Signal Rally

Cell signaling is often memorized as a list of steps, which hides how a message is received, relayed and turned into a response. This game follows two hormones from an endocrine cell through the bloodstream to a target cell. A protein hormone cannot cross the membrane, so it binds a G protein-coupled receptor; the activated G protein switches on adenylyl cyclase, which makes many cAMP molecules that activate protein kinase A, and PKA phosphorylates regulatory proteins that help release a factor already stored in vesicles. The steroid hormone instead crosses the bilayer and binds a cytoplasmic receptor; the complex enters the nucleus, where it switches on a gene whose product is made, packaged and secreted. Rival factors that block receptors loosely model how inhibitors can prevent reception.