Osmosis - Basic Explanations

Basic Explanations

Osmosis can occur when there is a partially permeable membrane, such as a cell membrane. When a cell is submerged in water, the water molecules pass through the cell membrane from an area of low solute concentration to high solute concentration (e.g. if the cell is submerged in saltwater, water molecules move out; if it is submerged in freshwater, however, water molecules move in); this is called osmosis. The cell membrane is selectively permeable, so only necessary materials are let into the cell and wastes are left out. The word 'osmosis' is particular to the diffusion of water molecules into the cell.

When the membrane has a volume of pure water on both sides, water molecules pass in and out in each direction at exactly the same rate; there is no net flow of water through the membrane.

Osmosis can be explained using the concept of thermodynamic free energy: the less concentrated solution contains more free energy, so its solvent molecules will tend to diffuse to a place of lower free energy in order to equalize free energy. Since the semipermeable membrane only allows solvent molecules to pass through it, the result is a net flow of water to the side with the more concentrated solution. Assuming the membrane does not break, this net flow will slow and finally stop as the pressure on the more concentrated side lessens and the movement in each direction becomes equal: this state is called dynamic equilibrium.

Osmosis can also be explained using the notion of entropy, from statistical mechanics. A system that has two solutions of different concentrations separated by a semipermeable membrane has less entropy than a similar system having two solutions of equal concentration. The system with the differing concentrations is said to be more ordered, and thus has less entropy. The second law of thermodynamics requires the presence of an osmotic flow that will take the system from an ordered state of low entropy to a disordered state of higher entropy. Thermodynamic equilibrium is achieved when the entropy gradient between the two solutions becomes zero.

Particle size has no bearing on osmotic pressure; this is the fundamental postulate of colligative properties.

Osmotic pressure is the main cause of support in many plants. The osmotic entry of water raises the turgor pressure exerted against the cell wall, until it equals the osmotic pressure, creating a steady state.

When a plant cell is placed in a hypertonic solution, the water in the cells moves to an area higher in solute concentration and the cell shrinks, and in doing so, becomes flaccid. This means the cell has become plasmolyzed – the cell membrane has completely left the cell wall due to lack of water pressure on it; the opposite of turgid.

Also, osmosis is responsible for the ability of plant roots to draw water from the soil. Since there are many fine roots, they have a large surface area, and water enters the roots by osmosis.

Osmosis can also be seen when potato slices are added to a high concentration of salt solution. The water from inside the potato moves to the salt solution, causing the potato to shrink and to lose its 'turgor pressure'. The more concentrated the salt solution, the bigger the difference in size and weight of the potato slice.

In unusual environments, osmosis can be very harmful to organisms. For example, freshwater and saltwater aquarium fish placed in water of a different salinity than that to which they are adapted to will die quickly, and in the case of saltwater fish, dramatically. Another example of a harmful osmotic effect is the use of table salt to kill leeches and slugs.

Suppose an animal or a plant cell is placed in a solution of sugar or salt in water.

1. If the medium is hypotonic — a dilute solution, with a higher water concentration than the cell — the cell will gain water through osmosis.
2. If the medium is isotonic — a solution with exactly the same water concentration as the cell — there will be no net movement of water across the cell membrane.
3. If the medium is hypertonic — a concentrated solution, with a lower water concentration than the cell — the cell will lose water by osmosis.

Essentially, this means that if a cell is put in a solution which has a solute concentration higher than its own, then it will shrivel up, and if it is put in a solution with a lower solute concentration than its own, the cell will expand and burst. Electronucleal exchange is the passive diffusion of cations and anions across a semi-permeable membrane according to electrical charge.

Chemical gardens demonstrate the effect of osmosis in inorganic chemistry.