Unit 6: The Endomembrane System and Protein Trafficking
How a cell addresses, folds, modifies, and ships every protein it makes to the exact compartment it belongs in, one vesicle at a time.
8 topics · 31 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Signal Sequences and Protein Sorting Decisions
- How it worksHow does a short stretch of amino acids at a protein's tip act as its zip code to a destination inside the cell?
- How it worksWalk me through how a signal recognition particle intercepts a ribosome making a secretory protein.
- Common mistakeWhy doesn't every protein made in the cytosol automatically end up outside the cell?
- Going furtherWhat happens to a protein's destination when its signal sequence is experimentally deleted?
Your turn1 of 2
A protein is being synthesized on a free ribosome in the cytosol. A student assumes that because translation happens in the cytosol, the finished protein will automatically be secreted outside the cell. Why is this wrong?Co-translational Translocation into the Rough ER
- How it worksHow does the signal recognition particle dock a translating ribosome onto the ER's translocon channel?
- How it worksWalk me through how the growing polypeptide threads through the translocon pore into the ER lumen as translation continues.
- Common mistakeWhy isn't every protein synthesized on a ribosome attached to the rough ER?
- Going furtherWhat happens to a membrane protein's orientation when a stop-transfer sequence halts it partway through the translocon?
Your turn1 of 2
A student assumes that because ribosomes can dock onto the rough ER, every ribosome in the cell eventually ends up attached there. Why isn't every protein synthesized on a ribosome attached to the rough ER?Protein Folding and Quality Control in the ER
- How it worksHow do ER chaperones like BiP recognize and hold onto an incompletely folded protein?
- How it worksHow does glycosylation act as a timer that tags a protein for another folding attempt or for destruction?
- Common mistakeWhy doesn't a protein that fails ER quality control simply get released into the cytosol unmodified?
- Ties togetherHow does ER-associated degradation connect the endomembrane system to the cytosol's own protein-destruction machinery?
Your turn1 of 2
A newly made protein in the ER fails quality control because it hasn't folded correctly. A student assumes the ER simply lets such proteins leak out into the cytosol unmodified once folding attempts stop. What actually happens instead?The Golgi Apparatus and Sequential Protein Modification
- How it worksHow do proteins move from the cis face to the trans face of the Golgi while enzymes modify them along the way?
- How it worksWalk me through how the Golgi sorts finished proteins into different vesicles addressed to different destinations.
- Common mistakeWhy isn't the Golgi apparatus just a passive tunnel that proteins pass through unchanged?
- Going furtherWhat happens to a protein's final sugar structure when a specific Golgi enzyme is missing?
Your turn1 of 2
A protein enters the Golgi apparatus at its cis face and exits at its trans face carrying a very different set of attached sugars than it had when it entered. A student assumes the Golgi is just a passive tunnel proteins pass through unchanged. Why is this wrong?Vesicle Coats: COPII, COPI, and Clathrin
- How it worksHow does a COPII coat select and concentrate cargo while pulling the ER membrane into a bud?
- How it worksHow does a clathrin coat form a lattice that pinches a vesicle off the plasma membrane?
- Common mistakeWhy doesn't a vesicle keep its coat all the way until it fuses with its target membrane?
- Going furtherWhat happens to ER-to-Golgi transport when the gene for a COPII coat protein is knocked out?
Your turn1 of 2
A vesicle buds off the ER wearing a COPII coat. A student assumes that coat stays on the vesicle all the way until it fuses with the Golgi. Why isn't that what actually happens?SNARE Proteins and the Specificity of Membrane Fusion
- How it worksHow do complementary v-SNARE and t-SNARE proteins zipper together to pull two membranes into fusion?
- Common mistakeWhy isn't vesicle fusion driven by chance collision between a vesicle and any nearby membrane?
- Going furtherWhat happens to neurotransmitter release when a toxin like botulinum cleaves a SNARE protein?
Your turn1 of 2
A vesicle budding from the ER reliably fuses only with the Golgi and never with a lysosome, even though both organelles are nearby in the cell. A student assumes this specificity comes down to which membrane the vesicle happens to bump into by chance. Why is this wrong?Lysosomes, Endocytosis, and Intracellular Digestion
- How it worksHow does a lysosome pump protons into itself to reach the acidic pH its enzymes need?
- How it worksWalk me through how a cell surrounds a bound ligand at its surface and pulls it inward as an endocytic vesicle.
- How it worksHow does an endosome mature into a lysosome as it acidifies and picks up digestive enzymes?
- Common mistakeWhy doesn't a lysosome's own membrane get digested by the enzymes it contains?
- Going furtherWhat happens to a cell's ability to clear damaged organelles when a lysosomal storage enzyme is missing?
Your turn1 of 2
A lysosome is packed with enzymes capable of digesting proteins, lipids, and nucleic acids, yet its own membrane remains intact. Why doesn't a lysosome digest itself with the enzymes it contains?Exocytosis and Regulated Secretion
- How it worksHow does a rise in intracellular calcium trigger a docked secretory vesicle to fuse with the plasma membrane?
- Common mistakeWhy isn't regulated secretion the same process as the constant, unregulated exocytosis that maintains the plasma membrane?
- Going furtherWhat happens to hormone release when the calcium signal that triggers vesicle fusion never arrives?
Your turn1 of 2
A pancreatic cell continuously inserts new membrane proteins into its plasma membrane through ongoing exocytosis, while separately holding thousands of insulin-packed vesicles in reserve until a specific signal arrives. A student assumes these are the same kind of exocytosis happening at different speeds. Why is this wrong?