**Osmosis is a special case of diffusion** in which only water moves across a membrane from the dilute side (higher water concentration) to the concentrated side (lower water concentration).

For osmosis, the measure of concentration changes from molarity to osmolarity. **Osmolarity is defined by osmoles of solute per litre of solution** e.g. 1 mol/L solution of KCl has a molarity (M) of 1, but its osmolarity (OsM) is 2, because KCl dissociates into K^{+} and Cl^{–} in water. However, a 1 M solution of glucose has an osmolarity of 1 OsM, because glucose is not an ionic compound and a molecule of glucose does not dissociate into smaller pieces in the presence of water.

**The osmolarity of a solution is given by adding the osmoles of all the solutes** present in the solution.

During osmosis, the rate of flow of water across the membrane depends on a number of factors including the permeability of the membrane to water, the area of the membrane, and the concentration difference between the two sides. These factors are summed up in the equation:

*flow = P _{water} × A × (C_{right} – C_{left})*

If ΔC is doubled, the flow of water would double, and so on.

**Eventually equilibrium is reached when concentrations on both sides are the same**; however, under equilibrium conditions, the amount of water on both sides will not be equal.

## Learning Outcomes

You should now be able to

- Determine the
**osmolarity**of a solution. - Determine
**which way water will flow**through a selectively permeable membrane, and find the**rate of flow**of water using a two-compartment model.

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