Chapter 15 Intracellular Compartments & Transport

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. 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 __________. 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.

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 signal. Proteins are transported into the Golgi apparatus via vesicles. The proteins transported into the endoplasmic reticulum by protein translocators are in their unfolded form.

​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. carbohydrate​ endosome ​lysosome disulfide bonds ​exocytic​ protein endocytic​ Golgi apparatus​ secretory endomembrane​ hydrogen bonds​ endoplasmic reticulum​ ionic bonds​ ​​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.

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.

Name a type of protein modification that can occur in the ER but not in the cytosol.

Proteins in the ER can undergo disulfide bond formation. (This does not occur in the cytosol because of its reducing environment.) (Signal-sequence cleavage and N-linked glycosylation are also acceptable answers.)

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. cytosol​ Golgi apparatus ​nucleus endoplasmic reticulum​ lysosome​ peroxisomes endosomes​ mitochondria​ plasma membrane ​​ 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.

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.

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.

a. Clathrin molecules are important for binding to and selecting cargoes for transport. Cargo binds to cargo receptors. Adaptin molecules capture cargo receptors, which bind to the appropriate cargo molecules for incorporation into the vesicle.

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 membrane-enclosed organelles; the other 90% of the cell volume is the cytosol. ​d.​ The nucleus is the only organelle that contains DNA.

a. In a typical cell, the area of the endoplasmic reticulum membrane far exceeds the endoplasmic reticulum membrane far exceeds the area of plasma membrane.

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.

a. It is bound by cytoplasmic proteins that direct the nuclear protein to the nuclear pore.

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 contain enzymes that help inactivate toxins. ​d.​ Proteins do not need to unfold to enter the peroxisome.

a. Most peroxisomal proteins are synthesized in the ER.

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.

a. Phagocytic cells are important in the gut to take up large particles of food.

Proteins that are fully translated in the cytosol and lack a sorting signal will end up in the ​a.​ cytosol. ​b.​ mitochondria. ​c.​ interior of the nucleus. ​d.​ nuclear membrane.

a. cytosol

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.

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.

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.

b. Cytosolic cholesterol levels will decrease relative to normal cells.

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.

b. Proteins that are misfolded are degraded in the ER lumen.

Which is 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 the smooth ER. ​c.​ Steroid hormones are synthesized on the smooth ER. ​d.​ The ER membrane is contiguous with the outer nuclear membrane.

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

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.

b. The nuclear membrane is thought to have arisen from the plasma membrane invaginating around the DNA.

Molecules to be packaged into vesicles for transport are selected by ​a.​ clathrin. ​b.​ adaptins. ​c.​ dynamin. ​d.​ SNAREs.

b. adaptins

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.

c. Nuclear transport receptors would be unable to release their cargo in the nucleus.

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 ER-associated protein translocators.

c. Proteins destined for the ER are translated by cytosolic ribosomes and are targeted to the ER when a signal sequence emerges during translation.

Cells have oligosaccharides displayed on their cell surface that are important for cell-cell recognition. Your friend discovered a transmembrane glycoprotein, GP1, on a pathogenic yeast cell that is recognized by human immune cells. He decides to purify large amounts of GP1 by expressing it in bacteria. To his purified protein he then adds a branched 14-sugar oligosaccharide to the asparagine of the only Asn-X-Ser sequence found on GP1 (Figure 15-32). Unfortunately, immune cells do not seem to recognize this synthesized glycoprotein. Which of the following statements is a likely explanation for this problem? ​a.​ The oligosaccharide should have been added to the serine instead of the asparagine. ​b.​ The oligosaccharide should have been added one sugar at a time. ​c.​ The oligosaccharide needs to be further modified before it is mature. ​d.​ The oligosaccharide needs a disulfide bond.

c. The oligosaccharide needs to be further modified before it is mature.

Most proteins destined to enter the endoplasmic reticulum ​a.​ are transported across the membrane after their synthesis is complete. ​b.​ are completely translated on free ribosomes in the cytosol. ​c.​ begin to cross the membrane while still being synthesized. ​d.​ remain within the endoplasmic reticulum.

c. begin to cross the membrane while still being synthesized.

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 was not listening carefully when he was told about the molecular defect of this cell line. He is 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

c. dynamin

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.

c. encoded in the amino acid sequence and sufficient for targeting a protein to its correct destination.

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

c. in the cytosol

Which of the following organelles is not part of the endomembrane system? ​a.​ Golgi apparatus ​b.​ the endosome ​c. ​mitochondria ​d. ​lysosomes

c. mitochondria Mitochondria are not part of the endomembrane system, which is thought to have arisen initially through invagination of the plasma membrane. Instead, mitochondria (and chloroplasts) are thought to have evolved from a bacterium that was engulfed by a primitive eukaryotic cell.

Which of the following is NOT a process that delivers material to the lysosome? ​a.​ pinocytosis ​b. ​phagocytosis ​c.​ transcytosis ​d. ​autophagy

c. transcytosis Transcytosis is the process by which the endosome delivers cargo across the cell to a different plasma membrane domain.

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.

d. Disulfide bonds form spontaneously within the ER because the lumen of the ER is oxidizing.

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

d. ER → Golgi → plasma membrane

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.

d. Once the signal sequence from Fuzzy has been cleaved, the signal peptide will be ejected into the ER membrane and degraded.

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

d. adaptins

Where are chloroplast proteins translated? ​a.​ in the cytosol ​b.​ in the chloroplast ​c.​ on the endoplasmic reticulum ​d.​ in both the cytosol and the chloroplast

d. in both the cytosol and the chloroplast Proteins in the chloroplast are translated in the cytosol and in the chloroplast. The chloroplast proteins that are encoded by the nuclear DNA are translated in the cytosol, and the sorting signals on the protein direct them to the chloroplast. The chloroplast proteins encoded by the chloroplast DNA are translated on ribosomes inside the chloroplast.

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.

d. the asparagine in the sequence Asn-X-Ser/Thr.

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.

d. transport vesicles.