11.1 Proteins serve a number of biological functions. (a) The light produced by fireflies is the result of a light-producing reaction between luciferin and ATP catalyzed by the enzyme luciferase. (b) The protein fibroin is the major structural component of spider webs. (c) Castor beans contain a highly toxic protein called ricin.

[Part a: Darwin Dale/Science Source. Part b: Rosemary Calvert/Image­state. Part c: Paroli Galperti /© Cuboimages/Photoshot.]