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5.16: The Endomembrane System - Biology

5.16: The Endomembrane System - Biology


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Learning Objectives

  • Describe the structure and function of the nucleus and nuclear membrane
  • Describe the structure, function, and components of the endomembrane system

The endomembrane system (endo = within) is a group of membranes and organelles (Figure 1) in eukaryotic cells that work together to modify, package, and transport lipids and proteins. It includes the nuclear envelope, lysosomes (which only appear in animal cells), vesicles, the endoplasmic reticulum, and Golgi apparatus, which we will cover shortly.

The Nucleus

Typically, the nucleus is the most prominent organelle in a cell (Figure 1). The nucleus (plural = nuclei) houses the cell’s DNA in the form of chromatin and directs the synthesis of ribosomes and proteins. Let us look at it in more detail (Figure 1).

The nuclear envelope is a double-membrane structure that constitutes the outermost portion of the nucleus (Figure 1). Both the inner and outer membranes of the nuclear envelope are phospholipid bilayers.

The nuclear envelope is punctuated with pores that control the passage of ions, molecules, and RNA between the nucleoplasm and the cytoplasm.

To understand chromatin, it is helpful to first consider chromosomes. Chromosomes are structures within the nucleus that are made up of DNA, the hereditary material, and proteins. This combination of DNA and proteins is called chromatin. In eukaryotes, chromosomes are linear structures. Every species has a specific number of chromosomes in the nucleus of its body cells. For example, in humans, the chromosome number is 46, whereas in fruit flies, the chromosome number is eight.

Chromosomes are only visible and distinguishable from one another when the cell is getting ready to divide. When the cell is in the growth and maintenance phases of its life cycle, the chromosomes resemble an unwound, jumbled bunch of threads, which is the chromatin.

We already know that the nucleus directs the synthesis of ribosomes, but how does it do this? Some chromosomes have sections of DNA that encode ribosomal RNA. A darkly staining area within the nucleus, called the nucleolus (plural = nucleoli), aggregates the ribosomal RNA with associated proteins to assemble the ribosomal subunits that are then transported through the nuclear pores into the cytoplasm.

The Endoplasmic Reticulum

The endoplasmic reticulum (ER) (Figure 1) is a series of interconnected membranous tubules that collectively modify proteins and synthesize lipids. However, these two functions are performed in separate areas of the endoplasmic reticulum: the rough endoplasmic reticulum and the smooth endoplasmic reticulum, respectively.

The hollow portion of the ER tubules is called the lumen or cisternal space. The membrane of the ER, which is a phospholipid bilayer embedded with proteins, is continuous with the nuclear envelope.

The rough endoplasmic reticulum (RER) is so named because the ribosomes attached to its cytoplasmic surface give it a studded appearance when viewed through an electron microscope.

The ribosomes synthesize proteins while attached to the ER, resulting in transfer of their newly synthesized proteins into the lumen of the RER where they undergo modifications such as folding or addition of sugars. The RER also makes phospholipids for cell membranes.

If the phospholipids or modified proteins are not destined to stay in the RER, they will be packaged within vesicles and transported from the RER by budding from the membrane (Figure 1). Since the RER is engaged in modifying proteins that will be secreted from the cell, it is abundant in cells that secrete proteins, such as the liver.

The smooth endoplasmic reticulum (SER) is continuous with the RER but has few or no ribosomes on its cytoplasmic surface (see Figure 1). The SER’s functions include synthesis of carbohydrates, lipids (including phospholipids), and steroid hormones; detoxification of medications and poisons; alcohol metabolism; and storage of calcium ions.

The Golgi Apparatus

We have already mentioned that vesicles can bud from the ER, but where do the vesicles go? Before reaching their final destination, the lipids or proteins within the transport vesicles need to be sorted, packaged, and tagged so that they wind up in the right place. The sorting, tagging, packaging, and distribution of lipids and proteins take place in the Golgi apparatus (also called the Golgi body), a series of flattened membranous sacs (Figure 2).

The Golgi apparatus has a receiving (cis) face near the endoplasmic reticulum and a releasing (trans) face on the side away from the ER, toward the cell membrane. The transport vesicles that form from the ER travel to the receiving face, fuse with it, and empty their contents into the lumen of the Golgi apparatus. As the proteins and lipids travel through the Golgi, they undergo further modifications. The most frequent modification is the addition of short chains of sugar molecules. The newly modified proteins and lipids are then tagged with small molecular groups so that they are routed to their proper destinations.

