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Cell Physiology

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Osmosis is a process by which solvent molecules move from a region of high concentration (dilute solution) to a region of low concentration (concentrated solution) through a semi permeable membrane.

Osmosis can be described as a special type of diffusion since it involves movement of solvent (water) particles from a region of high concentration to a region of low concentration.

Demonstration of Osmosis Using a visking Tubing


The visking tubing contains both sugar and water molecules. The beaker contains a higher concentration of water molecules than the visking tubing.

The water molecules diffused from the beaker (where they are highly concentrated) into the visking tubing (where they are lowly concentrated).

Even though there is a higher concentration of sugar molecules in the visking tubing, they were not able to diffuse out of the visking tubing due to their large molecular sizes. The visking tubing is semi permeable.

Other than visking tubing, dialysis tubing or cellophane are also other semi permeable membranes that can be used in this experiment.

Osmosis explained

When two separate solutions are separated by a semi permeable membrane, there will be movement of water molecules from their region of high concentration (dilute solution) to a region of low concentration (the highly concentrated solution) across the semi permeable membrane. The semi permeable membrane does not allow movement of solute particles across it.

The movement of the water molecules continues until the separate solutions have the same concentrations.

Solutions with the same concentrations are referred to as isotonic solutions. The solutions are said to be isotonic to each other.

A lowly concentrated solution (dilute solution) is referred to as a hypotonic solution. A hypotonic solution has less of the solute molecules but more of the solvent molecules.

A highly concentrated solution with more of the solute particles but less of the solvent particles is referred to as a hypertonic solution.

When isotonic solutions are separated with a semi permeable membrane, there will be no net movement of solvent molecules to any of the solutions since they have the same concentration of solvent molecules.

Osmotic pressure

When a concentrated solution is separated from distilled water by a semi permeable membrane, the concentrated solution will develop a force with which it draws water through the semi permeable membrane from the distilled water.

Osmotic pressure refers to the force with which a concentrated solution draws water to itself.

An osmometer is an instrument used to measure the osmotic pressure.

Osmotic potential

This is a measure of the pressure a solution would develop to withdraw water molecules from pure water when separated by a semi permeable membrane.

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