Vitamin A in Vision

Rod cell give us sensitivity towards low light but cone cells give us colour vision. Inside the rod cells there is a main pigment called rodopsin, which is a G-protein coupled receptor. The main structural unit in rhodopsin is 11-cis-retinal. When light falls on rhodopsin, it undergoes a transformational change, from 11-cis to all-trans retinal. Due to this transformational change an associated protein called Transducin gets activated. Then, the G-alpha subunit gets dissociated from transducin complex and moves in a membrane bound fashion to go and activate cyclic chain phosphodiesterase (cGMP phosphodiesterase). All this leads to an electrical change in the rod-cells which transduce signals to the higher order neurons of the visual system and that is how we can see.

Retinal in rhodopsin isomerises in light, This isomerisation sends signals to unpolarised optic nerve cells. The unpolarised nerve cells have an equal distribution of ions both on the outside and inside of cells.

Alcohol interferes with the conversion of retinol to retinaldehyde.

Xerophthalmia occurs due to Vitamin A deficiency. There is a WHO classification of various stages of Vitamin A deficiency.