COMBINED Cell & Molec Chapter 15, Molecular Biology Chapter 15 Practice Questions, SmartWork5 Chapter 10, Quiz 15.2: Protein Sorting + Vesicular Transport, Chapter 15, Chapter 15 Quiz (Cell Biology), Chapter 15: Endomembrane system, chegg questions

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Which does NOT normally occur at a nuclear pore complex? (A) A protein complex is imported into the nucleus. (B) A protein containing a nuclear localization signal enters the pore from the cytosol at the same time that a protein containing a nuclear export signal exits the pore from the nucleus. (C) An mRNA molecule is exported to the cytosol. (D) A nuclear import receptor is exported to the cytosol. (E) All of the above are common occurrences. (F) None of the above are common occurrences.

(E) All of the above are common occurrences.

Many viruses enter cells through receptor-mediated endocytosis. Which of the following strategies could be affective in blocking entry of this class of viruses into cells and could be used to treat viral infections?

-Block the function of adaptin -Block the receptor with an antibody Some viruses such as the human rhinovirus 2 gain entry into their host cells through receptor-mediated endocytosis by binding the LDL receptor on the surface of cells. Developing drugs that could block the entry of substrates into the cell by receptor-mediated endocytosis might be an effective treatment for things like colds caused by the rhinovirus or to treat other infections caused by viruses that enter the cell through receptor-mediated endocytosis. The receptor, adaptin, and clathrin are all required for viral entry so inhibition of any of these three would block receptor-mediated endocytosis. Increasing clathrin activity would not prevent viral entry. Actin filaments are not involved in receptor-mediated endocytosis, so blocking them would not affect viral entry.

Which statements are true of receptor-mediated endocytosis?

-It allows the internalization of extracellular substances in clathrin-coated vesicles. -It allows cholesterol-carrying low-density lipoproteins (LDLs) to be taken up by cells. -The process can be hijacked by viruses to gain entry into cells. In most animal cells, specific macromolecules can be taken up from the extracellular fluid via clathrin-coated vesicles. The macromolecules bind to complementary receptors on the cell surface and enter the cell as receptor-macromolecule complexes in clathrin-coated vesicles. This process, called receptor-mediated endocytosis, provides a selective concentrating mechanism that increases the efficiency of internalization of particular macromolecules more than 1000-fold compared with ordinary pinocytosis. Such is the case when animal cells import the cholesterol they need to make new membranes. Cholesterol is transported in the bloodstream bound to proteins in the form of particles called low-density lipoproteins, or LDL. Cholesterol-containing LDLs bound to receptors on the cell surface are ingested by receptor-mediated endocytosis and delivered to endosomes. In the acidic interior of endosomes, the LDL dissociates from its receptor. These empty receptors are returned, via transport vesicles, to the plasma membrane for reuse. The LDL is delivered to lysosomes, where it is broken down, allowing free cholesterol to enter the cytosol. Receptor-mediated endocytosis is also used to import many other essential metabolites: the vitamin B12 and iron required to make hemoglobin is taken up by immature red blood cells. At the same time, receptor-mediated endocytosis can also be exploited by viruses, such as the influenza virus, to gain entry into cells.

1 Histone per second

15-30 Match the components involved with ER transport with the appropriate cellular location. Locations can be used more than once, or not at all. Components Location 1. signal-recognition particle _____ A. cytosol 2. protein translocator _____ B. ER lumen 3. mRNA _____ C. ER membrane 4. SRP receptor _____ 5. active site of signal peptidase ____

1—A 2—C 3—A 4—C 5—B

A macrophage swallows 25% of its own volume of fluid each hour. This means that it removes 3% of its plasma membrane each minute, or 100% in about half an hour. At what rate is this membrane replaced by exocytosis? 3% per hour 25% per hour 50% per hour 100% per hour 200% per hour

200% per hour Because the cell's total surface area remains unchanged, as much membrane is added to the cell surface by exocytosis as is removed by endocytosis. To achieve this balance, the cell would need to add back 100% of its plasma membrane every half hour—or double its plasma membrane per hour.

What distinguishes proteins destined for regulated secretion? A) Their surface properties allow them to form aggregates that are packaged into secretory vesicles. B) They are cleaved from membrane domains in the Golgi apparatus prior to being packed into secretory vesicles. C) They are produced in small amounts and then concentrated in secretory vesicles. D) They bind to clathrin. E) They have a series of amino acids that acts as a tag that marks them for packaging into secretory vesicles.

Which of the following is true of lysosomes? A) Lysosomal enzymes are optimally active in the acidic conditions maintained within lysosomes. B) Lysosomes form from vesicles that pinch off from the endoplasmic reticulum. C) Lysosomes have a pH that is higher than that of the cytosol. D) Most of the lysosomal membrane proteins have glycosylated regions on the cytosolic side of the membrane. E) The products of digestion in lysosomes leave the lysosome by transport vesicles.

Mitochondria and peroxisomes do NOT have what in common? A) Proteins need to unfold to enter them. B) Their membranes contain a protein translocator. C) They are present in eukaryotic cells. D) They must import proteins to function.

A Some proteins enter the peroxisome via vesicles that bud from the endoplasmic reticulum; others enter via selective transport from the cytosol. Although the mechanism is still not fully understood, these proteins do not need to fully unfold before entering peroxisomes.

Which of the following is true? A) A common pool of ribosomes is used to synthesize both the proteins that stay in the cytosol and those that are destined for the ER. B) A special class of ribosomes embedded in the ER translates the proteins destined for that organelle. C) All ribosomes are attached to the ER when they begin synthesizing a protein. D) Polyribosomes translate only cytosolic proteins. E) Polyribosomes translate only those proteins that have an ER signal sequence.

A When a ribosome is making a protein that has an ER signal sequence, that signal sequence directs the ribosome to the endoplasmic reticulum (ER) membrane.

Which of the following is true?

A common pool of ribosomes is used to synthesize both the proteins that stay in the cytosol and those that are destined for the ER.

Which of the following is true? -A common pool of ribosomes is used to synthesize both the proteins that stay in the cytosol and those that are destined for the ER. -A special class of ribosomes embedded in the ER translates the proteins destined for that organelle. -All ribosomes are attached to the ER when they begin synthesizing a protein. -Polyribosomes translate only cytosolic proteins. -Polyribosomes translate only those proteins that have an ER signal sequence.

A common pool of ribosomes is used to synthesize both the proteins that stay in the cytosol and those that are destined for the ER.

Which organelle is essentially a small sac of digestive enzymes that functions in degrading worn-out organelles, as well as macromolecules

A lysosome

Which organelle contains enzymes used in a variety of oxidative reactions that break down lipids and destroy toxic

A peroxisome

In the process of translocating a polypeptide across the membrane of the endoplasmic reticulum, a stop-transfer sequence halts the process. What eventually becomes of the stop-transfer sequence?

A) It forms an α-helical membrane-spanning segment of the protein. B) It is cleaved from the protein. C) It is translocated into the lumen of the endoplasmic reticulum. D) It remains in the cytosol.

15-3 Name the membrane-enclosed compartments in a eukaryotic cell where each of the functions listed below takes place. A. photosynthesis B. transcription C. oxidative phosphorylation D. modification of secreted proteins E. steroid hormone synthesis F. degradation of worn-out organelles G. new membrane synthesis H. breakdown of lipids and toxic molecules

A. photosynthesis = chloroplast B. transcription = nucleus C. oxidative phosphorylation = mitochondrion D. modification of secreted proteins = Golgi apparatus and rough endoplasmic reticulum (ER) E. steroid hormone synthesis = smooth ER F. degradation of worn-out organelles = lysosome G. new membrane synthesis = ER H. breakdown of lipids and toxic molecules = peroxisome

Match the following structures used in receptor-mediated endocytosis with their functions.

Adaptin- mediates the contact bw the receptor and another component Clathrin- shapes the forming vesicle Receptor-mediated endocytosis occurs when cargo molecules to be imported into the cell are bound by a specific receptor in the plasma membrane. An adaptin protein binds to the receptor and acts as an adaptor or bridge between the receptor bound to cargo and the clathrin molecule. The growing vesicle is shaped by the clathrin molecules, which form a closed cage around the vesicle. After formation, the vesicle is uncoated and clathrin leaves. The vesicle is then transported to the proper intracellular location.

Which of the following is true of lysosomes?

An ATP-driven H+ pump in the lysosomal membrane maintains the organelle's pH.

Which organelle sorts ingested molecules and recycles some of them back to the plasma membrane?

An endosome

Which mechanism is used for degrading obsolete parts of a cell, such as a defective mitochondrion?

Autophagy

Which mechanism is used for degrading obsolete parts of a cell, such as a defective mitochondrion?

Autophagy In electron micrographs, mitochondria can be seen being digested inside lysosomes.

Which of the following is NOT true of receptor-mediated endocytosis? A) In receptor-mediated endocytosis, extracellular substances are internalized in clathrin-coated vesicles. B) In receptor-mediated endocytosis, internalized vesicles fuse with lysosomes, which then mature into endosomes.Correct! C) Low-density lipoproteins (LDLs) are taken into cells by receptor-mediated endocytosis. D) Some viruses are taken into the cell by receptor-mediated endocytosis.

