Endomembrane System: Protein Secretion - Online Tutor, Practice Problems & Exam Prep

The endomembrane system in eukaryotic cells comprises interconnected organelles, including the nucleus, endoplasmic reticulum (ER), and Golgi apparatus, facilitating protein secretion and cellular digestion. Protein synthesis begins in the nucleus, where DNA is transcribed into RNA, which is then translated into proteins by ribosomes on the rough ER. The Golgi apparatus processes and packages these proteins for secretion. The smooth ER synthesizes lipids and detoxifies substances, highlighting the system's diverse functions in maintaining cellular homeostasis.

Endomembrane System: Protein Secretion

In this video, we're going to begin our introduction to the endomembrane system and to one of its major functions, which is protein secretion. The endomembrane system can be defined as a group of membrane-bound organelles inside a eukaryotic cell. The root "endo" in endomembrane system means "inside," indicating that we're discussing membranes found within the cell. The system is composed of a group of organelles, all of which are membrane-bound, meaning their boundaries consist of membranes. The endomembrane system includes many different organelles that we will mention later in this table.

However, all these organelles that are part of the endomembrane system are interconnected by structures called vesicles. Vesicles are essentially tiny membrane bubbles that can transport materials internally. As we progress in our course, we will see examples of vesicles. The endomembrane system performs multiple functions, but we will primarily focus on two: protein secretion and cellular digestion.

In the example below, we have a table showing the organelles of the endomembrane system, with a green color coding linking those involved in protein secretion and a yellow color for those involved in cellular digestion. The cell membrane, not the focus of this part of the lesson, is shown with a white background. We'll discuss cell membranes more in a later lesson. The numbers displayed represent organelles of the endomembrane system, grouped based on their functions. Going forward, we will mainly focus on organelles involved in protein secretion, including the nuclear envelope or nucleus of the eukaryotic cell, the endoplasmic reticulum, the Golgi apparatus, and transport vesicles. Later, we will discuss organelles involved in cellular digestion, such as lysosomes, peroxisomes, and vacuoles, including central vacuoles.

This outline categorizes the organelles of the endomembrane system by their functions, and we will delve deeper into each as we progress in our course. That being said, I'll see you all in our next video.

Protein Secretion

So from our last lesson video, we know that one of the major functions of the endomembrane system is protein secretion. But what exactly does protein secretion even mean? Well, the word secretion is just a fancy name for a process by which a substance is released into the environment or released into the surroundings of the cell. And so, protein secretion is a process by which a protein is being released into the environment or released into the surroundings of the cell. And so, protein secretion involves several different organelles that interact with each other in a very specific order.

And so moving forward in our course, we're going to talk about each of the organelles that are involved with protein secretion. And so here, we're showing you an image of these organelles, but moving forward in our course, we're going to talk about each of the organelles in their own separate videos. And so, this includes the nucleus of the cell, as well as the endoplasmic reticulum and the Golgi apparatus that we see here. And so once again, we'll get to talk about each of these organelles moving forward in our course in their own separate video. So I'll see you all in our next video.

Nucleus

In this video, we're going to talk about the first organelle of the endomembrane system, which is the nucleus. Protein secretion starts in the nucleus because the nucleus of a eukaryotic cell stores the DNA of the cell. Recall that the DNA of the cell carries the recipe or the code for making the protein. That's exactly why protein secretion starts in the nucleus, because you can't secrete the protein until after you've made the protein. The nucleus can be defined as a rounded structure that contains and protects most of a eukaryotic cell's DNA.

Now there are three structures in the nucleus that you all should be aware of. Remember that the nucleus is part of the endomembrane system, which means that its border is a membrane itself. The border of the nucleus has a specific name that we refer to as the nuclear envelope. The nuclear envelope is really just defining the double membrane barrier that surrounds the nucleus, and acts as the barrier of the nucleus, basically separating the inside of the nucleus from the outside. Nuclear pores refer to tiny little pores or holes found in the nuclear envelope, which allow the entry and exit into and out of the nucleus.

