Chapter 15 - Intracellular Compartments and Protein Transport

Question 1

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

• endoplasmic reticulum
• endosome
• peroxisome
• mitochondrion
• Golgi apparatus

Answer 1

Endoplasmic reticulum

(The outer nuclear membrane is continuous with the membrane of the endoplasmic reticulum (ER), a system of interconnected membranous sacs and tubes that often extends throughout most of the cell.)

Question 2

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

• peroxisome
• nucleus
• mitochondrion
• Golgi apparatus
• endosome

Answer 2

Golgi apparatus

(Soluble proteins and pieces of membrane enter the Golgi network via transport vesicles derived from the ER. Proteins exit from the Golgi network in transport vesicles destined for either the cell surface or another organelle of the endomembrane system.)

Question 3

In a eukaryotic cell, the major membrane-enclosed organelles are surrounded by the _____, which is enclosed by the __________.

Answer 3

cytosol; plasma membrane

Question 4

The ______, which is surrounded by a double membrane perforated by nuclear pores, is generally the most prominent organelle in the cell.

Answer 4

nucleus

Question 5

The outer nuclear membrane is continuous with the __________, a system of interconnected membranes that often extends throughout most of the cell

Answer 5

Endoplasmic reticulum (ER)

Question 6

Large areas of the ER have ______ attached to its cytosolic surface via the proteins they are synthesizing, which are inserted into the ER membrane.

Answer 6

ribosomes

Question 7

_______ that are synthesizing cytosolic proteins remain unattached from the ER, floating free in the cytosol.

Answer 7

Ribosomes

Question 8

The ________, which looks like a flattened stack of membranous discs, is usually situated near the nucleus (but is not continuous with the nuclear envelope, as is the ER).

Answer 8

Golgi apparatus

Question 9

Small organelles called _______, often located near the plasma membrane, sort ingested material, some of which is passed on to spherical _______—either by fusion with preexisting lysosomes or by a maturation process that converts the endosome into a classical lysosome

Answer 9

endosomes; lysosomes

Question 10

_______ contain enzymes that produce hydrogen peroxide.

Answer 10

Peroxisomes

Question 11

________ are surrounded by a double membrane, with an inner membrane that is highly folded.

Answer 11

Mitochondria

Question 12

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

• None; these strategies are used only by eukaryotic cells.
• confining the proteins required for different metabolic processes within the plasma membrane
• confining proteins required for different metabolic processes within different membrane-enclosed compartments
• aggregating proteins into multicomponent complexes that form biochemical subcompartments with distinct functions
• None; prokaryotes do not regulate their metabolic processes.

Answer 12

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

(The formation of these subcompartments can involve the participation of scaffold proteins that bring together components involved in a particular reaction sequence, such as DNA synthesis.)

Question 13

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

• They do not communicate with one another.
• by excreting hormones and other small signaling molecules
• by fusing with one another
• by open pores that allow ions to exit and enter the organelles
• by small vesicles that bud off of one organelle and fuse with another

Answer 13

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

(In this way, transport vesicles carry soluble cargo proteins, as well as the proteins and lipids that are part of the vesicle membrane, from one organelle to another.)

Question 14

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

• endoplasmic reticulum
• peroxisome
• Golgi apparatus
• cytosol
• mitochondrion or chloroplast itself

Answer 14

Cytosol

(A few mitochondrial and chloroplast proteins are synthesized inside these organelles; however, most are made in the cytosol and subsequently imported.)

Question 15

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

• They are taken up by lysosomes.
• They are secreted.
• They remain in the cytosol.
• They are degraded by proteases.
• They are returned to their organelle of origin.

Answer 15

They remain in the cytosol.

(The fate of any protein molecule synthesized in the cytosol depends on its amino acid sequence, which can contain a sorting signal that directs the protein to the organelle in which it is required. Proteins that lack such signals remain as permanent residents of the cytosol.)

Question 16

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

• nuclear export receptors
• nuclear import receptors
• nuclear pore proteins
• cytosolic fibrils
• signal-recognition particles (SRPs)

Answer 16

Nuclear import receptors

(The nuclear localization signal on proteins destined for the nucleus is recognized by cytosolic proteins called nuclear import receptors. These receptors help direct a newly synthesized protein to a nuclear pore by interacting with the tentacle-like fibrils that extend from the rim of the pore into the cytosol.)