B Endosomes can mature to form lysosomes.

Oligosaccharide chains added in the ER can undergo further modification in which organelle(s)?

Both the cis and trans Golgi networks- Many of the oligosaccharide chains that are added to proteins in the ER undergo further modifications in the Golgi apparatus. On some proteins, for example, more complex oligosaccharide chains are created by a highly ordered process in which sugars are added and removed by a series of enzymes that act in a rigidly determined sequence as the protein passes through the Golgi stack. As would be expected, the enzymes that act early in the chain of processing events are located in cisternae close to the cis face, while enzymes that act late are located in cisternae near the trans face.

The interiors of the ER, Golgi apparatus, endosomes, and lysosomes communicate with each other in which of the following ways?

By small vesicles that bud off of one organelle and fuse with another

How do proteins travel from one cisterna to the next in the Golgi apparatus?

By transport vesicles that bud off from one cisterna and fuse with the next cisterna

Which statement about receptor-mediated endocytosis of LDL particles is true? A) LDL receptors are normally degraded in the lysosome along with LDL. B) LDL receptors are not taken up by receptor-mediated endocytosis unless they are carrying LDL. C) LDL receptors in the plasma membrane associate with clathrin-coated pits. D) Mutations that impair the endocytosis of LDL receptors cause a drop in blood cholesterol levels. E) The high pH inside endosomes allows LDL to dissociate from its receptor.

C LDL receptors are internalized in clathrin-coated vesicles.

Which of the following is NOT a role for the oligosaccharides on glycosylated proteins? A) They can guide the protein to the appropriate organelle by serving as a transport signal for packaging the protein into appropriate transport vesicles. B) They can protect the protein from degradation and hold it in the ER until it is properly folded. C) They can provide a source of energy for the cell. D) When displayed on the cell surface, oligosaccharides form part of the cell's carbohydrate layer. E) When displayed on the cell surface, they promote cell-cell recognition.

C The oligosaccharides on glycoproteins are not used to generate energy.

How do clathrin-coated vesicles select their cargo molecules?

Cargo receptors bind specifically to cargo proteins and to clathrin. Vesicles destined for different compartments have different types of protein coats. The cargo for these vesicles is selected by specifically binding to cargo receptors that interact with a specific type of protein coat.

Which type of protein binds to improperly folded or improperly assembled proteins in the ER, holding them there until proper folding occurs?

Chaperone proteins

Which type of protein binds to improperly folded or improperly assembled proteins in the ER, holding them there until proper folding occurs?

Chaperone proteins Chaperones prevent misfolded proteins from forming aggregates, which helps steer the proteins along the path to proper folding.

What protein can assemble into a basket-like network that gives budding vesicles their shape?

Clathrin

What proteins can assemble into a basketlike network that gives budding vesicles their shape?

Clathrin

What proteins can assemble into a basketlike network that gives budding vesicles their shape?

Clathrin The structure of clathrin dictates its assembly into cagelike baskets, even in the absence of the membrane vesicles that these baskets normally enclose.

What protein can assemble into a basketlike network that gives budding vesicles their shape?

Clathrin- Vesicles that bud from membranes usually have a distinctive protein coat on their cytosolic surface and are therefore called coated vesicles. The coat serves at least two functions: it helps shape the membrane into a bud and it captures molecules for onward transport. After budding from its parent organelle, the vesicle sheds its coat, allowing its membrane to interact directly with the membrane to which it will fuse. Cells produce several kinds of coated vesicles, each with a distinctive protein coat. The best-studied vesicles are those that have an outer coat made of the protein clathrin. These clathrin-coated vesicles bud from both the Golgi apparatus on the outward secretory pathway and from the plasma membrane on the inward endocytic pathway. At the plasma membrane, for example, each vesicle starts off as a clathrin-coated pit. Clathrin molecules assemble into a basketlike network on the cytosolic surface of the membrane, and it is this assembly process that starts shaping the membrane into a vesicle.

Proteins that lack a sorting signal remain as permanent residents of which part of a eukaryotic cell?

Cytosol With the exception of a few mitochondrial and chloroplast proteins, the synthesis of all proteins in the cell begins on ribosomes in the cytosol; proteins that lack a sorting signal are destined to remain there.

Which best describes a pathway that a protein might follow from synthesis to secretion?

Cytosol → ER → transport vesicle → Golgi apparatus → transport vesicle → plasma membrane The synthesis of all proteins—except for those made in mitochondria or chloroplasts—begins in the cytosol. From there, they enter the ER for folding, assembly, and initial glycosylation. Transport vesicles ferry proteins that have been properly folded and assembled to the Golgi apparatus for further modification and sorting. Proteins destined for secretion do not pass through endosomes. Instead, they leave the trans Golgi network either in transport vesicles, which fuse with the plasma membrane immediately, or in secretory vesicles, which await a signal to stimulate membrane fusion.

The interiors of the ER, Golgi apparatus, endosomes, and lysosomes communicate with each other in which of the following ways? A) By excreting hormones and other small signaling molecules B) By fusing with one another. C) By open pores that allow ions to exit and enter the organelles D) By small vesicles that bud off of one organelle and fuse with another E) They do not communicate with one another.

Which of the following statements is NOT true of mitochondrial proteins that are synthesized in the cytosol? A) Chaperone proteins help draw the proteins inside the mitochondrion. B) The proteins are unfolded as they are transported into the mitochondria. C) The proteins cross both the inner and outer mitochondrial membranes as they are imported. D) The proteins must be transported across the mitochondrial membranes while being synthesized. E) The proteins usually have a signal sequence at their N-terminus.

D Proteins enter the endoplasmic reticulum while they are being synthesized.

The final products of the digestion of macromolecules: A) are expelled from the lysosome by H+ pumps in the lysosomal membrane. B) are removed from the lysosome by transport vesicles that carry them to where they are needed. C) are secreted from the cell when lysosomes fuse with the plasma membrane. D) are transferred to the cytosol through transporters in the lysosomal membrane. E) are ultimately destroyed by lysosomal enzymes.

D These molecules can then be reused by the cell or excreted.

If a signal sequence is removed from an ER protein: A) the protein can enter any organelle other than the ER. B) the protein will be escorted to the ER by chaperone proteins. C) the protein will be immediately degraded. D) the protein will be unable to exit the ER. E) the protein will remain in the cytosol.

E Deleting the signal sequence from an endoplasmic reticulum (ER) protein converts it into a cytosolic protein.

Which membrane-enclosed organelles most likely evolved in a similar manner? A) Chloroplasts and peroxisomes B) Mitochondria and the ER C) Mitochondria and the Golgi apparatus D) Mitochondria and the nucleus E) The nucleus and the ER

E The nuclear membranes and the membranes of the endoplasmic reticulum (ER) most likely originated by invagination of the plasma membrane; the outer nuclear membrane is actually continuous with the rough ER.

Proteins destined for the Golgi apparatus, endosomes, lysosomes, and even the cell surface must pass through which organelle?

Where in the cell are some proteins initially decorated with an oligosaccharide tree on asparagine residues?

ER- Many of the proteins that enter the ER lumen or ER membrane are converted to glycoproteins in the ER by the covalent attachment of short, branched oligosaccharide side chains composed of multiple sugars. This process of glycosylation is carried out by glycosylating enzymes present in the ER but not in the cytosol. In the ER, individual sugars are not added one by one to the protein to create an oligosaccharide side chain. Instead, a preformed, branched oligosaccharide containing a total of 14 sugars is attached en bloc to all proteins that carry the appropriate site for glycosylation. The oligosaccharide is originally attached to a specialized lipid, called dolichol, in the ER membrane; it is then transferred to the amino (NH2) group of an asparagine side chain on the protein, immediately after a target asparagine emerges in the ER lumen during protein translocation. The addition takes place in a single enzymatic step that is catalyzed by a membrane-bound enzyme (an oligosaccharyl transferase) that has its active site exposed on the lumenal side of the ER membrane—which explains why cytosolic proteins are not glycosylated in this way. The addition of the 14-sugar oligosaccharide in the ER is only the first step in a series of further modifications before the mature glycoprotein reaches the cell surface. This oligosaccharide processing begins in the ER and continues in the Golgi apparatus.

Which of the following choices reflects the appropriate order of locations through which a protein destined for the plasma membrane travels?

ER---->Golgi---->plasma membrane

The movement of materials from the plasma membrane, through endosomes, and then to lysosomes describes which type of pathway?

Endocytic pathway

The movement of materials from the plasma membrane, through endosomes, and then to lysosomes, describes which type of pathway?

Endocytic pathway

Which organelle is the major site of new membrane synthesis in a cell?

Endoplasmic Reticulum

Proteins enter which organelle as they are being synthesized?

Endoplasmic reticulum

Which organelle sorts ingested molecules and recycles some of them back to the plasma membrane?

Endosome

Which cellular compartment acts as the main sorting station for extracellular cargo molecules taken up by endocytosis?

