Rods and cones have segments that are packed with membrane-rich disks. The membranes of these disks contain large quantities of a transmembrane protein called opsin. Each opsin molecule is associated with a much smaller molecule called retinal. In rod cells, the two-molecule complex is called rhodopsin. Each of the three types of cones (blue, red, or green) has its own specific kind of opsin. The retinal molecule is the light-sensitive portion of the complex. When retinal absorbs light, it changes shape. The change in the shape of retinal causes the surrounding opsin to change shape. The opsin, in turn, triggers a cascade of events that activates the cell. The rods and the three types of cones each have different opsins that determine the wavelengths of light that the retinal will best absorb in these cells. Rods are more light-sensitive than cones and are able to work under low-light conditions. Cones respond mainly to bright light and are responsible for color vision. For example, the type of opsin-retinal complex in the blue cones responds best to blue light. It also responds to yellow light, but less strongly. Intermediate wavelengths cause graded responses. Information from all of the photoreceptors enables the brain to differentiate many hues, using only three main types of color photoreceptors. The brain combines the signals from the three cones in much the same way as the three primary colors of light can be mixed to produce all the colors of the visible spectrum.