5.4 The Endomembrane System
Lysosome
a vesicle derived from the Golgi apparatus that contains enzymes that break down macromolecules such as proteins, nucleic acids, lipids, and complex carbohydrates
When are proteins produced on free ribosomes sorted?
after they are translated
Non-protein coding genes are typically found in: A) mitochondria. B) the nucleolus. C) the endoplasmic reticulum. D) the cytoplasm. E) areas of the nucleus other than the nucleolus.
B) the nucleolus.
Most elements of the endomembrane system are connected by: A) cytoskeleton B) vesicular trafficking C) continuous membranes
B) vesicular trafficking
Which base pairing is incorrect? A) G-C B) U-A C) A-G D) A-T
C) A-G
What is unique about proteins destined for cell membranes?
they contain a signal-anchor sequence (in addition to the amino-terminal signal sequence)
What happens to proteins produced on the rough ER sent?
they end up in the lumen of the endomembrane system, secreted out of the cell, or as a transmembrane protein
What is the internal pH of lysosomes?
5 -- acidic
Which of the following types of RNA molecule delivers amino acids to the ribosomes? A) transfer RNA B) messenger RNA C) ribosomal RNA
A) transfer RNA
Which base is not found in DNA? A) uracil B) cytosine C) guanine D) adenine E) thymine
A) uracil
What is the difference in the endomembrane system between plants and animals?
-the endomembrane system of plants is continuous between cells through plasmodesmata -in animals, the system includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, the plasma membrane, and the vesicles that move between them
What are the two main connections between organelles in a cell?
1) physically connected by membrane "bridges" 2) transiently connected by vesicles, small membrane-enclosed sacs that transport substances within a cell or from the interior to the exterior of the cell that form by budding off an organelle, taking with them a piece of the membrane and internal contents of the organelle from which they derive and fusing with another organelle or the plasma membrane, re-forming a continuous membrane and unloading their contents
How are transmembrane proteins made?
1. proteins with signal-anchor sequences are threaded through a channel in the ER membrane until the signal-anchor sequence is encountered 2. the ER channel releases the protein into the membrane 3. when translation is completed, the protein remains in the membrane
How does a protein produced by a free ribosome get to the RER?
1. the signal-recognition particle (SRP) binds to a signal sequence in the amino-terminal end of the growing polypeptide and halts translation 2. the SRP binds to the SRP receptor on the ER membrane 3. the SRP receptor brings the ribosome to a transmembrane channel; the SRP dissociates; protein synthesis resumes, and the growing polypeptide chain is threaded through the channel; a specific protease cleaves the signal sequence as it emerges in the lumen of the ER 4. the protein ends up in the lumen of the ER, where it may remain, be transported to the lumen of another organelle, or be secreted out of the cell
Consider the events that describe the progress of a protein that will be secreted from the cell. 1. SRP binds to the growing polypeptide chain and to the ribosome. 2. Translation resumes. 3. SRP binds to its receptor. 4. The signal sequence is cleaved. 5. Protein synthesis begins in the cytosol. 6. Translation pauses. Which of the sequences of events correctly describes the progress of a protein that will be secreted from the cell? A) 5 → 1 → 6 → 3 → 2 → 4 B) 5 → 6 → 1 → 3 → 2 → 4 C) 3 → 6 → 1 → 2 → 4 → 5
A) 5 → 1 → 6 → 3 → 2 → 4
What does RNA polymerase bind to in order to initiate transcription? A) DNA promoter region B) RNA promoter region C) start codon D) mRNA
A) DNA promoter region
Consider a protein that is targeted to be an integral membrane protein on the surface of a cell. It has a specific functional domain (domain Y) facing the cytoplasm of the cell. How will this protein be oriented in the Golgi? A) It will be embedded in the Golgi membrane with domain Y facing the cytoplasm of the cell. B) It will be completely within the Golgi lumen, not embedded in the membrane. C) It will be outside the Golgi as a peripheral membrane protein. D) It will be embedded in the Golgi membrane with domain Y facing the lumen of the E.R.