Question 17

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

• mitochondrion
• ER
• peroxisome
• nucleus

Answer 17

ER

(The ER serves as an entry point for proteins destined for many of the cell’s organelles, as well as for those that remain in the ER itself.)

Question 18

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 is cleaved from the protein.
• It remains in the cytosol.
• It is translocated into the lumen of the endoplasmic reticulum.
• It forms an α-helical membrane-spanning segment of the protein.
• It stops protein synthesis and causes the ribosome to be released back to the cytosol.

Answer 18

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

(The stop-transfer sequence anchors the protein in the membrane.)

Question 19

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

• endolytic pathway
• endosomal pathway
• secretory pathway
• endocytic pathway
• exocytic pathway

Answer 19

endocytic pathway

(This pathway carries materials inward, in the opposite direction of the exocytic pathway, which carries them outward toward the cell surface.)

Question 20

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

• clathrin
• SNAREs
• Rab proteins
• tethering proteins
• adaptin

Answer 20

SNAREs

(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.)

Question 21

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

• membranes via osmosis
• transport vesicles that bud from one cisterna and fuse with the next
• bridges that link the cisternae
• pores in the cisternal membranes
• transporters in the cisternal membranes

Answer 21

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

(Proteins also move through the Golgi by a maturation process in which the Golgi cisternae themselves migrate through the Golgi stack.)

Question 22

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

• Vinblastine will not affect the pathway because microtubules are not involved in secretion.
• Transport vesicles will only be brought to the plasma membrane, not to the Golgi apparatus.
• Transport vesicles will not be brought to either the Golgi apparatus or the plasma membrane.
• Transport vesicles will only be brought to the Golgi apparatus, not to the plasma membrane.

Answer 22

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

(Disruption of microtubules will disrupt all transport of vesicles around the cell.)

Question 23

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?

• nucleus → endoplasmic reticulum → cytosol → Golgi → transport vesicle → secreted
• endoplasmic reticulum → transport vesicle → Golgi → cytosol → secreted
• endoplasmic reticulum → transport vesicle → Golgi → transport vesicle → secreted
• Golgi → transport vesicle → endoplasmic reticulum → transport vesicle → secreted

Answer 23

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

(Secreted proteins travel through the endoplasmic reticulum, transport vesicles, the Golgi, and additional transport vesicles, and finally are secreted at the plasma membrane.)

Question 24

How are newly made lipids supplied to the plasma membrane?

• via enzymes that synthesize phospholipids, which are attached to the plasma membrane
• via lysosomes
• via the constitutive pathway of exocytosis
• via vesicles that bud from the ER and fuse with the plasma membrane
• via secretory vesicles produced by the regulated exocytosis pathway

Answer 24

via the constitutive pathway of exocytosis

(This steady supply of lipids enables the plasma membrane to expand prior to cell division and replaces old lipids in nonproliferating cells.)

Question 25

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.
• A block of sugar residues is added to the N-terminal signal sequence, creating a common, N-linked oligosaccharide.
• Only proteins phosphorylated on an asparagine residue become glycosylated.
• Sugar residues are added one at a time by a series of enzymes attached to the ER membrane.
• Only proteins bearing a dolichol residue become glycosylated.

Answer 25

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

(The enzyme, an oligosaccharyl transferase, removes the branched oligosaccharide chain from a dolichol lipid in the ER membrane and transfers it to a target asparagine in the protein.)

Question 26

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

• digest their own worn-out organelles
• secrete hormones and neurotransmitters
• consume large particles, such as microbes and cell debris
• engage in receptor-mediated endocytosis
• ingest extracellular fluid and macromolecules

Answer 26

Consume large particles, such as microbes and cell debris

This process is carried out mainly by specialized phagocytic cells.

Question 27

Which of the following is true of lysosomes?

• Lysosomes contain around 40 types of hydrolytic enzymes, which are optimally active at pH 7.2.
• Lysosomes have a pH that is higher than that of the cytosol.
• The products of digestion in lysosomes leave the lysosome by transport vesicles.
• An ATP-driven H+ pump in the lysosomal membrane maintains the organelle’s pH.
• Most of the lysosomal membrane proteins have glycosylated regions on the cytosolic side of the membrane.

