After lipids and proteins, carbohydrates are the third major component of biological membranes. Glycoproteins, proteins with bound carbohydrates, comprise nearly ninety percent of membrane carbohydrates. Glycolipids are the remaining ten percent. Both glycoproteins and glycolipids are found on the outer side of the membrane and perform diverse functions, including acting as antigens and receptors. Carbohydrates are attached to proteins through N-glycosidic and O-glycosidic bonds. N-glycosidic bond formation occurs between the amide group of asparagine and a sugar such as N-acetylglucosamine, whereas O-glycosidic bonds form between the hydroxyl group of serine or threonine and a sugar, such as N-acetylgalactosamine. Bound carbohydrates protect proteins from proteases, change their overall charge and increase their water-binding capacity. Glycolipids are derived from ceramide– a sphingolipid. Glucocerebroside or galactocerebroside is synthesized when glucose or galactose binds to ceramide. Gangliosides are formed when an oligosaccharide and sialic acid bind to ceramide. The negatively charged gangliosides regulate the concentration of positively charged calcium ions and thereby help in neurotransmission.