Phototransduction Biochem

Question 1

Photoreceptor cells

Answer 1

Rods: Night vision

• Rhodopsin
• High sensitivity, low resolution

Cones: Color detection

• Three opsins
• Low sensitivity, high resolution

Question 2

What is found concentrated at the Fovea

Answer 2

Cones

center field of vision, and is where there is the highest visual acuity

Question 3

What does light do to the rod photoreceptors

Answer 3

in the dark the cell is depolarized and releasing glutamate

In the presence of light, hyperpolarizes the cell because the rhodopsin molecules deactivate and the sodium channel closses stopping the release of glutamate to the ganglion cells

leads to depolarization of on center bipolar cells
and hyperpolarization of off center bipolar cells

Question 4

Modulation of the cGMP PDE by Gt

Answer 4

photon binds to retinal and turns it into trans retinal and activates the rhodopsin

rhodopsin then activates the transducin molecules by releasing GDP and binding GTP

then will ativate the PDE which will hydrolyze cGMP

• less cGMP means the sodium channels close
• hypepolarization of the cell and prevents voltage gated calcium to close

thus less calcium less glutamate released

leads to depolarization of the On center bipolar cells and hyperpolarization of the OFF center bipolar cells

Question 5

impact of cGMP

Answer 5

in light low cGMP levels close Na+ and Ca++ channels relowering their respective levels and leading to termnation of the phorotransduction cascade

this drop causes to increase the guanylate cyclase to increase levels of cGMP leading to recovery of the pathway

Question 6

Function of rhodopin kinase

Answer 6

phosphorylates the light activated rhodopsin allowing for the binding of arrestin which prevents the interaction with transducin (GPCR) thus blocking the signal

Question 7

How is retinal produced

Answer 7

from Vitamin A (retinol, retioic acid, and retinal

Question 8

Deficiency in Vitamin A

Answer 8

BItot spots, xerosis, keratomalacia

Nyctalopia (difficulty seeing in low light, insufficient rhodopsin)

Xerophthalmia: abnormal dryness of the conjunctiva and cornea of the eye with inflammation and ridge formation

Question 9

Retinoid cycle

Answer 9

Trans retinal is released from the activated opsin

Transport the rans retinal by ABCR then is converted to trans retinol via aRDH

transport of all trans retinol to the RPE via the iRBP

transfer to the CRBP of the RPE then to the LRAT that esterfies it

then the RPE65 will hydrolyze and isomerize the ester to a retinol

then the CRALBP carries it back as retinal

IRBP plays a role in getting it back

Question 10

Macular degeneration

Answer 10

Degeneration of the RPE and the retina

loss of central vision, poor vision at night

atrophy of the macula RPE

mutation in the retinoid cycle specifically the ABC transporters (ABC4)

impaired clearance of toxins and retinoid metabolites

Kale helps this (macular cartenoids)