RODS
Rod cells are able to function in dim light but do not perceive colour.
Rod cells are able to work in low light intensity because the cell can respond to a single photon of light (more sensitive). The brain requires fewer than 10 such responses to perceive the sensation of a flash of light.
Rod cells however are unable to distinguish colour therefore despite being more sensitive than cones to light they enable us to see at night but only in black and white.
A rod cell has an elongated structure with the outer segment specialized for photoreception. (See diagram on right) It is this segment that contains the many discs which are membrane enclosed sacks densely packed with photoreceptor molecules.
The photoreceptive molecule is rhodopsin
which consists of the protein opsin linked to 11-cis retinal
a prosthetic group.
A human retina contains about 125 million rod cells
The number of rods and cones vary over the surface of the retina.
In the very centre there are only cones this is where the vision is best.
