So far we've talked about three mechanisms of molecular transport. We've talked about diffusion, osmosis, active transport, and now we're going to talk about bulk transportation. So let's start by just kind of identifying some differences here. So far we have sort of identified that osmosis and diffusion Those are mechanisms of movement that move things toward equilibrium, and they don't require an input of energy. Active transport moves things away from equilibrium.
So in both, in all of these instances, in these first three instances, we describe the movement. in relation to the gradient. So you don't say we're moving things into the cell or out of the cell, you say we're moving things down the gradient or against the gradient because in different scenarios the direction of the gradient inside or outside the cell might be different, right? So we're describing movement based on the gradient.
Another commonality amongst all three of these is sort of what's being moved in general. So what I mean by that is We're moving smaller molecules, right? Really large things can't move via diffusion, osmosis, or active transport, regardless of what direction they're moving.
And we're moving individual molecules, right? So, for instance, if you have a molecule of, if you have a sodium ion, if it's moving via diffusion or active transport, it's just a single ion moving. individually at a time, not groups of them sort of moving across the membrane together.
This is kind of where two ways in which bulk transport differs. With bulk transport, we're not going to talk about movement in relation to the gradient. We are going to describe movement based on into or out of cell.
So the movement isn't going to be contingent on the gradient. It's going to be described based on whether the substance is coming into the cell or out of the cell. And as you can guess, this is opposite as well.
Bulk transportation would be used either for larger molecules, okay, so think maybe like proteins for instance, would be like a larger molecule that could not be moved via diffusion or active transport. Or if we're moving Many small molecules in bulk. So groups of smaller molecules that could potentially move individually via diffusion or active transport, moving in a conglomerate together across the membrane.
So that's kind of our definition of bulk transportation. It's the movement of either large molecules into or out of the cell, or several smaller molecules in mass. Now another way in which these differ is that bulk transportation utilizes vesicles instead of transport proteins.
So transport proteins are utilized in facilitated diffusion, osmosis, and active transport. The only one that doesn't use them is simple diffusion. But in bulk transportation, we're not going to use transport proteins because they're not going to be large enough to accommodate the things that we're moving across the membrane. Instead, we're going to use vesicles.
A vesicle... is essentially a little membrane-bound sac that is made up of phospholipid bilayers. So think of sort of a little circular arrangement, if you will. This is the head of my phospholipid. These are my tails.
You're going to have a circular formation of those, creating this sort of little pocket or little vehicle to move things across the cell membrane. Like active transport, bulk transportation does require an input of energy. And like I said, we're going to characterize the movement based on whether it's into or out of the cell.
Endocytosis is the word that we use for describing movement into the cell via bulk transportation, and exocytosis is referencing the movement of materials out of the cell. The way I always remember that is when I see E- In the context of exocytosis, I always think of the word exit, and that always helps me remember the two directions. In these images, essentially the movement and how to characterize this movement is the opposite in these two. In endocytosis, some material is going to butt up against the cell membrane.
So in this image, these blue lines are the cell membrane. And remember, that's a phospholipid bilayer. They're not showing you all of that, but wherever you see this blue, you know, imagine two layers thick of phospholipids with the heads facing outwards and the tails facing inwards. So the material butts up against the edge of the cell, the membrane starts to pinch inward around the material, and it eventually pinches in all of the way, creating this vesicle right in here. This is a vesicle, this little circular membrane-bound sac that is carrying the contents of whatever was brought into the cell.
And then that vesicle will move to wherever that material is needed within the cell. So maybe it's going to the smooth endoplasmic reticulum, or maybe it's going to a lysosome. Wherever the destination is, it'll travel to that location within the cell. Sort of the opposite is true when we have exocytosis, a vesicle. holding contents that needs to be expelled from the cell moves towards the membrane.
The vesicle fuses with the membrane of the cell, and that opens up and then deposits the material outside of the cell. And then that vesicle, right, the vesicle that the material was in and that got deposited, then becomes a part of the cell membrane. Now, we're going to focus a little bit on a couple of different types of endocytosis.
The three forms of endocytosis we're going to focus on are phagocytosis, penocytosis, and receptor-mediated endocytosis. Let's start with phagocytosis first. A lot of times this is referred to as cell eating.
So it's the taking in of a large solid mass. An example of this would be a white blood cell engulfing some sort of foreign material. So maybe we'll say engulfing bacteria.
So in this image, this is your white blood cell, and it's getting ready to... could take in via endocytosis, phagocytosis specifically, whatever this foreign material is. And then once it takes it in, white blood cells have lots of lysosomes to help dismantle whatever the foreign material is and keep it from harming the individual. Penocytosis is sort of referred to as cell drinking.
In essence, what happens with penocytosis is that You are taking in many dissolved molecules. So you have many molecules that are dissolved in liquid water. And what happens is many of those small solutes are brought in at one time.
So rather than these solutes diffusing across the membrane individually, you're taking in a conglomerate of them. Whatever is dissolved in the water in this space is going to be brought into the cell. And then lastly, receptor-mediated endocytosis. Receptor-mediated endocytosis is specific in what's brought into the cell, in that on the exterior of the cell membrane are all of these little tiny receptors, which in the image are indicated with their purple, and they look like little sort of cups extended from the cell exterior.
The receptors on the exterior of the cell are called the endocytosis. are very specific to certain types of molecules. And what will happen is you sort of have a lock and key effect. If you'll notice, the molecules extending from certain other cells or certain substances will match these receptors.
When those receptors are stimulated, when they're filled, when enough of them become filled, then the membrane pinches in around that substance and brings it into the cell. This is really good for bringing in things that are sparse within the cell. So if there isn't a very high concentration of whatever this is, penocytosis would be pretty ineffective, right?
Like, for instance, let's say that I have a cell right here, and let's say I have all of these different substances around it. I'll use different colors. Let's say that the cell wants to take in whatever the green dots represent. If we were utilizing pinocytosis and just kind of taking in whatever is dissolved in the water around it, it's going to take a while for us to accumulate the green dots because they're pretty spread out. They're not highly concentrated in the liquid around this.
So we'll have to take in a whole bunch of whatever represents the red and the yellow in order to get at the green dots. The receptor-mediated endocytosis allows for us to take in more specifically just whatever it is that we're trying to get, because the cell won't pinch in until enough receptors in a certain area have been matched with the extending molecules from whatever it is that they're trying to take in. So it's kind of a way to be a bit more specific about what's taken into the cell, especially good for things that...
Yes. aren't super plentiful around and allowing them to be specific about what is taken into the cell.