Endocytosis and Exocytosis: Videos & Practice Problems

Endocytosis and exocytosis are crucial processes for transporting large biomolecules across cell membranes. Endocytosis involves the engulfment of macromolecules via lipid vesicles, with three main types: phagocytosis (cell eating), pinocytosis (cell drinking), and receptor-mediated endocytosis, which uses specific receptor proteins. In contrast, exocytosis allows molecules like hormones and neurotransmitters to exit the cell by vesicle fusion with the plasma membrane, releasing contents into the extracellular space. Understanding these mechanisms is essential for grasping cellular transport dynamics.

Endocytosis and Exocytosis

In this video, we're going to begin our lesson on endocytosis and exocytosis. Up until this point in our course, we've been talking about the membrane transport of very small molecules. But what about large molecules? How do they get across a cell's plasma membrane? Well, large biomolecules, for example, large proteins, large carbohydrates, or nucleic acids like DNA, are simply too large to diffuse through membranes or even through protein channels. And so they're not able to diffuse through the methods that we've talked about so far. Instead, large macromolecules will be transported across cell membranes via either the process of endocytosis or the process of exocytosis. We'll get to talk a lot more about endocytosis and exocytosis as we move forward in our course.

Below here, we're showing you a map, a snippet of our lesson on membrane transport, focusing on bulk transport or the transport of very large molecules. These large molecules can either be transported via endocytosis to enter the cell or exocytosis to exit the cell. We're going to start off our lesson by focusing on endocytosis, entering the cell. There are three types of endocytosis that we're briefly going to touch on, which are phagocytosis (cell eating), pinocytosis (cell drinking), and receptor-mediated endocytosis, which is a form of pinocytosis.

Later, after we talk about endocytosis, we'll discuss exocytosis, exiting the cell. But for now, this concludes our introduction to endocytosis and exocytosis. We'll get to learn more about these processes as we move forward in our course. So, I'll see you all in our next video.

Endocytosis Allows Entry to the Cell

In this video, we're going to focus on endocytosis and how endocytosis allows entry into the cell. The "en" in endocytosis is going to be very helpful to remind you about what it does, and that's because endocytosis is defined as macromolecule engulfment by the cell membrane allowing for entry into the cell via a lipid vesicle. You can see the "en" in endocytosis is for the "en" in engulfment and the "en" in entry. The big idea here is that molecules are going to be brought into the cell with endocytosis. Now there are 3 main types of endocytosis that you all should know.

The first type is phagocytosis. Phagocytosis is when a large solid material is being taken into the cell via endocytosis. Because it is a solid material, it's commonly known as cell eating. The second type of endocytosis that you all should know is pinocytosis. Pinocytosis is defined as when a small liquid material is being taken into the cell by endocytosis. Because the materials are liquid, it's commonly known as cell drinking.

The third and final type of endocytosis that you all should know is receptor-mediated endocytosis. Receptor-mediated endocytosis is just a special form of pinocytosis, and that's why we have this little indentation here to show that receptor-mediated endocytosis is just a special type of pinocytosis. It's a specific form of pinocytosis that uses receptor proteins. So let's take a look at our image down below to get a better understanding of these ideas.

On the far left, notice that we're showing you the first type of endocytosis, phagocytosis. Notice that the outside of the cell is over here and the inside of the cell is over here, and notice that a bacterium is being brought into the cell here, and when it's brought into the cell, it ends up inside of a lipid vesicle. Recall, vesicles are just these little membrane bubbles, and it will bring in these large solid materials such as the bacteria here.

Moving on, pinocytosis is over here in the middle and notice that the outside is over here and the inside of the cell is over here, and so these small liquid materials are being brought into the cell via a lipid vesicle. This is pinocytosis or cellular drinking.

The final type of endocytosis that you all should know is receptor-mediated endocytosis, which is really just a form of pinocytosis, so cellular drinking. You can see the same blue molecules embedded in the membrane at these positions here. The receptors are proteins that have a specific binding attraction to these blue liquid molecules. Because it's a form of endocytosis, it's going to be brought into the cell in a lipid vesicle. The big idea here is that receptor-mediated endocytosis uses these receptors, these orange receptors.

This here concludes our introduction to endocytosis and how endocytosis, the "en," is going to remind you that it allows for entry into the cell. We'll be able to talk about exocytosis in our next video. So I'll see you all there.

Endocytosis and Exocytosis Example 1

Alright. So here we have an example problem that says white blood cells of our immune systems engulf bacteria using which type of endocytosis? And we've got these four potential answer options down below. Now, after reading through these options, we know from our last lesson video that phagocytosis and pinocytosis are definitely types of endocytosis that we talked about in our last lesson video. But osmosis is definitely not one of the types of endocytosis. And, also, when we look at option d, it says receptor-mediated exocytosis. But receptor-mediated exocytosis is not a type of endocytosis, and so receptor-mediated endocytosis would be a type of endocytosis, but receptor-mediated exocytosis is not. And so, we did not talk about that in our last lesson video. So we can eliminate option c and option d. So now we're between either option a or option b. And so, notice it's saying white blood cells of our immune systems engulf bacteria. And bacteria is going to be a solid. And because bacteria is a solid, it's going to be involved with cellular eating. And if it were a liquid, then it would be involved with cellular drinking. And recall from our last lesson video that it's phagocytosis that is involved with cellular eating, eating essentially solid materials like bacteria. And so pinocytosis is involved with cellular drinking, but again, bacteria are not liquids, they are solids. And so option b is not going to be correct, and option a is the correct answer to this example problem. So that concludes this example, and I'll see you all in our next video.

