Practice question

Sea urchin eggs release diffusible peptides into surrounding jelly that establish chemical gradients guiding sperm navigation. Resact, a fourteen amino acid peptide purified from Arbacia punctulata jelly, serves as potent chemoattractant for conspecific sperm, while speract functions in Strongylocentrotus purpuratus. Resact binds to transmembrane guanylate cyclase receptor on sperm flagellum, boosting cGMP, activating cyclic nucleotide-gated K+ channels, causing hyperpolarization, intracellular alkalization, and periodic CatSper calcium influxes that steer flagellum toward higher gradient. Bindin mediates adhesion after arrival, hyalin supports embryonic adhesion, not chemotactic directionality.