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

Endomembrane System: Protein Secretion

In this video, we're going to begin our introduction to the endomembrane system and to one of the major functions of the endomembrane system, which is protein secretion. The endomembrane system can be defined as a group of membrane-bound organelles that are inside of a eukaryotic cell. The root "endo" in endomembrane system means inside. You can see that we're talking about membranes that are found on the inside of the cell. The system here is really just referring to the fact that it's made up of a group of organelles, and all of these organelles are membrane-bound, meaning that their boundaries are made up of membranes.

The endomembrane system includes many different organelles that we're going to mention down below in this table. However, all of these organelles that are part of the endomembrane system are interconnected to each other by structures called vesicles. Vesicles is just a fancy name for these tiny little membrane bubbles that can carry materials on the inside. All of these organelles that are part of the endomembrane system are interconnected by these vesicles, and we'll get to see examples of vesicles as we move forward in our course.

The endomembrane system has multiple functions, and we're not going to talk about all of the different functions. We're mainly going to focus on 2 of its functions. Those include protein secretion, as well as cellular digestion. Notice that down below in our example here, we have a table that shows the organelles of the endomembrane system. Notice that behind them, we have this green color coding that links the organelles that we're going to talk about in our course moving forward that are involved with protein secretion. In yellow, we have the organelles that are involved with cellular digestion that we're going to talk about moving forward in our course. Over here with this white background, we have the cell membrane, which we're not going to focus on in this particular portion of our lesson. We'll talk more about cell membranes later in a different lesson.

What you'll see is that all of these numbers that you see here are showing you organelles that are part of the endomembrane system, and we have them grouped based on their functions, whether they're involved with protein secretion or cellular digestion. Moving forward in our course, we're going to focus mainly on the organelles that are involved with protein secretion, which include the nuclear envelope or the nucleus of the eukaryotic cell, the endoplasmic reticulum of the eukaryotic cell, the Golgi apparatus, as well as transport vesicles, which we mentioned again are these little membrane bubbles. Later in our course, we'll get to talk more about the organelles of the endomembrane system that are involved in cellular digestion, which include lysosomes and peroxisomes, as well as vacuoles, such as central vacuoles and things like that.

This here really is outlining the organelles of the endomembrane system and grouping them by their functions, and we'll get to talk more about each of these organelles that are on here as we move forward in our course. So 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 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.

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, referred to as the nuclear envelope. The nuclear envelope is defined as the double membrane barrier that surrounds the nucleus, acting as the barrier of the nucleus, basically separating the inside of the nucleus from the outside.

Nuclear pores refer to tiny pores or holes found in the nuclear envelope. These tiny nuclear pores allow the entry and exit into and out of the nucleus. They control and monitor what leaves the nucleus and also what enters. The nucleolus is a small dense structure inside the nucleus, specifically 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 below to get a better understanding of the eukaryotic nucleus. On the left-hand side, we are showing our representation of the eukaryotic cell, and notice the nucleus of the cell. If we zoom in to the nucleus, what you'll notice is that it is made up of a membrane, a double membrane. This double membrane acts as the barrier of the nucleus. The gray structures you see are the nuclear envelope, the double membrane that surrounds the nucleus and acts as its barrier. Notice that in the nuclear envelope, there are holes, the nuclear pores. This hole in the nuclear envelope is a nuclear pore.

Inside the nucleus, we have DNA. These structures represent DNA, and most of the eukaryotic cell's DNA is found inside the nucleus. Also, within the nucleus at the core, there's another small dense structure, the nucleolus, where ribosomes are going to be assembled.

The reason protein secretion process starts in the nucleus is because the nucleus contains the DNA of the cell. The DNA needs to be transcribed into RNA, and then the RNA needs to be translated into a protein by the ribosome. There will be a lot more details on this process, including transcription and translation, later in our course. For now, note that protein secretion starts in the nucleus because, in order for a protein to be made, the DNA 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. 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. The endoplasmic reticulum, or the ER, is a part of the endomembrane system, which means that its boundary is going to be a membrane. So it's no surprise that the endoplasmic reticulum is going to be a membranous structure. The endoplasmic reticulum, or ER, is a membranous structure but is actually continuous with the nuclear envelope, which is the membrane of the nucleus. The endoplasmic reticulum is going to have multiple functions, and we'll be able to talk about some of those functions down below here. Because the endoplasmic reticulum is a membranous structure, the membranous structure acts as a barrier, separating the outside of the endoplasmic reticulum from its inside. The internal space or the compartment of the endoplasmic reticulum has a specific name, which we call the ER lumen. The ER lumen is again the internal space or compartment inside the endoplasmic reticulum. There are 2 main types of the endoplasmic reticulum that you all should know, and we have them numbered down below, number 1 and number 2.

