Membrane proteins

Membrane proteins are proteins associated to their lipid bayer in some way, to provide that membrane with its characteristic functional properties. The amount and types of proteins in a membrane are highly variable, for example in a typical plasma membrane about 50% mass is membrane proteins, whereas in plasma membranes involved in ATP production there is around 75% mass. 

There are many ways that the membrane proteins can be associated to the membrance. Transmembrane proteins are directly placed in the lipid bilayer and contain a hydrophobic region within the phospholipids and hydrophillic regions in the cytosol and outside the cell. However some membrane proteins are non covalently bonded to other proteins embedded in the lipid bilayer. Alberts et al (2008). Molecular Biology of the Cell. 5th ed. New York: Garland Science. 629-630.