Photopigments of Rods and Cones Flashcards
Rods and cones contain special photopigments each containing what?
A protein: opsin and a chromophore.
What is a chromophore?
A light-sensitive molecule.
What is the photopigment of rods?
rhodopsin
What are the photopigment of cones?
color pigments or iodipsins or cone opsins.
What is the opsin protein in rods called?
Scotopsin
What is the opsin portion in cones called?
Photopsin
What sets scotopsins and photopsin apart?
They have different amino acid composition and they absorb different wavelength of light.
Chromorphore is the same or different in rods and cones?
They are the same.
What is the chromophore composition?
an aldehyde of vitamin A (retinaldehyde)
11-cis retinal isomer of retinal is what?
The chromophore
What is rhodopsin?
Scotopsin plus 11-cis retinal
What are the functions of the visual pigments?
They act as photoreceptors and they are required for vision.
What do the visual pigments detect?
Detection of shape, depth, and color.
The absorption of light energy by rhodopsin causes what?
a raid transformation fo 11-cis retinal to all trans retinal.
What binds less avidly to opsin ?
Trans retinal
What does all trans retinal isomer cause?
Conformational changes in scotopsin
What activates the transducin ( a G-Protein)?
The trans retinal isomer becomes free and moves laterally in the plasma membrane and then activates it.
What occurs during the regeneration of rhodopsin?
All trans retinal is converted to 11-cis retinal.
How is all trans retinal converted to 11-cis retinal?
by retinal isomerase
Regenerated 11 cis retinal automatically recombines with scotopsin to form what?
Rhodopsin.
When rhodopsin is activated it is what?
Decomposing rhodopsin
When rhodopsin is deactivated it is what?
regenerated rhodopsin
Where is vitamin A present?
In the cytoplasm of the rods and cones as well as in retinal pigment epithelial cells.
What does vitamin A convert to?
11-cis retinal as needed.
What occurs to Vitamin A for storage?
excess retinal is converted to retinol (vit A)
What is night blindness (nyctalopia)?
the inability to see in relatively low light.
What is a major cause of night blindness?
Vitamin A deficiency
What is vitamin A deficiency?
Less retinal is formed than needed
At night or with low light the retinal levels are what?
inadequate for vision in a subject that has night blindness.
The dark is referred to what type of state?
unexcited state.
The unexcited state Na+ conductance into outer segment of rods is what?
Greater than that of a typical sensory cells.
What does the cyclic guanosine monophosphate regulate?
The transmembrane conductance of sodium in photoreceptor cells.
In the dark what occurs to the cytoplasmic levels of cGMP?
They increase
What occurs when cGMP levels increase?
The cGMP regulated channels become activated and increase Na+ influx.
The membrane potential in an unexcited state in a low light environment is what?
More positive
The membrane potential of that in any other sensory cells in an unexcited state is what?
More negative.
The excitation of rods by light results in what?
hyper polarization not depolarization
excitation in other sensory receptors are usually caused by?
depolarization
In bright light the activation of rhodopsin causes what?
stimulation of the G protein called transducin
The activation of transducin leads to what?
The stimulation of phosphodiesterase
What does phosphodiesterase do in Rod hyper polarization in light?
hydrolyzes cGMP to 5’GMP
The activation of photopigments in cones is similar to what?
Rods
What does the activation in cones require?
More photons for the same response in as rods.
Cones are 30-300 fold less what then rods?
Less sensitive to light than rods.
The extent in amplification in con excitation cascade is what?
much less than in rods.
