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 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.
in the biology of the cell.
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
Observations
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Blood Laboratory | Red cell fragility > Osmotic hemolysis |
Cell membranes are semipermeable barriers, and osmotic gradients are established between intracellular and extracellular fluids which can cause water to flow into and out of the cells. The amount of osmotic pressure depends upon the difference between the concentration of non-diffusible ions on each side of the membrane. | |
The theoretical background for this exercise is to be reviewed in your text book. | |
The intracellular fluid of erythrocytes is a solution of salts, glucose, protein and hemoglobin. A 0.9% NaCl solution is said to be isotonic: when blood cells reside in such a medium, the intracellular and extracellular fluids are in osmotic equilibrium across the cell membrane, and there is no net influx or efflux of water. | |
When subjected to hypertonic media (e.g. 1.8% NaCl), the cells lose their normal biconcave shape, undergoing collapse (leading to crenation) due to the rapid osmotic efflux of water. | On the other hand, in a hypotonic environment (e.g. 0.4% NaCl or distilled water), an influx of water occurs: the cells swell, the integrity of their membranes is disrupted, allowing the escape of their hemoglobin (hemolysis) which dissolves in the external medium. |
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In this experiment, we make use of the property that the osmotic fragility (or susceptibility to hemolysis) of erythrocytes is not uniform, and the number of cells undergoing hemolysis depends on the degree of hypotonicity of the extracellular medium. The concentration of liberated hemoglobin in each test medium is an index of the extent of osmotic hemolysis. Your task is to examine the relationship between extent of hemolysis and osmolarity of the medium in which the erythrocytes are suspended. | |
To continue with the procedure of erythrocyte fragility, click here | |