Answer 27

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

(The lysosome is maintained at an acidic pH by a pump in its membrane that hydrolyzes ATP to transport H+ into the lumen.)

Question 28

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

• lysosomes
• endosomes
• Golgi apparatus
• transport vesicles
• clathrin-coated vesicles

Answer 28

Endosomes

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

Question 29

What is the function of cargo?

Answer 29

Molecules packaged into vesicle for transport

Question 30

What is the function of the receptor?

Answer 30

Captures the correct cargo molecules

Question 31

What is the function of adaptin?

Answer 31

Mediates contact between the receptor and another component

Question 32

What is the function of clathrin?

Answer 32

Shapes the forming vesicle

Question 33

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.
• Vesicles would be larger, increasing the cargo endocytosed.
• All movement of molecules into and out of the cell would cease.
• Endocytosis would be unaffected, since adaptors and receptors can still interact.

Answer 33

Vesicles cannot form properly without a clathrin cage, thus inhibiting endocytosis.

(Clathrin is responsible for shaping the forming vesicle during endocytosis. If clathrin forms an open-ended lattice, the vesicle will not properly form and endocytosis will be inhibited.)

Question 34

True or False:

When cargo binds to the receptor, the adaptin binds the receptor and clathrin, facilitating the formation of the vesicle and consequently endocytosis.

Answer 34

True

Question 35

Which membrane-enclosed organelles most likely evolved in a similar manner?

• chloroplasts and peroxisomes
• the nucleus and the ER
• mitochondria and the Golgi apparatus
• mitochondria and the nucleus
• mitochondria and the ER

Answer 35

The nucleus and the ER

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

Question 36

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

• plasma membrane
• nuclear inner membrane
• smooth ER
• rough ER
• lysosome

Answer 36

Rough ER

(The rough ER is folded up to form an extensive maze of interconnected spaces. This organelle can, in some cases, compose about half of the total membrane present in the cell.)

Question 37

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 cytosol 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?

• the head only
• the tail only
• 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.

Answer 37

The tail only

(Only the preparations that have at least one tail are able to enter the nucleus after being injected into the cytoplasm (column on the right).)

Question 38

Trypanosomes are single-celled parasites that cause sleeping sickness when they infect humans. Trypanosomes taken from infected humans are known to store the enzymes needed to carry out some of the reactions of glycolysis in an organelle that resembles the peroxisome. In contrast, trypanosomes taken from tsetse flies—the intermediate host—carry out glycolysis entirely in the cytosol.

Investigators at a pharmaceutical company decide to follow up on this observation to design a potential new therapeutic. They determine that in trypanosomes from tsetse flies, one of the glycolytic enzymes, phosphoglycerate kinase (PGK), is present entirely in the cytosol, whereas in parasites taken from humans, 90% of the PGK activity is in a peroxisome-like compartment and only 10% is in the cytosol.

When the investigators clone the PGK genes, they discover that the parasites have three forms, each of which differs slightly from the others. They design probes that hybridize specifically to the mRNAs from each gene and then use these probes to determine which genes are expressed by trypanosomes from humans (H) and which are expressed by trypanosomes from tsetse flies (F).

Shown here is a gel in which mRNAs purified from the two different trypanosomes have been separated by size and exposed to probes that recognize the three different forms of the PGK gene (genes 1, 2, and 3).

Based on these results, which gene most likely encodes the peroxisomal form of PGK?

• gene 3
• both genes 1 and 2
• both genes 1 and 3
• gene 1
• gene 2

Answer 38

Gene 3

(Gene 3 is expressed exclusively in trypanosomes from humans, so it most likely encodes the form of PGK that is localized to the peroxisome-like organelle.)

Question 39

True or False:

By acting as a start-transfer sequence, the added N-terminal signal sequence would effectively turn the protein upside down; the N-terminal signal sequence would later be removed by a signal peptidase.

Answer 39

True

Question 40

List the steps used to transport proteins into mitochondria in the proper order.

Answer 40

1. The mitochondrial protein is synthesized in the cytosol.
2. The receptor on the mitochondrial membrane binds the signal sequence on the protein.
3. The protein is delivered to the translocation apparatus on the mitochondria.
4. The protein is passed through the translocation apparatus.
5. The signal sequence is removed by signal peptidase and the protein folds into its final.