Endosomes

Which cellular compartment acts as the main sorting station for extracellular cargo molecules taken up by endocytosis?

Endosomes Endosomes can send material back to the plasma membrane, to a different domain of the plasma membrane, or on to lysosomes for degradation.

Which organelle sorts ingested molecules and recycles some of them back to the plasma membrane?

Endosomes- Eukaryotic cells are continually taking up fluid, along with large and small molecules, by the process of endocytosis. Certain specialized cells are also able to internalize large particles and even other cells. The material to be ingested is progressively enclosed by a small portion of the plasma membrane, which first buds inward and then pinches off to form an intracellular endocytic vesicle. The ingested materials, including the membrane components, are delivered to endosomes, from which they can be recycled to the plasma membrane or sent to lysosomes for digestion.

Which cellular compartment acts as the main sorting station for extracellular cargo molecules taken up by endocytosis?

Endosomes- Just as the Golgi network acts as the main sorting station in the outward secretory pathway, the endosomal compartment serves this function in the inward endocytic pathway. The acidic environment of the endosome (pH 5 to 6) plays a crucial part in the sorting process by causing many (but not all) receptors to release their bound cargo. The routes taken by receptors once they have entered an endosome differ according to the type of receptor: (1) most are returned to the same plasma membrane domain from which they came, as is the case for the LDL receptor discussed earlier; (2) some travel to lysosomes, where they are degraded; and (3) some proceed to a different domain of the plasma membrane, thereby transferring their bound cargo molecules across the cell from one extracellular space to another, a process called transcytosis.

15-60 For each of the following sentences, fill in the blanks with the best word or phrase selected from the list below. Not all words or phrases will be used. Each word or phrase should be used only once. Eukaryotic cells are continually taking up materials from the extracellular space by the process of endocytosis. One type of endocytosis is __________________, which uses __________________ proteins to form small vesicles containing fluids and molecules. After these vesicles have pinched off from the plasma membrane, they will fuse with the __________________, where materials that are taken into the vesicle are sorted. A second type of endocytosis is __________________, which is used to take up large vesicles that can contain microorganisms and cellular debris. Macrophages are especially suited for this process, as they extend __________________ (sheetlike projections of their plasma membrane) to surround the invading microorganisms. chaperone Golgi apparatus pseudopods cholesterol mycobacterium rough ER clathrin phagocytosis SNARE endosome pinocytosis transcytosis

Eukaryotic cells are continually taking up materials from the extracellular space by the process of endocytosis. One type of endocytosis is *pinocytosis*, which uses *clathrin* proteins to form small vesicles containing fluids and molecules. After these vesicles have pinched off from the plasma membrane, they will fuse with the *endosome*, where materials that are taken into the vesicle are sorted. A second type of endocytosis is *phagocytosis*, which is used to take up large vesicles that can contain microorganisms and cellular debris. Macrophages are especially suited for this process, as they extend *pseudopods* (sheetlike projections of their plasma membrane) to surround the invading microorganisms.

Which of the following is a difference between exocytic and endocytic pathways?

Exocytic pathways often start with synthesis of proteins, whereas endocytic pathways involve breaking down macromolecules like proteins. Endocytic pathways and exocytic pathways both use transport vesicles to move lipids, membrane components, proteins, and soluble molecules from the outside of the cell to inside of the cell or vice versa. Endocytic pathways bring molecules in from the outside of the cell into an endosome, which can then mature into a lysosome. Endocytosed molecules do not travel to the Golgi in vesicles.

Eukaryotic cells continually ingest bits of their plasma membrane, along with small amounts of extracellular fluid. The lost pieces of membrane are replaced by the process of:

Exocytosis As much membrane is added to the cell surface by exocytosis as is removed by pinocytosis.

Which of the following pathways does NOT deliver materials directly to lysosomes? Autophagy Endocytosis Exocytosis Phagocytosis Pinocytosis

Exocytosis Exocytosis carries material from the Golgi apparatus to the plasma membrane.

Eukaryotic cells continually ingest bits of their plasma membrane, along with small amounts of extracellular fluid. The lost pieces of membrane are replaced by the process of:

Exotycosis

Complementary Rab proteins on transport vesicles and target membranes bind to one another to allow transport vesicles to dock selectively at their appropriate target membranes. t/f

False

The signal sequence that directs a growing polypeptide chain to enter the ER membrane is always removed by a transmembrane signal peptidase. t/f

False

True or False? The signal sequence that directs a growing polypeptide chain to enter the ER membrane is always removed by a transmembrane signal peptidase.

False For some transmembrane proteins, transfer into the endoplasmic reticulum (ER) is triggered by an internal, rather than an N-terminal, signal sequence; these are not removed from the polypeptide, but remain embedded in the membrane.

True or False? Complementary Rab proteins on transport vesicles and target membranes bind to one another to allow transport vesicles to dock selectively at their appropriate target membranes.

False Rab proteins on the surface of each vesicle bind to corresponding tethering proteins on the cytosolic surface of the target membrane.

True or False? Complementary Rab proteins on transport vesicles and target membranes bind to one another to allow transport vesicles to dock selectively at their appropriate target membranes.

False: Rab proteins on the surface of each vesicle bind to corresponding tethering proteins on the cytosolic surface of the target membrane.

Which of the following organelles is not surrounded by a double membrane?

Golgi Apparatus

Which of the following organelles is not surrounded by a double membrane?

Golgi apparatus

Which organelle receives proteins and lipids from the endoplasmic reticulum, modifies them, and then dispatches them to other destinations in the cell?

Golgi apparatus

Which of the following is NOT true of receptor-mediated endocytosis?

In receptor-mediated endocytosis, internalized vesicles fuse with lysosomes, which then mature into endosomes.

In which cellular location would you expect to find ribosomes translating mRNAs that encode ribosomal proteins?

In the cytosol

Most mitochondrial and chloroplast proteins are made in which compartment of the cell?

In the cytosol

In the process of translocating a polypeptide across the membrane of the endoplasmic reticulum, a stop transfer sequence halts the process. What eventually becomes of the stop transfer sequence?

It forms an /alpha-helical membrane-spanning segment of the protein.

In the process of translocating a polypeptide across the membrane of the endoplasmic reticulum, a stop transfer sequence halts the process. What eventually becomes of the stop transfer sequence?

It forms an alpha-helical membrane-spanning segment of the protein

As a polypeptide is being translocated across the membrane of the endoplasmic reticulum, a stop-transfer sequence can halt the process. What eventually becomes of this stop-transfer sequence?

It forms an α-helical membrane-spanning segment of the protein.

In the process of translocating a polypeptide across the membrane of the endoplasmic reticulum, a stop-transfer sequence halts the process. What eventually becomes of the stop-transfer sequence?

It forms an α-helical membrane-spanning segment of the protein.

Which is true of the constitutive exocytosis pathway?

It operates continually in all eukaryotic cells. In all eukaryotic cells, a steady stream of vesicles buds from the trans Golgi network and fuses with the plasma membrane in the process of exocytosis. This constitutive exocytosis pathway supplies the plasma membrane with newly made lipids and proteins, enabling the plasma membrane to expand prior to cell division and refreshing old lipids and proteins in nonproliferating cells. The constitutive pathway also carries soluble proteins to the cell surface for secretion. Some of these proteins remain attached to the cell surface, some are incorporated into the extracellular matrix, and still others diffuse into the extracellular fluid to nourish or signal other cells. Entry into the constitutive pathway does not require a particular signal sequence like those that direct proteins to endosomes or back to the ER. And proteins secreted by the constitutive pathway do not aggregate and are therefore carried automatically to the plasma membrane.

What would happen to a protein that is engineered to contain both a nuclear localization signal and a nuclear export signal?

It would shuttle in and out of the nucleus.

Which statement about receptor-mediated endocytosis of LDL particles is true?

LDL receptors in the plasma membrane associate with clathrin-coated pits. Cholesterol-containing LDLs, which are secreted by the liver, bind to receptors located on the surface of cells. These receptors are recognized by adaptin proteins and then internalized by clathrin-coated vesicles. The vesicles lose their coat and fuse with endosomes, whose acidic internal environment promotes dissociation of LDL from its receptor. The LDL particles are then delivered to lysosomes, where they are degraded to release their cholesterol. Cholesterol then escapes into the cytosol, where it can be used to synthesize new membrane. Meanwhile, back in the endosome, the remaining LDL receptors are removed by transport vesicles that return them to the plasma membrane, where they can be reused to capture and internalize more LDL cholesterol. Some individuals inherit a defective version of the gene encoding the LDL receptor protein; in some cases, the receptors are missing, while in others, they are present but nonfunctional. In either case, because the cells are deficient in taking up LDL, cholesterol accumulates in the blood and predisposes the individuals to develop atherosclerosis.

Which of the following is true of lysosomes?

Lysosomal enzymes are optimally active in the acidic conditions maintained within lysosomes

Which of the following statements about peroxisomes is false?

