Phototransduction Cascade Explained | MCAT Biology

[00:00:01]Last video, we looked at how light from the sun can enter the eye-- and this is the eye.

[00:00:09]So it enters the eye, hits the back of the eye, and somehow through a set of steps, it gets converted into neural impulse.

[00:00:20]And the impulse gets sent to the brain for you to make sense of the information.

[00:00:26]So we looked at something called the retina in the last video.

[00:00:31]And we talked about how the retina is made up of a bunch of different cells.

[00:00:37]And the two main cells are rods and cones.

[00:00:41]So we're going to look at just the rods in this video.

[00:00:45]So the rod, which is shaped like this.

[00:00:50]Let's give him a little smiley face because he's happy.

[00:00:53]And what he does is that as soon as light is presented to him, he basically takes that light and then converts it into a neural impulse.

[00:01:04]So normally, he is turned on.

[00:01:06]When there's no light, the rod is turned on.

[00:01:09]But when light is present, it actually turns him off.

[00:01:13]So with light, he's turned off.

[00:01:15]So how does this happen?

[00:01:17]So this occurs through this thing called the phototransduction cascade.

[00:01:27]So the phototransduction cascade is the set of steps that occurs at the molecular level that basically takes this rod and turns him off.

[00:01:36]And in turning him off, he's actually able to turn on a bunch of other cells that eventually lets the brain know, hey, there's light here.

[00:01:43]So it allows the brain to realize that there's light and allows it to comprehend what's going on make sense of the world.

[00:01:51]So let's go ahead and examine this phototransduction cascade.

[00:01:55]So let's give ourselves a little bit of space here, just look at it in a little bit more detail.

[00:02:04]So let's look at just a part of the rod.

[00:02:07]So I'm going to redraw the rod over here.

[00:02:10]And let's just look at this part of the rod, just this very top little bit of the rod.

[00:02:15]And I'm going to draw it a lot bigger, so that we can really make sense of what's going on here.

[00:02:24]So inside the rod, which we just made much bigger, there are a bunch of these little disks.

[00:02:29]So they are really thin little disks, and they're just stacked on top of one another, and there are hundreds of them.

[00:02:35]And basically they are like this.

[00:02:36]They just go stacked on top of one another, and they fill up the entire rod.

[00:02:41]Same thing with cones but we're just going to look at the rods here.

[00:02:45]So inside of these little disks, there are a whole bunch of different proteins all interspersed throughout the disks.

[00:02:54]So this protein that I'm drawing in red-- let me just go ahead and draw it a lot bigger, so we're going to go ahead and blow this protein up.

[00:03:03]And it's basically this multimeric protein, so it consists of a bunch of different subunits.

[00:03:09]There are actually seven subunits, so there are 5, 6, and 7.

[00:03:15]So there are all these subunits that make up this little protein right here.

[00:03:19]So this protein as a whole is called rhodopsin.

[00:03:24]And it's called rhodopsin because it's in a rod.

[00:03:26]If it were in a cone, it would be called cone opsin, but it's basically the same protein.

[00:03:30]And sitting inside of this protein is a small molecule, and it sits just inside.

[00:03:38]Let's go ahead and just zoom in a little bit, so we can make a little bit more sense of what we're looking at here.

[00:03:47]Let's go ahead and zoom in over here.

[00:03:49]So this little molecule is just sitting inside of rhodopsin, and it's kind of bent.

[00:03:56]You can see here how I drew it just a little bit bent.

[00:04:00]So this little molecule is called retinal.

[00:04:06]And in this bent confirmation, we call it 11-cis retinal.

[00:04:12]And so what happens is light comes in from the sun, goes in through the pupil, hits the retina, and then hits this little rod.

[00:04:20]Some of the light will actually hit this molecule.

[00:04:23]So it'll hit rhodopsin, and it'll actually hit this molecule.

[00:04:26]So here's the light wave, it comes in, and it just hits the molecule right there.

[00:04:31]So an interesting thing happens when the light hits the molecule right at this region.

[00:04:36]And what it actually causes, is it causes the retinal to actually change confirmation.