(Mitochondrial proteins are synthesized in the cytosol and then passed through the translocation apparatus into the mitochondria. This contrasts with protein import into the ER, which happens co-translationally.)

Question 41

ATP is important for chaperone protein function. Why would protein import into mitochondria be disrupted if ATP were depleted from inside mitochondria?

• The signal sequence would not be recognized on the mitochondrial protein.
• The translocation apparatus would be unable to function without ATP hydrolysis.
• The protein would be blocked from entering the translocation apparatus.
• The protein could slip back out of the mitochondria during transport.

Answer 41

The protein could slip back out of the mitochondria during transport.

(Chaperones require ATP to help pull proteins into the mitochondrial matrix and prevent their exit back through the translocation channel.)

Question 42

How does the nuclear pore restrict the passage of large molecules that do not bear the correct nuclear localization signal?

• The pores remain closed until they are stimulated by the binding of proteins with the proper localization signal.
• The cytosolic fibrils obstruct access to the pore and can only be parted by nuclear import receptors.
• Inbound proteins are captured by the nuclear basket and released by GTP hydrolysis.
• The hydrophobic interior of the pore repels proteins that lack the correct nuclear localization signal.
• Nuclear pore proteins contain disordered segments that form a gel-like meshwork inside the pore.

Answer 42

Nuclear pore proteins contain disordered segments that form a gel-like meshwork inside the pore.

(These disordered segments form a soft, tangled meshwork—like a kelp forest—that fills the center of the channel, preventing the passage of large molecules but allowing small, water-soluble molecules to pass freely and nonselectively between the nucleus and the cytosol.)

Question 43

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?

• budding of vesicles from the endoplasmic reticulum
• docking of vesicles to target membranes
• fusion of vesicles with target membranes
• entry of proteins with ER signal sequences into the ER lumen

Answer 43

Fusion of vesicles with target membranes

(SNAREs help mediate vesicle membrane fusion (see image). In the absence of vesicle fusion, vesicle-stored neurotransmitters cannot be released into synaptic clefts, leading to paralysis.)

Question 44

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.

Based on these observations, where is the second peptide removed from the pro-insulin precursor protein?

• Golgi apparatus
• immature secretory vesicles
• mature secretory vesicles
• ER
• lysosomes

Answer 44

Immature secretory vesicles

(The appearance of yellow fluorescence in the immature secretory vesicles indicates that both the precursor protein and mature insulin are present, which suggests that the cleavage is taking place here.)

Question 45

Researchers studying yeast discovered that, for some mutants, when the temperature at which the cells are grown is elevated from 25ºC to 37ºC, their secretory pathway no longer functions and the cells grow dense with unsecreted protein.

When these cells are examined microscopically, they can be divided into groups that vary in terms of where the unsecreted proteins accumulate. In some of the mutants, proteins accumulate in the ER; in others, the Golgi; in others, they accumulate in vesicles near the plasma membrane.

What is the likely explanation for this difference in appearance?

• Different temperature-sensitive mutations disrupt protein synthesis.
• Different temperature-sensitive mutations promote an increase in protein synthesis.
• The temperature-sensitive mutant proteins accumulate in different compartments.
• Different temperature-sensitive mutations disrupt the integrity of cell membranes.
• Different temperature-sensitive mutations affect different stages of the transport process.

Answer 45

Different temperature-sensitive mutations affect different stages of the transport process.

(This approach was used by one research group to identify at least 23 different genes required for the transport of proteins from their site of synthesis to their secretion at the cell surface.)

Question 46

To determine whether a signal sequence directs proteins to a particular organelle, researchers prepare two versions of the same protein: one version contains the signal sequence, while the other lacks it. They label the protein that contains the signal sequence with a radioactive marker, and then incubate both of the proteins with the organelle of interest.

After allowing enough time for any of the proteins to be transported into the organelle, a protease is added to the mixture.

If the signal sequence is the correct one for the selected organelle, what would the researchers likely see?

• The radioactive label would be associated with one particular protein fragment.
• The radioactive label would be associated with a collection of protein fragments.
• The radioactive label would be associated with the protease.
• The radioactive label would be destroyed.
• The radioactive label would be associated with an intact protein.

