In many cell types secretory vesicles are continually fusing with the plasma membrane. The membrane surface of the vesicles is added to the plasma membrane. Despite the addition of this extra area of membrane to the plasma membrane, the surface area of the cell remains essentially stable. Explain. (3) I Trace a molecule of a lysosomal protein from its point of synthesis on a ribosome in the cytosol, to its ?nal destination in a lysosome (2). 0 Explain recycling of marmose-6 phosphate receptors .(2)

1.     Upon fusion of the secretory vesicles with the plasma membrane and addition of the membrane surface of the vesicles to the plasma membrane, the area of the plasma membrane increases. However, the increase in the plasma membrane surface area is temporal. This is because reuptake of membrane components occurs by endocytosis, which results in restoration of the normal surface area of the plasma membrane. Regions of the plasma membrane bud in and fusion with internal membranes occur to facilitate recycling of the membrane components. This explains why the membrane surface area remains relatively stable even after exocytosis.

2.     The synthesized lysosomal protein is transported to the trans Golgi network via the Golgi apparatus by cisternal migration. Cisternal migration implies transport of the protein by binding to cis-Golgi apparatus. The lysosomal protein is tagged with a mannose-6-phosphate receptor, by the formation of a receptor-ligand complex between mannose-6-phosphate receptor and a newly synthesized protein. Transport vesicles specific to the lysosomal protein bud from the trans Golgi network and are used to deliver the tagged protein to the lysosome.

3.     Mannose 6-phosphate receptor facilitates delivery of newly synthesized proteins to the lysosome. Upon reaching the lysosome, low pH condition triggers the dissociation of a mannose 6-phosphate receptor from its ligand. This results in recycling of the mannose 6-phosphate receptor sequences.