120 questions spanning Diversity, Membranes, Transport, Intracellular Compartments, Protein Sorting, and Recycling. Weighted toward the distinctions that separate an A from a B: the verified lecture-vs-textbook corrections, second-order reasoning, and cross-topic synthesis. This is not a re-run of the per-topic Chapter Exams — it tests whether the pieces connect. Verified against Alberts, Molecular Biology of the Cell, 7th ed.
Coverage: Topics 2–6 (Ch. 1, 10, 11, 12, 13, 14, 3, 6)Format: 120 × Scantron MCQ · one submissionScored only on submit — then copy your review list
correct answer + mechanism distractor = named misconception▲ marks a verified lecture-vs-textbook correction
§How to Use This Exam
The six Chapter Exams already gave you 192 topic-level questions. This one is deliberately different: fewer gimme-recall items, more "why," "predict," and "how do these connect." If you can clear this at 85%+, you are ready for the real Midterm 1 mock exam.
Strategy built into the question mix: every verified correction from the topic files is re-tested here, because those are exactly the high-confidence-wrong traps a well-written final exploits. The final 12 questions (§Integration) test cross-topic connections directly — the single highest-value section, since integration is what a cumulative exam is for and what per-topic study misses.
Do it in one sitting, no feedback until submit — that's the real exam condition. Answer all 120, submit once, then work the copy-pasteable review list against the linked topic files. Unanswered questions score as wrong, same as the real Scantron.
◎Coverage Map & Correction Watchlist
Where the questions sit, and the trap in each block. If any correction below isn't reflexive, that's your first place to review.
Topic 2 · Diversity — Q1–18
Three domains, universal features, viruses, endosymbiosis intro. ▲ Margulis proposed endosymbiosis 1967 (not 1981); Plasmodium is a protozoan, not a virus.
Topic 3 · Membranes — Q19–36
Fluid mosaic, fluidity buffering, asymmetry, rafts. ▲ The hydrophobic effect is entropy-driven (water ordering), not an attractive "hydrophobic bond" between tails.
Topic 4 · Transport — Q37–54
Channels vs carriers, pumps, gradients. ▲ K⁺ leak channels dominate resting potential (pump <10%); ▲ K⁺ selectivity is by carbonyl-oxygen geometry, not pore size.
Topic 5a · Compartments — Q55–72
Mitochondria, oxphos, mtDNA, nucleus. ▲ Chloroplasts have three membrane systems (not two); ▲ glycolysis is cytosolic; Margulis 1967 recurs.
Topic 5b · Sorting — Q73–90
Import routes, topology, coats, secretion. ▲ ER→Golgi vesicles are COPII-coated (not clathrin); the cleaved ER signal peptide stays in the translocon.
Topic 6 · Recycling — Q91–108
Lysosome/peroxisome/proteasome, ubiquitin. ▲ Ubiquitin: ~3 residues differ (not "one"); the proteasome yields peptides, not amino acids.
§ Integration — Q109–120
Cross-topic synthesis: endosymbiosis→mtDNA→antibiotics, M6P→I-cell, translocon vs proteasome, proton gradients across topics, signal-by-shape.
Scoring target
≥90% = exam-ready. 75–89% = close specific gaps. <75% = the misses will cluster; rework those topics and retake. Review list tells you exactly where.
✔Midterm 1 Mock Exam
120 questions. No feedback until you submit. Answer everything, then score and copy the review list.
Topic 2 — Diversity of Living Organisms (Q1–18)
1. Which feature is shared by all three domains of life and therefore taken as evidence of common ancestry?
Peptidoglycan walls are a bacterial trait; Archaea and many eukaryotes lack it. (T2)
Only Eukarya have a nucleus; Bacteria and Archaea do not. A universal feature must be present in all three. (T2)
Mitochondria are a eukaryotic (endosymbiotic) acquisition; prokaryotes make ATP without them. (T2, T5a)
Answer as lectured Ribosomal translation and a near-universal genetic code are shared across all life — a core argument for a single common ancestor. (T2)
