Each golgi cisternae stack is organized as a series of processing compartments which has a Cis face and a Trans face . The proteins from the ER are transported to the cis face which is the entry face and then move through the cisternae to the trans face which is the exit face,where glycosylation is completed. Both faces are connected by a network of cisternal structures called the cis golgi network and the trans golgi network . Both networks are responsible for the sorting of proteins and either of them can send proteins back to the ER or send them on to the lysosomes, secretory vessels or on to the cell surface. It is during the period between the cis and trans networks that oligosaccharides are processed and proteoglycans are assembled.
Oligosaccharides are of two types, high mannose and complex .
The high mannose form has no sugars added to it in the golgi apparatus. The complex form on the other hand contains more than two N-acetylglucosamines and a variable quantity of galactose, sialic acid, and occasionally frucose. Of these, sialic acid is the only negatively charged sugar added to the glycoprotein. Complex oligosaccharides are formed by trimming the original form of oligosaccharide formed in the ER, and adding sugars to it. These proteins are modified in stages as they move from cisternae to cisternae across the stack and each oligosaccharide must be processed by an enzyme prior to being accepted as a substrate.