Practice question

Sea urchin eggs release small soluble peptides into surrounding seawater to actively guide sperm navigation. Speract modulates intracellular Ca2+ to increase flagellar beat frequency aiding vicinity search, while resact is definitive 14-amino-acid chemotactic peptide purified from Arbacia punctulata jelly that attracts sperm directionally. Resact binds receptor guanylate cyclase and elicits rapid cGMP-dependent Ca2+ bursts that transiently increase curvature and steer swimming up concentration gradient, analogous to bacterial chemotaxis. Bindin is sperm adhesion protein, hyalin mediates blastomere adhesion. Resact thus provides precise directional information orienting sperm toward egg center, greatly increasing encounter efficiency during broadcast spawning.