Final answer:
The last sugar residue added to the core oligosaccharide in the cytoplasm during glycoprotein synthesis is a complex sugar such as glucose, galactose, or mannose, completed in the Golgi apparatus.
Step-by-step explanation:
The last sugar residue added in the cytoplasm to the core oligosaccharide during glycoprotein synthesis in the endomembrane system is typically a complex sugar, such as a monosaccharide like glucose, galactose or mannose. These sugars are added in the Golgi apparatus as the final step in oligosaccharide formation. The core oligosaccharides begin their synthesis in the rough endoplasmic reticulum (RER), where partial glycans are linked to proteins, forming glycoproteins. As they transit through the Golgi vesicles, terminal glycosylation occurs, which is the addition of the final sugar residues. When vesicles from the trans-Golgi network fuse with the plasma membrane, these glycoproteins, with their now complete oligosaccharide chains, are presented on the cell surface, playing critical roles in cell recognition and adhesion.