Biological Membranes: The cell membrane and the sub-cellular membranes are grouped together as biological membranes. The sub-cellular membranes are the membranes that surround the vacuole and cell organelles such as membranes of endoplasmic reticulum and Golgi complex, nuclear membrane, membranes around chloroplasts and mitochondria. Significance of Biomembranes: Compartmentalization– Biological membranes compartmentalize the cytosol of the