Lipoproteins commonly occur in the membranes of mitochondria, endoplasmic reticulum, nucleus, myelin sheath of nerves, chloroplasts and bacterial membranes. The lipid components, such as triacyl glycerol, phospholipid and cholesterol, are joined to the protein moiety consisting of non – polar amino acids, A specific example is retinol which is bound to the proteins opsin forming 11 – cis – retinol . Lipovitelline is also a lipoprotein found in the egg yolk.

The lipopolysaccharides are highly complex compounds which are present in the surface layer of E.coli and gram-negative bacteria. They are organized in the outer layer and the peripheral part of the Lipopolysaccharides contains the O-antigenic determinants consisting of tetra or pentasaccharide units to form high molecular weight polysaccharides.

Brain tissue contains protein – lipid complex called proteolipids which are insoluble in water but soluble in organic solvents. The protein portion of this lipid has high content of hydrophobic amino acids, while the lipid portion is made up of equal parts of phosphoglycerides and cerebrosides . such lipids occur in the myelin sheath.