Final answer:
The fertilization of an egg by a sperm triggers an increase in cytosolic Ca2+ in the egg through the fast block and slow block mechanisms.
Step-by-step explanation:
The fertilization of an egg by a sperm triggers an increase in cytosolic Ca2+ in the egg through two mechanisms: the fast block and the slow block. In the fast block mechanism, the binding of the first sperm causes a near instantaneous change in sodium ion permeability of the oocyte plasma membrane, depolarizing it and preventing fusion of additional sperm. This depolarization leads to an influx of calcium ions, which triggers the slow block mechanism. In the slow block mechanism, cortical granules fuse with the oocyte plasma membrane and release inhibiting proteins and mucopolysaccharides. The inhibiting proteins cause the release of any attached sperm and destroy the oocyte's sperm receptors, while the mucopolysaccharides form a fertilization membrane that prevents further sperm binding.