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Transit Across Membrane: Active B


Transit across the membranes, we're looking at active transit, part B. And so far in the last couple of lessons, we were looking at the ways in which things like, well ions and small molecules cross over the membrane either passively or with some extra push. Now we're looking at ways that larger things get across. So that would be both directions and there are variety of ways this happens.

The two basic ways are exocytosis, endocytosis, this is also part of active transport. Endocytosis is actually divided into three major ways. Exocytosis refers to the ways in which larger molecules, and so that might be proteins, groups of molecules, are transported from the cell to the extracellular environment.

How things get spit out, so to speak. And this happens via vesicles that are formed within the cell. These vesicles allow well sort of watery substances to get across the fatty bilayer. Some things that are pushed out of the cell frequently would be, well proteins I already mentioned and it's shown here in the drawing.

But also hormones, it could be antibodies, starches. So bigger glucose configuration or glycogen. And that happens through these vesicles. The vesicles also have the purpose of bringing things up to the cell membrane, so rather than having things expelled, these vesicles are used to transport the things that comprise the membrane from different organelles in the centre of the cell up to the membrane.

Things that build the structure and function, these different integral proteins that make up the membrane. Well these all have to be built somewhere. They're built from within the cell, so these vesicles form that function as well. In this step 1 here, we have what's called trafficking. And this is just the movement itself.

In exocytosis, it's the movement of the vesicle up to the membrane. And what number 2 here shows, is tethering and docking. That's when the vesicle makes contact with the membrane. It becomes attached to the membrane and then from there we have fusion and we have release. The vesicle merges with, and it actually becomes part of the membrane itself.

And then that allows the substances to be released on the other side. What we're seeing in this diagram is what's called complete fusion. And that's because the vesicle after this process no longer exists. It merges with the membrane, it's part of the membrane, it's never gonna be a vesicle again. The actual pieces that built up the vesicle are likely to be recycled for future vesicles but that initial vesicle in that shape does not exist anymore.

There's another kind of fusion and release that's called kiss and run. It's not is shown here but it actually looks kinda more like this stage. Because the vesicle gets really close to the membrane and then just creates a tiny little hole, like it's nano meter sized. Releases what it needs to release and then it retreats back into the cell. And that's really efficient because it allows that vesicle to be used again for other purposes.

This kiss and run or what's often called just K-R fusion is really important when it comes to neuro transmitters. Because a nerve terminals, these vesicles are really important and we need a lot of them, they have to fire really quickly. So it's essential that a single vesicle is set up in such a way that it can be used more times.

It can multiple journeys rather than this whole process of medium to reassemble itself again each time it makes another trip. And then we have endocytosis and this is bringing large molecules or groups, collections of substances, into the cell and it also uses a vesicle. This picture makes it look like we just have the exact same process as exocytosis in reverse, it can look kind of that way, but there are actually variety of types of endocytosis.

The three main kinds I'd showed you in the outline, I'm going to go through each of those and you will see that these are pretty different from one another. So let's start with pinocytosis. This is also sometimes called cell drinking, or I think I've heard it called liquid endocytosis. And it's a situation in which the cell takes in molecules across the water in big quantities.

From the extra-cellular environment, but it takes them in the water that the molecules or substances are floating in from the local micro environment. In pinocytosis, this membrane invaginates, it then pinches off and it brings in the particles through this vesicle that's now formed. At that point we have lysosomes, and the lysosomes come, they bind to the vesicles. And because these lysosomes produce hydrolytic enzyme mixtures, they can cut through the water.

They can easily break down the substances that are coming through these little buckets of water. And also break down whatever happens to be floating in it, or at least make those things available to be broken down maybe by other portions or regions. And so, the advantage of pinocytosis isn't actually the ingestion of water. And so, when we say cell drinking, I think that's a little misleading cuz it sounds like it's some way for the cell to get water into it.

It's actually the enzymes and the nutrients that the cell's trying to get through this process. We already covered how the cell gets water in and out. That was in passive transport and it's through things like osmosis. And we looked at the different tonicity types. The point is pinocytosis is a really quick and efficient way to grab a lot of things all at once and haul them into the cell.

So the process of pinocytosis is it's nonspecific. It's nonspecific to the molecules that are held within the water. It just brings in the water and whatever happens to be with it. And it's very different from receptor mediated endocytosis which is extremely selective and is really a process that's designed to bring in very specific substances.

