Chapter 15: Intracellular Compartments and Protein Transport

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.

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. The nuclear localization signal typically contains positively charged amino acids, not hydrophobic ones [choice (b)]. Proteins are not unfolded as they enter the nucleus [choice (c)]. Proteins are actively transported in and out of the nucleus and do not diffuse through the nuclear pores [choice (d)].

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.

(a) Most peroxisomal proteins are synthesized in the ER. Although peroxisomes can get some membrane-embedded proteins from the ER, most peroxisomal proteins are imported from the cytosol.

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. Although some unicellular eukaryotes ingest food particles by phagocytosis, phagocytosis is not involved in digestion in the animal gut.

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.

(a) The membrane of a secretory vesicle will fuse with the plasma membrane when it discharges its contents to the cell's exterior. Vesicles for regulated exocytosis bud from the trans Golgi network and accumulate at the plasma membrane until the appropriate signal has been received [choice (b)]. There are no signal sequences for proteins destined for exocytosis [choice (c)]. Those proteins that are to be secreted by regulated exocytosis aggregate in the trans Golgi network as a result of the acidic pH and high Ca2+ concentrations [choice (d)]; those proteins that do not aggregate are packed into transport vesicles for constitutive exocytosis.

Figure Q15-31 shows the organization of a protein that resides on the ER membrane. The N- and C-termini of the protein are labeled. Boxes 1, 2, and 3 represent membrane-spanning sequences. Non-membrane-spanning regions of the protein are labeled "X," "Y," and "Z." Figure Q15-31 Once this protein is fully translocated, where will region Y be? (a) in the cytoplasm (b) in the ER lumen (c) inserted into the ER membrane (d) degraded by signal peptidase

(a) in the cytoplasm

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.

(a) sorting sequence, which typically contains the positively charged amino acids lysine and arginine. The nuclear import receptor interacts with the fibrils of the nuclear pore [choice (c)] and Ran [choice (d)].

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

(a) the cytosol. Proteins produced in the cytosol that lack sorting signals remain in the cytosol. Proteins produced in the cytosol and destined for the mitochondria [choice (b)] or the interior of the nucleus [choice (c)] will have a sorting signal to direct the protein to its proper location. Proteins destined for the nuclear membrane [choice (d)] are not translated in the 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. Once a protein is bound to the import receptor, the protein—in a complex that includes the protein translocator—will diffuse along the outer membrane until it reaches a specialized site where the inner and outer membranes contact each other, and will then be translocated simultaneously across the inner and outer membranes.

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. Normally, cholesterol dissociates from the LDL receptor in the acidic environment of the endosomes and is released into the cytosol. If the drug prevents cholesterol from dissociating from the LDL receptor in acidic conditions, cholesterol may not become released into the cytosol, and thus cytosolic cholesterol levels are likely to decrease relative to those in normal cells. There is no reason to believe that the LDL receptor will remain on the plasma membrane [choice (c)], because the cytosolic region of the receptor is not directly altered by the drug. Vesicle uncoating is also unlikely to be altered [choice (d)], because this occurs after vesicles are pinched off from the membrane.

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. Proteins that are misfolded are exported from the ER into the cytosol, where they are degraded.

Figure Q15-34 shows the organization of a protein that normally resides in the plasma membrane. The boxes labeled 1 and 2 represent membrane-spanning sequences and the arrow represents a site of action of signal peptidase. Given this diagram, which of the following statements must be true? Figure Q15-34 (a) The N-terminus of this protein is cytoplasmic. (b) The C-terminus of this protein is cytoplasmic. (c) The mature version of this protein will span the membrane twice, with both the N- and C-terminus in the cytoplasm. (d) None of the above.

(b) The C-terminus of this protein is cytoplasmic. The mature version of this protein will span the membrane once, with membrane-spanning segment 2 in the membrane and the C-terminus facing the cytoplasm.

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 Lysosomes are part of the endomembrane system and are not thought to have come from the engulfment of an ancient prokaryotic cell [choice (a)]. Bacteria use their plasma membrane for ATP production [choice (c)]. Chloroplasts and mitochondria have their own DNA and do not share [choice (d)].

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

(b) adaptins.

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.

(b) will fuse with only one type of membrane. An individual vesicle may contain more than one type of protein in its lumen [choice (a)], all of which will contain the same sorting signal (or will lack specific sorting signals). Endocytic vesicles [choice (c)] generally move away from the plasma membrane. The vesicle membrane will not necessarily contain the same lipid and protein composition as the donor organelle, because the vesicle is formed from a selected subsection of the organelle membrane from which it budded [choice (d)].

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.

(c) Activation of the UPR occurs when receptors in the cytoplasm sense misfolded proteins. The receptors for the unfolded proteins are on the ER membrane, and they sense the misfolded proteins using their luminal domains.

Which of the following statements is true? (a) The signal sequences on mitochondrial proteins are usually at the C-terminus. (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.

(c) Chaperone proteins in the mitochondria facilitate the movement of proteins across the outer and inner mitochondrial membranes. The signal sequences on a protein destined for the mitochondria are on its N-terminus [choice (a)]. Although some mitochondrial proteins are synthesized inside the mitochondria from the mitochondrial genome, most mitochondrial proteins are encoded by genes in the nucleus and imported into the mitochondria after synthesis in the cytosol [choice (b)]. Mitochondrial proteins are unfolded as they enter the mitochondria through protein translocators[choice (d)].

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.

