Question

Photosensitive compound in the human eye is made up of
(a) Guanosine and retinol
(b) Opsin and retinal
(c) Opsin and retinol
(d) Transducin and Retinene

Answer 1

Hint: The photosensitive compound in the human eye is made up of a protein while the other is an aldehyde which is a derivative of vitamin A. These are located on the outermost layer of the retina.

Complete step by step answer:
The retina is a layer of nervous tissue that covers the posterior two- thirds of the eyeball, which when stimulated by light, initiates the sensation of vision. The light ray focuses on the retina through the cornea and lens generating potentials in its photosensitive compounds made up of rods and cones. The human eye contains opsin, a protein, and retinal, an aldehyde of vitamin A. Light stimulates the dissociation of the retinal from opsin, thus causing changes in the structure of the opsin.
- The retina has three layers of the cell from the outermost to innermost- photoreceptor cells made up of rods and cones, bipolar cells, and ganglion cells.
- Disassociation of photosensitive compounds in photoreceptor cells of retina results in membrane permeability changes.
- Membrane permeability changes cause activation of a regulatory protein known as transducin. It activates another enzyme ‘phosphodiesterase’ which brings about the hyperpolarization or generation of an action potential in the ganglion cells of the retina through bipolar cells.
- Bipolar cells take the signal from photoreceptor cells. These cells form the middle layer of the retina and pass the action potential to ganglion cells.
- Ganglion cells form the innermost layer of the retina and carry visual information to the brain. It forms the optic nerve fibers which later exit the retina through the blind spot.
- The action potentials are transmitted to the visual area of the occipital lobe of the cerebral hemisphere.
- The nerve impulses are analyzed and an erect image is recognized based on early memory and recognition.
So, the correct answer is ‘(b) Opsin and retinal.’

Note:
- The human eye can perceive only the visible spectrum of the electromagnetic radiation which ranges between the wavelength of 380- 760 nm.
- The human eyes also have a remarkable power of accommodation. It does so by changing the convexity of the lens.