Finally, the modified and tagged proteins are packaged into vesicles that bud from the opposite face of the Golgi. While some of these vesicles, transport vesicles, deposit their contents into other parts of the cell where they will be used, others, secretory vesicles, fuse with the plasma membrane and release their contents outside the cell.

The amount of Golgi in different cell types again illustrates that form follows function within cells. Cells that engage in a great deal of secretory activity (such as cells of the salivary glands that secrete digestive enzymes or cells of the immune system that secrete antibodies) have an abundant number of Golgi.

In plant cells, the Golgi has an additional role of synthesizing polysaccharides, some of which are incorporated into the cell wall and some of which are used in other parts of the cell.

Practice Question

Why does the cis face of the Golgi not face the plasma membrane?

[practice-area rows=”2″][/practice-area]
[reveal-answer q=”453156″]Show Answer[/reveal-answer]
[hidden-answer a=”453156″]Because that face receives chemicals from the ER, which is toward the center of the cell.[/hidden-answer]


Typically, the nucleus is the most prominent organelle in a cell (Figure 1). The nucleus (plural = nuclei) houses the cell’s DNA in the form of chromatin and directs the synthesis of ribosomes and proteins. Let us look at it in more detail (Figure 1).

Figure 1. The outermost boundary of the nucleus is the nuclear envelope. Notice that the nuclear envelope consists of two phospholipid bilayers (membranes)—an outer membrane and an inner membrane—in contrast to the plasma membrane, which consists of only one phospholipid bilayer. (credit: modification of work by NIGMS, NIH)

The nuclear envelope is a double-membrane structure that constitutes the outermost portion of the nucleus (Figure 1). Both the inner and outer membranes of the nuclear envelope are phospholipid bilayers.

The nuclear envelope is punctuated with pores that control the passage of ions, molecules, and RNA between the nucleoplasm and the cytoplasm.

To understand chromatin, it is helpful to first consider chromosomes. Chromosomes are structures within the nucleus that are made up of DNA, the hereditary material, and proteins. This combination of DNA and proteins is called chromatin. In eukaryotes, chromosomes are linear structures. Every species has a specific number of chromosomes in the nucleus of its body cells. For example, in humans, the chromosome number is 46, whereas in fruit flies, the chromosome number is eight.

Chromosomes are only visible and distinguishable from one another when the cell is getting ready to divide. When the cell is in the growth and maintenance phases of its life cycle, the chromosomes resemble an unwound, jumbled bunch of threads, which is the chromatin.

We already know that the nucleus directs the synthesis of ribosomes, but how does it do this? Some chromosomes have sections of DNA that encode ribosomal RNA. A darkly staining area within the nucleus, called the nucleolus (plural = nucleoli), aggregates the ribosomal RNA with associated proteins to assemble the ribosomal subunits that are then transported through the nuclear pores into the cytoplasm.


4.4 The Endomembrane System and Proteins

In this section, you will explore the following questions:

  • What is the relationship between the structure and function of the components of the endomembrane system, especially with regard to the synthesis of proteins?

Connection for AP ® Courses

In addition to the presence of nuclei, eukaryotic cells are distinguished by an endomembrane system that includes the plasma membrane, nuclear envelope, lysosomes, vesicles, endoplasmic reticulum, and Golgi apparatus. These subcellular components work together to modify, tag, package, and transport proteins and lipids. The rough endoplasmic reticulum (RER) with its attached ribosomes is the site of protein synthesis and modification. The smooth endoplasmic reticulum (SER) synthesizes carbohydrates, lipids including phospholipids and cholesterol, and steroid hormones engages in the detoxification of medications and poisons and stores calcium ions. Lysosomes digest macromolecules, recycle worn-out organelles, and destroy pathogens. Just like your body uses different organs that work together, cells use these organelles interact to perform specific functions. For example, proteins that are synthesized in the RER then travel to the Golgi apparatus for modification and packaging for either storage or transport. If these proteins are hydrolytic enzymes, they can be stored in lysosomes. Mitochondria produce the energy needed for these processes. This functional flow through several organelles, a process which is dependent on energy produced by yet another organelle, serves as a hallmark illustration of the cell’s complex, interconnected dependence on its organelles.

Information presented and the examples highlighted in the section support concepts and Learning Objectives outlined in Big Idea 2 and Big Idea 4 of the AP ® Biology Curriculum Framework. The Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP ® Biology course, an inquiry-based laboratory experience, instructional activities, and AP ® exam questions. A Learning Objective merges required content with one or more of the seven Science Practices.

