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 in the rough ER. These proteins are modified in the Golgi apparatus before being packaged into vesicles 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 the major functions of the endomembrane system, which is protein secretion. The endomembrane system can be defined as a group of membrane-bound organelles inside of a eukaryotic cell. The root endo- in endomembrane system means inside, and we're talking about membranes found inside the cell. The system here refers to the fact that it's made up of a group of organelles, all of which are membrane-bound, meaning their boundaries are made of membranes. The endomembrane system includes many different organelles, which we will mention later in this table. However, all of these organelles are interconnected to each other by structures called vesicles. Vesicles are tiny membrane bubbles that can carry materials inside. All of these organelles are interconnected by these vesicles, and we'll see examples of vesicles as we move forward in our course.

The endomembrane system has multiple functions, but we will mainly focus on two of its functions: protein secretion and cellular digestion. Notice in our example below, we have a table that shows the organelles of the endomembrane system. There is green color coding behind the organelles involved with protein secretion and yellow behind those involved with cellular digestion. With a white background, we have the cell membrane, which we will not focus on in this part of our lesson. We will discuss cell membranes later in a different lesson. However, all these numbers indicate organelles that are part of the endomembrane system, grouped based on their functions, whether they're involved with protein secretion or cellular digestion.

Moving forward in our course, we will focus mainly on the organelles involved with protein secretion, which include the nuclear envelope or the nucleus of the eukaryotic cell, the endoplasmic reticulum, the Golgi apparatus, and transport vesicles, which are the little membrane bubbles we mentioned. Later in our course, we will talk more about the organelles of the endomembrane system involved in cellular digestion, which include the lysosomes and peroxisomes, as well as vacuoles such as central vacuoles and others.

This content outlines the organelles of the endomembrane system and groups them by their functions, helping us understand 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 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.

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 going to be a membrane itself. The border of the nucleus has a specific name that we refer to as the nuclear envelope. The nuclear envelope defines the double membrane barrier that surrounds the nucleus, and again acts as the barrier of the nucleus, basically separating the inside of the nucleus from the outside. Nuclear pores refer to tiny pores or holes that are found in the nuclear envelope. These tiny nuclear pores in the nuclear envelope allow the entry and exit into and out of the nucleus. They can control and monitor what leaves the nucleus and also what enters the nucleus.

The nucleolus refers to a small dense structure inside the nucleus, and this is 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. Over here on the left-hand side, we are showing you our representation of the eukaryotic cell, and notice right here, we are showing you the nucleus of the cell.

If we zoom in here to the nucleus of the cell, you'll notice it is made up of a membrane, a double membrane. This double membrane, which you see here, acts as the barrier of the nucleus. The grey structures that you see here are referring to the nuclear envelope. We can label this as the nuclear envelope, the double membrane that surrounds the nucleus and acts as its barrier, basically what you see surrounding the nucleus here. Notice that in the nuclear envelope, there are these pores that you see throughout, these pores, if you will, the nuclear pores. This hole that you see right here in the nuclear envelope is referring to the nuclear pore.

Notice that inside the nucleus, we have DNA. These structures represent DNA, and most of the eukaryotic cell's DNA is found inside of the nucleus, which is important to keep in mind. Also, within the nucleus right here at the core, there's another small dense structure inside the nucleus, where ribosomes are being assembled. This is specifically the nucleolus. The nucleolus, a structure within the inside of the nucleus itself, is involved with building or assembling ribosomes.

The reason once again that the protein secretion process starts in the nucleus is that the nucleus contains the DNA of the cell. The DNA of a cell, found in the nucleus, is transcribed into RNA, and then the RNA is translated into a protein. The protein, recall, is being built by the ribosome. There's going to be a lot more details on this process that we're mentioning right here, later in our course. We'll talk more about this idea of transcription or being transcribed as well as 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. 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 2nd organelle of the endomembrane system, which is the endoplasmic reticulum. The endoplasmic reticulum is commonly abbreviated with the letters ER, and you'll see this abbreviation ER throughout our lesson moving forward a lot. The endoplasmic reticulum, or the ER, once again is a part of the endomembrane system which means that the endoplasmic reticulum is a membranous structure whose border is made of a membrane, and the endoplasmic reticulum, or the ER, is actually a membranous structure that is continuous with the nuclear envelope, which you might recall from some of our previous lesson videos, is really just another membrane structure that makes up the perimeter of the nucleus. The endoplasmic reticulum, or the ER, is an extension of the nucleus since it is continuous with the nuclear envelope.