2. In what year did Lynn Margulis propose the endosymbiotic theory of organelle origin?
1981 is when her book appeared; the theory was proposed in 1967. (T5a)
Answer as lectured Margulis proposed endosymbiosis in 1967; her well-known book followed in 1981. Conflating the book date with the proposal date is the common error. (T2, T5a)
The proposal predates this by decades (1967). (T5a)
1953 is Watson–Crick's DNA structure, unrelated to endosymbiosis. (T2)
3.Plasmodium, the agent of malaria, is correctly classified as a:
It belongs to Eukarya, not Archaea. (T2)
Answer as lecturedPlasmodium is a single-celled eukaryotic protozoan parasite — it has a nucleus and organelles, unlike a virus. (T2)
Malaria is often loosely called "viral," but Plasmodium is a eukaryotic protozoan, not a virus. (T2)
It is a eukaryote (protozoan), not a prokaryotic bacterium. (T2)
4. Why are viruses not classified as living cells?
Viruses are nucleic acid plus protein (some with a lipid envelope); the issue is no ribosomes/metabolism. (T2)
Answer as lectured Viruses have no protein-synthesis machinery or metabolism of their own; they hijack a host, so they fail the criteria for a cell. (T2)
Viruses do carry nucleic acid (DNA or RNA); what they lack is ribosomes and metabolism. (T2)
Viruses are far smaller than cells; size isn't the disqualifier. (T2)
5. Archaea are grouped as a separate domain from Bacteria primarily because:
Answer as lectured Despite prokaryotic morphology, Archaea differ from Bacteria at the molecular level and share features with Eukarya, justifying a separate domain. (T2)
Archaea are single-celled prokaryotes. (T2)
Archaea are prokaryotes without a nucleus; the distinction is molecular, not nuclear. (T2)
Many Archaea are extremophiles; regardless, the classification rests on molecular differences. (T2)
6. The "C-value paradox" refers to the observation that:
Answer as lectured Some simple organisms have larger genomes than complex ones; total DNA amount doesn't track complexity or gene number cleanly. (T2)
The paradox is that this expected correlation fails. (T2)
Genome sizes vary enormously; the paradox is the lack of correlation with complexity. (T2)
That's not what C-value refers to; it concerns genome size vs complexity. (T2)
7. The central dogma of molecular biology describes information flow as:
Information flows DNA→RNA→protein; proteins are the endpoint, not the template. (T2)
RNA is the obligatory intermediate; there's no direct DNA→protein route. (T2)
Answer as lectured Genetic information is transcribed from DNA to RNA and translated into protein — the universal core flow. (T2)
The standard flow starts from DNA; RNA→DNA occurs only in special cases (e.g. retroviruses). (T2)
8. A researcher wants to study a simple multicellular animal with a fully mapped cell lineage and nervous system. The best-established model is:
E. coli is a single-celled bacterium — no cell lineage or nervous system. (T2)
Yeast is a great model but unicellular, with no nervous system. (T2)
Arabidopsis is the model plant — no animal nervous system or cell-lineage map. (T2)
Answer as lecturedC. elegans has a completely mapped cell lineage and connectome, making it the classic simple-animal model. (T2)
9. The single strongest structural difference between a prokaryotic and a eukaryotic cell is:
Both have DNA; eukaryotes enclose it in a nucleus. (T2)
All cells have a plasma membrane; the nucleus is the distinguishing organelle. (T2)
Both have ribosomes (different sizes); the nucleus is the key difference. (T2)
Answer as lectured The defining split is the presence of a nucleus (and other membrane organelles) in eukaryotes. (T2)
10. Horizontal gene transfer complicates the "tree of life" because:
HGT complicates the tree's topology but doesn't negate common ancestry. (T2)
Answer as lectured HGT moves genes across branches, blurring a strictly branching tree — especially among prokaryotes. (T2)
HGT is especially common among prokaryotes. (T2)
HGT is horizontal (across lineages), distinct from vertical inheritance. (T2)
11. Which observation is the most direct line of evidence that mitochondria arose by endosymbiosis?
They retain their own DNA — that's the evidence. (T5a)
They arise only from pre-existing mitochondria (they can't be built de novo), consistent with endosymbiotic origin. (T5a)
Mitochondria are in nearly all eukaryotes, not just plants. (T5a)
Answer as lectured A residual bacterial-style genome and ribosomes are hallmark evidence of a free-living bacterial ancestor. (T2, T5a)
12. The near-universality of the genetic code across all domains is best explained by:
Answer as lectured A shared code across all life points to inheritance from a single common ancestor. (T2)
The code is inherited vertically from a common ancestor, not swapped between organisms. (T2)
The code is largely a "frozen accident," not chemically forced; universality reflects shared ancestry. (T2)
Repeated independent origin of an identical code is far less parsimonious than common descent. (T2)
13. Compared with a bacterium, a typical eukaryotic cell is:
Answer as lectured Eukaryotic cells are generally much larger and use organelles to compartmentalize functions. (T2, T5a)
Eukaryotes have an extensive cytoskeleton. (T2)
They differ substantially in both size and internal organization. (T2)
Eukaryotic cells are typically larger and more complex. (T2)
14. Natural selection acts most directly on:
Answer as lectured Selection requires heritable, fitness-affecting variation; that's what changes allele frequencies over generations. (T2)
Acquired, non-heritable traits aren't passed on, so selection can't act on them across generations. (T2)
Selection acts on organisms/populations via heritable variation, not on ATP. (T2)
Without variation there is nothing for selection to favor. (T2)