Big Idea 2 Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis.
Enduring Understanding 2.B Growth, reproduction and dynamic homeostasis require that cells create and maintain internal environments that are different from their external environments.
Essential Knowledge 2.B.3 Eukaryotic cells maintain internal membranes that partition the cell into specialized regions.
Science Practice 6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices.
Learning Objective 2.13 The student is able to explain how internal membranes and organelles contribute to cell functions.
Big Idea 4 Biological systems interact, and these systems and their interactions possess complex properties.
Enduring Understanding 4.A Interactions within biological systems lead to complex properties.
Essential Knowledge 4.A.2 The structure and function of subcellular components, and their interactions, provide essential cellular processes.
Science Practice 6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices.
Learning Objective 4.5 The student is able to construct explanations based on scientific evidence as to how interactions of subcellular structures provide essential functions.

Teacher Support

Students will need help in visualizing the endomembrane system. For example, explain how the interior membrane surface of a vesicle will face the outside of the cell, once the vesicle fuses with the plasma membrane. Use rubber bands to simulate vesicles and mark the inside with a sharpie or a pen. Follow the ink marks as the “vesicle” rubber band fuses with the cell membrane (cut the rubber band to facilitate the fusion being modeled.)

Students may think that all ribosomes are attached to the rough endoplasmic reticulum. Stress that there are free ribosomes as well. They are found in the cytosol where they are involved in the synthesis of cytosolic proteins, which remain within the cytosol. Free and bound ribosomes are identical in structure . Individual ribosomes cycle between free and membrane-bound positions as needed.

Mitochondria and chloroplasts also contain ribosomes which resemble those of prokaryotes. This observation is one of the arguments in favor of the endosymbiotic theory.

“Smooth endoplasmic reticulum is not as important as the rough endoplasmic reticulum.” No, both endomembrane networks play important roles in the life of a cell.

The Science Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards:
[APLO 4.6]

The Endoplasmic Reticulum

The endomembrane system (endo = “within”) is a group of membranes and organelles (Figure 4.18) in eukaryotic cells that works together to modify, package, and transport lipids and proteins. It includes the nuclear envelope, lysosomes, and vesicles, which we’ve already mentioned, and the endoplasmic reticulum and Golgi apparatus, which we will cover shortly. Although not technically within the cell, the plasma membrane is included in the endomembrane system because, as you will see, it interacts with the other endomembranous organelles. The endomembrane system does not include the membranes of either mitochondria or chloroplasts.

Visual Connection

  1. The vesicle travels from the endoplasmic reticulum to get embedded in plasma membrane.
  2. The vesicle travels from the Golgi to the plasma membrane to release the protein outside.
  3. The vesicle travels from the endoplasmic reticulum to the plasma membrane, and returns to the Golgi apparatus to get modified.
  4. The vesicle moves from the endoplasmic reticulum into the cytoplasmic area, remaining there.

The endoplasmic reticulum (ER) (Figure 4.18) is a series of interconnected membranous sacs and tubules that collectively modifies proteins and synthesizes lipids. However, these two functions are performed in separate areas of the ER: the rough ER and the smooth ER, respectively.

The hollow portion of the ER tubules is called the lumen or cisternal space. The membrane of the ER, which is a phospholipid bilayer embedded with proteins, is continuous with the nuclear envelope.

Rough ER

The rough endoplasmic reticulum (RER) is so named because the ribosomes attached to its cytoplasmic surface give it a studded appearance when viewed through an electron microscope (Figure 4.19).

Ribosomes transfer their newly synthesized proteins into the lumen of the RER where they undergo structural modifications, such as folding or the acquisition of side chains. These modified proteins will be incorporated into cellular membranes—the membrane of the ER or those of other organelles—or secreted from the cell (such as protein hormones, enzymes). The RER also makes phospholipids for cellular membranes.

If the phospholipids or modified proteins are not destined to stay in the RER, they will reach their destinations via transport vesicles that bud from the RER’s membrane (Figure 4.18).

Since the RER is engaged in modifying proteins (such as enzymes, for example) that will be secreted from the cell, you would be correct in assuming that the RER is abundant in cells that secrete proteins. This is the case with cells of the liver, for example.

Smooth ER

The smooth endoplasmic reticulum (SER) is continuous with the RER but has few or no ribosomes on its cytoplasmic surface (Figure 4.18). Functions of the SER include synthesis of carbohydrates, lipids, and steroid hormones detoxification of medications and poisons and storage of calcium ions.