Most peroxisomal proteins are synthesized in the ER

15-58 For each of the following sentences, choose one of the two options enclosed in square brackets to make a correct statement. New plasma membrane reaches the plasma membrane by the [regulated/constitutive] exocytosis pathway. New plasma membrane proteins reach the plasma membrane by the [regulated/constitutive] exocytosis pathway. Insulin is secreted from pancreatic cells by the [regulated/constitutive] exocytosis pathway. The interior of the trans Golgi network is [acidic/alkaline]. Proteins that are constitutively secreted [aggregate/do not aggregate] in the trans Golgi network.

New plasma membrane reaches the plasma membrane by the *constitutive* exocytosis pathway. New plasma membrane proteins reach the plasma membrane by the *constitutive* exocytosis pathway. Insulin is secreted from pancreatic cells by the *regulated* exocytosis pathway. The interior of the trans Golgi network is *acidic*. Proteins that are constitutively secreted *do not aggregate* in the trans Golgi network.

Which proteins bind to nuclear localization signals on newly synthesized proteins?

Nuclear import receptors

Which proteins bind to nuclear localization signals on newly synthesized proteins?

Nuclear import receptors! Nuclear import receptors interact with proteins bearing nuclear localization signals and with proteins that are part of the nuclear pore.

What is true of protein glycosylation in the ER?

Oligosaccharides are added by an enzyme that has its active site on the lumenal side of the ER membrane.

What is true of protein glycosylation in the ER?

Oligosaccharides are added by an enzyme that has its active site on the lumenal side of the ER membrane. In the ER, individual sugars are not added one by one to the protein to create an oligosaccharide side chain. Instead, a preformed, branched oligosaccharide containing a total of 14 sugars is attached en bloc to all proteins that carry the appropriate site for glycosylation. The oligosaccharide is originally attached to a specialized lipid, called dolichol, in the ER membrane; it is then transferred to the amino (NH2) group of an asparagine side chain on the protein, immediately after a target asparagine emerges in the ER lumen during protein translocation. The addition takes place in a single enzymatic step that is catalyzed by a membrane-bound enzyme (an oligosaccharyl transferase) that has its active site exposed on the lumenal side of the ER membrane—which explains why cytosolic proteins are not glycosylated in this way. A simple sequence of three amino acids, of which the target asparagine is one, defines which sites in a protein receive the oligosaccharide. Oligosaccharide side chains linked to an asparagine NH2 group in a protein are said to be N-linked, and this is by far the most common type of linkage found on glycoproteins.

Which organelle contains enzymes used in a variety of oxidative reactions that break down lipids and destroy toxic molecules?

Peroxisome

Cells ingest large particles by:

Phagocytosis

15-9 For each of the following sentences, fill in the blanks with the best word or phrase selected from the list below. Not all words or phrases will be used. Use each word or phrase only once. Plasma membrane proteins are inserted into the membrane in the __________________. The address information for protein sorting in a eukaryotic cell is contained in the __________________ of the proteins. Proteins enter the nucleus in their __________________ form. Proteins that remain in the cytosol do not contain a __________________. Proteins are transported into the Golgi apparatus via __________________. The proteins transported into the endoplasmic reticulum by __________________ are in their __________________ form. amino acid sequence Golgi apparatus sorting signal endoplasmic reticulum plasma membrane transport vesicles folded protein translocators unfolded

Plasma membrane proteins are inserted into the membrane in the *endoplasmic reticulum*. The address information for protein sorting in a eukaryotic cell is contained in the *amino acid sequence* of the proteins. Proteins enter the nucleus in their *folded* form. Proteins that remain in the cytosol do not contain a *sorting signal*. Proteins are transported into the Golgi apparatus via *transport vesicles*. The proteins transported into the endoplasmic reticulum by *protein translocators* are in their *unfolded* form.

In the unfolded protein response, the accumulation of misfolded proteins in the ER serves as a signal for the cell to do which of the following?

Produce more ER

15-37 For each of the following sentences, fill in the blanks with the best word or phrase selected from the list below. Not all words or phrases will be used. Use each word or phrase only once. Proteins are transported out of a cell via the __________________ or __________________ pathway. Fluids and macromolecules are transported into the cell via the __________________ pathway. All proteins being transported out of the cell pass through the __________________ and the __________________. Transport vesicles link organelles of the __________________ system. The formation of __________________ in the endoplasmic reticulum stabilizes protein structure. carbohydrate Golgi apparatus disulfide bonds hydrogen bonds endocytic ionic bonds endomembrane lysosome endoplasmic reticulum protein endosome secretory exocytic

Proteins are transported out of a cell via the *secretory* or *exocytic* pathway. Fluid and macromolecules are transported into the cell via the *endocytic* pathway. All proteins being transported out of the cell pass through the *endoplasmic reticulum* and the *Golgi apparatus*. Transport vesicles link organelles of the *endomembrane* system. The formation of *disulfide bonds* in the endoplasmic reticulum stabilizes protein structure.

Which best describes a potential pathway followed by a protein destined for secretion? A) Cytosol → ER → Golgi apparatus → transport vesicle → endosome → secretory vesicle → plasma membrane B) Cytosol → ER → secretory vesicle → plasma membrane C) Cytosol → ER → transport vesicle → Golgi apparatus → transport vesicle → plasma membraneCorrect! D) ER → Golgi apparatus → secretory vesicle → plasma membrane E) ER → Golgi apparatus → transport vesicle → endosome → secretory vesicle → plasma membrane

Proteins destined for secretion leave the trans Golgi network either in transport vesicles, which fuse with the plasma membrane immediately, or in secretory vesicles, which await a signal to stimulate membrane fusion.

What is one of the main differences in the behavior of the proteins in a vesicle destined for constitutive secretion, and the proteins in the vesicle destined for regulated secretion?

Proteins in the regulated secretion vesicle tend to aggregate and become highly concentrated in the ionic conditions in the vesicle. Constitutive secretion vesicles contain lipids and proteins that are continuously supplying the plasma membrane with new components. Proteins in regulated secretion vesicles form concentrated aggregates so that when they are released in response to a signal, the levels of the protein can rapidly increase.

Mitochondria and peroxisomes do NOT have what in common?

Proteins need to unfold to enter them

Mitochondria and peroxisomes do NOT have what in common?

Proteins need to unfold to enter them.

Which of the following statements about the endoplasmic reticulum (ER) is false?

Proteins to be delivered to the ER lumen are synthesized on smooth ER.

In a typical human secretory cell, which of the following membranes has the largest surface area?

Rough ER The rough endoplasmic reticulum (ER) can, in some cases, comprise about half of the total membrane present in the cell.

Which proteins play a central role in the fusion of a vesicle with a target membrane?

SNARE proteins

Which proteins play a central role in the fusion of a vesicle with a target membrane?

SNARE proteins SNARE proteins on the vesicle interact with SNARE proteins in the target membrane to help vesicles dock; by winding around each other tightly, SNARE proteins pull the membrane bilayers close enough to allow their lipids to flow together.

Which proteins play a central role in the fusion of a vesicle with a target membrane?

SNAREs

Which proteins play a central role in the fusion of a vesicle with a target membrane?

SNAREs- Rab proteins, tethering proteins, and SNAREs help direct transport vesicles to their target membranes. Interaction between Rab proteins on the vesicle and tethering proteins on the target membrane provide the initial recognition. A v-SNARE on the vesicle then binds to a complementary t-SNARE on the target membrane, ensuring that the transport vesicle docks at an appropriate target membrane. Following vesicle docking, SNARE proteins catalyze the fusion of the vesicle and target membranes. Once appropriately triggered, the tight pairing of v-SNAREs and t-SNAREs draws the two lipid bilayers into close apposition. The force of the SNAREs winding together squeezes out any water molecules that remain trapped between the two membranes, allowing their lipids to flow together to form a continuous bilayer.

In muscle cells, which organelle sequesters Ca2+ from the cytosol?

Smooth ER Because it's so specialized in muscle cells, this form of smooth endoplasmic reticulum (ER) is called the sarcoplasmic reticulum.

15-5 For each of the following sentences, fill in the blanks with the best word or phrase selected from the list below. Not all words or phrases will be used. Use each word or phrase only once. The __________________ makes up about half of the total cell volume of a typical eukaryotic cell. Ingested materials within the cell will pass through a series of compartments called __________________ on their way to the __________________, which contains digestive enzymes and will ultimately degrade the particles and macromolecules taken into the cell and will also degrade worn-out organelles. The __________________ has a cis and trans face and receives proteins and lipids from the __________________, a system of interconnected sacs and tubes of membranes that typically extends throughout the cell. cytosol Golgi apparatus nucleus endoplasmic reticulum lysosome peroxisomes endosomes mitochondria plasma membrane

The *cytosol* makes up about half of the total cell volume of a typical eukaryotic cell. Ingested materials within the cell will pass through a series of compartments called *endosomes* on their way to the *lysosome*, which contains digestive enzymes and will ultimately degrade the particles and macromolecules taken into the cell and will also degrade worn-out organelles. The *Golgi apparatus* has a cis and trans face and receives proteins and lipids from the *endoplasmic reticulum*, a system of interconnected sacs and tubes of membranes that typically extends throughout the cell.

