Sensitivity to light—photosensitivity—confers on the simplest animals the ability to orient to the sun and sky and gives more complex animals rapid and extremely detailed information about objects in their environment. It is not surprising that both simple and complex animals can sense and respond to light. What is remarkable is that across the entire range of animal species, evolution has conserved the same basis for photosensitivity: a family of pigments called rhodopsins. In this animation we will describe how rhodopsin molecules respond when stimulated by light energy and how that response is transduced into neural signals.


In this animation we examined the function of rod cells in the human eye. The other class of photoreceptor—the cone cell—functions in a similar way. Rod cells are responsible for highly sensitive black-and-white vision and cone cells are responsible for the less sensitive color vision.

The human retina has three kinds of cone cells, each containing slightly different opsin molecules. The three cone opsins and the single rod opsin all differ in the wavelengths of light they absorb best. Although the same 11-cis-retinal group is the light absorber in all, its molecular interactions with opsin determine the spectral sensitivity of the rhodopsin molecule. Because different wavelengths of light are differentially absorbed by the different visual pigments, the brain can interpret the relative inputs from the different classes of cones as a full range of color. Color blindness results from the absence or dysfunction of one or more classes of cone cells. The most common form is red–green color blindness, which occurs in about 10 percent of men of European descent.

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Textbook Reference: Concept 34.4 Sensory Processes Provide Information on an Animal’s External Environment and Internal Status