water molecules), to the solution which is more concentrated (has more solute molecules). This is because as in ordinary diffusion the molecules move to even-out any difference in concentration.

However, because of the semi-permeable membrane, which does not allow the larger solute molecules to cross; only the water molecules can move. The water molecules will continue to cross the semi-permeable membrane until equilibrium is reached, where the two solutions are of equal concentration.

The greater the difference in the number of water molecules on either side of the membrane, the greater the water potential. Water potential is represented by the Greek letter Ψ. pure water has a water potential of 0kPa. As substances dissolve in the water, the water potential drops and becomes negative. Water moves by osmosis to the region with the lowest water potential.

When a cell is placed in a hypotonic solution, one that is less concentrated (higher water potential) the solution enters the cell therefore decreasing the water potential. However if a cell is placed in a hypertonic solution, one that is more concentrated (lower water potential) the water leaves the cell. If a cell is placed in a isotonic solution, solutions on either side of membrane have equal concentration, there is not water movement.

When an animal cell is placed in a hypertonic solution the cell will gain water, swell and then burst. However when placed in a hypotonic solution the cell shrinks as it looses water.

In plant cells, when placed in a hypotonic solution, water enters causing the volume to increase meaning the pressure inside the cell also increases. This stops more water entering and also stops the cell from bursting. When a plant cell is full with water it is called turgid.

Also in plant cells, when it is placed in a hypertonic solution, the cell will loose water. The cell membrane will shrink and pull away from the cell wall. This process is called plasmolysis and the cell is said to be plamolysed.