Practice question

Unfertilized sea urchin egg vitelline membrane lies closely apposed to plasma membrane with little perivitelline space. Upon calcium-dependent cortical granule exocytosis, contents including hyalin, proteases, sulfated mucopolysaccharides, and ovoperoxidase are released. Proteases cleave linkers between vitelline membrane and plasma membrane, osmotic swelling by glycosaminoglycans elevates membrane outward, and cross-linking enzymes harden it via di-tyrosine bonds. Modified vitelline membrane becomes fertilization envelope raised above egg surface with perivitelline space between. This conversion changes soft vitelline layer into tough barrier preventing supernumerary sperm entry and protecting embryo before hatching occurs.