[00:04:42]So the light provides enough energy that the retinal goes from this bent confirmation, and it causes it to look more like this.

[00:04:51]I'll draw it over here.

[00:04:54]I'll draw a little carbon ring, so it becomes straight.

[00:04:58]So it went from being bent to being straight.

[00:05:01]So the light does this, and basically goes from being 11-cis retinal to all-trans retinal.

[00:05:08]So when the retinal changes shape, it actually causes the rhodopsin molecule to also change shape.

[00:05:16]So the two are really closely linked, so when the retinal changes shape, the rhodopsin changes shape.

[00:05:21]So let's go ahead and pretend that maybe it looks like this.

[00:05:25]So this region that I'm shading in right here is what the new rhodopsin looks like after the retinal has changed shape after the light hit it.

[00:05:34]So rhodopsin now changes shape, and that basically begins this big cascade of events.

[00:05:42]So what happens next is there's a molecule-- and I'm going to draw that molecule right here in green-- and it's made up of three different parts.

[00:05:50]So there's this alpha subunit, there's a beta subunit, and there's a gamma subunit.

[00:05:56]And this molecule as a whole is called transducin.

[00:06:18]So transducin, basically as soon as retinal changes shape and causes rhodopsin to change shape, transducin breaks away from rhodopsin.

[00:06:27]And the alpha subunit actually comes over here to another part of the disk and binds to a protein called phosphodiesterase, which I'll just draw as this little box over here.

[00:06:40]So this little protein is called cyclic GMP phosphodiesterase, or PDE for short.

[00:06:48]So PDE, basically what it does-- let's go ahead and zoom out a little bit.

[00:06:55]What PDE does when it's activated is it takes cyclic GMP, which is floating all around the cell-- it's a little molecule, it's tiny-- and it basically takes the cyclic GMP and converts it into just regular GMP.

[00:07:10]So this basically reduces the concentration of cyclic GMP and increases the concentration of GMP.

[00:07:18]And the reason that this is important is because there's another channel over here, so there's a whole bunch of these sodium channels and they're all over the cell.

[00:07:31]So they're just a whole bunch of them, and basically what they let the cell do is they allow the cell to take in sodium from the outside.

[00:07:41]So let's just say there's a little sodium ion, and it allows it to come inside the cell.

[00:07:46]So in order for this sodium channel to be open, it actually needs cyclic GMP to be bound to it.

[00:07:54]So as long as cyclic GMP is bound, the channel is open.

[00:07:58]But as the concentration of cyclic GMP decreases because of the phosphodiesterase, it actually causes sodium channels to close.

[00:08:08]So now we have a closing of sodium channels, and now we basically have less sodium entering the cell.

[00:08:15]And as less sodium enters the cell, it actually causes the cell to hyperpolarize and turn off.

[00:08:22]So as the sodium channels close, it actually causes the rods to turn off.

[00:08:30]So basically, without light, the rods are on because these sodium channels are open.

[00:08:35]Sodium is flowing through, and the rods are turned on.

[00:08:38]They can actually produce an action potential and activate the next cell and so on.

[00:08:43]But as soon as they're turned off, what happens is very interesting because-- let's just look at this rod over here.

[00:08:52]So what happens is really interesting, because there's this other cell over here that is called the bipolar cell.

[00:08:57]And we'll just give it a kind of a neutral base, because it's bipolar.

[00:09:01]This cell, there are actually two different variants, so there are on-center and there are off-center bipolar cells.

[00:09:12]So the on-center bipolar cells normally are being turned off when this rod cell is turned on.

[00:09:19]But as we mentioned, due to the phototransduction cascade, the rods turn off, which actually turns on the bipolar cell.

[00:09:27]So basically, on-center bipolar cells get turned on with light and get turned off when there's no light.

[00:09:33]So that's how they get their names.

[00:09:35]So when the bipolar cell gets turned on, it activates a retinal ganglion cell, which then sends an axon to the optic nerve, and then into the brain.

[00:09:45]And so that process is known as the phototransduction cascade, and it basically allows your brain to recognize that there's light entering the eyeball.