Alright. So now that we've covered molecular transport of really small molecules, including passive transport and active transport, we're finally going to move on to the last section of our map of the lesson on membrane transport. Now, the macro prefix and macromolecules really just mean large, and so macromolecules include really large proteins, polysaccharides, and DNA molecules as well. These macromolecules are simply just way too large to diffuse through membranes or channels, and so they will not be able to use the mechanisms of molecular transport that we talked about in our previous lesson videos, including the forms of passive transport or the forms of active transport that we discussed. Instead, these macromolecules must be transported across cell membranes using the process of either endocytosis or exocytosis, as you can see down below in our map. We'll be able to define endocytosis and exocytosis a little bit later in our course. But for now, I'd like to foreshadow for you guys that integral membrane proteins, very specific ones called fusion proteins, are really important to the process of endocytosis and exocytosis. These fusion proteins are integral membrane proteins that help fuse membranes, as their name implies, in both the process of endocytosis and exocytosis. Moving forward in our course, we're only going to talk about fusion proteins in the context of exocytosis, more specifically in the process of neurotransmitter release. However, it's important to keep in mind that fusion of membranes also takes place in endocytosis, and there are fusion proteins that we're not going to talk about that are involved in the process of endocytosis as well. Of course, because we're exploring our map using the leftmost branches first, in our next video we're going to explore endocytosis and the different types of endocytosis. So, I'll see you guys in that video.
Endocytosis & Exocytosis - Online Tutor, Practice Problems & Exam Prep
Endocytosis & Exocytosis
Video transcript
Endocytosis & Exocytosis
Video transcript
In this video, we're going to introduce endocytosis. And really the biggest takeaway from this video is that endocytosis allows entry into the cell. Endocytosis is defined as macromolecule engulfment by the cell membrane, allowing entry into the cell as a lipid vesicle. The "en" in endocytosis can remind you of the "en" in engulfment and the "en" in entry to the cell. There are three main types of endocytosis that you should be somewhat familiar with, and we will introduce those three below. The numbers that you see above correspond with the numbers in our image below.
The first type of endocytosis is phagocytosis. Phagocytosis occurs when large and solid materials are taken up by the process of endocytosis, also known as cellular eating. It is called cellular eating because the materials that are brought in are large and solid; it's almost like the cell is eating something. The second type of endocytosis is pinocytosis. Pinocytosis occurs when small and liquid materials are taken up by the process of endocytosis, also known as cellular drinking. It is called drinking because the materials brought into the cell are small and liquid.
The third and final type of endocytosis that you should know is a specific type of pinocytosis, which is why we have this indentation to show that it is really just a subtype of the previous type. This third type is called receptor-mediated endocytosis. It is a specific form of pinocytosis that uses receptor proteins. We'll be able to see examples of each of these in our image below.
On the far left, we have an example of phagocytosis. You can see the cell's plasma membrane; the blue background represents the outside of the cell, and the other background represents the inside of the cell. During the process of phagocytosis, large and solid materials, such as an entire bacterium and multiple bacteria, can be brought into the cell, as you can see by this arrow, in the form of a lipid vesicle. Phagocytosis is a process utilized by some of our white blood cells and can be involved in immunity, essentially protecting us from harmful bacteria. Phagocytosis could also be a general process used to bring nutrients into the cell, another reason why it's referred to as cellular eating.
In the middle, we have the process of pinocytosis, and the biggest difference is the types of molecules that are brought in. Notice here what we have are very small and liquid-type materials being brought into the cell, hence it's a type of cellular drinking. The third and final type of endocytosis is receptor-mediated endocytosis. Notice that it is really just a specific type of pinocytosis. You can see that the materials are very similar, but the biggest difference is that there are receptor proteins. These orange structures embedded in the membrane as receptors are involved in the endocytotic process that allows entry into the cell.
We'll be able to get a little bit of practice with these concepts in our next video, and then we'll talk about exocytosis. So, I'll see you there.
Which of the following is FALSE about endocytotic movements?
Endocytosis & Exocytosis
Video transcript
So now that we've covered endocytosis, in this video we're going to introduce exocytosis. And really, the biggest takeaway of this video is that exocytosis allows exiting from the cell. And so exocytosis is really defined as vesicle fusion with the cell membrane allowing the vesicle contents to exit the cell into the extracellular space. And so you can think that the EX in exocytosis is for the EX in exit and the EX of extracellular space. Now many biological processes rely on exocytosis, and so neurotransmitters and zymogens of digestive enzymes are just specific examples of substances that are released by exocytosis. And so if we take a look at our image down below, notice that this here represents a cell's plasma membrane. And so over here you can see the white background represents the outside of the cell or the extracellular space. And of course, this other background over here represents the inside of the cell. And so here what you can see is that there's a vesicle filled with some cellular contents, perhaps neurotransmitters or perhaps zymogens of digestive enzymes, and this vesicle filled with contents is making its way towards the cell's plasma membrane, and ultimately what it does is it begins to fuse with the cell's plasma membrane until it is fully fused with the cell's plasma membrane and the vesicle contents are released to the outside of the cell, to the extracellular space. And so you can see here that the vesicle contents here are exiting the cell, and again, that's the biggest takeaway of this video. And so this here concludes our introduction to exocytosis and we'll be able to get some practice applying these concepts in our next couple of videos. So I'll see you guys there.
Which of the following is NOT a true statement regarding exocytosis?
Label the following phrases based on if they apply to A) Only Endocytosis, B) Only Exocytosis, or C) Both:
a) Decreases the surface area of the plasma membrane: _________.
b) Increases the surface area of the plasma membrane: _________.
c) Secretes large molecules out of the cell: _________.
d) Brings molecules into the cell: _________.
e) Requires cellular energy: _________.
Problem Transcript
Which means of particle transport is shown in the figure below?