GCD 3033 - Exam 2

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

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

Which of the following statements about membrane-enclosed organelles is true?

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

Name the membrane-enclosed compartments in a eucaryotic cell where each of the functions listed below takes place: (1) Photosynthesis, (2) Transcription, (3) Oxidative phosphorylation, (4) Modification of secreted proteins, (5) Steroid hormone synthesis, (6) Degradation of worn-out organelles, (7) New membrane synthesis, (8) Breakdown of lipids and toxic molecules

(1) Chloroplast; (2) Nucleus; (3) Mitochondrion; (4) Golgi apparatus and rough ER; (5) Smooth ER; (6) Lysosome; (7) ER; (8) Peroxisome

Which of the following organelles are not part of the endomembrane system?

mitochondria

Where are proteins in the chloroplast synthesized?

in both the cytosol and the chloroplast

Which of the following statements is true?

Chaperone proteins in the mitochondria facilitate the movement of proteins across the outer and inner mitochondrial membranes.

Most proteins destined to enter the endoplasmic reticulum

begin to cross the membrane while still being synthesized.

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?

all of the above

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

in the cytosol

What is the role of the nuclear localization sequence in a nuclear protein?

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

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?

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

You are interested in Fuzzy, a soluble protein that functions within the ER lumen. Given that information, which of the following statement must be true?

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

The figure below 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?

The C-terminus of this protein is cytoplasmic.

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

ER -> Golgi -> plasma membrane

Which of the following statements about vesicle budding from the Golgi is false?

Clathrin molecules are important for binding to and selecting cargos for transport.

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?

dynamin

Which of the following protein families are not involved in directing transport vesicles to the target membrane?

adaptins

An individual transport vesicle

will fuse with only one type of membrane.

N-linked oligosaccharides on secreted glycoproteins are attached to

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

Which of the following statements about disulfide bond formation is false?

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

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 (see figure). Unfortunately, immune cells do not seem to recognize this synthesized glycoprotein. Which of the following statements is a likely explanation for this problem?

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

Which of the following statements about the unfolded protein response (UPR) is false?

Activation of the UPR occurs when receptors in the cytoplasm sense misfolded proteins.

Which of the following statements about secretion is true?

The membrane of a secretory vesicle will fuse with the plasma membrane when it discharges its contents to the cell’s exterior.

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?

Cytosolic cholesterol levels will decrease relative to normal cells.

How did the nuclear envelope, ER, Golgi, endosomes, and lysosomes evolve?

They developed through invagination of the plasma membrane.

How did mitochondria and chloroplasts evolve according to endosymbiotic theory?

They evolved from bacteria that were engulfed by primitive eukaryotic cells.

Which organelles constitute the endomembrane system?

The ER, Golgi apparatus, peroxisomes, endosomes, and lysosomes.

What determines the destination of a protein synthesized in the cytosol?

Its amino acid sequence, which can encode a specific sorting signal.

What happens to a cytosolic protein that lacks a sorting signal?

It remains permanently in the cytosol.

What are the three major mechanisms for protein transport into organelles?

Transport through nuclear pores, protein translocators in membranes, and transport vesicles.

How do nuclear pores selectively restrict the movement of macromolecules?

Unstructured regions of pore proteins form a soft, tangled meshwork that blocks large molecules while allowing small molecules to freely diffuse.

What sequence directs proteins to the nucleus and what amino acids characterize it?

Nuclear localization signal, which typically contains several positively charged lysine or arginine residues.

How does the monomeric GTPase Ran regulate nuclear import?

Ran-GTP binds the import receptor inside the nucleus to trigger cargo release, then exits to the cytosol where GTP hydrolysis causes Ran-GDP to dissociate.

What requirement must be met for proteins to cross the membranes of mitochondria and chloroplasts?

Proteins must unfold as they pass through protein translocators across both inner and outer membranes.

What two protein components target an ER signal sequence to the ER membrane?

Signal recognition particle (SRP) and an ER-membrane-bound SRP receptor.

What enzyme removes the signal sequence once a protein is translocated into the ER lumen?

Signal peptidase.

How is a single-pass transmembrane protein inserted into the ER membrane?

A hydrophobic stop-transfer sequence pauses translocation, gets released into the lipid bilayer, and forms a membrane-spanning alpha-helix.

What cytosolic protein coat initiates vesicle formation by creating a basketlike network?

Clathrin.

Which GTPase ring structure pinches off clathrin-coated vesicles from the membrane neck?

Dynamin.