They basically can control and monitor what leaves the nucleus and also what enters. The nucleolus is a small dense structure on the inside of the nucleus, where ribosomes are assembled. Recall from our previous lesson videos that ribosomes are important for making proteins, which is another reason why the nucleus is the first step of protein secretion because it is involved with making ribosomes. Let's take a look at our image down below to get a better understanding of the eukaryotic nucleus. On the left hand side, we show our representation of the eukaryotic cell, displaying the nucleus of the cell.

If we zoom in to the nucleus of the cell, you'll notice it is made up of a double membrane, which acts as the barrier of the nucleus. The gray structures here refer to the nuclear envelope. Notice that in the nuclear envelope, there are holes, or nuclear pores, throughout.

This hole seen here in the nuclear envelope is a nuclear pore. Notice that inside the nucleus, we have DNA, and most of the eukaryotic cell's DNA is found inside the nucleus. Within the nucleus, at the core, there's another small dense structure, where ribosomes are going to be assembled. This is the nucleolus, a structure within the inside of the nucleus itself that is involved with assembling ribosomes. The reason, once again, that protein secretion process starts in the nucleus is because the nucleus contains the DNA of the cell. The DNA in the nucleus is transcribed into RNA, and then the RNA is translated into a protein. The protein, recall, is built by the ribosome.

There's gonna be a lot more details on this process that we're mentioning right here, later in our course. We'll talk more about the idea of transcription and translation, later in our course. For now, what you should note is that protein secretion starts in the nucleus because, in order for a protein to be made, the DNA in the nucleus needs to be transcribed into RNA, and then the RNA needs to be translated into the protein. Once the protein is made, it can then be secreted. This concludes our introduction to the nucleus, and we'll be able to talk more about the other organelles of the endomembrane system moving forward in our course.

So I'll see you guys in our next video.

Which part of the nucleus is responsible for assembling ribosomes and ribosomal RNA?

• A. The mitochondria.
• B. Nuclear Envelope.
• C. Nuclear pores.
• D. Nucleolus.

Endoplasmic Reticulum

In this video, we're going to talk about the second organelle of the endomembrane system, which is the endoplasmic reticulum. Now, the endoplasmic reticulum is commonly abbreviated as just ER for short. And so the endoplasmic reticulum or the ER, is a part of the endomembrane system, which means that its boundary is going to be a membrane. And so it's no surprise that the endoplasmic reticulum is going to be a membranous structure. Now the endoplasmic reticulum or ER, it is a membranous structure, but it's actually continuous with the nuclear envelope, which is the membrane of the nucleus.

And, the endoplasmic reticulum is actually going to have multiple functions, and we'll be able to talk about some of those functions down below here. Now because the endoplasmic reticulum is a membranous structure, the membranous structure is going to act as a barrier to the endoplasmic reticulum, separating the outside of the endoplasmic reticulum from the inside of the endoplasmic reticulum. And the internal space or the internal compartment of the endoplasmic reticulum has a specific name that we call the ER lumen. And so the ER lumen is again the internal space or compartment on the inside of the endoplasmic reticulum. Now really there are 2 main types of endoplasmic reticulum that you all should know, and we have them numbered down below, number 1 and number 2.

And so the first type of endoplasmic reticulum that you should know is the rough endoplasmic reticulum or the rough ER for short. And the rough ER is sometimes abbreviated as just the RER for the rough endoplasmic reticulum. Now, the rough endoplasmic reticulum or the rough ER or RER is going to be a lot closer to the nucleus, so it directly extends off of the nuclear envelope. And as its name implies here with the rough, the rough endoplasmic reticulum has a rough surface that is, coated with ribosomes. And so the reason the rough ER is called the rough ER is because its surface is rough or at least it appears to be rough because it has ribosomes coating the surface and attached to the surface of the ER, the rough ER itself.