Most mitochondrial and chloroplast proteins are made in...

The Cell Cytosol! Although both organelles contain their own genomes and make some of their own proteins, most mitochondrial and chloroplast proteins are encoded by genes in the nucleus and are imported from the cell cytosol.

Where in the cell are some proteins initially decorated with an oligosaccharide tree on asparagine residues?

The ER The reaction is catalyzed by a membrane-bound oligosaccharyl transferase that faces the endoplasmic reticulum lumen; the oligosaccharide can be further modified in the Golgi apparatus.

Which organelle cannot receive proteins directly from the cytosol?

The Golgi apparatus

Which organelle receives proteins and lipids from the endoplasmic reticulum, modifies them, and then dispatches them to other destinations in the cell?

The Golgi apparatus

Which organelle cannot receive proteins directly from the cytosol?

The Golgi apparatus Proteins are delivered to the Golgi apparatus from the endoplasmic reticulum from other components of the endomembrane system.

When a vesicle fuses with the plasma membrane, which way will the monolayer that was exposed to the interior of the vesicle face?

The cell exterior- Most cell membranes are asymmetric and have distinct "inside" and "outside" faces: the cytosolic monolayer always faces the cytosol, while the noncytosolic monolayer is exposed to either the cell exterior—in the case of the plasma membrane—or the interior space (lumen) of an organelle. This asymmetry is preserved as membranes, in the form of vesicles, which bud from one organelle and fuse with another or with the plasma membrane.

Proteins that lack a sorting signal remain as permanent residents of which part of a eucaryotic cell?

The cytosol

Proteins that lack a sorting signal remain as permanent residents of which part of a eukaryotic cell?

The cytosol

Proteins that lack a sorting signal remain as permanent residents of which part of the eukaryotic cell?

The cytosol

Many proteins are glycosylated in:

The endoplasmic reticulum

Proteins in the cytosol that are destined for other organelles must first enter the:

The endoplasmic reticulum

The outer membrane of the nucleus is continuous with the membrane of which other organelle?

The endoplasmic reticulum

Which organelle is the major site of new membrane synthesis in a cell?

The endoplasmic reticulum

Proteins in the cytosol that are destined for other organelles must first enter...

The endoplasmic reticulum Proteins destined for the Golgi apparatus, endosomes, and lysosomes, as well as proteins destined for the cell surface or for the endoplasmic reticulum (ER) itself, all first enter the ER from the cytosol.

Proteins have to unfold during their transport across the membranes of all but one of the following organelles. Which one?

The nucleus

Proteins have to unfold during their transport across the membranes of all but which of the following organelles?

The nucleus

Ricin is one of the most powerful toxins known. The protein consists of two subunits: the A chain is an enzyme that inhibits translation and the B chain is a lectin that binds to carbohydrates on the cell surface. What is the most likely mechanism by which ricin enters the cell?

The protein is internalized by endocytosis. Ricin is a powerful toxin produced by the castor bean plant. Less than 2 mg injected into the bloodstream will kill an adult human. The protein is a heterodimer composed of an A chain, which inhibits protein translation, and a B chain, which binds to carbohydrates and glycoproteins on the cell surface. Because ricin is a large protein, it must be taken into the cell via endocytosis. The toxin is internalized by both receptor-mediated endocytosis and pinocytosis. It is then transported via endosomes to the Golgi apparatus and from there into the ER. It escapes from the ER to the cytosol by partially unfolding in the ER lumen, triggering its release to the cytosol for degradation. Once in the cytosol, the A chain refolds and exerts its toxic influence on protein synthesis.

If a signal sequence is removed from an ER protein:

The protein remains in the cytosol

You have attached green fluorescent protein (GFP) to the carboxy terminal end of a secreted yeast protein. You express this protein in normal yeast cells, secretory mutant A cells, and secretory mutant B cells (see image). Using fluorescent microscopy, you observe the expected results, with protein secretion in normal cells, ER accumulation in mutant A, and Golgi apparatus accumulation in mutant B. You also express the GFP-fusion protein in double-mutant yeast cells containing mutations in both the gene underlying mutant A and the gene underlying mutant B. What is the correct location and explanation for where the GFP-fusion protein will accumulate in these A and B double-mutant yeast cells?

The protein will accumulate in the ER because that is an earlier step in the secretory pathway. Yeast cells containing mutations in the genes underlying both secretory mutant A and secretory mutant B will look like secretory A mutants, with secreted proteins accumulating in the ER. This is because secreted proteins pass through the ER before moving on to the Golgi apparatus. If a mutation traps proteins in the ER, those proteins cannot proceed to the next step.

Investigators have engineered a gene that encodes a protein bearing an ER signal sequence followed by a nuclear localization signal. What would be the likely fate of that protein?

The protein will be recognized by an SRP and enter the ER.Correct!

A transmembrane protein is inserted into a cell membrane as shown. If an ER signal sequence were added to its N-terminus, how would that change the way the protein is arranged? -It would not change the arrangement of the protein. -The protein would be arranged as depicted in A. -The protein would be arranged as depicted in B. -The protein would be arranged as depicted in C. -The protein would be arranged as depicted in D. https://s3.amazonaws.com/RM_Images/20160117001300562_raw.jpg

The protein would be arranged as depicted in C.

Which of the following statements is NOT true of mitochondrial proteins that are synthesized in the cytosol?

The proteins are transported across the mitochondrial membranes while being synthesized

Which of the following statements is NOT true of mitochondrial proteins that are synthesized in the cytosol?

The proteins must be transported across the mitochondrial membranes while being synthesized.

In the endocrine cells of the adrenal gland, steroid hormones are synthesized in....?

The smooth ER Most lipids are synthesized in the smooth endoplasmic reticulum (ER).

In a classic experiment designed to study nuclear transport, investigators added a dye molecule to the subunits of a protein called nucleoplasmin, which is involved in chromatin assembly. They then injected the intact protein or combinations of its subunits into the cytoplasm of a frog oocyte or into its nucleus. The results of the experiment are shown in the diagram, where red indicates the location of the labeled protein. Based on these results, which part of the nucleoplasmin protein bears a nuclear localization signal? -Both the head and the tail -Neither the head nor the tail -No conclusion about the nuclear localization signal can be drawn from the data. -The head -The tail https://s3.amazonaws.com/RM_Images/20160117001258605_raw.jpg

The tail

In the unfolded protein response, the accumulation of misfolded proteins in the ER serves as a signal for the cell to do which of the following? A) Destroy the misfolded proteins B) Export the misfolded proteins to the cytosol C) Glycosylate the misfolded protein D) Produce more ER E) Send the misfolded proteins to the Golgi apparatus for destruction

The unfolded protein response also slows protein synthesis and prompts the cell to produce more chaperones and other quality-control proteins.

What distinguishes proteins destined for regulated secretion?

Their surface properties allow them to form aggregates that are packaged into secretory vesicles. Proteins destined for regulated secretion have special surface properties that cause them to aggregate with one another under the ionic conditions (acidic pH and high Ca2+) that prevail in the trans Golgi network. The aggregated proteins are packaged into secretory vesicles, which pinch off from the network and await a signal instructing them to fuse with the plasma membrane. Proteins secreted by the constitutive pathway, on the other hand, do not aggregate and are therefore carried automatically to the plasma membrane. Selective aggregation has another function: it allows secretory proteins to be packaged into secretory vesicles at concentrations much higher than the concentration of the unaggregated protein in the Golgi lumen. This increase in concentration can reach 200-fold, enabling secretory cells to release large amounts of the protein promptly when triggered to do so.

Proteins entering the cisGolgi network can do which of the following?

They can either move onward through the Golgi stack or be returned to the ER.

Which of the following is NOT a role for the oligosaccharides on glycosylated proteins?

They can provide a source of energy for the cell.

What distinguishes proteins destined for regulated secretion?

They have special surface properties that cause them to form aggregates that are packaged into secretory vesicles

What happens to proteins with no signal sequence that are made in the cytosol?

They remain in the cytosol.

The drug vinblastine disrupts microtubule polymerization. How would adding vinblastine to a cell affect the constitutive secretory pathway?

Transport vesicles will not be brought to either the Golgi apparatus or the plasma membrane.

The drug vinblastine disrupts microtubule polymerization. How would adding vinblastine to a cell affect the constitutive secretory pathway?

Transport vesicles will not be brought to either the Golgi apparatus or the plasma membrane. Microtubules are used as tracks for the transport of organelles and vesicles around the cell. Transport vesicles that bud off the endoplasmic reticulum are transported along microtubules to the Golgi apparatus. From the Golgi, vesicles are transported along microtubules to the plasma membrane, where the vesicle contents are released out of the cell.

Membrane-bound ribosomes and free ribosomes are structurally and functionally identical; they differ only in the proteins they are making at a particular time. t/f

True

The constitutive exocytosis pathway of the Golgi apparatus operates continually in all eucaryotic cells

True

Vesicle budding is driven by the assembly of a protein coat.