The first type of endoplasmic reticulum that you should know is the rough endoplasmic reticulum, or the rough ER for short, sometimes abbreviated as RER. The rough endoplasmic reticulum, or rough ER or RER, is going to be a lot closer to the nucleus, directly extending off of the nuclear envelope. As its name implies, with the rough, the rough endoplasmic reticulum has a rough surface that is coated with ribosomes. The rough ER is called the rough ER 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. Recall that ribosomes are structures that build proteins, and these ribosomes that are coated on the surface or attached to the surface of the rough ER are going to build newly built proteins. The newly built proteins built by these ribosomes are going to fold and get modified in the rough ER lumen or the internal space or compartment of the rough endoplasmic reticulum.

The second type of endoplasmic reticulum that you all should know is the smooth ER, or SER for smooth endoplasmic reticulum. The smooth endoplasmic reticulum or SER is farther from the nucleus, extending off of the membrane of the rough ER itself. 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. Because the smooth ER does not have any ribosomes, it's ribosome-free, then it's not going to 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. 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.

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 is part of the endomembrane system because it has a membrane, called the nuclear envelope, surrounding it. The endoplasmic reticulum, which is shown here in blue and in orange, is just a continuous extension from the nuclear envelope, as we mentioned above. There are 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 the rough endoplasmic reticulum here is in the bluish color surrounding the nucleus. We can go ahead and label this as the rough endoplasmic reticulum or the rough ER. Recall that the rough ER is rough because it has a rough surface that is coated with ribosomes. 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, including this little dot right here. Those are ribosomes. And remember, the ribosomes are important for building proteins. 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.

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. We can go ahead and label this as the smooth ER, the smooth endoplasmic reticulum. Notice that the surface of the smooth endoplasmic reticulum looks pretty smooth because it does not have any ribosomes. It's ribosome-free. Instead of making proteins, it's going to be important for making lipids and detoxifying drugs and poisons. Extending off of these, the endoplasmic reticulum, made up of membranes, these little membrane bubbles can form 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. 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.

This concludes our introduction to the endoplasmic reticulum and the two main types, the rough ER and the smooth ER. 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. These molecules that are transported by vesicles can be transported to the Golgi apparatus. 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. The stacks of flat membrane sacs have a scientific name that is cisternae. Cisternae is just the scientific name referring to those flat membranous sacs. The Golgi apparatus's 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. Once again, the Golgi apparatus is capable of receiving vesicles that are transported. Remember, the vesicles are transporting molecules that are being synthesized by the ER, either proteins or lipids. The Golgi apparatus receives those vesicles and then it modifies the vesicle contents, which would again either be proteins or lipids. After it modifies the vesicle contents, it will actually repackage the modified contents back into vesicles for export from the Golgi apparatus. Once again, this is why the Golgi apparatus functions as a processing center because it receives vesicles, modifies the vesicle contents, and then repackages the contents for export either out of the cell or to a different location within the cell. 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. Some of the vesicles that are being shipped from the Golgi apparatus can actually fuse with the cell membrane for secretion. The Golgi apparatus here is going to be one of the last organelles involved with getting a better understanding here. This pink structure that you see right here is the Golgi apparatus. You can see its position relative to other organelles that we already talked about in our previous lesson videos such as the nucleus, the rough endoplasmic reticulum, and the smooth endoplasmic reticulum. Here we're talking about the Golgi apparatus. Here's a different view of the Golgi apparatus and some of these other organelles. Notice it's a bird's eye view where we have the nucleus. Here, we're showing you only the smooth ER for simplicity's sake. The 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. The rough ER recall is studded with ribosomes. Those ribosomes make proteins, and those proteins can end up on the inside of vesicles, these little tiny membrane bubbles. This here is representing a vesicle that's carrying a protein because it's coming from the rough ER. These 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. Here, the receiving end is going to be the cis end or the cis face of the Golgi apparatus. 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 shipping end of the Golgi apparatus or the trans end of the Golgi apparatus. 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. This means that vesicles are going to be created at this end and these vesicles are carrying the modified contents. These vesicles can travel to the cell membrane as we indicated up above. They can travel and fuse with the cell membrane for secretion. At this point traveling to the cell membrane, the contents can be exported and released to the outside of the cell. Once again, each of these flat sacs that you see here, these flat membranous sacs, they're referred to as cisternae. The cisternae, once again, are the flat membranous sacs of the Golgi apparatus. This concludes our introduction to the Golgi apparatus and the organelles that are involved with protein secretion. 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.