Now recall that ribosomes are structures that build proteins, And so these ribosomes that are coded to the surface or attached to the surface of the rough ER are going to build, newly built, proteins, build newly built proteins. And the newly built proteins that are built by these ribosomes coated on the surface of the rough ER are going to fold and get modified on the rough ER lumen or the internal space or compartment of the rough endoplasmic reticulum. Now the second type of endoplasmic reticulum that you all should know is the smooth ER or for short, the SER for smooth endoplasmic reticulum. And the smooth endoplasmic reticulum or SER is farther away from the nucleus, so it's extending off of the membrane of the rough ER itself. And as its name implies with the smooth, the smooth ER has a smooth surface, and that is because it has a ribosome free surface, meaning that there are no ribosomes on the surface of the smooth ER.

Now because the smooth ER does not have any ribosomes, it's ribosome free, then it's not gonna be associated with building proteins. Instead, the smooth ER is going to be, building or synthesizing lipids, and it's also important for detoxifying drugs and poisons. So let's take a look at our example image down below to further distinguish between the rough ER and the smooth ER. So once again, on the left hand side over here, we're showing you our eukaryotic cell representation, and we're zooming in specifically into this region right here, in, this box. And so what you'll notice is the nucleus, which we already covered in our last lesson video, is right here in this image.

This is the nucleus, and the nucleus we know is part of the endomembrane system because it has a membrane, surrounding it called the nuclear envelope. And the endoplasmic reticulum, which is shown here in blue and in orange, is really just a continuous extension from the nuclear envelope as we mentioned up above. It's continuous with the nuclear envelope. Now there's 2 types of endoplasmic reticulum. We know that there's the rough endoplasmic reticulum and the smooth endoplasmic reticulum.

The rough endoplasmic reticulum is closer to the nucleus, and so the rough endoplasmic reticulum here is in bluish color here. It's in the bluish color surrounding the nucleus. And so we can go ahead and label this as the rough endoplasmic reticulum or the rough ER. Now recall that the rough ER is rough because it has a rough surface that is coated with ribosomes. So it has a rough ribosome-coated surface.

So when we take a look down below, notice that all of these little blue dots that we see here on the rough ER represent ribosomes. Okay. Including this little dot right here. Those are ribosomes. And remember, the ribosomes are important for building proteins.

And so, the newly built proteins that are made, they're actually going to fold and get modified on the inside of the rough endoplasmic reticulum, inside of the lumen of the endoplasmic reticulum. And so, if these ribosomes here, these little blue dots, if they're making proteins, those proteins will end up on the inside of the endoplasmic reticulum or the lumen. Now extending off of the membrane of the rough ER, we have this reddish structure that we're showing you here, which is actually the smooth ER or the SER. And so we can go ahead and label this as the smooth ER, the smooth endoplasmic reticulum. And notice that the surface of the smooth endoplasmic reticulum looks pretty smooth because it does not have any ribosomes.

It's free. It's ribosome free. And so instead of making proteins, it's gonna be important for making lipids and detoxifying drugs and poisons. And so, extending off of these endoplasmic reticulum, they because they're made up of membranes, these little membrane bubbles are capable of forming from both the smooth ER, as well as from the rough ER too, even though we're not showing you any membrane bubbles budding from the rough ER. They can come from the rough ER too.

But these little bubbles that you see here, these little vesicles, these little membrane bubbles, they're important for carrying materials throughout and interconnecting all of the organelles of the endomembrane system. Now, this here concludes our introduction to the endoplasmic reticulum and the two main types, the rough ER and the smooth ER. And we'll be able to get some practice applying these concepts as we move forward in our course and continue to talk more about organelles of the endomembrane system as well. So I'll see you all in our next video.

Which of the following is most likely to have the greatest concentration of smooth endoplasmic reticulum?

• A. A cell that secretes enzymes.
• B. A cell that destroys pathogens.
• C. A cell that makes steroid hormones.
• D. A cell that engages in photosynthesis.
• E. A cell that manufactures proteins.

Golgi Apparatus

In this video, we're going to talk about the 3rd organelle of the endomembrane system, which is the Golgi apparatus. Now molecules that are synthesized or built in the endoplasmic reticulum such as proteins in the rough ER and lipids in the smooth ER, these molecules are transported by vesicles or tiny little membrane bubbles. And these molecules that are transported by vesicles, they can be transported to the Golgi apparatus. And so the Golgi apparatus is really just a stack of flat membranous sacs, and because it is membranous, it's no surprise that it's part of the endomembrane system. Now the stacks of flat membrane sacs have a scientific name that is cisternae.