(c) Nuclear pores have water-filled passages that small, water-soluble molecules can pass through in a nonselective fashion. mRNAs and proteins can move through the same nuclear pore [choice (a)]. Nuclear import receptors bind to proteins in the cytosol and transit with them across the nuclear pore into the nucleus [choice (b)]. Nuclear pores are made up of many copies of multiple proteins [choice (d)].

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. When Ran-GTP binds to the nuclear transport receptor, cargo is released. If Ran could not exchange its GDP for GTP, this would not happen. Ran-GTP is not needed for cargo binding, for nuclear entry, or for interactions with the nuclear pore fibrils during nuclear import.

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.

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.

(c) The GTP hydrolysis of the Rab proteins provides the energy for membrane fusion. Rab proteins are important for docking, but are not involved in the catalysis of membrane fusion.

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 Q15-48). Unfortunately, immune cells do not seem to recognize this synthesized glycoprotein. Which of the following statements is a likely explanation for this problem? Figure Q15-48 (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. Oligosaccharides are usually further modified by enzymes in the ER and the Golgi before the glycoprotein is inserted into the plasma membrane. The other choices are untrue, and thus are not good explanations. Oligosaccharides are added to the Asn and not the Ser [choice (a)] and are added as a branched 14-sugar oligosaccharide [choice (b)]. Disulfide bonding occurs between cysteines of proteins and not in sugars [choice (d)].

Figure Q15-57 shows the orientation of the Krt1 protein on the membrane of a Golgi-derived vesicle that will fuse with the plasma membrane. Figure Q15-57 Given this diagram, which of the following statements is true? (a) When this vesicle fuses with the plasma membrane, the entire Krt1 protein will be secreted into the extracellular space. (b) When this vesicle fuses with the plasma membrane, the C-terminus of Krt1 will be inserted into the plasma membrane. (c) When this vesicle fuses with the plasma membrane, the N-terminus of Krt1 will be in the extracellular space. (d) When this vesicle fuses with the plasma membrane, the N-terminus of Krt1 will be cytoplasmic.

(c) When this vesicle fuses with the plasma membrane, the N-terminus of Krt1 will be in the extracellular space. The orientation of Krt1 as the vesicle fuses with the plasma membrane is shown in Figure A15-57. The darker-colored lines in the membrane represent the membranes contributed by the vesicle during fusion.

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.

(c) begin to cross the membrane while still being synthesized. Proteins destined to enter the endoplasmic reticulum have an N-terminal signal sequence that leads to the docking of the ribosome synthesizing the protein onto the ER and the entry of the protein across the ER membrane as the polypeptide chain is being synthesized.

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

(c) cis Golgi network.

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

(c) dynamin Given that coated pits can form but no vesicle budding is seen, dynamin is the most likely answer. Since coated pits are formed, clathrin and adaptin are unlikely to be the answer, because they are involved in the initial shaping of the vesicle into the pit [choices (a) and (d)]. Rab proteins are involved in the recognition of the transport vesicle with its target membrane and not with vesicle budding [choice (b)].

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. Signal sequences are found within the amino acid sequence of proteins. They are sometimes removed when the protein is at the correct destination [choice (d)], but not all are removed. For example, nuclear import signals are not removed once a protein is inside the nucleus. A protein translocator resides in the membrane and helps transport soluble proteins across the membrane [choice (b)], but does not add signal sequences to proteins.

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 Ribosomes are cytoplasmic proteins and thus their protein components are translated in the cytosol.

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. An enzyme in the ER lumen catalyzes disulfide bond formation.

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

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.

(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.

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. ER signal sequences are typically at the N-terminus [choice (a)]. More than one ribosome can bind to an mRNA molecule [choice (b)]. Hydrophobic stop-transfer sequences are found on membrane-inserted proteins and not on soluble proteins [choice (c)].

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.

(d) Some of the proteins in the lumen of the ER can end up in the plasma membrane. Plasma membrane proteins come from proteins in the ER membrane, not from the ER lumen.

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 Adaptins are involved in vesicle budding and are removed during the uncoating process, and thus should not be present when the vesicle reaches its target.

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

(d) all of the above The rough ER consists of ER membranes and polyribosomes that are in the process of translating and translocating proteins into the ER membrane and lumen. Thus, all proteins that end up in the lysosome, Golgi apparatus, or plasma membrane, or are secreted, will be encoded by the RNAs associated with the rough ER.

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

(d) in both the cytosol and the chloroplast Proteins in the chloroplast are synthesized in the cytosol and in the chloroplast. The chloroplast proteins that are encoded by the nuclear DNA are synthesized in the cytosol, and the sorting signals on the protein direct them to the chloroplast. The chloroplast proteins encoded by the chloroplast DNA are synthesized 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. Proteins destined for transport vesicles will be translated on ribosomes associated with the endoplasmic reticulum.

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

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 area of plasma membrane. the area of the endoplasmic reticulum membrane is 20-30 times that of the plasma membrane in a typical cell. Chloroplasts and mitochondria are also surrounded by a double membrane [choice (b)]. The cytosol is about half the volume of a typical eukaryotic cell, with membrane-enclosed organelles making up the other half of the volume [choice (c)]. Chloroplasts and mitochondria also carry their own genome, whereas the nucleus carries the genome of the organism [choice (d)].

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.

b) Proteins to be delivered to the ER lumen are synthesized on smooth ER. Proteins to be delivered to the ER lumen are synthesized on rough ER; these areas appear "rough" because ribosomes are attached to the cytosolic surface of these ER regions.

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

c) 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.