Intracellular Transport Flashcards
(25 cards)
Which of the following organelles is the site of steroid hormone synthesis in endocrine cells?
smooth endoplasmic reticulum
The endoplasmic reticulum (ER) is the location of synthesis for proteins (rough ER) and many lipids including steroid hormones (smooth ER).
Which of the following organelles is surrounded by a single membrane?
Golgi apparatus
The nucleus, mitochondria, and chloroplasts are each surrounded by a double membrane, and organelles like the Golgi apparatus, lysosomes, and the endoplasmic reticulum have a single membrane.
Approximately what percentage of the volume of a typical eukaryotic cell is comprised of cytosol?
50%
The volume of a typical eukaryotic cell is about 50% cytosol and the remaining 50% is occupied by membrane-bound organelles.
Which of the organelles is NOT a part of the endomembrane system?
nucleus
The endomembrane system is a set of organelles that extensively communicate via vesicle budding. The organelles in the system include the endoplasmic reticulum, the Golgi apparatus, endosomes, lysosomes, and peroxisomes.
Which of the following compartments receives proteins directly from cytosol?
peroxisomes
Proteins destined for the nucleus, mitochondrion, peroxisomes, and chloroplasts are transported from the cytosol. Proteins destined for the Golgi, lysosomes, and endosomes are transported into the endoplasmic reticulum as they are being synthesized and are then transported via vesicles to their final destination.
What would the final destination be for a protein bearing both an ER sorting signal and a nuclear localization signal?
endoplasmic reticulum
An endoplasmic reticulum sorting signal directs a protein to the ER while it is still being synthesized, thus an ER sorting signal would supersede a nuclear localization signal. A nuclear localization signal imports a completely synthesized protein from the cytosol to the nucleus.
Fully folded proteins can be transported into which of the following organelles?
nucleus
Transporters in the membranes of mitochondria and chloroplasts unfold the protein during transfer. Proteins destined for the endoplasmic reticulum are transported as they are being synthesized, thus the proteins are unfolded. Nuclear transport occurs on folded proteins.
Nuclear pores restrict larger molecules from traversing the membrane due to their
interwoven meshwork of protein fibrils.
A nuclear pore is a large passage in the nuclear membrane whose opening is filled with protein fibrils that allow passage of small, water-soluble molecules. Larger molecules are impeded from entering by this meshwork of protein fibrils (unless they have a nuclear localization or export sequence).
Which of the following accurately describes a step in GTP-driven nuclear transport?
Binding of Ran-GTP to the receptor releases the cargo protein.
GTP hydrolysis ensures that nuclear import occurs in the proper direction. This is accomplished by a small GTPase protein called Ran. In the nucleus it is in its GTP-bound form and binds to nuclear import receptors, causing them to release their cargo. It is then shuttled back to the cytosol with the nuclear import receptor, and upon entry, hydrolysis of GTP is stimulated, and leads to release of the receptor to bind more cargo.
Proteins encoded by nuclear genes and destined for the mitochondrial matrix are
in possession of a signal sequence for targeting to the mitochondria.
Mitochondrial proteins that are encoded by genes in the nucleus are synthesized on free ribosomes in the cytosol. A signal sequence in the proteins is recognized by import machinery that transports the unfolded protein across both membranes at once.
A single-pass transmembrane protein destined for one of the organelles in the endomembrane system would be marked by what type of signal sequence?
a cleaved N-terminal ER signal sequence and an internal stop-transfer sequence
Proteins that are destined for the ER lumen have an N-terminal signal sequence—a hydrophobic region that, once cleaved, releases the protein to the interior of the ER. If the protein also contains an internal stop-transfer sequence, the transfer of the protein is halted once it reaches this hydrophobic region, and it remains embedded in the ER membrane. The N-terminal signal sequence is still cleaved to release the N-terminal end from the membrane.
Which of the following is a difference between exocytic and endocytic pathways?
Exocytic pathways often start with synthesis of proteins, whereas endocytic pathways involve breaking down macromolecules like proteins.
Endocytic pathways and exocytic pathways both use transport vesicles to move lipids, membrane components, proteins, and soluble molecules from the outside of the cell to inside of the cell or vice versa. Endocytic pathways bring molecules in from the outside of the cell into an endosome, which can then mature into a lysosome. Endocytosed molecules do not travel to the Golgi in vesicles.
How do clathrin-coated vesicles select their cargo molecules?
Cargo receptors bind specifically to cargo proteins and to clathrin.
Vesicles destined for different compartments have different types of protein coats. The cargo for these vesicles is selected by specifically binding to cargo receptors that interact with a specific type of protein coat.
In which process do Rab proteins function?
vesicle tethering
Rab proteins are a family of small GTPases that are specific for each type of organelle and vesicle. Rab proteins on vesicles are recognized by tethering proteins on the target membrane and help capture and tether the vesicle for later docking and fusion.
Which of the following is a covalent modification that occurs mainly in the ER?
formation of disulfide bonds
Two major covalent modifications occur in the ER. Certain proteins need disulfide bonds to form between cysteines to stabilize their structure, and some proteins have a branched oligosaccharide added to them, which is then matured in the Golgi into the final structure glycoprotein structure.
How are misfolded proteins and incompletely assembled proteins retained in the ER?
Chaperone proteins bind them and prevent their entry into vesicles.
Misfolded proteins or incompletely assembled protein complexes are bound by chaperone proteins that help guide their proper folding. This prevents them from leaving the ER or from aggregating and causing issues while folding is attempted.
What is one of the main differences in the behavior of the proteins in a vesicle destined for constitutive secretion, and the proteins in the vesicle destined for regulated secretion?
Proteins in the regulated secretion vesicle tend to aggregate and become highly concentrated in the ionic conditions in the vesicle.
Constitutive secretion vesicles contain lipids and proteins that are continuously supplying the plasma membrane with new components. Proteins in regulated secretion vesicles form concentrated aggregates so that when they are released in response to a signal, the levels of the protein can rapidly increase.
Which of the endocytic pathways involves the ingestion of large particles or microorganisms and is performed mainly by specialized cells?
phagocytosis
The endocytic pathways are phagocytosis and pinocytosis. Pinocytosis involves the intake of molecules and fluid and is active in all cells. Phagocytosis is a process by which microorganisms or large particles are ingested by a specialized phagocytic cell, such as those in the immune system.
Which of the following pathways helps selectively concentrate substances to be ingested by their binding to proteins on the cell surface?
receptor-mediated endocytosis
Receptor-mediated endocytosis is a form of pinocytosis where specific molecules are concentrated on the cell surface by their binding to receptors. The receptors are then taken up in a clathrin-coated vesicle for delivery to the endosome.
Lysosomes contain ____________ enzymes that can break down diverse macromolecules, cell parts, and microorganisms.
hydrolytic
Lysosomes are a compartment in the cell where ingested cell particles, organelles, or macromolecules can be digested for recycling. The macromolecules from ingestion or from these cell parts are broken down into their building blocks by hydrolytic enzymes and are then exported out of the lysosome for reuse.
Proteins that are fully translated in the cytosol do not end up in _______.
transport verhicles
Proteins destined for transport vesicles will be translated on ribosomes associated with the endoplasmic reticulum.
Proteins that are fully translated in the cytosol and lack a sorting signal will end up in ____.
the cytosol
Signal sequences that direct proteins to the correct compartment are _________.
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?
in the cytosol