And so cisternae is just the scientific name that's referring to those flat membranous sacs. Now the Golgi apparatus, its function is to act as a processing center, because it's able to receive packages, modify the contents of the packages, and then deliver the packages. And so, it's acting like a processing center in that way. Once again, the Golgi apparatus is capable of receiving vesicles that transport molecules that are being synthesized by the ER, either proteins or lipids. So the Golgi apparatus receives those vesicles and then it modifies the vesicle contents, which would again either be proteins or lipids.

And then after it modifies the vesicle contents, it will actually repackage the modified contents back into vesicles for export from the Golgi apparatus. And so once again, this is why the Golgi apparatus functions as a processing center because it receives vesicles, it modifies the vesicle contents, and then it repackages the contents for export either out of the cell or to a different location within the cell. And so this means that the Golgi apparatus is going to have a receiving end where it receives vesicles and that's going to be the receiving end of the Golgi apparatus is referred to as the cis end of the Golgi apparatus. And then the shipping end, which is basically the end that repackages and exports, the shipping end of the Golgi apparatus is going to be the trans end of the Golgi apparatus. Now some of the vesicles that are being shipped from the Golgi apparatus can actually fuse with the cell membrane for secretion, And so this is why the Golgi apparatus here is gonna be one of the last organelles that are involved with protein secretion.

And so let's take a look here at our image down below at the Golgi apparatus to get a better understanding here. And so the Golgi apparatus in this image is this pink structure that you see right here. This is the Golgi apparatus. And so you can see its position, relative to other organelles that are around that we already talked about in our previous lesson videos, such as the nucleus, the rough endoplasmic reticulum, and the smooth endoplasmic reticulum. So here we're talking about the Golgi apparatus.

So here's a different view here of the Golgi apparatus and some of these other organelles. Notice it's a bird's eye view where here we have the nucleus. Here we're showing you only the smooth ER for some simplicity's sake. I'm sorry, the rough ER just for simplicity's sake. We're only showing you the rough ER, the one that's here in blue.

And, the rough ER recall is studded with ribosomes. And, those ribosomes make proteins, and those proteins can end up on the inside of vesicles, these little tiny membrane bubbles. And so this, here is representing a vesicle that's carrying a protein because it's coming from the rough ER. And so these vesicles, they're capable of fusing with the Golgi apparatus, which we're showing you right here. So the vesicles, the receiving end of the Golgi apparatus, recall the receiving end of the Golgi apparatus is going to be the cis end of the Golgi apparatus.

So here the receiving end is going to be the cis end or the cis face of the Golgi apparatus. And then once those contents make it to the Golgi apparatus, they get modified and they make their way through the Golgi apparatus until they get to the other side of the Golgi apparatus, which is going to be the shipping end of the Golgi apparatus or the trans end of the Golgi apparatus. And so down below right here, this end over here is going to be the trans end of the Golgi apparatus or the trans face of the Golgi apparatus. And so, this means that vesicles are going to be created at this end and these vesicles are carrying the modified contents. And these vesicles, they can travel to the cell membrane as we indicated up above.

They can travel and fuse with the cell membrane for secretion. And so at this point traveling to the cell membrane, the contents can be exported and released to the outside of the cell. And so once again, each of these flat sacs that you see here, these flat membranous sacs, they're referred to as cisternae, like this. So the cisternae, once again, are the flat membranous sacs of the Golgi apparatus. And so this here concludes our introduction to the Golgi apparatus and the organelles that are involved with protein secretion.

And so we'll be able to get some practice applying the concepts that we've learned in our next video here. So I'll see you all there.

Which of the following is the most common pathway taken for a newly synthesized protein to be secreted?

• A. Rough ER → Golgi→ Transport vesicle → Nucleus.
• B. Golgi → Rough ER → Lysosome → Transport vesicle → Cell membrane.
• C. Rough ER → Golgi → Transport vesicle → Cell membrane.
• D. Rough ER → Lysosome → Transport vesicle → Cell membrane.