The difference between pinocytosis and receptor-mediated endocytosis is that ________.

• A. Pinocytosis brings only water molecules into the cell, receptor-mediated endocytosis brings in other molecules also
• B. Pinocytosis increases the surface area of the plasma membrane, receptor-mediated endocytosis decreases it
• C. Pinocytosis is nonselective, receptor-mediated endocytosis offers more selectivity
• D. Pinocytosis can concentrate substances from the extracellular fluid, receptor-mediated endocytosis cannot

Answer: (C)

Exocytosis Allows Exiting from the Cell

So in our last lesson video, we talked about endocytosis and how it allows for entry into the cell. And so in this video, we're going to talk about the complete opposite of endocytosis, which is exocytosis, and how exocytosis allows exiting from the cell. And so the EX in exocytosis is going to be very helpful for you all to remember exactly what exocytosis is about. And so exocytosis can be defined as vesicle fusion with the cell membrane, allowing the contents of the vesicle to exit the cell into the extracellular space or the outside space outside of a cell. And so you can think the ex in exocytosis is for the ex in exit and the ex in extracellular space. Now, there are many different types of molecules that can undergo exocytosis, but a few examples include hormones, neurotransmitters, and digestive enzymes. And this is just a small subset of all the examples of molecules that can undergo exocytosis, allowing exiting from the cell. So let's take a look at our image down below to get a better understanding of this. And notice the inside of the cell is over here on the right-hand side, and the outside of the cell is over here on the left-hand side. And so notice that, originally, these red molecules that you see here that are representing hormones are inside of a vesicle, inside of a membrane bubble here on the inside of a cell. And this membrane bubble, this vesicle, can start to fuse with the cell's plasma membrane here. And when that vesicle fully fuses, it can release the contents that used to be on the inside of the vesicle. And so ultimately, what we get is the vesicle contents are being released into the outside of the cell. And so these contents are exiting the cell, via exocytosis. And so this here concludes our introduction to exocytosis and how it allows exiting from the cell, and we'll be able to get some practice applying these concepts as we move forward in our course. So I'll see you all in our next video.

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 removing large waste particles that cannot be recycled by the cell.

Answer: (A)

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 processes used by cells to transport large molecules across the cell membrane. Endocytosis involves the engulfment of macromolecules into the cell via lipid vesicles. There are three main types: phagocytosis (cell eating), pinocytosis (cell drinking), and receptor-mediated endocytosis, which uses specific receptor proteins. In contrast, exocytosis is the process by which molecules such as hormones, neurotransmitters, and digestive enzymes exit the cell. This occurs through the fusion of vesicles with the plasma membrane, releasing their contents into the extracellular space. Both processes are essential for maintaining cellular function and homeostasis.

What are the three types of endocytosis?

The three types of endocytosis are phagocytosis, pinocytosis, and receptor-mediated endocytosis. Phagocytosis, also known as cell eating, involves the engulfment of large solid materials, such as bacteria, into the cell. Pinocytosis, or cell drinking, involves the uptake of small liquid materials into the cell. Receptor-mediated endocytosis is a specific form of pinocytosis that uses receptor proteins to selectively bring in specific molecules. These receptors bind to their target molecules and facilitate their entry into the cell via lipid vesicles.

How does receptor-mediated endocytosis differ from pinocytosis?

Receptor-mediated endocytosis is a specialized form of pinocytosis. While pinocytosis involves the non-specific uptake of extracellular fluid and dissolved solutes, receptor-mediated endocytosis is highly specific. It uses receptor proteins on the cell membrane to bind and internalize specific molecules. These receptors recognize and bind to particular ligands, such as hormones or nutrients, and facilitate their entry into the cell via vesicles. This specificity allows the cell to efficiently uptake necessary molecules while avoiding unnecessary or harmful substances.

What role does exocytosis play in cellular function?

Exocytosis plays a crucial role in cellular function by allowing the cell to expel molecules into the extracellular space. This process is essential for the secretion of hormones, neurotransmitters, and digestive enzymes. During exocytosis, vesicles containing these molecules fuse with the plasma membrane, releasing their contents outside the cell. This mechanism is vital for communication between cells, the regulation of metabolic processes, and the maintenance of cellular homeostasis. Without exocytosis, cells would be unable to effectively interact with their environment or perform many of their essential functions.

What types of molecules are typically transported via exocytosis?

Exocytosis typically transports a variety of molecules, including hormones, neurotransmitters, and digestive enzymes. Hormones are chemical messengers that regulate physiological processes, while neurotransmitters are involved in transmitting signals between nerve cells. Digestive enzymes are crucial for breaking down food substances in the digestive tract. These molecules are packaged into vesicles within the cell, which then fuse with the plasma membrane to release their contents into the extracellular space. This process is essential for intercellular communication, digestion, and the regulation of various bodily functions.