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.


I can’t understand why in Pearsons book “Biology a global aproach” it’s written the other way around.
-The transduction of visual information to the nervous system
begins with the light-induced conversion of cis-retinal to
trans-retinal in rods and cones. Like other cis-trans pairs, these
isomers of retinal differ only in the spatial arrangement of
atoms at a carbon-carbon double bond (see Figure 4.7).
As shown in Figure 50.17, trans-retinal and cis-retinal
differ in shape. This shift in shape activates the visual pigment (in rods, rhodopsin), which activates a G protein,
which in turn activates the enzyme phosphodiesterase. The
substrate for this enzyme in rods and cones is cyclic GMP,
which in the dark binds to sodium ion (Na+
) channels and
keeps them open (Figure 50.18a). When the enzyme hydrolyzes cyclic GMP, Na+
channels close, and the cell becomes
hyperpolarized (Figure 50.18b). The signal transduction
pathway then shuts off as enzymes convert retinal back to
the cis form, inactivating the visual pigment.

as i understand here it says that namely change of retinal activates rhodopsin

Hey! I think you might have a slight misunderstanding of Pearson’s

Rhodopsin Activation in the Visual Cycle: Rhodopsin, found in rod cells, is a complex of the protein opsin and the chromophore 11-cis retinal. When light hits rhodopsin, the 11-cis retinal is photoisomerized to all-trans retinal. This change in the shape of the retinal molecule triggers a conformational change in the opsin protein, leading to the activation of rhodopsin.
Pearson’s Book Description: The description in the Pearson’s Biology book is consistent with this process. It states that the conversion of cis-retinal to trans-retinal in rods and cones is what activates the visual pigment (rhodopsin in rods). This is the initial step in the phototransduction pathway, where the isomerization of retinal from the cis to the trans form leads to a series of biochemical changes within the cell, ultimately resulting in the conversion of light into an electrical signal.

Thank you! I also made a little reaserch and found the right answern :slight_smile:

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