Endocytosis & Exocytosis: Videos & Practice Problems

Macromolecules, such as proteins and polysaccharides, cannot diffuse through cell membranes and require endocytosis and exocytosis for transport. Endocytosis involves engulfing materials, categorized into phagocytosis (cellular eating of solids), pinocytosis (cellular drinking of liquids), and receptor-mediated endocytosis (specific pinocytosis using receptors). Exocytosis allows vesicle contents, like neurotransmitters, to exit the cell by fusing with the plasma membrane. Understanding these processes is crucial for grasping cellular transport mechanisms and their roles in biological functions.

Endocytosis & Exocytosis

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

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?

• A. Phagocytosis could be used against invading microorganisms.
• B. Ingestion of plasma membrane along with extracellular fluid is a kind of pinocytosis.
• C. Phagocytosis is a means of feeding in some eukaryotes and is important in animal nutrition.
• D. The materials engulfed will be enclosed by a small portion of the ER membrane.

Answer: (D)

Endocytosis & Exocytosis

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?

• A. It forms intracellular vesicles from inward folding of the plasma membrane.
• B. It requires fusion of vesicles with the plasma membrane.
• C. It secretes large molecules out of the cell.
• D. It is responsible for neurotransmitter release of acetylcholine to stimulate muscle contractions.

Answer: (A)

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: _________.

Alright. So this practice problem wants us to label the following phrases that we see below based on if they apply to, a, only endocytosis, b, only exocytosis, or c, both endocytosis and exocytosis. And so looking at a here, it says decreases the surface area of the plasma membrane. And so recall from our previous lesson videos that in endocytosis, part of the plasma membrane actually pinches off from the plasma membrane and becomes a vesicle inside of the cell. And so that is certainly going to decrease the surface area of the plasma membrane. But in exocytosis, recall that an intracellular vesicle actually fuses with the plasma membrane and increases the surface area of the plasma membrane. And so what we're saying here is that decreasing the surface area of the plasma membrane only applies to a, only endocytosis, whereas increasing the surface area of the plasma membrane only applies to b, exocytosis. Now c says, secreting large molecules out of the cell. And, of course, if molecules are leaving the cell, then they are exiting the cell, and that means that they are leaving via exocytosis. And so this is going to be, of course, b. Bringing molecules into the cell means that they are entering the cell and that is going to apply to endocytosis which is only endocytosis, so a. And then e here says, require cellular energy, which is actually going to apply to both endocytosis and exocytosis, and so c here is actually the correct answer here. Now what we'll be able to see later when we talk about specific types of exocytosis such as the release of neurotransmitters, we'll be able to see that this is going to require cellular energy. But for now here this concludes our practice problem and I'll see you guys in our next video.

Which means of particle transport is shown in the figure below?

• A. Exocytosis.
• B. Endocytosis.
• C. Facilitated diffusion.
• D. Simple diffusion.

Answer: (B)

Here’s what students ask on this topic:

What is the difference between endocytosis and exocytosis?

Endocytosis and exocytosis are cellular processes for transporting macromolecules across the cell membrane. Endocytosis involves the engulfment of materials into the cell, forming a vesicle. It includes phagocytosis (cellular eating of solids), pinocytosis (cellular drinking of liquids), and receptor-mediated endocytosis (specific pinocytosis using receptors). Exocytosis, on the other hand, involves the fusion of vesicles with the cell membrane to release their contents outside the cell. This process is crucial for the secretion of substances like neurotransmitters and digestive enzymes. Both processes are essential for maintaining cellular functions and homeostasis.

What are the three types of endocytosis?

The three types of endocytosis are phagocytosis, pinocytosis, and receptor-mediated endocytosis. Phagocytosis, or cellular eating, involves the uptake of large, solid materials like bacteria. Pinocytosis, or cellular drinking, involves the uptake of small, liquid materials. Receptor-mediated endocytosis is a specific form of pinocytosis that uses receptor proteins to selectively bring in specific molecules. Each type of endocytosis plays a unique role in cellular function and nutrient uptake.

How does receptor-mediated endocytosis differ from pinocytosis?

Receptor-mediated endocytosis is a specific type of pinocytosis that involves receptor proteins on the cell membrane. These receptors bind to specific molecules, allowing the cell to selectively take in these molecules. In contrast, pinocytosis is a non-specific process where the cell engulfs extracellular fluid and its dissolved solutes. Receptor-mediated endocytosis ensures that only target molecules are internalized, making it a more selective and efficient process compared to the general uptake in pinocytosis.

What role do fusion proteins play in exocytosis?

Fusion proteins are integral membrane proteins that facilitate the fusion of vesicles with the cell membrane during exocytosis. These proteins help the vesicle membrane merge with the plasma membrane, allowing the vesicle contents to be released into the extracellular space. This process is crucial for the secretion of substances like neurotransmitters and digestive enzymes. Fusion proteins ensure that vesicle fusion is precise and efficient, playing a vital role in cellular communication and function.

Why is exocytosis important for neurotransmitter release?

Exocytosis is essential for neurotransmitter release because it allows the vesicles containing neurotransmitters to fuse with the presynaptic membrane and release their contents into the synaptic cleft. This release is crucial for transmitting signals between neurons. Without exocytosis, neurotransmitters would not be able to exit the neuron and communicate with adjacent neurons, impairing neural communication and overall brain function.