 Dear students, in this topic we shall discuss some basics of photoreception in animals. The photoreception involves the transduction of photons of light into the electrical signals. These signals can be interpreted by the nervous system. The photoreceptors which are involved in photoreception possess light-sensitive carotenoid pigments, retinol and 3-dehydro-retinol. These two pigments are found in all animal kingdom as photoreceptor molecules. These carotenoids are associated with opsin proteins and form rhodopsins. Rhodopsins absorb photons of light and produce generator potentials. Dear students, the complexity and arrangement of photoreceptive structures varies greatly within the animal kingdom. The simplest photoreceptive structures found in some protozoa, for example, euglena, are called the eye spots or stigma. A stigma is a bright red-colored organelle. It has carotenoid pigments in it. It gives a sense of light and dark to the animal. It cannot form an image. This sense helps in phototexes of these animals. The multicellular photoreceptive structures consist of a cup-like depression, which contains photoreceptive cells. These simplest structures are called the eye cops or oceli. They are found in some niderians and in flatworms, for example, planaria. These simple multicellular structures cannot form images and they provide the animal only a sense of direction. Dear students, the photoreceptive structures of higher invertebrates and vertebrates are image-forming eyes. Eyes give an animal a more precise information about its surroundings. The eyes of vertebrates have cornea and lens, which focus light on to the sensory surface of the eye, which is called retina. These eyes form sharp images. Dear students, the eyes have visual pigments, which capture the light and convert it into or transduce it into the electrical signal. This visual transduction is based on the presence of a very highly conserved set of proteins in Animal Kingdom, which are called the opsin proteins. These opsin proteins are found in the cell membranes of all photoreceptor cells. An opsin molecule has seven transmembrane domains. It is a large molecule with seven transmembrane domains. These opcins provide an optical pathway to capture the photons within the photoreceptor cells. The opsin proteins are coupled to the light-absorbing photopigment retinol. This forms the functional pigment molecule, which is called the rhodopsin. The photopigments are the molecules which are structurally altered by the absorption of photons. When the photons reach them, as a result, structural changes appear in these photopigments. As a result, they activate a cascade of associated molecules and a cascade of reactions that results in the opening of ion channels and generate graded receptor potentials.