Detection of Light by Mammals – A Level Biology B Edexcel Revision

Detection of Light by Mammals

Introduction

Light detection by mammals relies on specialised cells in the eye known as photoreceptors.
Two main types of photoreceptors play distinct roles: rods and cones.

Rods are sensitive to low light intensity and are important for night vision.
Cones are less sensitive but allow for colour vision. There are three types: red, green and blue cones, each sensitive to a different wavelength of light.

Light entering the eye is focused onto the retina, a layer at the back of the eye containing photoreceptor cells.
Photoreceptors convert light energy into an electrical signal which is transmitted to the brain via the optic nerve.
This conversion is initiated by the absorption of light by a pigment known as rhodopsin in rods and iodopsin in cones, leading to a series of chemical reactions resulting in the creation of an electrical signal.

Electrical signals are sent from photoreceptors to bipolar cells, then to ganglion cells, and finally along the optic nerve to the visual cortex of the brain.
The brain interprets these signals, allowing us to perceive attributes of light such as colour, intensity and direction.

The transition from bright light to darkness (and vice versa) requires the eyes to adapt for optimal vision.
In bright light, rods become overstimulated and cones are primarily used for vision. As light levels decrease, rods gradually regain their sensitivity, enabling vision in low light.
This transition takes time, and is responsible for the temporary difficulty we have seeing when moving from a brightly lit space to a darker one.

Accurate light detection is crucial for a range of behaviours including navigation, finding food, and avoidance of predators.
Understanding the mechanism of light detection can aid in the detection and treatment of vision disorders in mammals.