In muscle cells, a specialized SER called the sarcoplasmic reticulum is responsible for storage of the calcium ions that are needed to trigger the coordinated contractions of the muscle cells.

Link to Learning

You can watch an excellent animation of the endomembrane system here.

  1. The endomembrane system processes and ships proteins specified by the nucleus. In the nucleus, DNA is used to make RNA which exits the nucleus and enters the cytoplasm of the cell. The ribosomes on the rough ER use the RNA to create the different types of protein needed by the body.
  2. The endomembrane system processes and ships proteins specified by the nucleus. From the nucleus, DNA exits and enters the cytoplasm of the cell. The ribosomes on the rough ER use the DNA to create the different types of protein needed by the body.
  3. The endomembrane system processes and ships proteins specified by the nucleus. In the nucleus, DNA is used to make RNA which exits the nucleus and enters the cytoplasm of the cell. The smooth ER uses the RNA to create the different types of protein needed by the body.
  4. The endomembrane system processes and ships proteins specified by the nucleus. In the nucleus, DNA is used to make RNA which exits the nucleus and enters the cytoplasm of the cell. The ribosomes on the smooth ER use the RNA to create the different types of protein needed by the body.

Career Connection

Cardiologist

Heart disease is the leading cause of death in the United States. This is primarily due to our sedentary lifestyle and our high trans-fat diets.

Heart failure is just one of many disabling heart conditions. Heart failure does not mean that the heart has stopped working. Rather, it means that the heart can’t pump with sufficient force to transport oxygenated blood to all the vital organs. Left untreated, heart failure can lead to kidney failure and failure of other organs.

The wall of the heart is composed of cardiac muscle tissue. Heart failure occurs when the endoplasmic reticula of cardiac muscle cells do not function properly. As a result, an insufficient number of calcium ions are available to trigger a sufficient contractile force.

Cardiologists (cardi- = “heart” -ologist = “one who studies”) are doctors who specialize in treating heart diseases, including heart failure. Cardiologists can make a diagnosis of heart failure via physical examination, results from an electrocardiogram (ECG, a test that measures the electrical activity of the heart), a chest X-ray to see whether the heart is enlarged, and other tests. If heart failure is diagnosed, the cardiologist will typically prescribe appropriate medications and recommend a reduction in table salt intake and a supervised exercise program.

The Golgi Apparatus

We have already mentioned that vesicles can bud from the ER and transport their contents elsewhere, but where do the vesicles go? Before reaching their final destination, the lipids or proteins within the transport vesicles still need to be sorted, packaged, and tagged so that they wind up in the right place. Sorting, tagging, packaging, and distribution of lipids and proteins takes place in the Golgi apparatus (also called the Golgi body), a series of flattened membranes (Figure 4.20).

The receiving side of the Golgi apparatus is called the cis face. The opposite side is called the trans face. The transport vesicles that formed from the ER travel to the cis face, fuse with it, and empty their contents into the lumen of the Golgi apparatus. As the proteins and lipids travel through the Golgi, they undergo further modifications that allow them to be sorted. The most frequent modification is the addition of short chains of sugar molecules. These newly modified proteins and lipids are then tagged with phosphate groups or other small molecules so that they can be routed to their proper destinations.

Finally, the modified and tagged proteins are packaged into secretory vesicles that bud from the trans face of the Golgi. While some of these vesicles deposit their contents into other parts of the cell where they will be used, other secretory vesicles fuse with the plasma membrane and release their contents outside the cell.

In another example of form following function, cells that engage in a great deal of secretory activity (such as cells of the salivary glands that secrete digestive enzymes or cells of the immune system that secrete antibodies) have an abundance of Golgi.

In plant cells, the Golgi apparatus has the additional role of synthesizing polysaccharides, some of which are incorporated into the cell wall and some of which are used in other parts of the cell.

Career Connection

Geneticist

Many diseases arise from genetic mutations that prevent the synthesis of critical proteins. One such disease is Lowe disease (also called oculocerebrorenal syndrome, because it affects the eyes, brain, and kidneys). In Lowe disease, there is a deficiency in an enzyme localized to the Golgi apparatus. Children with Lowe disease are born with cataracts, typically develop kidney disease after the first year of life, and may have impaired mental abilities.