How are target membranes identified by incoming transport vesicles?

Rab proteins on the vesicle surface are recognized by corresponding tethering proteins on the target membrane.

How do SNARE proteins catalyze membrane fusion?

v-SNAREs on vesicles bind t-SNAREs on target membranes and wrap tightly together to pull lipid bilayers together.

How does N-linked glycosylation occur in the ER lumen?

A 14-sugar preformed oligosaccharide is transferred from the lipid dolichol to an asparagine side chain on the protein.

What four-amino-acid sequence retains proteins in the ER?

An ER retention signal located at the C-terminus.

What happens to misfolded proteins in the ER that fail to fold correctly with chaperone assistance?

They are exported to the cytosol and degraded by the proteasome.

What triggers the Unfolded Protein Response (UPR) and what does it induce?

Accumulation of misfolded proteins in the ER triggers the UPR to expand ER volume and produce more chaperone proteins.

How do constitutive and regulated exocytosis differ?

Constitutive exocytosis operates continuously without a signal sequence, whereas regulated exocytosis requires a specific external stimulus to release aggregated vesicles.

How do phagocytosis and pinocytosis compare in vesicle size?

Phagocytosis uses large vesicles (>250 nm) for large particles, while pinocytosis uses small vesicles (<150 nm) for fluids and small molecules.

How are lysosomal enzymes targeted to lysosomes during Golgi processing?

By the covalent attachment of a mannose-6-phosphate sugar group.

What is autophagy?

The degradation of cytosolic proteins or damaged organelles enclosed within a double-membrane autophagosome that fuses with a lysosome.

CO2 and O2 are water-soluble molecules that diffuse freely across cell membranes. (True/False)

True

Transporters are similar to channels, except that they are larger, allowing folded proteins as well as smaller organic molecules to pass through them. (True/False)

False (Transporters move small solutes/molecules via conformational changes, not folded proteins)

Cells expend energy in the form of ATP hydrolysis so as to maintain ion concentrations that differ from those found outside the cell. (True/False)

True

Although extracellular environment has high Na+ and intracellular has high K+, both are neutralized by anions. What is the principal extracellular anion?

Cl– (Chloride)

Which molecule is more likely to diffuse through the lipid bilayer: amino acids or benzene?

Benzene

Which molecule is more likely to diffuse through the lipid bilayer: Cl– or ethanol?

Ethanol

Which molecule is more likely to diffuse through the lipid bilayer: H2O or O2?

O2

Which uncharged polar molecule has the LOWEST rate of diffusion across a synthetic membrane: glucose, water, glycerol, or ethanol?

Glucose

Fill in the blanks: A molecule moves down its concentration gradient by [A] transport, but requires [B] transport to move up. Transporter proteins and channels provide a [C] pathway. [D] bind solutes specifically and change conformation, while [E] discriminate mainly by size and electrical charge.

A: passive | B: active | C: hydrophilic | D: transporter proteins | E: ion channels

Channels discriminate by size and charge. In addition to Na+, which ion would you expect to freely diffuse through a Na+ channel? (Mg2+, H+, K+, or Cl–)

H+ (Proton)

Transporters, in contrast to channels, work by _____________.

Specific binding to solutes

Pumps harness energy to move solutes across membranes against their concentration gradient. This transport is called _____________.

Active transport

Transporters undergo transitions between different conformations, depending on whether the substrate-binding pocket is empty or occupied. (True/False)

True

The electrochemical gradient for K+ across the plasma membrane is small. Therefore, movement of K+ from inside to outside is driven solely by its concentration gradient. (True/False)

True

The net negative charge on the cytosolic side of the membrane enhances the rate of glucose import into the cell by a uniporter. (True/False)

False (Glucose is uncharged, so membrane potential does not affect its transport rate)

Active transport requires energy. Which of the following is NOT a common mechanism for active transport? (Na+-coupled, K+-coupled, ATP-driven, light-driven)

K+-coupled

What happens immediately after the Na+-K+ pump hydrolyzes ATP?

The pump is phosphorylated.

Ca2+ pumps in the plasma membrane and ER are important for _____________.

Preventing Ca2+ from altering the activity of molecules in the cytosol

Fill in the blanks: Passive transport of uncharged molecules is determined by [A] gradient. For charged molecules, [B] is also considered, forming the [C] gradient. [D] transporters use downhill movement of one solute to drive another uphill. Same direction = [E]; opposite = [F].

