Question

Which of the following events takes place FIRST in phototransduction in response to a flash of light.

A. The cGMP level drops.
B. The cGMP level rises.
C. The Ca2+ level drops.
D. Retinal isomerizes to an all-trans configuration.
E. Gt exchanges its bound nucleotide.

Answer 1

The first event in phototransduction is the isomerization of retinal to an all-trans configuration when light hits the photoreceptors.

Step-by-step explanation:

The event that takes place first in phototransduction in response to a flash of light is D. Retinal isomerizes to an all-trans configuration. When light strikes the photoreceptors, the retinal changes its shape from a bent (cis) form to a straight (trans) isomer, known as all-trans-retinal.

This photoisomerization triggers a cascade of events that ultimately leads to changes in the levels of intracellular molecules such as cGMP and Ca2+, but those occur subsequent to the isomerization of retinal.