A) It will be embedded in the Golgi membrane with domain Y facing the cytoplasm of the cell.
Which enzyme is made up of both RNA and protein? A) RNA polymerase B) nucleosome C) ribosome
A) RNA polymerase
The sequence for a portion of the DNA template strand and the corresponding mRNA that is being transcribed is shown here. mRNA 5'-GCAGCC-3' DNA template 3'-GCGCGTCGGTACA-5' Which base will the RNA polymerase add next to the growing mRNA molecule? A) adenine B) thymine C) cytosine D) guanine E) uracil
A) adenine
The controlled release of cellular material stored in membrane-bound vesicles to the outside of the cell is an example of: A) exocytosis. B) phagocytosis. C) endocytosis. D) transcytosis.
A) exocytosis.
Consider a protein that is targeted to be excreted to the outside of the plasma membrane. Where would this protein be located in the endoplasmic reticulum? A) in the lumen (inside) of the E.R. B) outside the E.R. as a peripheral membrane protein C) embedded as an integral membrane protein in the E.R. membrane
A) in the lumen (inside) of the E.R.
Which of the answer choices is not considered to be a part of the endomembrane system? A) mitochondria B) Golgi C) plasma membrane D) lysosome E) endoplasmic reticulum
A) mitochondria
What components make up the ribosome? Select all that apply? A) protein B) carbohydrate C) RNA D) DNA
A) protein C) RNA
Which of the following mRNA processing events occurs in the nucleus of human cells? (Select all that apply.) A) splicing of exons B) poly-A tail addition C) excision of introns D) 5' cap addition
A) splicing of exons B) poly-A tail addition C) excision of introns D) 5' cap addition
Disorders of which organelle are often associated with defects in transport from compartment to compartment, resulting in poor sorting of protein components within the cell? A) the Golgi apparatus B) nucleus C) plasma membrane D) plasmids E) endoplasmic reticulum
A) the Golgi apparatus
Which sequence accurately describes the path traveled by a new protein from when it first starts to be translated to its release from the cell? A) nuclear envelope → ER → vesicle → Golgi → plasma membrane → external environment B) cytosol → ER → Golgi → vesicle → plasma membrane → external environment C) cytosol → Golgi → ER → vesicle → plasma membrane → external environment D) plasma membrane → ER → vesicle → Golgi → cytosol → external environment E) nucleus → ER → Golgi → vesicle → plasma membrane → external environment
B) cytosol → ER → Golgi → vesicle → plasma membrane → external environment
For each of the following organelles or structures, check the corresponding box if the organelle or structure is named or discussed in the animation. (Select all that apply.) A) lysosomes B) the nucleus C) mitochondria D) the Golgi apparatus E) ribosomes
B) the nucleus C) mitochondria E) ribosomes
Does protein expression begin with transcription or translation? A) translation B) transcription
B) transcription
Several sets of percentage of DNA bases that make up the genome are listed below. Which set is correct? A) A: 10%, T: 30%, G: 40%, C: 20% B) A: 30%, T: 40%, G: 20%, C: 10% C) A: 30%, T: 30%, G: 20%, C: 20% D) A: 20%, T: 20%, G: 50%, C: 10%
C) A: 30%, T: 30%, G: 20%, C: 20%
The number of hydrogen bonds between two molecules affects how much energy is needed to break those two molecules apart. The more hydrogen bonds there are, the more energy that is required. Which of the following base pairs requires the most energy to break apart? A) A-T B) T-A C) G-C D) A-U
C) G-C
Where are the ribosomes that translate proteins to be secreted from the cell? A) attached to the Golgi apparatus B) in the nucleolus C) attached to the rough endoplasmic reticulum D) in the cytosol
C) attached to the rough endoplasmic reticulum
A protein with an internal signal sequence is most likely to be located in: A) the cytoplasm. B) a chloroplast. C) the nucleus. D) a mitochondrion. E) the extracellular space.