Lowe disease is a genetic disease caused by a mutation on the X chromosome. The X chromosome is one of the two human sex chromosomes, as these chromosomes determine a person's sex. Females possess two X chromosomes while males possess one X and one Y chromosome. In females, the genes on only one of the two X chromosomes are expressed. Females who carry the Lowe disease gene on one of their X chromosomes are carriers and do not show symptoms of the disease. However, males only have one X chromosome and the genes on this chromosome are always expressed. Therefore, males will always have Lowe disease if their X chromosome carries the Lowe disease gene. The location of the mutated gene, as well as the locations of many other mutations that cause genetic diseases, has now been identified. Through prenatal testing, a woman can find out if the fetus she is carrying may be afflicted with one of several genetic diseases.

Geneticists analyze the results of prenatal genetic tests and may counsel pregnant women on available options. They may also conduct genetic research that leads to new drugs or foods, or perform DNA analyses that are used in forensic investigations.

Lysosomes

In addition to their role as the digestive component and organelle-recycling facility of animal cells, lysosomes are considered to be parts of the endomembrane system. Lysosomes also use their hydrolytic enzymes to destroy pathogens (disease-causing organisms) that might enter the cell. A good example of this occurs in a group of white blood cells called macrophages, which are part of your body’s immune system. In a process known as phagocytosis or endocytosis, a section of the plasma membrane of the macrophage invaginates (folds in) and engulfs a pathogen. The invaginated section, with the pathogen inside, then pinches itself off from the plasma membrane and becomes a vesicle. The vesicle fuses with a lysosome. The lysosome’s hydrolytic enzymes then destroy the pathogen (Figure 4.21).

Science Practice Connection for AP® Courses

Activity

Homemade Cell Project. Using inexpensive and common household items, create a model of a specific eukaryotic cell (e.g., neuron, white blood cell, plant root cell, or Paramecium) that demonstrates how at least three organelles work together to perform a specific function.

Think About It

A certain cell type functions primarily to synthesize proteins for export. What is the most likely route the newly made protein takes through the cell? Justify your prediction.

Teacher Support

The activity is an application of Learning Objective 2.13 and Science Practice 6.2 and Learning Objective 4.6 and Science Practice 1.4 because students are asked to create a model that describes various organelles in a specific cell type and then describe how organelles work together to perform a characteristic function of the cell.

The Think About It question is an application of Learning Objective 4.4 and Science Practice 6.4 because students are making a prediction about the interactions of subcellular organelles in performing a specific function.

The path is ribosomes→rough ER→vesicle→Golgi apparatus → vesicle and release. Use information in the text.

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    Biology 171

    By the end of this section, you will be able to do the following:

    • List the components of the endomembrane system
    • Recognize the relationship between the endomembrane system and its functions

    The endomembrane system (endo = “within”) is a group of membranes and organelles ((Figure)) in eukaryotic cells that works together to modify, package, and transport lipids and proteins. It includes the nuclear envelope, lysosomes, and vesicles, which we have already mentioned, and the endoplasmic reticulum and Golgi apparatus, which we will cover shortly. Although not technically within the cell, the plasma membrane is included in the endomembrane system because, as you will see, it interacts with the other endomembranous organelles. The endomembrane system does not include either mitochondria or chloroplast membranes.


    If a peripheral membrane protein were synthesized in the lumen (inside) of the ER, would it end up on the inside or outside of the plasma membrane?

    The Endoplasmic Reticulum

    The endoplasmic reticulum (ER) ((Figure)) is a series of interconnected membranous sacs and tubules that collectively modifies proteins and synthesizes lipids. However, these two functions take place in separate areas of the ER: the rough ER and the smooth ER, respectively.

    We call the ER tubules’ hollow portion the lumen or cisternal space. The ER’s membrane, which is a phospholipid bilayer embedded with proteins, is continuous with the nuclear envelope.

    Rough ER

    Scientists have named the rough endoplasmic reticulum (RER) as such because the ribosomes attached to its cytoplasmic surface give it a studded appearance when viewing it through an electron microscope ((Figure)).


    Ribosomes transfer their newly synthesized proteins into the RER’s lumen where they undergo structural modifications, such as folding or acquiring side chains. These modified proteins incorporate into cellular membranes—the ER or the ER’s or other organelles’ membranes. The proteins can also secrete from the cell (such as protein hormones, enzymes). The RER also makes phospholipids for cellular membranes.

    If the phospholipids or modified proteins are not destined to stay in the RER, they will reach their destinations via transport vesicles that bud from the RER’s membrane ((Figure)).

    Since the RER is engaged in modifying proteins (such as enzymes, for example) that secrete from the cell, you would be correct in assuming that the RER is abundant in cells that secrete proteins. This is the case with liver cells, for example.