Now, the endoplasmic reticulum, or the ER, actually has multiple functions, and we'll get to talk about some of those functions down below in our lesson. Again, the endoplasmic reticulum, or the ER, is a membranous structure whose border is made of a membrane, and that membrane serves as a barrier to separate the outside of the endoplasmic reticulum from the inside of the endoplasmic reticulum, and scientists refer to the inside of the endoplasmic reticulum as the ER lumen. The ER lumen can be defined as the internal space or compartment of the ER.

Now, there are actually 2 main types of endoplasmic reticulum that you should be familiar with, and we have those numbered down below: number 1 and number 2. The first type of endoplasmic reticulum that you should be aware of is the rough endoplasmic reticulum, or the rough ER, and the rough ER is commonly abbreviated as the RER for short. The rough endoplasmic reticulum is actually closer to the nucleus than the second type of endoplasmic reticulum, which you can see is the smooth ER. The rough ER, as its name implies, is going to have a rough ribosome-coated surface, which means that the rough ER is going to have a surface that is coated or studded with many ribosomes, making the surface of the rough ER appear to be rough. Recall from our previous lesson videos that ribosomes are non-membranous organelles that perform translation, the process of making proteins. Ribosomes make proteins, and because ribosomes are coating the surface of the rough ER, the rough ER will also be associated with these newly built proteins. Newly built proteins will fold and can be modified in the rough ER lumen or inside the rough ER.

The second main type of ER is going to be the smooth ER, commonly abbreviated as the SER. The smooth ER, or the SER, is going to be further from the nucleus compared to the rough ER, and, as its name implies, the smooth ER is going to have a smooth surface that is ribosome-free. The smooth ER does not have any ribosomes on its surface, and so its surface appears to be smooth in comparison to the rough ER, which has lots of ribosomes on its surface, making the surface rough. Because the smooth ER lacks ribosomes, it is ribosome-free and is not going to be associated with making proteins. Instead, the smooth ER is going to be associated with synthesizing lipids as well as detoxifying drugs and poisons.

If we take a look at our image down below, we can further distinguish between the rough ER and the smooth ER. Notice on the left-hand side over here we're showing our representation of a eukaryotic cell, and what you'll notice is that if we zoom into the cell here, that it is going to have, if we zoom into this region, that's what this image is showing you. You can see that we have the nucleus here in the center, and you'll notice that we have the rough ER here in a darker shade of red so we can go ahead and label it as the rough ER. Then in this lighter-colored structure over here, this is actually going to be the smooth ER. So we can label it as so. All of these little blue dots that you see throughout our image all over, all of these little blue dots, as you can see indicated by this blue arrow right here, these are going to be the ribosomes. Notice that the rough ER appears to have a rough surface because it is studded and coated with these ribosomes. Whereas the smooth ER, which is over here, notice that it is not coated with those ribosomes, and so its surface appears to be smooth.

This here concludes our brief introduction to the endoplasmic reticulum, the smooth ER, the rough ER, and the smooth ER, and we'll be able to get some practice applying these concepts and learn more as we move forward. So I'll see you all in our next video.

Golgi Apparatus

In this video, we're going to introduce the third organelle of the endomembrane system, which is the Golgi apparatus. It's important to note that molecules synthesized or built in the endoplasmic reticulum (ER), such as proteins built in the rough ER and lipids built in the smooth ER, are transported by vesicles (little membrane bubbles) to the Golgi apparatus. The Golgi apparatus is essentially a stack of flat membranous sacs, and being membranous, it is part of the endomembrane system. These flat membranous sacs are specifically referred to as cisternae. The term cisternae refers to the flat membranous sacs that make up the Golgi apparatus.

The Golgi apparatus functions as a processing center. It receives vesicles, modifies the contents of those vesicles, then repackages and distributes the modified contents to different parts of the cell or outside the cell. The Golgi apparatus has a receiving end, called the cis end, and a shipping end, called the trans end. Some vesicles are shipped from the Golgi apparatus and fuse with the cell membrane, allowing for the secretion of their contents. Secretion, in this context, is the release of substances into the environment of the cell. This explains why the Golgi apparatus is one of the final organelles involved in protein secretion.

