IMAT 2020 Q26 [Stimulated Rod Cells]

Which one of the following will be caused by an appropriate stimulus of a rod cell in the retina of a healthy human eye?

A. rhodopsin will form an opsin and retinol
B. an opsin and retinol will form rhodopsin
C. rhodopsin will form an opsin and retinal
D. rhodopsin will form opsonin and retinol
E. an opsin and retinal will form rhodopsin


The correct answer is C.

This question refers to the visual cycle. What we mean by visual cycle is the transformation of a photon (from light) to electrical signals inside the retina. Therefore, it can also be called visual phototransduction.

The visual cycle and photoreceptors

Like the name suggests, it is a process that begins with the detection of light by photoreceptors. In the retina, there are 2 different types of photoreceptors : rod cells and cone cells.

Both have different functions :

  • Cone cells are responsible for color vision and function best in bright light.

  • Rod cells, as opposed to cone cells, work best in dim light and thus permit night vision. They contain rhodopsin, which is a light-sensitive receptor protein.

Those photoreceptors contains 11-cis retinal, which is a form of vitamin A. It is linked to opsin and makes it able to activate the signaling pathways. Both 11-cis retinal and opsin are called visual pigments.

How is rhodopsin involved ?

Rhodopsin is another visual pigment. When stimulated by light, and through the activation of the visual cycle, it produces trans-retinal, which is then transformed into cis retinal after the action of an isomerase.

Additionally, it transforms into metarhodopsin-II, which is then transformed into opsin.

Therefore, rhodopsin will form an opsin and retinal.