    Smooth ER

    The smooth endoplasmic reticulum (SER) is continuous with the RER but has few or no ribosomes on its cytoplasmic surface ((Figure)). SER functions include synthesis of carbohydrates, lipids, and steroid hormones detoxification of medications and poisons and storing calcium ions.

    In muscle cells, a specialized SER, the sarcoplasmic reticulum, is responsible for storing calcium ions that are needed to trigger the muscle cells’ coordinated contractions.

    Cardiologist Heart disease is the leading cause of death in the United States. This is primarily due to our sedentary lifestyle and our high trans-fat diets.

    Heart failure is just one of many disabling heart conditions. Heart failure does not mean that the heart has stopped working. Rather, it means that the heart can’t pump with sufficient force to transport oxygenated blood to all the vital organs. Left untreated, heart failure can lead to kidney failure and other organ failure.

    Cardiac muscle tissue comprises the heart’s wall. Heart failure occurs when cardiac muscle cells’ endoplasmic reticula do not function properly. As a result, an insufficient number of calcium ions are available to trigger a sufficient contractile force.

    Cardiologists (cardi- = “heart” -ologist = “one who studies”) are doctors who specialize in treating heart diseases, including heart failure. Cardiologists can diagnose heart failure via a physical examination, results from an electrocardiogram (ECG, a test that measures the heart’s electrical activity), a chest X-ray to see whether the heart is enlarged, and other tests. If the cardiologist diagnoses heart failure, he or she will typically prescribe appropriate medications and recommend a reduced table salt intake and a supervised exercise program.

    The Golgi Apparatus

    We have already mentioned that vesicles can bud from the ER and transport their contents elsewhere, but where do the vesicles go? Before reaching their final destination, the lipids or proteins within the transport vesicles still need sorting, packaging, and tagging so that they end up in the right place. Sorting, tagging, packaging, and distributing lipids and proteins takes place in the Golgi apparatus (also called the Golgi body), a series of flattened membranes ((Figure)).


    We call the Golgi apparatus’ the cis face. The opposite side is the trans face. The transport vesicles that formed from the ER travel to the cis face, fuse with it, and empty their contents into the Golgi apparatus’ lumen. As the proteins and lipids travel through the Golgi, they undergo further modifications that allow them to be sorted. The most frequent modification is adding short sugar molecule chains. These newly modified proteins and lipids then tag with phosphate groups or other small molecules in order to travel to their proper destinations.

    Finally, the modified and tagged proteins are packaged into secretory vesicles that bud from the Golgi’s trans face. While some of these vesicles deposit their contents into other cell parts where they will be used, other secretory vesicles fuse with the plasma membrane and release their contents outside the cell.

    In another example of form following function, cells that engage in a great deal of secretory activity (such as salivary gland cells that secrete digestive enzymes or immune system cells that secrete antibodies) have an abundance of Golgi.

    In plant cells, the Golgi apparatus has the additional role of synthesizing polysaccharides, some of which are incorporated into the cell wall and some of which other cell parts use.

    Geneticist Many diseases arise from genetic mutations that prevent synthesizing critical proteins. One such disease is Lowe disease (or oculocerebrorenal syndrome, because it affects the eyes, brain, and kidneys). In Lowe disease, there is a deficiency in an enzyme localized to the Golgi apparatus. Children with Lowe disease are born with cataracts, typically develop kidney disease after the first year of life, and may have impaired mental abilities.

    A mutation on the X chromosome causes Lowe disease. The X chromosome is one of the two human sex chromosomes, as these chromosomes determine a person’s sex. Females possess two X chromosomes while males possess one X and one Y chromosome. In females, the genes on only one of the two X chromosomes are expressed. Females who carry the Lowe disease gene on one of their X chromosomes are carriers and do not show symptoms of the disease. However, males only have one X chromosome and the genes on this chromosome are always expressed. Therefore, males will always have Lowe disease if their X chromosome carries the Lowe disease gene. Geneticists have identified the mutated gene’s location, as well as many other mutation locations that cause genetic diseases. Through prenatal testing, a woman can find out if the fetus she is carrying may be afflicted with one of several genetic diseases.

    Geneticists analyze prenatal genetic test results and may counsel pregnant women on available options. They may also conduct genetic research that leads to new drugs or foods, or perform DNA analyses for forensic investigations.

