Notes
Phototransduction
Sections
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
- Cis-isomer of retinal
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.
Full-Length Text
- Here, we'll learn the physiology of phototransduction.
- Start a table, denote that phototransduction is the process wherein light particles are converted to electrical signal.
To begin, let's sketch an overview of the formation and breakdown of rhodopsin, which is the pigment for rod photoreceptors.
- First, show that retinal (specifically the cis-isomer) combines with opsin to form rhodopsin.
- Retinal is derived from Vitamin A.
- Now, show that light particles shine on rhodopsin and activate it, and then it breaks down into the trans-isomer of retinal and opsin.
- Then, show that retinal isomerizes back to the cis-form (the inactivated form).
- It is then, once again, able to bind to opsin to form rhodopsin.
Now, let's take a closer look at rhodopsin formation and breakdown.
- First, we'll draw the cis-isomer of retinal.
- Although it isn't necessary for you to draw out its chemistry, we show it here, so we can see the bend its carbon chain.
- This distinguishes the cis-isomer from the trans-isomer.
- Draw a bent line underneath the molecule to remember that the cis isomer is bent.
- Now, draw the trans-isomer – show that its carbon chain is straight.
- Draw a straight line underneath the molecule to remember that the trans isomer is straight.
- Next, show that opsin comprises a 7 alpha-helix transmembrane bundle – this bundle passes across the photoreceptor disc membrane, which we'll draw, soon, which is why we call it a transmembrane bundle.
- Then, show that when these combine through a chemical reaction, they form inactive rhodopsin.
- The opsin is bound to nitrogen for reasons beyond the scope of this tutorial.
- Next show that when light is introduced, rhodopsin takes on its trans form (it's straight chain form).
- Indicate that this form is active!
- Finally, show that rhodopsin breaks down to the trans-isomer of retinal and also opsin.
- Then, show that the trans-form of retinal isomerizes back to the cis-isomer, which is again ready to combine with opsin.
Now, let's show the process of phototransduction that occurs along the photoreceptor cell membrane.
- First, draw a photoreceptor cell.
- Specify its outer segment, which is the site of the rod disc.
- Then, draw a segment of the cell membrane.
- Specify intracellular and extracellular.
- Along the cell membrane, first draw inactive rhodopsin – again, it's a transmembrane protein, so it passes across the membrane.
- Then, draw activated rhodopsin.
- Step 1: Indicate that light activates rhodopsin.
- Next, draw transducin (a G-protein).
- Step 2: Indicate that activated rhodopsin activates transducin.
- Then, draw phosphodiesterase (PDE).
- Step 3: Indicate that transducin activates phosphodiesterase.
- Now, draw an open sodium channel.
- Show cGMP attached to it and that the channel is open.
- Indicate that sodium ions enter the cell, which serves to make the intracellular space more positive (less hyperpolarized).
- Draw the transducin/PDE combination.
- Step 4: Show that it converts cGMP to GMP.
- Next, re-draw the sodium channel but as closed.
- Show that sodium cannot enter the channel.
- Step 5: Indicate that the drop in cGMP causes the sodium channels to close.
- Lastly, draw a membrane potential graph and show that..
- 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.