Final answer:
In the Golgi apparatus, Galactose often replaces mannose in glycan chains during glycosylation. Galactose is a key constituent of glycolipids in the brain and is a C4-epimer of D-glucose, while Mannose is a C2-epimer.
Step-by-step explanation:
In the Golgi apparatus, Galactose often replaces mannose in glycan chains during the process of glycosylation. Galactose, which is an important monosaccharide also known as a brain sugar, is a constituent of glycolipids found in the brain and myelin sheath of nerve cells. Furthermore, Galactose and mannose are both diastereomers of D-glucose and differ from it by the configuration of only one chiral center; Galactose is a C4-epimer, while Mannose is a C2-epimer of D-glucose.
Oligosaccharides, which begin their synthesis in the rough endoplasmic reticulum, undergo further glycosylation in the Golgi. This is where galactose can be added to glycoproteins and glycolipids such as galactocerebrosides, which are a type of glycosphingolipid. Thus, the answer to the question of what replaces mannose in glycan chains in the Golgi is B. Galactose.