Back
ditki Logo
medical & biological sciences
All Access Pass - 1 FREE Month!
Institutional email required, no credit card necessary.
Phototransduction
Free One-Month! Institutional (.edu or .org) email required
Log in or start your One-Week Free Trial!

Phototransduction

phototransduction
  • Phototransduction is the process wherein light particles are converted to electrical signal.
rhodopsin formation and breakdown - overview
Rhodopsin is the pigment for rod photoreceptors.
  • Retinal (cis-isomer) combines with opsin to form rhodopsin.
Retinal is derived from Vitamin A.
  • Light particles shine on rhodopsin and activate it. Rhodopsin breaks down into:
  • Retinal (trans-isomer) and opsin.
  • Retinal (trans-isomer) isomerizes back to the cis-form (the inactivated form). It is then, once again, able to bind to opsin to form rhodopsin.
rhodopsin formation and breakdown - Details
The bend in its carbon chain distinguishes the cis-isomer from the trans-isomer.
  • Trans-isomer of retinal
Its carbon chain is straight.
  • Opsin
Comprises a 7 alpha-helix transmembrane bundle – this bundle passes across the photoreceptor disc membrane; hence, it's a "transmembrane bundle."
  • Inactive Rhodopsin
Cis-isomer of retinal + Opsin -> When these combine through a chemical reaction, they form inactive rhodopsin.
  • Active Rhodopsin
    • When light is introduced, rhodopsin takes on its trans form (it's straight chain form): this form is active!
    • Rhodopsin then breaks down to the trans-isomer of retinal and also opsin.
    • The trans-form of retinal isomerizes back to the cis-isomer, which is again ready to combine with opsin.
phototransduction along the photoreceptor cell membrane
Site of activity is a photoreceptor cell, in its outer segment, which is the site of the rod disc.
  • Step 1: Light activates rhodopsin.
  • Step 2: Activated rhodopsin activates transducin (a G-protein).
  • Step 3: Transducin activates phosphodiesterase (PDE).
  • Step 4: The transducin/PDE combination converts cGMP to GMP.
When cGMP attaches to a sodium channel it is open. Sodium ions enter the cell, which serves to make the intracellular space more positive (less hyperpolarized). When the transducin/PDE combination converts cGMP to GMP, the sodium channel closes. Sodium cannot enter the channel.
  • Step 5: The drop in cGMP causes the sodium channels to close.
  • Step 6: The photoreceptor cell membrane potential becomes more negative (hyperpolarizes) because sodium ions cannot enter the cell ->So it becomes less excitable.
Consider what occurs when you enter a dark room on a sunny day. It takes time for your eyes to "re-adjust" – your photoreceptors are hyperpolarized from the saturation of light in the environment.

Related Tutorials