C) the nucleus.
For the macromolecules listed here, select the correct order for the proper flow of genetic information. 1) RNA 2) protein 3) DNA A) 1 > 2 > 3 B) 2 > 3 > 1 C) 1 > 3 > 2 D) 3 > 1 > 2
D) 3 > 1 > 2
Which tRNA anticodon sequence is the correct match to the mRNA codon listed below? Note: codon-anticodon base-pairing is antiparallel. 5'-CGA-3' A) 3'-CGA-5' B) 3'-TCG-5' C) 3'-GCT-5' D) 3'-GCU-5'
D) 3'-GCU-5'
Many cell functions involve communication between cells via molecular signals that must be sent from one cell to be received by a target cell. Suppose the signal to be sent is a small protein. Where would you expect to find the protein in the Golgi? A) It would be an integral membrane protein with its functional domain on the cytoplasmic side of the membrane. B) It would be an integral membrane protein with its functional domain on the lumen side of the membrane. C) It would be on the exterior of the Golgi. D) It would be in the lumen of the Golgi.
D) It would be in the lumen of the Golgi.
Which portion of DNA contains the genetic information "read" by the RNA polymerase and transmitted into mRNA? A) sugar B) backbone C) phosphodiester bond D) base
D) base
The genetic information carried by DNA is in the ___ and their ___. A) phosphates; phosphodiester bonds B) sugars; atoms C) base-pairing; hydrogen bonds D) bases; sequence
D) bases; sequence
In which eukaryotic organelle are various macromolecules broken down and destroyed? A) the Golgi apparatus B) endoplasmic reticulum C) mitochondria D) lysosome
D) lysosome
Which protein level best matches the description "blocks on a string"? A) tertiary B) secondary C) quaternary D) primary
D) primary
The protein shown being synthesized during the animation is: A) a histone protein. B) chaperone. C) RNA polymerase II. D) myosin. E) actin.
E) actin.
How does a protein end up free in the cytosol, embedded in the plasma membrane, or secreted from the cell?
Proteins produced on free ribosomes in the cytosol are directed to their final destination through particular amino acid sequences called signal sequences. These proteins are sorted after they have been translated. Proteins destined for the nucleus, mitochondria, and chloroplasts have specific signal sequences (e.g., nuclear localization signals will direct the protein to the nucleus). Proteins with no signal sequence remain in the cytosol. Proteins produced by ribosomes on the rough endoplasmic reticulum end up in the lumen of the endomembrane system or embedded in its membrane. They may also be secreted out of the cell. These proteins are sorted as they are translated. They are initially translated by a ribosome in the cytosol, but a signal sequence in the growing protein directs the ribosome to a channel on the rough ER. AS the protein is translated, it is threaded through the channel., these proteins are destined for the ER lumen, Golgi apparatus, lysosomes, or for secretion outside the cell. If the protein contains an additional signal sequence called a signal anchor sequence, it remains in the ER membrane as it is synthesized, rather than passing entirely into the ER lumen.
Ribosomes
a complex structure of RNA and protein, bound to the cytosolic face of the RER in the cytoplasm, on which proteins are synthesized
Vesicles
a small membrane-enclosed sac that transports substances within the cell
What is the significance of the distinctive shapes that sugars contribute to glycoproteins and glycolipids?
allow them to be recognized specifically by other cells and molecules in the external environment
Signal-Recognition Particle (SRP)
an RNA-protein complex that binds with part of a polypeptide chain and marks the molecule for incorporation into the endoplasmic reticulum (eukaryotes) or the plasma membrane (prokaryotes)
When are proteins synthesized by ribosomes on the rough ER sorted?
as they are being translated
Where is the signal sequence located if the protein is destined for mitochondria or chloroplasts?