    Lysosomes

    In addition to their role as the digestive component and organelle-recycling facility of animal cells, lysosomes are part of the endomembrane system. Lysosomes also use their hydrolytic enzymes to destroy pathogens (disease-causing organisms) that might enter the cell. A good example of this occurs in macrophages, a group of white blood cells which are part of your body’s immune system. In a process that scientists call phagocytosis or endocytosis, a section of the macrophage’s plasma membrane invaginates (folds in) and engulfs a pathogen. The invaginated section, with the pathogen inside, then pinches itself off from the plasma membrane and becomes a vesicle. The vesicle fuses with a lysosome. The lysosome’s hydrolytic enzymes then destroy the pathogen ((Figure)).


    Section Summary

    The endomembrane system includes the nuclear envelope, lysosomes, vesicles, the ER, and Golgi apparatus, as well as the plasma membrane. These cellular components work together to modify, package, tag, and transport proteins and lipids that form the membranes.

    The RER modifies proteins and synthesizes phospholipids in cell membranes. The SER synthesizes carbohydrates, lipids, and steroid hormones engages in the detoxification of medications and poisons and stores calcium ions. Sorting, tagging, packaging, and distributing lipids and proteins take place in the Golgi apparatus. Budding RER and Golgi membranes create lysosomes. Lysosomes digest macromolecules, recycle worn-out organelles, and destroy pathogens.

    Art Connections

    (Figure) If a peripheral membrane protein were synthesized in the lumen (inside) of the ER, would it end up on the inside or outside of the plasma membrane?

    (Figure) It would end up on the outside. After the vesicle passes through the Golgi apparatus and fuses with the plasma membrane, it turns inside out.

    Free Response

    In the context of cell biology, what do we mean by form follows function? What are at least two examples of this concept?

    “Form follows function” refers to the idea that the function of a body part dictates the form of that body part. As an example, compare your arm to a bat’s wing. While the bones of the two correspond, the parts serve different functions in each organism and their forms have adapted to follow that function.

    In your opinion, is the nuclear membrane part of the endomembrane system? Why or why not? Defend your answer.

    Since the external surface of the nuclear membrane is continuous with the rough endoplasmic reticulum, which is part of the endomembrane system, then it is correct to say that it is part of the system.

    Glossary


    Endomembrane system

    The endomembrane system (endo = &ldquowithin&rdquo) is a group of membranes and organelles (Figure 17.2) in eukaryotic cells that works together to modify, package, and transport lipids and proteins. It includes the nuclear envelope as well as:

    • Endoplasmic reticulum
    • Golgi apparatus
    • Endosomes
    • Lysosomes
    • Vacuoles
    • Peroxisomes

    Figure 17.2: Interaction of the endomembrane systems.

    Although not technically within the cell, the plasma membrane is included in the endomembrane system because, it interacts with the other endomembranous organelles. The endomembrane system does not include the mitochondria. The system of intra cellular membranes is designed to move proteins through both the secretory pathway (constitutive or regulated) and the endocytic pathways.

    The endoplasmic reticulum (ER)

    The endoplasmic reticulum (ER) (Figure 17.2) is a series of interconnected membranous sacs and tubules that collectively modifies proteins and synthesizes lipids. However, these two functions take place in separate areas of the ER: the rough ER and the smooth ER, respectively.

    Smooth ER

    The smooth endoplasmic reticulum (SER) is continuous with the rough ER (RER) but has few or no ribosomes on its cytoplasmic surface. SER functions include synthesis of carbohydrates, lipids, and steroid hormones detoxification of medications and poisons and storing calcium ions. In muscle cells, a specialized SER, the sarcoplasmic reticulum, is responsible for storing calcium ions that are needed to trigger the muscle cellsʼ coordinated contractions.

    Rough ER

    Scientists have named the rough endoplasmic reticulum (RER) as such because the ribosomes attached to its cytoplasmic surface give it a studded appearance when viewing it through an electron microscope.

    Ribosomes transfer their newly synthesized proteins into the RERʼs lumen where they undergo structural modifications, such as folding or acquiring side chains. These modified proteins incorporate into cellular membranes the ER or other organellesʼ membranes. The proteins can also be secreted from the cell (such as protein hormones, enzymes). The RER also makes phospholipids for cellular membranes.

    If the phospholipids or modified proteins are not destined to stay in the RER, they will reach their destinations via transport vesicles that bud from the RERʼs membrane.

    Glycosylation

    Nearly all RER-synthesized proteins are glycosylated with short branched oligosaccharides. This occurs in a N-linked fashion on asparagine residues.

    Protein degradation

    If proteins arenʼt folded properly this can contribute to a host of disease processes related to mis-folding events. Typically, folding is facilitated in the ER using chaperones (BiP) but if the protein is altered (due to mutation) this can lead to aggregation. Accumulation of BiP can initiate the unfolded protein response (UPR) (Figure 17.3).

