Final answer:
The Golgi apparatus is responsible for adding carbohydrate modifications to proteins, producing glycoproteins and glycolipids that are essential for cell membrane structure and function.
Step-by-step explanation:
The cellular organelle responsible for adding carbohydrate modifications to proteins is the Golgi apparatus. Proteins synthesized in the rough endoplasmic reticulum (RER) are first modified by attachment of a core glycoside. As these proteins transit through the Golgi apparatus, they undergo further glycosylation where additional sugar molecules are attached, culminating in the formation of glycoproteins or glycolipids that play crucial roles in cell-to-cell communication and membrane structure. The Golgi not only modifies proteins and lipids with carbohydrates but also labels and packages them for different destinations such as the plasma membrane or extracellular space. In plant cells, the Golgi also specializes in synthesizing polysaccharides, with some being incorporated into the cell wall.