What are Photoreceptors?-Definition, Structure and Types

Photoreceptors are the cells in the retina that are responsible for vision. They contain a large amount of tightly packed membrane that contains the photopigment rhodopsin or a related molecule. This allows them to respond to light and send signals to the brain that create the sensation of sight.

What are Photoreceptors?

Photoreceptor cells are specialized light-sensitive nerve cells found throughout our bodies. They play an important role in detecting changes in light levels around us, such as when entering or leaving a room or tunnel.

Photoreception

“Photoreception” refers to the ability of these cells to detect photons (the smallest unit of visible light) and convert them into electrical signals that can then be processed by other neurons.

Structure & Composition of Photoreceptor

All photoreceptors have photopigments. These pigments are carotenoids and rhodopsin. These pigments absorb photons of light energy and generate a generator potential. Different types of photoreceptors exist in different animals.

Stigmata, or stigma, are present in certain types of protozoa, such as euglena. These protozoa have chlorophyll that possesses stigma. Stigmata is a group of bright red photoreceptor granules. These granules are carotenoid pigments. The part of the protozoan that detects light is the swelling at the base of the flagellum.

The stigma acts as a shield for this light-sensitive area. It helps the photoreceptor to detect light coming from a particular direction. In this way, the photoreceptor and stigma enable Euglena to orient itself towards the light. Because of this ability, Euglena can store water in areas that receive maximum light exposure.

Types Of Photoreceptors

There are two main types of photoreceptors;

1. Unicellular Photoreceptors
2. Multicellular Photoreceptors

Unicellular Photoreceptors

Some animals like earthworms have simple unicellular photoreceptor cells. They are scattered over the epidermis. Or they concentrated on particular areas of the body.

Multicellular Photoreceptors

There are three types of multicellular photoreceptors: ocelli, compound eyes, and complex eyes.

Ocellus

The ocellus is a light-sensitive organ that helps an organism detect changes in light intensity and direction. This important sense organ is found in many different types of animals and is made up of several light-sensitive cells that are backed by a layer of pigment.

Many examples of chemical change occur in the pigment that produces an electrical signal when light hits these cells. This signal is then sent to another part of the body for interpretation. Ocelli are very important for survival as they help animals orient themselves and avoid predators or other dangers.

Compound Eyes

The compound eye consists of many units called ommatidia (ommatidium). Compound eyes are present in arthropods. They also occur in some annelids and bivalve mollusks.