True

Vesicle budding is driven by the assembly of a protein coat. t/f

True

True or False? The constitutive exocytosis pathway of the Golgi apparatus operates continually in all eukaryotic cells.

True Cells specialized for secretion operate an additional, regulated exocytosis pathway.

In which process do Rab proteins function?

Vesicle tethering- Rab proteins are a family of small GTPases that are specific for each type of organelle and vesicle. Rab proteins on vesicles are recognized by tethering proteins on the target membrane and help capture and tether the vesicle for later docking and fusion.

Scientists have modified a clathrin molecule so that it still assembles but forms an open-ended lattice instead of a closed spherical cage. How would this clathrin molecule affect endocytosis in cells?

Vesicles cannot form properly without a clathrin cage, thus inhibiting endocytosis. Clathrin proteins are responsible for forming the closed spherical cage around the forming vesicle. This cage helps shape and build the vesicle. When clathrin is mutated, it will not form the proper cage and instead will make a flat sheet as an open-ended lattice. This flat sheet will not form the spherical vesicle and so endocytosis will not occur. Any processes that require clathrin to form the vesicle in the cell will be blocked. Other transport mechanisms that do not rely upon clathrin will still function normally.

Which molecule is displaced when a vesicle and its target membrane fuse?

Water

Which molecule is displaced when a vesicle and its target membrane fuse?

Water must be displaced from the hydrophilic surfaces of the membranes to allow their lipids to flow together and intermix.

Imagine that 32 million histone octamers are required to package the DNA in a human cell. How many histone proteins must pass through each nuclear pore complex per second in a cell that has 3000 nuclear pores and divides once per day? A) 1 Histone per second B) 2 Histones per second C) 3000 Histones per second D) About 0.1 histone per second (or 1 histone every 10 seconds) E) About 0.5 histone per second (or 1 histone every 2 seconds)

When the cell divides, its daughters will each receive half of the histones packaged in the parent DNA. Thus the cell will need to produce and transport 32 x 106 histone octamers. Each of these octamers is comprised of eight histone proteins. So 256 million histones will need to pass through the 3000 nuclear pores per day. That would be about 85,333 histones through each pore per day. There are 86,400 seconds in a day, which means that each nuclear pore complex must transport about 1 histone molecule per second, on average, over the course of a day.

An individual transport vesicle _______________.

Will fuse with only one type of membrane.

15-12 Proteins that are fully translated in the cytosol and lack a sorting signal will end up in ____. (a) the cytosol. (b) the mitochondria. (c) the interior of the nucleus. (d) the nuclear membrane.

15-14 What is the role of the nuclear localization sequence in a nuclear protein? (a) It is bound by cytoplasmic proteins that direct the nuclear protein to the nuclear pore. (b) It is a hydrophobic sequence that enables the protein to enter the nuclear membranes. (c) It aids in protein unfolding so that the protein can thread through nuclear pores. (d) It prevents the protein from diffusing out of the nucleus through nuclear pores.

15-16 A large protein that passes through the nuclear pore must have an appropriate _________. (a) sorting sequence, which typically contains the positively charged amino acids lysine and arginine. (b) sorting sequence, which typically contains the hydrophobic amino acids leucine and isoleucine. (c) sequence to interact with the nuclear fibrils. (d) Ran-interacting protein domain.

15-2 Which of the following statements about membrane-enclosed organelles is true? (a) In a typical cell, the area of the endoplasmic reticulum membrane far exceeds the area of plasma membrane. (b) The nucleus is the only organelle that is surrounded by a double membrane. (c) Other than the nucleus, most organelles are small and thus, in a typical cell, only about 10% of a cell's volume is occupied by membraneenclosed organelles; the other 90% of the cell volume is the cytosol. (d) The nucleus is the only organelle that contains DNA.

15-21 Which of the following statements about peroxisomes is false? (a) Most peroxisomal proteins are synthesized in the ER. (b) Peroxisomes synthesize phospholipids for the myelin sheath. (c) Peroxisomes produce hydrogen peroxide. (d) Vesicles that bud from the ER can mature into peroxisomes.

15-38 Which of the following statements about vesicle budding from the Golgi is false? (a) Clathrin molecules are important for binding to and selecting cargoes for transport. (b) Adaptins interact with clathrin. (c) Once vesicle budding occurs, clathrin molecules are released from the vesicle. (d) Clathrin molecules act at the cytosolic surface of the Golgi membrane.

15-56 Which of the following statements about secretion is true? (a) The membrane of a secretory vesicle will fuse with the plasma membrane when it discharges its contents to the cell's exterior. (b) Vesicles for regulated exocytosis will not bud off the trans Golgi network until the appropriate signal has been received by the cell. (c) The signal sequences of proteins destined for constitutive exocytosis ensure their packaging into the correct vesicles. (d) Proteins destined for constitutive exocytosis aggregate as a result of the acidic pH of the trans Golgi network.

15-61 Which of the following statements about phagocytic cells in animals is false? (a) Phagocytic cells are important in the gut to take up large particles of food. (b) Phagocytic cells scavenge dead and damaged cells and cell debris. (c) Phagocytic cells can engulf invading microorganisms and deliver them to their lysosomes for destruction. (d) Phagocytic cells extend pseudopods that surround the material to be ingested.

Lysosomal enzymes are directed to lysosomes by which of the following?

a phosphorylated sugar group- The specialized digestive enzymes and membrane proteins of the lysosome are synthesized in the ER and transported through the Golgi apparatus to the trans Golgi network. While in the ER and the cis Golgi network, the enzymes are tagged with a specific phosphorylated sugar group (mannose 6-phosphate) so that when they arrive in the trans Golgi network they can be recognized by an appropriate receptor, the mannose 6-phosphate receptor. This tagging and recognition permits the lysosomal enzymes to be sorted and packaged into transport vesicles, which bud off and deliver their contents to lysosomes via endosomes.

Which of these strategies do prokaryotic cells use to isolate and organize their chemical reactions?

aggregating proteins into multicomponent complexes that form biochemical subcompartments with distinct functions

Vesicle budding is driven by which of the following?

assembly of a protein coat- Vesicle budding is driven by the assembly of a protein coat on the cytosolic surface of a membrane. The protein coat serves at least two functions: it helps shape the membrane into a bud and it captures molecules for onward transport. After budding from its parent organelle, the vesicle sheds its coat, allowing its membrane to interact directly with the membrane to which it will fuse.

15-1 Which of the following statements about the endoplasmic reticulum (ER) is false? (a) The ER is the major site for new membrane synthesis in the cell. (b) Proteins to be delivered to the ER lumen are synthesized on smooth ER. (c) Steroid hormones are synthesized on the smooth ER. (d) The ER membrane is contiguous with the outer nuclear membrane.

15-20 Which of the following statements about transport into mitochondria and chloroplasts is false? (a) The signal sequence on proteins destined for these organelles is recognized by a receptor protein in the outer membrane of these organelles. (b) After a protein moves through the protein translocator in the outer membrane of these organelles, the protein diffuses in the lumen until it encounters a protein translocator in the inner membrane. (c) Proteins that are transported into these organelles are unfolded as they are being transported. (d) Signal peptidase will remove the signal sequence once the protein has been imported into these organelles.

15-39 Molecules to be packaged into vesicles for transport are selected by ________. (a) clathrin. (b) adaptins. (c) dynamin. (d) SNAREs.

15-42 An individual transport vesicle ________. (a) contains only one type of protein in its lumen. (b) will fuse with only one type of membrane. (c) is endocytic if it is traveling toward the plasma membrane. (d) is enclosed by a membrane with the same lipid and protein composition as the membrane of the donor organelle.

15-51 Which of the following statements about the protein quality control system in the ER is false? (a) Chaperone proteins help misfolded proteins fold properly. (b) Proteins that are misfolded are degraded in the ER lumen. (c) Protein complexes are checked for proper assembly before they can exit the ER. (d) A chaperone protein will bind to a misfolded protein to retain it in the ER.

15-64 You are working in a biotech company that has discovered a small-molecule drug called H5434. H5434 binds to LDL receptors when they are bound to cholesterol. H5434 binding does not alter the conformation of the LDL receptor's intracellular domain. Interestingly, in vitro experiments demonstrate that addition of H5434 increases the affinity of LDL for cholesterol and prevents cholesterol from dissociating from the LDL receptor even in acidic conditions. Which of the following is a reasonable prediction of what may happen when you add H5434 to cells? (a) Cytosolic cholesterol levels will remain unchanged relative to normal cells. (b) Cytosolic cholesterol levels will decrease relative to normal cells. (c) The LDL receptor will remain on the plasma membrane. (d) The uncoating of vesicles will not occur.

15-8 Which of the following statements is true? (a) Lysosomes are believed to have originated from the engulfment of bacteria specialized for digestion. (b) The nuclear membrane is thought to have arisen from the plasma membrane invaginating around the DNA. (c) Because bacteria do not have mitochondria, they cannot produce ATP in a membrane-dependent fashion. (d) Chloroplasts and mitochondria share their DNA.

