Question

Colour blindness is due to defect in ………………..
A. Cones
B. Rods
C. Rods and cones
D. Rhodopsin

Answer 1

There are two types of cell present in the eye. They are rod cells and cone cells. These cells help to see the world around us. If one of these cells impaired, we have difficulty in vision. Colour Blindness is one such disorder that is characterized by the inability of an individual to distinguish different colours.

Complete answer:
Cone cells are colour sensitive cells found in vertebrate eyes. They are responsible for coloured vision. The cone cells react differently to lights of different wavelengths.
There are three types of cone cells – L-cones for light of longer wavelengths, M-cones for light of medium wavelength and S-cones for short wavelength light. These cells are present in greater numbers in the fovea centralis of the eye as compared to the retina.
Cone cells have a synaptic terminal, large number of mitochondria and are nucleated. The membrane forms invaginations stacked in the form of discs, in which the photo pigments are present as transmembrane proteins.
Rod cells are photoreceptor cells, present in the retina of the eye. These cells function better in low light and have no role to play in coloured vision. Rod cells are more numerous than the cone cells. The basic structure of these cells comprises a synapse, inner and outer segments. Nucleus and organelles are present in the inner segment, while the outer segment is responsible for absorption of light. Rod cells are sensitive to low light and are desensitized when exposed to high proton concentration for longer periods.

Rhodopsin is a transmembrane G-protein coupled receptor protein that is primarily responsible for vision. It is found in the rod cells of the retina and belongs to the opsin family of proteins. Rhodopsin is a strong absorber of blue green light and appears to be reddish purple in colour. It is also known as ‘Visual Purple’. Mutation of the gene synthesizing Rhodopsin causes retinitis pigmentosa which is responsible for loss of vision.
Therefore, the correct option is (A).

Note: There are two types of cell present in the eye. They are rod cells and cone cells, responsible for vision in low light and bright light respectively. The light sensitivity of rod cells to light are more than the cone cells and are present in the periphery of the eye. Cones are present towards the center of the eye and are fewer in number than the rod cells. Impairment in one of these cells can cause erroneous eye conditions like Colour Blindness. There are various stages of Colour Blindness. People with total blindness find it difficult to adjust to very bright environments and have impaired visual accuracy.