If we take a look at our image below, we can get a better understanding of the Golgi apparatus. On the left-hand side, we are showing you an image from our previous lesson videos. You can see the nucleus of the cell here, surrounded by the endoplasmic reticulum, the rough ER and the smooth ER. In this image, we are introducing the Golgi apparatus. On the right-hand side, we are showing you a bird's-eye view, a different vantage point of the same organelles. You'll notice the nucleus of the cell and the rough ER, studded with ribosomes, which make proteins that end up in the lumen of the rough ER and then in vesicles that transport proteins to the Golgi apparatus.

The Golgi apparatus has a receiving end, where it receives packages called the cis end or the cis face of the Golgi, and these packages are modified throughout the Golgi apparatus until they reach the trans end or the trans face of the Golgi. This is the shipping end where the modified contents are repackaged into vesicles. These vesicles can either be distributed to different locations within the cell or travel to the cell membrane where they fuse with the cell membrane and release their contents. Each of these flat membranous sacs is named cisterna. This is the pathway for protein secretion: Proteins are made by ribosomes in the rough ER, go to vesicles, then to the cis face of the Golgi, be modified in the Golgi, go to the trans face of the Golgi, be repackaged into vesicles, and then make their way to the cell membrane where those proteins are secreted.

This concludes our brief introduction to the Golgi apparatus, and we'll be able to get some practice applying these concepts as we move forward. I'll see you all in our next video.

Here’s what students ask on this topic:

What is the endomembrane system and what are its main functions?

The endomembrane system is a group of membrane-bound organelles within eukaryotic cells that work together to modify, package, and transport lipids and proteins. The main components include the nucleus, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, and vesicles. This system is crucial for protein secretion and cellular digestion. Protein synthesis begins in the nucleus, where DNA is transcribed into RNA. The RNA is then translated into proteins by ribosomes in the rough ER. These proteins are modified in the Golgi apparatus before being packaged into vesicles for secretion. The smooth ER synthesizes lipids and detoxifies substances, highlighting the system's diverse functions in maintaining cellular homeostasis.

How does protein secretion occur in the endomembrane system?

Protein secretion in the endomembrane system begins in the nucleus, where DNA is transcribed into RNA. The RNA is then translated into proteins by ribosomes located on the rough endoplasmic reticulum (ER). These newly synthesized proteins enter the lumen of the rough ER, where they undergo folding and modifications. The proteins are then packaged into vesicles and transported to the Golgi apparatus. In the Golgi, the proteins are further modified, sorted, and repackaged into new vesicles. Finally, these vesicles fuse with the cell membrane, releasing the proteins into the extracellular environment. This process ensures that proteins are correctly synthesized, modified, and delivered to their intended destinations.

What roles do the rough and smooth endoplasmic reticulum play in the cell?

The rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER) have distinct roles in the cell. The RER is studded with ribosomes, giving it a rough appearance, and is primarily involved in protein synthesis. Ribosomes on the RER translate RNA into proteins, which then enter the RER lumen for folding and modification. The SER, on the other hand, lacks ribosomes and has a smooth appearance. It is involved in lipid synthesis, detoxification of drugs and poisons, and calcium ion storage. Together, the RER and SER contribute to the synthesis and processing of essential biomolecules, maintaining cellular function and homeostasis.

What is the function of the Golgi apparatus in protein secretion?

The Golgi apparatus functions as a processing and packaging center in protein secretion. Proteins synthesized in the rough endoplasmic reticulum (ER) are transported to the Golgi apparatus in vesicles. The Golgi apparatus consists of stacked membranous sacs called cisternae. At the cis face (receiving end), the Golgi receives vesicles containing proteins. These proteins are then modified, sorted, and repackaged as they move through the cisternae. At the trans face (shipping end), the modified proteins are packaged into new vesicles, which can be transported to various cellular destinations or secreted outside the cell. This ensures that proteins are correctly processed and delivered to their intended locations.

How are vesicles involved in the endomembrane system?

Vesicles are small, membrane-bound sacs that play a crucial role in the endomembrane system by transporting materials between organelles. They facilitate the movement of proteins and lipids from the endoplasmic reticulum (ER) to the Golgi apparatus and from the Golgi to other cellular destinations or the cell membrane for secretion. Vesicles bud off from one organelle, carrying specific cargo, and fuse with the membrane of the target organelle, delivering their contents. This vesicular transport ensures that biomolecules are efficiently and accurately distributed within the cell, maintaining cellular organization and function.