at the amino end
What is the significance of the signal-anchor sequence?
it is hydrophobic, so it can diffuse laterally in the lipid bilayer, and signals when the protein complex should stop being threaded through the channel in the ER membrane; the ribosome dissociates from the channel and the signal-anchor sequence brings the protein and the ribosome with it to continue translation without the channel
Where is the signal sequence located if the protein is targeted to the nucleus?
it's located internally (a nuclear localization signal), that allows proteins to move through pores in the nuclear envelope
Nuclear Envelope
the cell structure, composed of two membranes, inner and outer, that defines the boundary of the nucleus; each is a lipid bilayer with associated proteins
Which organs have extensive rough ER?
the gut that secretes digestive enzymes and the pancreas that produces insulin
Endoplasmic Reticulum
the organelle involved in the synthesis of proteins and lipids
Golgi Apparatus
the organelle that modifies proteins and lipids produced by the endoplasmic reticulum and acts as a sorting station as they move to their final destinations
Rough Endoplasmic Reticulum (RER)
the part of the endoplasmic reticulum with attached ribosomes
Smooth Endoplasmic Reticulum
the portion of the endoplasmic reticulum that lacks ribosomes
Protein Sorting
the process by which proteins end up where they need to be in the cell to perform their function
Endocytosis
the process in which a vesicle buds off from the plasma membrane, bringing material from outside the cell into that vesicle, which can then fuse with other membranes
Exocytosis
the process in which a vesicle fuses with the plasma membrane and empties its contents into the extracellular space or delivers proteins to the plasma membrane
Cisternae
the series of flattened membrane sacs that make up the Golgi apparatus
Nuclear Localization Signals
the signal sequence for the nucleus that enables proteins to move through pores in the nuclear envelope
What is the impact of sugars on protein in a glycoprotein?
the sugars attached to the protein can protect the protein from enzyme digestion by blocking access to the peptide chain; thus glycoproteins form a relatively flexible and protective coating over the plasma membrane
What is the significance of a signal sequence?
they allow proteins to be recognized and sorted
What are the two places a protein is produced?
free ribosomes in the cytosol and membrane-bound ribosomes on the rough ER
Lumen
in eukaryotes, the continuous interior of the endoplasmic reticulum; in plants, a fluid-filled compartment enclosed by the thylakoid membrane; generally, the interior of any tube-like structure
Signal-Anchor Sequence
in protein sorting, an amino acid sequence in a polypeptide chain that embeds the chain in the membrane
What happens if a protein synthesized on free ribosomes does not have a signal sequence?
it remains in the cytosol
Nuclear Pores
one of many protein channels in the nuclear envelope that act as gateways that allow molecules to move into and out of the nucleus and are thus essential for the nucleus to communicate with the rest of the cell
What happens when traffic moves in the reverse direction from the Golgi apparatus to the ER?
proteins that were accidentally moved forward can be retrieved and/or membrane components can be recycled
What are the three primary functions of the Golgi apparatus?
1) it further modifies proteins and lipids produced by the ER 2) it acts as a sorting station as these proteins and lipids move to their final destinations 3) it is the site of synthesis of most of the cell's carbohydrates
In which ribosome site would you find the uncharged tRNA? A) E site B) A site C) P site
A) E site
Shown here is a portion of the final mRNA sequence. Which sequence is from the corresponding DNA template strand? mRNA 5'-CCAUGUUCGAAUGGCUUG-3' A) 5'-CCAUGUUCGAAUGGCUUG-3' B) 5'-CAAGCCATTCGAACATGG-3' C) 5'-CCATGTTCGAATGGCTTG-3' D) 5'-CAAGCCAUUCGAACAUGG-5'
B) 5'-CAAGCCATTCGAACATGG-3'
All genes in DNA code for proteins. A) True B) False
B) False
Consider a protein that is targeted to be excreted to the outside of the plasma membrane. Where would this protein be located in the Golgi? A) outside the Golgi as a peripheral membrane protein B) embedded as an integral membrane protein in the Golgi membrane C) in the lumen (inside) of the Golgi
C) in the lumen (inside) of the Golgi
tRNA genes are transcribed by: A) DNA polymerase B) spliceosome C) aminoacyl tRNA synthetase E) RNA polymerase
E) RNA polymerase
Signal Sequences
an amino acid sequence that directs a protein to its proper cellular compartment
What are the names and functions of the major organelles of the endomembrane system in eukaryotic cells?
