4.3: Osmosis
Osmosis is a special case of diffusion in which water molecules pass through a selectively permeable membrane, but larger molecules do not. Osmosis proceeds from a region of high water concentration, across a semi-permeable membrane, to a region of lower water concentration until equilibrium is reached.
A solute is a solid substance, such as salt or sugar that is dissolved in a solvent. Water is usually the solvent in living systems.
A typical animal cell contains a salt concentration of 0.9%. A solution of equal solute concentration is referred to as
isotonic
. A cell placed in an isotonic environment will experience movement of water inside and outside the cell, but there will be no change
in the biology of the cell.
A hypertonic solution contains a high solute concentration with respect to cells. For example, a solution containing 10% salt is hypertonic. When a cell is placed in a hypertonic environment, there is a net movement of water to the outside of the cell (from the higher water environment inside the cell). The cell shrinks in response.
A solution of low solute concentration is referred to as hypotonic . A solution containing 0.5% salt is hypotonic with respect to the cell. When a cell is placed in a hypotonic environment, there is a net movement of water into the cell. The cell swells in response.
Hypertonic, hypotonic, and isotonic are relative terms and are used only when comparing two different solutions.
Osmosis in eggplant and potato cells
Materials
- Thin slice of eggplant
- Two slices of potato pre-cut
- NaCl (table salt)
- 2 test tubes
- 10% NaCl solution
- 1 piece of weigh paper or plastic
Procedure
- Obtain a thin slice of eggplant. Sprinkle the eggplant with salt. Place on a piece of plastic or weigh paper. Incubate at room temperature for approximately 10 minutes.
-
Obtain two pieces of peeled potato, approximately 2 cm X 0.25cm. Label two test tubes with a wax marker at the 5 cm point
Tube 1: Add distilled water to the 5 cm mark
Tube 2: Add 10% sodium chloride to the 5 cm mark
Add a potato piece to each tube and incubate at room temperature for ~15 minutes Pour off the solution and feel each potato piece. Rinse test-tubes thoroughly with water to remove traces of salt and potato starch