An association of lipids and proteins in a system that combines aspects of the properties of both types of molecule. Most membrane systems (see plasma membrane
) are composed primarily of lipoprotein. The major lipids in plant membranes are phosphoglycerides and glycolipids. (Glycolipids are especially abundant in the chloroplast membranes.) The protein component is not limited to any one type of protein but membrane proteins often have a slightly modified structure to improve their interaction with lipids. For example, there are relatively few disulphide bridges in membrane proteins. The forces binding lipid and protein together in a lipoprotein system are not conventional chemical bonds. The most important binding forces are hydrophobic interactions but polar and electrostatic forces are also important.