Phospholipid bilayers are embedded with proteins, which may be either permanently or temporarily attached to the membrane

Integral Proteins

• Integral proteins penetrate the phospholipid bilayer to remain permanently attached to the membrane
  

• These transmembrane proteins cannot be readily isolated without disrupting the bilayer (e.g. via detergents)
  

• Examples of integral membrane proteins include glycoproteins, ion channels, carrier proteins and protein pumps
  

Peripheral Proteins

• Peripheral proteins are only temporarily associated with one side of a membrane (can be removed by polar solvents)
  

• They are either attached to integral proteins, linked to the polar heads of the bilayer or held in place by the cytoskeleton or extracellular matrix
  

• Examples of peripheral proteins are receptor complexes involved in cell signalling (e.g. G proteins)
  

The composition of a membrane protein is determined by its function within the cell

• Non polar amino acids (hydrophobic) will associate with the lipid bilayer, while polar amino acids (hydrophilic) will face the aqueous solutions
  

• The inner surface of a protein channel will be lined with polar amino acids to facilitate the passage of specific polar or charged molecules
  

Types of Membrane Proteins

Membrane Protein Functions