A: concentration | B: membrane potential | C: electrochemical | D: coupled | E: symport | F: antiport

Which best describes the behavior of a gated channel in response to a given stimulus?

It opens more frequently in response to a given stimulus.

The ion selectivity of a channel depends solely on the charge of the amino acids lining the pore. (True/False)

False (Selectivity depends on both size/structure of the selectivity filter and charge)

Most ion channels are gated, allowing them to open and close in response to a specific stimulus. (True/False)

True

The stimulation of auditory nerves depends on channels in auditory hair cells. Which gating mechanism do these cells use?

Stress-gated (mechanically-gated)

The acetylcholine receptor in skeletal muscle cells is a _______ ion channel.

Ligand-gated

_______ ion channels are found in the hair cells of the mammalian cochlea.

Stress-gated

_______ ion channels respond to changes in membrane potential.

Voltage-gated

Many receptors for neurotransmitters are _______ ion channels.

Ligand-gated

When exposed to a threshold membrane potential, how does a population of voltage-gated channels behave?

Some channels remain closed and some open completely.

K+ leak channels open and close randomly in resting plasma membranes. What do they accomplish in a resting cell?

They keep the electrochemical gradient for K+ at zero (establishing resting potential).

When using the Nernst equation, which assumption is INCORRECT? (A: 37°C, B: Membrane primarily permeable to Na+, C: Interior negative at rest, D: K+ principal positive ion inside)

B: "The plasma membrane is primarily permeable to Na+" (It is primarily permeable to K+ at rest).

If Na+ channels open in a cell at rest, how will the resting membrane potential be affected?

It becomes more positive (depolarizes).

Neurotransmitters are released into the synaptic cleft after fusion of synaptic vesicles with the presynaptic membrane. (True/False)

True

Action potentials are usually mediated by voltage-gated Ca2+ channels. (True/False)

False (Action potentials in neurons are primarily mediated by voltage-gated Na+ channels)

Voltage-gated Na+ channels become automatically inactivated shortly after opening, preventing backward propagation of action potentials. (True/False)

True

Voltage-gated K+ channels open immediately in response to local depolarization, reducing the action potential magnitude. (True/False)

False (Voltage-gated K+ channels open with a slight delay [delayed rectifiers] to help repolarize the cell after peak)

Which channel type is required for neurotransmitter secretion at the presynaptic nerve terminal?

Voltage-gated Ca2+ channels

[Action Potential Curve] What is indicated by the initial slight rise in membrane potential before reaching threshold?

Effect of a depolarizing stimulus

[Action Potential Curve] What measurement is indicated by the baseline membrane potential before stimulus (~ -70mV)?

Resting membrane potential

[Action Potential Curve] What level is indicated by the dashed line that triggers an all-or-none response?

Threshold potential

[Action Potential Curve] What is indicated by the large transient upward spike in membrane potential?

Action potential

Fill in the blanks: Action potential is a wave of [A] triggered when potential becomes [B] negative. Propagated by [C]-gated channels. Potential changes from [D] to [E]. Travels along [F] to terminals. Signals received at [G].

A: depolarization | B: less | C: voltage | D: negative | E: positive | F: axon | G: dendrites

What type of neurotransmitter is released at neuromuscular junctions between motor neurons and muscle cells?

Acetylcholine

By what mechanism do inhibitory neurotransmitters prevent postsynaptic action potentials?

By opening Cl– channels (hyperpolarizing the postsynaptic cell)

Fill in the blanks: Neurons communicate at [A]. Receptors are ligand-gated channels in the [B] cell membrane responding to signals from the [C] cell. They convert [D] signals into [E] ones. Release is stimulated by voltage-gated [F] channels.

A: synapses | B: postsynaptic | C: presynaptic | D: chemical | E: electrical | F: Ca2+ channels

In a squid giant axon experiment, doubling internal K+ (via KOAc) while maintaining electrical balance makes resting potential…

More negative (hyperpolarizes the membrane)

In a squid giant axon experiment, doubling extracellular Na+ causes the action potential to be…

Larger (increases peak amplitude)

In a squid giant axon experiment, replacing half of extracellular NaCl with choline chloride causes the action potential to be…

Smaller (decreases peak amplitude)

Which small water-soluble molecules can freely diffuse across the lipid bilayer?

CO2 and O2

What are the two general classes of membrane transport proteins?

Transporters and Channels

How do transporters move small solutes across membranes?

By changing their shape (conformation)

How do channels allow solutes to cross the membrane?

They form small hydrophilic pores that allow solutes to pass by passive diffusion