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Ans) Photoreceptors are sensory cells found in the Retina of eye, which are light detectors, as the name suggest photoreceptors. These cells absorbs light falling on them and convert them into electrical signals to pass to the brain for decodation of the visual image and this process is called Phototransduction.

Photoreceptors in vertebrates are of two types Rods and cones which are found on the outermost layer of retina. Rods and cones have different shape but similar structure like rods are shaped like long rod like structure and cones have a tapered end unlike rods. Rods are numerous in number than cones and are also more sensitive to light than cones. Cones on the other hand are more sensitive to colour and that too for three colours Red , Blue and green but rods are not sensitive to colour.

Both rods and cones are long and thin but cones have pointing tapered one end and rods are flat rod like. Like nerve cells they have dendrites at one end and photoreceptor cells are thus called as modified nerve cells. They have Axon terminal which is close to the visual field and releases neurotransmitter called glutamate. To the back they have cell body containing all cell organelles. Both these cells have an extra inner segment which is a specialized part containing full of Mitochondria ( which is the power house of cell and provides ATP or energy for the sodium potassium ion channel pump). Finally the outermost segment that is closest to the brain and which absorbs light and these are in form of cilia like structure containing disks filled with Opsin ; a protein molecule which forms rhodopsin ( visual pigment). In rods the disks are closed but in cones the disks are partially open to the outer surrounding fluid.

Photoreceptor cells being specialized types of neuron or nerve cell have great biological importance as they convert light into signals to further stimulate biological processes. Rods are extremely sensitive and gets triggered by single photon of very low light for visual effects, Cones on the other hand works well when there is bright light and hence is helpful in colour detection unlike rods , as in very low or dim light the colours cannot be detected and only rods function then. However in bright light all three types of cone cells like red , blue and green light wavelength work accordingly. Cones convert opsin to Photopsin unlike rods that convert opsin to Rhodopsin. Photopsin further reacts to different light frequencies to produce colour of the object. Hence both photoreceptor cells performs functions related to vision of light and colour.

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