Rhodopsin, the receptor protein in rod cells, crosses the disk membrane seven times; its odd shape is shared by three receptor proteins in cone cells. Retinal (which absorbs light) is shown in purple. The other colored balls represent amino acids that make up the rhodopsin structure.

Illustration: Eade Creative Services Inc./George Eade illustrator (from figure 28-4 page 405, Principles of Neuroscience by Eric Kandel, James H. Schwartz and Thomas M. Jessell ©1991 Elsevier Science Publishing Company, Inc.)