Osmosis

Osmosis

• Osmisis - The diffusion of water molecules across a partially permeable membrane, from an area of higher water potential to an area of lower water potenial. 
• Water potential - the potential of water molecules to diffuse out of or into a solution.

Osmosis is a special form of diffusion which only involves water molecules. It requires the presence of a partially permeable membrane, this is permeable to water molecules and other small molecules but not to larger molecules.

Solutions and water potential

• Solute - any substance that is dissolved in a solvent. e.g. sugar or salt
• Solvent - this does the dissolving. e.g. water

When you add a solute and a solvent you get a solution. Water potential is represented by ψ or psi (pronounced sigh), it's also measured in units of pressure which are kilopascals (kPa). Water potential is the pressure created by water molecules. Under standard conditions of temp (25 degrees celcilus) and pressure (100kPa), pure water has a water potential of zero.

If you  add a solute to pure water it will lower the water potential. The water potential of a solution must always be less than zero, a negative value. The more concentrated a solution, i.e. the more solute that is added, the lower its water potential. Water will then move by osmosis from a region of higher water potential (less negative) to a region of lower water potential (more negative). The highest value of water potential is zero so all the other values are negative, the more negative the value is, the lower the water potential.

You can use the understanding of water potential in order to find the water potential of cells or tissues, do this by:

• Placing the cells/tissue in a series of solutions of different water potentials
• If there's no net gain or loss of water from the cells/tissues then the water potential inside the cells/tissues must be the same as that solution

Osmosis and…