Cylinders or discs of fresh potato are often used to investigate osmosis in living cells. To carry out this type of experiment, you need to:
- cut equal-sized pieces of potato
- blot with tissue paper and weigh
- put pieces into different concentrations of sucrose solution for a few hours
- remove, blot with tissue paper and reweigh
The percentage change in mass can be calculated for each piece of potato:
A piece of potato has a mass of 2.5 g at the start and 3.0 g at the end.
percentage change in mass = (3.0 – 2.5) ÷ 2.5 × 100 = 0.5 ÷ 2.5 × 100 = +20%
The plus sign shows that it has gained mass - it will have gained water by osmosis.
A piece of potato has a mass of 2.5 g at the start and 2.0 g at the end.
percentage change in mass = (2.0 – 2.5) ÷ 2.5 × 100 = –0.5 ÷ 2.5 × 100 = –20%
The minus sign shows that it has lost mass - it will have lost water by osmosis.
A graph of change in mass (vertical axis) against concentration of sucrose (horizontal axis) can be plotted.
Where the line crosses the horizontal axis at 0% change in mass, the sucrose concentration is equal to the concentration of the contents of the potato cells. The sucrose concentration is isotonic with the cells' cytoplasm, so there is no net movement of water by osmosis.
Biology IA: Osmosis 002223-0028 2
Osmosis is a fundamental concept in the study of biology. All cells use the principles of osmosis to transport water in and out of themselves. It is also very important in osmoregulation; a process which
regulates the osmotic pressure of an organism’s fluids in order to maintain a homeostatic
environment. Osmotic pressure is essential for support in plants. Entry of water in the cell raises the turgor pressure exerted against the cell wall, making it turgid and enabling it to stand upright. Plant cells are usually in hypotonic environments, where the fluid in the cell is more concentrated than that outside the cell, so water enters in. This is illustrated below.
Figure 2: Plant cells in hypertonic, isotonic and hypotonic environments.
Figure 2 also shows that if plant cells are in a hypertonic environment, then all the water will leave the cell, making it plasmolyzed
. This causes the cytoplasm to be pinched away from the cell wall, and the cell can no longer function. If plants are in isotonic environment, they are not turgid, but flaccid; they tend to wilt. Osmosis is also responsible for the ability of plant roots to draw water from the soil. Roots are adapted for this because of the numerous root hair cells; they increase the surface area to volume ratio, making the absorption highly effective. Animal cells also use osmosis to transport water in and out, but the consequences in this case are different due to the absence of cell walls.
Villarreal, M. 2007.
Turgor pressure on plant cells
. [image online] Available at: http://en.wikipedia.org/wiki/File:Turgor_pressure_on_plant_cells_diagram.svg[Accessed: 8 Jan 2014].
shrinking of the cytoplasm away from the wall of a living cell due to outward osmotic flow of water