    Figure 17.3: Unfolded protein response in the RER.

    E3 ubiquitin ligase is often responsible for tagging aggregates with ubiquitin which targets the protein the the proteasome. The proteasome consists of two subunits (19S and 20S) to make a functional 26S proteasome. Inside the proteasome, the polypeptide chains are cleaved back to their native amino acids and can be reused in other translational events. However, if the aggregates accumulate in some instances they can contribute to any number of neurodegenerative disorders.

    Golgi

    When proteins exit the RER they are trafficked to the Golgi where they will incur further post-translational modifications and will translocate to their final destination. These modifications will include ʻpruningʼ of large oligosaccharides which were attached in the RER, glycosylation, sulfation and phosphorylation. Additionally, some proteins require Golgi associated cleavage to produce a mature protein ready for trafficking.

    The Golgi is divided into Trans and Cis networks.

    • Cis Golgi is close the ER and sorts proteins back to the ER.
    • Trans Golgi sorts proteins into vesicles bound for the plasma membrane and intracellular vesicles.

    In the golgi, O-linked glycosylation happens and most mannose residues are removed. This is done by a large family of enzymes known as glycosyltranferases.


    Coevolution of Eukaryote-like Vps4 and ESCRT-III Subunits in the Asgard Archaea

    The emergence of the endomembrane system is a key step in the evolution of cellular complexity during eukaryogenesis. The endosomal sorting complex required for transport (ESCRT) machinery is essential and required for the endomembrane system functions in eukaryotic cells. Recently, genes encoding eukaryote-like ESCRT protein components have been identified in the genomes of Asgard archaea, a newly proposed archaeal superphylum that is thought to include the closest extant prokaryotic relatives of eukaryotes. However, structural and functional features of Asgard ESCRT remain uncharacterized. Here, we show that Vps4, Vps2/24/46, and Vps20/32/60, the core functional components of the Asgard ESCRT, coevolved eukaryote-like structural and functional features. Phylogenetic analysis shows that Asgard Vps4, Vps2/24/46, and Vps20/32/60 are closely related to their eukaryotic counterparts. Molecular dynamics simulation and biochemical assays indicate that Asgard Vps4 contains a eukaryote-like microtubule-interacting and transport (MIT) domain that binds the distinct type 1 MIT-interacting motif and type 2 MIT-interacting motif in Vps2/24/46 and Vps20/32/60, respectively. The Asgard Vps4 partly, but much more efficiently than homologs from other archaea, complements the vps4 null mutant of Saccharomyces cerevisiae, further supporting the functional similarity between the membrane remodeling machineries of Asgard archaea and eukaryotes. Thus, this work provides evidence that the ESCRT complexes from Asgard archaea and eukaryotes are evolutionarily related and functionally similar. Thus, despite the apparent absence of endomembranes in Asgard archaea, the eukaryotic ESCRT seems to have been directly inherited from an Asgard ancestor, to become a key component of the emerging endomembrane system.IMPORTANCE The discovery of Asgard archaea has changed the existing ideas on the origins of eukaryotes. Researchers propose that eukaryotic cells evolved from Asgard archaea. This hypothesis partly stems from the presence of multiple eukaryotic signature proteins in Asgard archaea, including homologs of ESCRT proteins that are essential components of the endomembrane system in eukaryotes. However, structural and functional features of Asgard ESCRT remain unknown. Our study provides evidence that Asgard ESCRT is functionally comparable to the eukaryotic counterparts, suggesting that despite the apparent absence of endomembranes in archaea, eukaryotic ESCRT was inherited from an Asgard archaeal ancestor, alongside the emergence of endomembrane system during eukaryogenesis.

    Keywords: Asgard archaea ESCRT endomembrane system eukaryogenesis evolution.

    Figures

    Phylogenetic and amino acid sequence…

    Phylogenetic and amino acid sequence analysis of the ESCRT-III-related subunits in archaea and…

    Phylogenetic and structural analysis of…

    Phylogenetic and structural analysis of the Asgard Vps4. (A) Unrooted maximum likelihood phylogenetic…

    Comparison of the Vps4 (surface…

    Comparison of the Vps4 (surface representation, gray) in complex with ESCRT-III subunits (ribbon…

    Functional complementation of Saccharomyces cerevisiae…

    Functional complementation of Saccharomyces cerevisiae vps4 null mutants by Asgard Vps4. (A) Complementation…


    Watch the video: The Endomembrane System (June 2022).


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