Then Stryer explained how rhodopsin , the light-sensitive receptor protein in the disk membranes of rod cells, announces the arrival of this tiny pulse of light to the signaling machinery inside the cell. Stryer had found that rhodopsin could do this only with the help of an intermediary, called a G-protein, which belonged to a family of proteins that was already known to biochemists from their study of how cells respond to hormones and growth factors. Nathans immediately realized this meant that the structure of rhodopsin itself might be similar to that of receptors for hormones. His mind began racing with possibilities. Until then, Nathans had been studying the genetics of fruit flies.
Rod cells function only in dim light and are blind to color.
Although you can see shapes fairly well, colors are completely absent. It is remarkable how few people realize that they do without color vision in dim light. Even though the fovea is essential for fine vision, it is less sensitive to light than the surrounding retina. Thus, if we wish to detect a faint star at night, we must gaze slightly to the side of the star in order to project its image onto the more sensitive rods, as the star casts insufficient light to trigger a cone into action.