Answer 46

The radioactive label would be associated with an intact protein.

(The labeled protein, which contains the appropriate signal sequence, would be transported into the organelle, where it would be protected from digestion by the protease.)

Question 47

Three separate pathways make up the unfolded protein response in the ER.

What are the characteristics of the unfolded protein responses in the IRE1 pathway?

Answer 47

• Leads to removal of an intron from a specific RNA and translation of the mRNA
• Contains both kinase and RNAse domains

Question 48

Three separate pathways make up the unfolded protein response in the ER.

What are the characteristics of the unfolded protein responses in the PERK pathway?

Answer 48

-Phosphorylates a translation initiation factor, leading to a global reduction in translation

Question 49

Three separate pathways make up the unfolded protein response in the ER.

What are the characteristics of the unfolded protein responses in the ATF6 pathway?

Answer 49

• Requires cleavage of the protein

- Travels to the Golgi apparatus and the nucleus before activating transcription

Question 50

Three separate pathways make up the unfolded protein response in the ER.

What are the characteristics of the unfolded protein responses in all 3 pathways?

Answer 50

• Activates transcription of specific genes

- Increases the ability of the ER to fold more proteins

Question 51

The three pathways of the unfolded protein response differ in importance in different cell types, enabling cells to tailor the response to their individual needs. You join a lab that studies the relative importance of the UPR in different cell types. Your advisor gives you a new cell culture and directs you to determine which of the three pathways is the most important for that cell type. You first treat the cells with a kinase inhibitor. Given the results in Figure A, which pathway(s) might be important in these cells?

• IRE1
• IRE1 and PERK
• ATF6
• IRE1 and ATF6
• PERK

Answer 51

IRE1 and PERK

(Both IRE1 and PERK are protein kinases and would be inhibited by a kinase inhibitor. They also could be involved in the response in this cell line.)

Question 52

You complete a further experiment by treating your cells with an RNAse inhibitor and get the results shown in Figure B. Given the results of Figure A and Figure B, what pathway(s) is/are important for this cell line?

• ATF6
• PERK
• IRE1
• IRE1 and PERK
• IRE1 and ATF6

Answer 52

IRE1

(IRE1 has kinase and RNAse domains. Inhibition of either domain blocks pathway activation and will lower the unfolded protein response in this cell line.)

Question 53

True or False:

The three different pathways that make up the unfolded protein response in the ER use distinct mechanisms to activate transcription of genes to increase the protein-folding capacity of the ER.

Answer 53

True

Question 54

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

• LDL receptors: form bridges between the LDL particle and adaptin
• lysosome: releases LDL from the receptor
• clathrin: forms the coated vesicle
• adaptin: binds to the specific receptors and recruits clathrin

Answer 54

lysosome: releases LDL from the receptor

(The lysosome is not where LDL is released from the receptor. The LDL is released from the receptor in the endosome before being transported to the lysosome for release of cholesterol.)

Question 55

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?
Choose one or more:

• Block the function of adaptin.
• Increase the activity of clathrin.
• Block the actin filaments.
• Block the receptor with an antibody.

Answer 55

• Block the function of adaptin.
• Block the receptor with an antibody.

(For a virus to enter a cell via receptor-mediated endocytosis, the virus needs to bind the receptor and then adaptin binds the receptor to recruit clathrin. Blocking either the receptor or adaptin would block receptor-mediated endocytosis and viral entry.)

Question 56

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.
• The A chain stimulates autophagy.
• The B chain interacts with SNAREs.
• The protein enters through pore complexes in the plasma membrane.
• The A chain binds to clathrin.

Answer 56

The protein is internalized by endocytosis.

Ricin is taken up by endocytosis, most likely mediated by its binding to cell-surface receptors.

Question 57

Which statements are true of receptor-mediated endocytosis?
Choose one or more:

• The process can be hijacked by viruses to gain entry into cells.
• It allows hemoglobin to be taken up by immature red blood cells.
• 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.
• Internalized endocytic vesicles fuse with lysosomes, which can return empty receptors to the plasma membrane.

Answer 57

• The process can be hijacked by viruses to gain entry into cells.
• 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.

(Receptor-mediated endocytosis is used to take up cholesterol, in the form of LDL, via clathrin-coated vesicles. The process can also be hijacked by viruses, including influenza.)