It's also called clathrin-dependent endocytosis. And it works through receptors clathrin and adapter proteins. Here we have the receptor for some particular kind of molecule, maybe a hormone, and this pinkish reddish dot right here is an adapter protein. This one happens to be AP-2. There are all different kinds of adaptor proteins.

The purpose, or we could say the definition, of adaptor proteins they facilitate interactions and signalling pathways, and they often do so by linking up in certain configurations. And through that they can form coordinated communication across the cell. Some of these adapter proteins carry material between organelles within the cell.

So for instance AP-4 would be an example of that. The one we're looking at here, AP-2, it's task is to recognize and bind with the clathrin. And it needs to do so in such a way as to form all the vesicles that are needed to bring particles and so, what happens is that the AP-2 associated with the plasma membrane.

And up here at what's called the budding sites, we have these bunches or these complexes of AP-2 proteins. And they're connected to these integral proteins, these black ones, and then also of course to the clathrin, down here. Clathrin is also a protein, and it's specific to creating vesicles. Not just the vesicles that are used in endocytosis, but lots of different vesicles within a cell are made of clathrin.

It has a really sturdy structure to it. And I've included a picture here. I know I just covered up some of the drawing but we'll look at a close up of this drawing in the next slide. Clathrins can't bind to the cargo directly. It attaches one foot at a time.

So, maybe one of its feet is gonna be attached to the adapter protein. The other two feet are going to attach to other clathrin in the environment, in the area, and they have this kind of angular shape. We see the knees here, the ankles and so that allows it to form this lattice type of kind of shell or encasing. It's been described as like a soccer ball when all the pieces are put together or I think of those climbing structures on playgrounds with the bars, the ones that are shaped like a hemisphere and clathrin is, like I said, it's pretty sturdy and it's used over and over again.

Once it's formed one vesicle, it gets broken down, it forms others. So let's look at it a little bit closer. The molecule here, or we could call it a ligand, and the receptor here in black, as well as the adaptor protein and the clathrin. All four of these link together in strata and then the entire string invaginates, breaks off within the cell.

And, like I said before, once inside the cell, the clathrin as well as the adaptor proteins can be reassembled, can be used for other purposes. Phagocytosis is very different. It doesn't look similar at all. It's also known sometimes as cell eating. And in this case I think that term, that kind of colloquial term does fit.

It really does give the right impression, unlike pinocytosis, cell drinking. Phagocytosis, its more like a movement. The cell itself, the cell that's phagocytosing, is moving around to find targets to eat, to engulf. And the cell that's doing this movement is called a phagocyte. They target large items that they want to ingest.

Which would be in this case we're looking at this, the idea is this blue item is foreign to the cell. It's trying to engulf that cell or that item. It could be an entire cell. Sometimes the cells are engulfing entire cells. And then because of this engulfing the entire shape of the phagocytes can change.

It doesn't always, it really depends on how big of the substance is bringing in. But a lot times, they're pretty sizeable. And here we're kind of looking at a two stage process. This is the stage where there's a binding that's occurring. The phagocyte has the receptors. The target is doing the binding, and then in number 2, the shape of the phagocyte has changed and the targeted substances is internalized.

Phagocytosis is frequently carried out by immune cells. They detect foreign pathogens, they might be looking for cells that are dying or dead. It's often a way of cleaning things up within the cell. Here's the process that occurs in a little bit more detail. The target, such as this microbe, binds to the receptor of the phagocyte.

So that's step 1 up here, binding an absorbing, in step 2, we have the formation of a phagosome and that's just the sphere. And I honestly don't know why they didn't just, when they drew this picture of this blue cell, why didn't just draw it facing the other way so the numbers lined up with cell but that's OK. I can't flip the picture on my end because then all the letters and numbers and things would be backwards.

So, we'll just do some crossover work. In step 3, the phagosome and lysosome, seeing those guys again, form a phagolysosome. And that's just this connection between the two. They actually merge with one another, as you can see. The particles are being broken down.

And then at 5, we have the release of the microbial products on the other side. This is 5, and if this looks a little bit like exocytosis to you, that's exactly what's going on in step 5. These numbers are arbitrary. I mean they want you to know the general order of course, and they would want you to know the difference between a phagocyte, a phagosome, a phagolysosome.

But whenever I'm showing you this step process, with the exception of a few examples, the steps themselves, the number themselves don't matter at all. So don't worry about that, just know the terms, know the general process.

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