How do the interiors of the ER, Golgi apparatus, endosomes, and lysosomes communicate with each other?

by small vesicles that bud off of one organelle and fuse with another

How do the interiors of the ER, Golgi apparatus, endosomes, and lysosomes communicate with each other?

by small vesicles that bud off of one organelle and fuse with another- Transport from the ER to the Golgi apparatus—and from the Golgi apparatus to other compartments of the endomembrane system—is carried out by the continual budding and fusion of transport vesicles. This vesicular transport extends outward from the ER to the plasma membrane, where it allows proteins and other molecules to be secreted by exocytosis, and it reaches inward from the plasma membrane to lysosomes, allowing extracellular molecules to be imported by endocytosis. Together, these pathways thus provide routes of communication between the individual organelles within the endomembrane system and between the interior of the cell and its surroundings.

15-13 Signal sequences that direct proteins to the correct compartment are _________. (a) added to proteins through post-translational modification. (b) added to a protein by a protein translocator. (c) encoded in the amino acid sequence and sufficient for targeting a protein to its correct destination. (d) always removed once a protein is at the correct destination.

15-15 Which of the following statements about nuclear transport is true? (a) mRNAs and proteins transit the nucleus through different types of nuclear pores. (b) Nuclear import receptors bind to proteins in the cytosol and bring the proteins to the nuclear pores, where the proteins are released from the receptors into the pores for transit into the nucleus. (c) Nuclear pores have water-filled passages that small, water-soluble molecules can pass through in a nonselective fashion. (d) Nuclear pores are made up of many copies of a single protein.

15-18 Your friend works in a biotechnology company and has discovered a drug that blocks the ability of Ran to exchange GDP for GTP. What is the most likely effect of this drug on nuclear transport? (a) Nuclear transport receptors would be unable to bind cargo. (b) Nuclear transport receptors would be unable to enter the nucleus. (c) Nuclear transport receptors would be unable to release their cargo in the nucleus. (d) Nuclear transport receptors would interact irreversibly with the nuclear pore fibrils.

15-19 Which of the following statements is true? (a) The signal sequences on mitochondrial proteins are usually at the Cterminus. (b) Most mitochondrial proteins are not imported from the cytosol but are synthesized inside the mitochondria. (c) Chaperone proteins in the mitochondria facilitate the movement of proteins across the outer and inner mitochondrial membranes. (d) Mitochondrial proteins cross the membrane in their native, folded state.

15-23 Most proteins destined to enter the endoplasmic reticulum _________. (a) are transported across the membrane after their synthesis is complete. (b) are synthesized on free ribosomes in the cytosol. (c) begin to cross the membrane while still being synthesized. (d) remain within the endoplasmic reticulum.

15-25 In which cellular location would you expect to find ribosomes translating mRNAs that encode ribosomal proteins? (a) the nucleus (b) on the rough ER (c) in the cytosol (d) in the lumen of the ER

15-29 Which of the following statements is true? (a) Proteins destined for the ER are translated by a special pool of ribosomes whose subunits are always associated with the outer ER membrane. (b) Proteins destined for the ER translocate their associated mRNAs into the ER lumen where they are translated. (c) Proteins destined for the ER are translated by cytosolic ribosomes and are targeted to the ER when a signal sequence emerges during translation. (d) Proteins destined for the ER are translated by a pool of cytosolic ribosomes that contain ER-targeting sequences that interact with ERassociated protein translocators.

15-41 Your friend has just joined a lab that studies vesicle budding from the Golgi and has been given a cell line that does not form mature vesicles. He wants to start designing some experiments but wasn't listening carefully when he was told about the molecular defect of this cell line. He's too embarrassed to ask and comes to you for help. He does recall that this cell line forms coated pits but vesicle budding and the removal of coat proteins don't happen. Which of the following proteins might be lacking in this cell line? (a) clathrin (b) Rab (c) dynamin (d) adaptin

15-43 Which of the following statements about vesicular membrane fusion is false? (a) Membrane fusion does not always immediately follow vesicle docking. (b) The hydrophilic surfaces of membranes have water molecules associated with them that must be displaced before vesicle fusion can occur. (c) The GTP hydrolysis of the Rab proteins provides the energy for membrane fusion. (d) The interactions of the v-SNAREs and the t-SNAREs pull the vesicle membrane and the target organelle membrane together so that their lipids can intermix

15-52 Which of the following statements about the unfolded protein response (UPR) is false? (a) Activation of the UPR results in the production of more ER membrane. (b) Activation of the UPR results in the production of more chaperone proteins. (c) Activation of the UPR occurs when receptors in the cytoplasm sense misfolded proteins. (d) Activation of the UPR results in the cytoplasmic activation of gene regulatory proteins.

15-54 Vesicles from the ER enter the Golgi at the ______. (a) medial cisternae. (b) trans Golgi network. (c) cis Golgi network. (d) trans cisternae.

15-6 Which of the following organelles is not part of the endomembrane system? (a) Golgi apparatus (b) the nucleus (c) mitochondria (d) lysosomes

The low pH inside endosomes:

causes internalized receptors to release their cargo.

The low pH inside endosomes leads to what outcome?

causing many internalized receptors to release their cargo- Low-density lipoproteins (LDLs) bind to LDL receptors on the cell surface and are internalized in clathrin-coated vesicles. The vesicles lose their coat and then fuse with endosomes. In the acidic environment of the endosome, LDL dissociates from its receptors. The LDL ends up in lysosomes, where it is degraded to release free cholesterol; the LDL receptors—usually empty—are returned to the plasma membrane via transport vesicles to be used again. An LDL receptor typically makes one round trip into the cell and back every 10 minutes, making a total of several hundred trips over its 20-hour life-span.

Vesicles from the ER enter the Golgi at the:

cis Golgi Network

Phagocytosis is a process by which cells do which of the following?

consume large particles, such as microbes and cell debris

Most mitochondrial and chloroplast proteins are made within which part of the cell?

cytosol

15-10 Where are proteins in the chloroplast synthesized? (a) in the cytosol (b) in the chloroplast (c) on the endoplasmic reticulum (d) in both the cytosol and the chloroplast

15-11 Proteins that are fully translated in the cytosol do not end up in _______. (a) the cytosol. (b) the mitochondria. (c) the interior of the nucleus. (d) transport vesicles.

15-24 After isolating the rough endoplasmic reticulum from the rest of the cytoplasm, you purify the RNAs attached to it. Which of the following proteins do you expect the RNA from the rough endoplasmic reticulum to encode? (a) soluble secreted proteins (b) ER membrane proteins (c) plasma membrane proteins (d) all of the above

15-27 You are interested in Fuzzy, a soluble protein that functions within the ER lumen. Given that information, which of the following statements must be true? (a) Fuzzy has a C-terminal signal sequence that binds to SRP. (b) Only one ribosome can be bound to the mRNA encoding Fuzzy during translation. (c) Fuzzy must contain a hydrophobic stop-transfer sequence. (d) Once the signal sequence from Fuzzy has been cleaved, the signal peptide will be ejected into the ER membrane and degraded.

15-28 Which of the following statements about a protein in the lumen of the ER is false? (a) A protein in the lumen of the ER is synthesized by ribosomes on the ER membrane. (b) Some of the proteins in the lumen of the ER can end up in the extracellular space. (c) Some of the proteins in the lumen of the ER can end up in the lumen of an organelle in the endomembrane system. (d) Some of the proteins in the lumen of the ER can end up in the plasma membrane.

15-36 Which of the following choices reflects the appropriate order of locations through which a protein destined for the plasma membrane travels? (a) lysosome -> endosome -> plasma membrane (b) ER -> lysosome -> plasma membrane (c) Golgi -> lysosome -> plasma membrane (d) ER -> Golgi -> plasma membrane

15-40 Which of the following protein families are not involved in directing transport vesicles to the target membrane? (a) SNAREs (b) Rabs (c) tethering proteins (d) adaptins

15-45 N-linked oligosaccharides on secreted glycoproteins are attached to ________. (a) nitrogen atoms in the polypeptide backbone. (b) the serine or threonine in the sequence Asn-X-Ser/Thr. (c) the N-terminus of the protein. (d) the asparagine in the sequence Asn-X-Ser/Thr.

15-47 Which of the following statements about disulfide bond formation is false? (a) Disulfide bonds do not form under reducing environments. (b) Disulfide bonding occurs by the oxidation of pairs of cysteine side chains on the protein. (c) Disulfide bonding stabilizes the structure of proteins. (d) Disulfide bonds form spontaneously within the ER because the lumen of the ER is oxidizing.

15-49 Different glycoproteins can have a diverse array of oligosaccharides. Which of the statements below about this diversity is true? (a) Extensive modification of oligosaccharides occurs in the extracellular space. (b) Different oligosaccharides are covalently linked to proteins in the ER and the Golgi. (c) A diversity of oligosaccharyl transferases recognizes specific protein sequences, resulting in the linkage of a variety of oligosaccharides to proteins. (d) Oligosaccharide diversity comes from modifications that occur in the ER and the Golgi of the 14-sugar oligosaccharide added to the protein in the ER.