The nucleus stores the cell's genetic information and is the site of transcription. The endoplasmic reticulum (ER) is the organelle in which proteins and lipids are synthesized. The Golgi apparatus modifies protein and lipids produced by the ER and acts as a sorting station as those molecules move to their final destinations. Lysosomes contain enzymes that break down macromolecules such as proteins, nucleic acids, lipids, and complex carbohydrates.
B) B
Where is the anticodon located? A) A B) B C) C
Do all cell have some rough ER?
Yes, for the production of transmembrane and organelle proteins
Endomembrane System
a cellular system that includes the nuclear envelope, the endoplasmic reticulum, the Golgi apparatus, lysosomes, the plasma membrane, and the vesicles that move between them
What is glycosylation?
a chemical modification that occurs predominantly in the Golgi apparatus in which sugars are covalently linked to lipids or specific amino acids of proteins, forming glycolipids or glycoproteins
Which proteins would be synthesized on the rough endoplasmic reticulum and processed in Golgi apparatus? A) Transcription factors that bind to DNA sequences in the nucleus. B) Lysosomal enzymes that break down proteins in the lumen of the lysosome. C) Cytoskeletal proteins actin and tubulin, which are found in the cytoplasm. D) DNA polymerase that functions during DNA replication.
B) Lysosomal enzymes that break down proteins in the lumen of the lysosome.
Which of the following reads the nucleotide sequence of a gene and synthesizes the corresponding primary transcript? A) DNA polymerase. B) RNA polymerase. C) ribosome.
B) RNA polymerase.
The primary transcript is _________ to the DNA template strand. (Select all that apply.) A) identical B) antiparallel C) complementary
B) antiparallel C) complementary
Where does transcription of mRNA begin? A) at the 5' end of the gene on the DNA template strand B) at the AUG start codon on the mature mRNA strand C) at the 3' end of the gene on the DNA template strand
B) at the AUG start codon on the mature mRNA strand
For the numbered steps below, select the option that places them in the correct order. 1) The ribosome binds to the mRNA and uses tRNAs to translate mRNA into the corresponding amino acid polypeptide sequence. 2) The spliceosome removes introns. 3) The primary structure of the polypeptide chain undergoes hierarchical foldings to form the tertiary structure. 4) RNA polymerase binds to the promoter region of a gene and initiates transcription. A) 1 > 2 > 3 > 4 B) 4 > 2 > 1 > 3 C) 3 > 1 > 4 > 2 D) 2 > 1 > 4 > 3
B) 4 > 2 > 1 > 3
Consider a protein that is targeted to be an integral membrane protein on the surface of a cell. It has a specific functional domain (domain X) facing the external environment of the cell. How will this protein be oriented in the E.R.? A) It will be embedded in the E.R. membrane with domain X facing the cytoplasm of the cell. B) It will be embedded in the E.R. membrane with domain X facing the lumen of the E.R. C) It will be completely within the E.R. lumen, not embedded in the membrane. D) It will be outside the E.R. as a peripheral membrane protein.
B) It will be embedded in the E.R. membrane with domain X facing the lumen of the E.R.
Where does the expression of a protein begin? A) in the nucleolus B) in the nucleus C) in the cytoplasm
B) in the nucleus
Which process is carried out by the spliceosome? A) exon removal B) intron removal C) transcription D) translation E) DNA replication
B) intron removal
