Semi-permeable membrane

Semi-permeable membrane is a type of biological or synthetic, polymeric membrane that will allow certain molecules or ions to pass through it by diffusion and occasionally specialized "facilitated diffusion".

Overview

The rate of passage depends on the pressure, concentration, and temperature of the molecules or solutes on either side, as well as the permeability of the membrane to each solute. Depending on the membrane and the solute, permeability may depend on solute size, solubility, properties, or chemistry. How the membrane is constructed to be selective in its permeability will determine the rate and the solute(s) that pass through it. Typically, the membrane will allow small molecules and not allow larger ones to pass through into the cell. This property is critical for the cell as it generally must block the passage of proteins or RNA molecules that need to stay within the cell.

Biological semi-permeable membrane

An example of a biological semi-permeable membrane is the lipid bilayer, on which is based the plasma membrane that surrounds all biological cells. A group of phospholipids (consisting of a phosphate head and two fatty acid tails) arranged into a double layer, the phospholipid bilayer is a semi-permeable membrane that is very specific in its permeability. The hydrophilic phosphate heads are in the outside layer and exposed on either side to water and the hydrophobic fatty acid tails are the middle of the membrane. The arrangement of phospholipids and proteins is described as the fluid mosaic model.

Synthetic semi-permeable membrane

Synthetic semi-permeable membranes include dialysis membranes and reverse osmosis membranes, widely used in industry for microfiltration, ultrafiltration, nanofiltration, reverse osmosis, dialysis, pervaporation, gas separation, electrodialysis, and fuel cells. The pores of such membranes are so small that they are invisible to the naked eye.

See also

• Osmosis
• Diffusion
• Active transport
• Passive transport
• Cell membrane
• Lipid bilayer

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