The movement of materials from the plasma membrane, through endosomes, and then to lysosomes describes which type of pathway?

endocytic pathway

The outer membrane of the nucleus is continuous with the membrane of which other organelle?

endoplasmic reticulum

You complete a pulse-chase experiment to monitor the secretion of a protein from the cell. Which of the following correctly lists the order of locations of the protein during the chase period?

endoplasmic reticulum → transport vesicle → Golgi → transport vesicle → secreted

During a pulse-chase experiment with secreted proteins, the proteins are synthesized for a short "pulse" time with radioactive or fluorescent amino acids to label the proteins. During the "chase" period, unlabeled amino acids are added, so any additional proteins synthesized are not labeled. The labeled proteins can then be monitored over time. You complete a pulse-chase experiment to monitor the secretion of a protein from the cell. Which of the following correctly lists the order of locations of the protein during the chase period?

endoplasmic reticulum → transport vesicle → Golgi → transport vesicle → secreted Secreted proteins are synthesized at the rough endoplasmic reticulum, where ribosomes can be found. During constitutive secretion, proteins accumulate at random locations in the endoplasmic reticulum membrane network and are packaged into transport vesicles. The vesicles fuse into transport intermediates that travel along microtubule tracks to the Golgi apparatus. From the Golgi, proteins are again packaged into new transport vesicles for travel along additional microtubules to the plasma membrane, where they are secreted to the outside of the cell.

Which cellular compartment acts as the main sorting station for extracellular cargo molecules taken up by endocytosis?

endosomes

Botulism is a potentially fatal foodborne disease caused by the bacterium Clostridium botulinum. C. botulinum produces different toxins, several of which are proteases that cleave neuronal SNARE proteins. What normal process is blocked by cleavage and inhibition of SNARE proteins?

fusion of vesicles with target membranes- Many different proteins help vesicles dock and fuse with the correct target membrane (see below). Rabs interact with tethering proteins during the docking phase, bringing v-SNAREs and t-SNAREs into close proximity. The SNAREs then intertwine, aiding vesicle fusion with the target membrane. In neurons, the t-SNARE synaptosomal nerve-associated protein 25 (SNAP-25) is important for the fusion of neurotransmitter-containing vesicles with the plasma membrane.

The ER signal sequence on a growing polypeptide chain is recognized by a signal-recognition particle (SRP) in the cytosol. This interaction:

guides the ribosome and its polypeptide to the ER membrane

The ER signal sequence on a growing polypeptide chain is recognized by a signal recognition particle (SRP) in the cytosol. This interaction:

guides the ribosome and its polypeptide to the ER membrane.

The ER signal sequence on a growing polypeptide chain is recognized by a signal-recognition particle (SRP) in the cytosol. This interaction:

guides the ribosome and its polypeptide to the ER membrane.

Insulin is synthesized in the form of a precursor protein that requires cleavage of two different peptide segments before the mature protein is secreted from β cells in the pancreas. The first peptide is removed when the protein enters the lumen of the ER. To find out when the second cleavage event takes place, investigators prepare a pair of antibodies: one recognizes the pro-insulin precursor, the other the mature insulin protein. They tag the antibody that binds to the precursor protein with a red fluorescent marker; the antibody that binds to mature insulin is tagged with a green fluorescent marker. When both markers are present, the sample fluoresces yellow. The investigators then incubate an isolated β cell with both antibodies at the same time and monitor the fluorescence in its various membrane-bound compartments. The data are shown in the table below. (Google this question to get the table!) Based on these observations, where is the second peptide removed from the pro-insulin precursor protein?

immature secretory vesicles- The fluorescence associated with the ER and Golgi apparatus is red, which suggests that only the precursor protein is present and no appreciable cleavage has yet taken place. However, the fluorescence in mature secretory vesicles is green, indicating that by the time the protein is aggregated for secretion, cleavage is complete. The fluorescence in immature secretory vesicles is yellow, which means that both the mature protein and its precursor are present at once. (For fluorescent markers, the combination of red and green produces yellow.) Therefore, cleavage must be taking place in these immature vesicles. The absence of fluorescence in lysosomes indicates that the protein is not being routed to this compartment for processing or degradation. And the lack of fluorescence in the nucleus and mitochondria should not come as a surprise.

Most mitochondrial and chloroplast proteins are made:

in the cell cytosol

Which of the following components of receptor-mediated endocytosis of LDL is incorrectly matched with its function?

lysosome: releases LDL from the receptor When LDL binds to the specific LDL receptor on the plasma membrane, an adaptin binds to the receptor on the cytosol side. Adaptin recruits clathrin proteins, which form the coated vesicle. After vesicle formation, the clathrin is released and the naked transport vesicle is transported to the endosome. The endosome is the specific compartment where the LDL is released from the receptor as a result of the compartment's low pH. The empty LDL receptors are then recycled back to the plasma membrane, where the process can repeat. The LDL particles travel from the endosome to the lysosome where the LDL is broken down, finally releasing cholesterol to the cytosol for use in the cell.

Phagocytic cells include:

macrophages and neutrophils.

Which proteins bind to nuclear localization signals on newly synthesized proteins?

nuclear import receptors

Proteins with no signal sequence that are made in the cytosol:

remain in the cytosol.

If a phospholipid is located in the outer layer of the bilayer in a vesicle, where will it end up when the vesicle fuses with the plasma membrane?

the cytosolic face of the bilayer- When a vesicle fuses with the plasma membrane, the lipids on the outside of the vesicle will end up in the phospholipid layer on the inside (cytosolic face) of the plasma membrane. Lipids on the inside of the vesicle membrane will face the extracellular fluid in the plasma membrane.

Proteins in the cytosol that are destined for other organelles must first enter:

the endoplasmic reticulum

All of the carbohydrates in the plasma membrane face the cell exterior. Which direction do the carbohydrates on internal cell membranes face?

the lumen of the vesicle or organelle- The lipids that show the most dramatically lopsided distribution in cell membranes are the glycolipids, which are located mainly in the plasma membrane and only in the noncytosolic half of the bilayer. The sugar groups of these membrane lipids face the cell exterior, where they form part of a continuous coat of carbohydrate (called the glycocalyx), which surrounds and protects animal cells. Glycolipid molecules acquire their sugar groups in the Golgi apparatus, where the enzymes that engineer this chemical modification are confined. These enzymes are oriented such that sugars are added only to lipid molecules in the noncytosolic half of the bilayer. Once a glycolipid molecule has been created in this way, it remains trapped in this monolayer, as there are no flippases that transfer glycolipids to the cytosolic side. Thus, when a glycolipid molecule is finally delivered to the plasma membrane, it displays its sugars to the exterior of the cell. On internal cell membranes, the noncytosolic half of the lipid bilayer faces the lumen of the vesicle or organelle. For an internal cell membrane, half of the bilayer that faces toward the plasma membrane would face the cytosol, as would the part of the membrane that faces the direction of the cell exterior.

Proteins have to unfold during their transport across the membranes of all but which of the following organelles? Chloroplasts Mitochondria The endoplasmic reticulum The nucleus

the nucleus Nuclear proteins are directed to nuclear pores by nuclear import receptors.

If a signal sequence is removed from an ER protein:

the protein will remain in the cytosol.

Through which of the following do proteins travel from one cisterna to the next in the Golgi apparatus?

transport vesicles that bud from one cisterna and fuse with the next

How are newly made lipids supplied to the plasma membrane?

via the constitutive pathway of exocytosis

How are newly made lipids supplied to the plasma membrane?

via the constitutive pathway of exocytosis- In all eukaryotic cells, a steady stream of vesicles buds from the trans Golgi network and fuses with the plasma membrane in the process of exocytosis. This constitutive exocytosis pathway supplies the plasma membrane with newly made lipids and proteins, enabling the plasma membrane to expand prior to cell division and refreshing old lipids and proteins in nonproliferating cells. The regulated exocytosis pathway also adds phospholipids to the plasma membrane; however, this pathway only operates in cells specialized for secretion.

Which molecule is displaced when a vesicle and its target membrane fuse?

water- Whereas docking requires only that the two membranes come close enough for the SNAREs protruding from the two lipid bilayers to interact, fusion requires a much closer approach: the two bilayers must come within 1.5 nanometers (nm) of each other so that their lipids can intermix. For this close approach, water must be displaced from the hydrophilic surfaces of the membranes—a process that is energetically highly unfavorable and thus prevents membranes from fusing randomly. All membrane fusions in cells must therefore be catalyzed by specialized proteins that assemble to form a fusion complex that provides the means to cross this energy barrier. For vesicle fusion, the SNARE proteins themselves catalyze the process: when fusion is triggered, the v-SNAREs and t-SNAREs wrap around each other tightly, thereby acting like a winch that pulls the two lipid bilayers into close proximity.

Voir tous les ensembles d'études

COMBINED Cell & Molec Chapter 15, Molecular Biology Chapter 15 Practice Questions, SmartWork5 Chapter 10, Quiz 15.2: Protein Sorting + Vesicular Transport, Chapter 15, Chapter 15 Quiz (Cell Biology), Chapter 15: Endomembrane system, chegg questions

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