Cell Biology: Essential Principles of Membrane Transport
Cellular Compartments and Nuclear Transport
Like the lumen of the ER, the interior of the nucleus is topologically equivalent to the outside of the cell. (True)
Researchers in a biotechnology company have 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.)
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.)
Membrane Structure and Protein Synthesis
On average, how many amino acids engaged in predominantly hydrophobic alpha-helices does it take to cross the hydrophobic part of the membrane? (18 – 22 amino acids)
The endoplasmic reticulum is localized all over the cytoplasm. (True)
To what residue of a polypeptide are N-linked oligosaccharide chains attached as that polypeptide enters the rough ER lumen through the translocator? (Asparagine)
Through which of the following ways can an integral membrane protein possibly associate with a cellular membrane? (All)
In which cellular location would you expect to find ribosomes translating mRNAs that encode ribosomal proteins? (Cytosol)
How are integral membrane proteins thought to enter the lipid bilayer? (The aqueous translocator channel appears to have a gate that continuously opens and closes, giving each nascent polypeptide segment a chance to partition itself into the lipid bilayer’s hydrophobic core.)
New membrane phospholipids are synthesized by enzymes bound to the cytosolic side of the endoplasmic reticulum membrane.
Which of the following would yield the most highly mobile phospholipid? (16 carbons with 2 double bonds)
All of the common phospholipids—phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and sphingomyelin—carry a positively charged moiety on their polar head group, but none carry a net positive charge. (True)
What are the differences between ribosomes that make secretory proteins and those that make proteins intended for the cytosol? (There are no differences between them.)
Which of the following proteins are unlikely the precursors of the inner mitochondrial protein? (Proteins initially located in the lumen of the endoplasmic reticulum.)
Imagine that you’ve engineered a plasmid that expresses a GFP-fused protein, which contains an N-terminal endoplasmic reticulum (ER) signal sequence, a nuclear localization signal, a nuclear export signal, and a C-terminal peroxisomal targeting sequence. Where would you expect to find the protein after its synthesis in a mammalian cell? (Endoplasmic reticulum)
What are the two major sites within a cell at which protein synthesis is generally thought to occur? (Cytosolic surface of rough ER and free ribosomes)
Which of the following motions of a lipid molecule in a lipid bilayer rarely occurs? (Flip-flop)
A signal-recognition particle (SRP), which directs the ER signal sequence to a specific receptor in the rough ER membrane, is a complex containing six protein subunits and one RNA molecule. (True)
Unlike soluble cytosolic proteins, integral membrane proteins are more difficult to purify. Which of the following substances is most commonly used to help purify an integral membrane protein? (Detergent)
Ion Channels and Transport Mechanisms
A channel that opens in response to changes in ionic charge across a membrane is called a voltage-gated channel.
Transport by transporters can be either active or passive, whereas transport by channels is always passive. (True)
A neuron’s repetitive firing rate is limited by an absolute refractory period, during which a new action potential cannot be generated. Which event is chiefly responsible for this limit? (Inactivation of voltage-gated Na+ channels)
Hydropathy plots are useful for identifying hydrophobic polypeptide segments that are long enough to span a membrane as an alpha helix or as a beta sheet. (False)
Imagine a small synthetic vesicle made from pure phospholipids enclosing an interior lumen containing 1 mM glucose and 1 mM sodium chloride. If the vesicle is placed in pure water, which of the following happens faster? (H2O diffuses in)
Ca2+ pumps in the plasma membrane and endoplasmic reticulum are important for preventing Ca2+ from altering the activity of molecules in the cytosol.
An ion channel mediates passive transport, is ion-selective, is typically several orders of magnitude faster than a transporter, and is usually gated. (All)
In the Na+/glucose cotransporter, Na+ ions moving down their gradient drive the transport of glucose against its gradient.
The sodium-potassium pump makes the cell interior more negative by pumping 3 sodium ions out of the cell for every 2 potassium ions pumped in.
Like FRAP (fluorescence recovery after photobleaching), FRET (fluorescence resonance energy transfer) is also used to study the dynamics of molecules in a living cell. (True)
At equilibrium across a biological membrane, ions often are not at the same concentration on both sides. (True)
Protein transport into mitochondria is co-translational. (False)
N-linked glycosylation can be carried out co-translationally, possibly at multiple asparagine residues on the same protein molecule. (True)
Which of the following is the reason why an aquaporin does not pass ions (such as Na+) through? (The Na+ ions do not favorably interact with the wall of the pore and therefore cannot be dehydrated.)
The plasma membrane is highly impermeable to all charged molecules. (False)
A transport system that moves one solute into the cell and another one out of the cell during a single cycle accompanied by the expenditure of energy through ATP hydrolysis could be called active transport.