15. All cellular membranes across all domains share which basic architecture?
Membranes are lipid bilayers with proteins, not protein monolayers. (T3)
The defining membrane feature is the lipid bilayer. (T3)
Membranes are lipids/proteins, not DNA. (T3)
Answer as lectured The lipid-bilayer-plus-protein plan is universal, another shared feature of cellular life. (T2, T3)
16. Which statement about the three-domain system is correct?
They are separate domains, distinguished by molecular differences. (T2)
Plants/animals/fungi are eukaryotic kingdoms, not domains. (T2)
Viruses aren't cells and aren't a domain of cellular life. (T2)
Answer as lectured Life is divided into these three domains based on molecular phylogeny. (T2)
17. A key reason the same fundamental cell-biology principles apply from yeast to humans is that:
They transfer precisely because of evolutionary conservation. (T2)
Genomes differ; it's the conserved core mechanisms that transfer. (T2)
They're vastly diverged; conservation of key pathways is the point. (T2)
Answer as lectured Deep conservation is exactly why model organisms are informative about human biology. (T2)
18. Which pairing of organism and category is correct?
Plasmodium is eukaryotic; E. coli is a bacterium, not an archaeon. (T2)
Answer as lecturedPlasmodium is a eukaryotic protozoan; E. coli is a prokaryotic bacterium. (T2)
Plasmodium isn't a virus, and E. coli isn't a eukaryote. (T2)
Neither is a virus. (T2)
Topic 3 — Membrane Structure and Function (Q19–36)
19. The phospholipid bilayer forms spontaneously in water because phospholipids are:
The polar/charged part is the head group; the tails are nonpolar. (T3)
Answer as lectured The dual character drives spontaneous bilayer assembly that buries the tails away from water. (T3)
Fully hydrophilic molecules would dissolve, not form a bilayer core. (T3)
If they were fully hydrophobic they wouldn't form organized bilayers with hydrophilic surfaces. (T3)
20. The hydrophobic effect that drives membrane assembly is fundamentally driven by:
"Hydrophobic bond" is colloquial shorthand; there's no special attractive bond — water-entropy gain drives association. (T3)
The overall entropy increases (water becomes less ordered); that's what makes it favorable. (T3)
Answer as lectured Clustering nonpolar tails frees ordered water, raising entropy — the effect is entropy-driven, not an attractive bond between tails. (T3)
Tails aren't covalently linked; the driving force is the entropic hydrophobic effect. (T3)
21. Increasing the proportion of unsaturated fatty acid tails in a membrane makes it:
Saturation affects fluidity, not a switch to total impermeability. (T3)
Unsaturation lowers the transition temperature, keeping membranes fluid. (T3)
Answer as lectured Cis double bonds introduce kinks that disrupt packing, raising fluidity. (T3)
Kinks reduce packing and increase fluidity. (T3)
22. Cholesterol is best described as a fluidity buffer because it:
Answer as lectured Cholesterol dampens the extremes — stiffening fluid membranes and fluidizing cold ones — buffering against temperature swings. (T3)
At low temperature it prevents stiffening — buffering both ways. (T3)
Cholesterol is a major modulator of membrane fluidity. (T3)
It also decreases fluidity at high temperature; that's why it's a buffer. (T3)
23. Which movement of a phospholipid within a bilayer is the slowest and generally requires an enzyme?
Lateral diffusion is rapid and needs no enzyme. (T3)
Answer as lectured Moving a polar head across the hydrophobic core is very unfavorable and slow; flippases catalyze it. (T3)
Rotation is very rapid, not the slow step. (T3)
Tail flexing is rapid; flip-flop is the slow, enzyme-assisted motion. (T3)
24. Membrane carbohydrates (glycolipids/glycoproteins) are found:
Membrane sugar distribution is strictly asymmetric. (T3)
Answer as lectured Sugars are added in the ER/Golgi lumen and always face away from the cytosol — a fixed asymmetry. (T3, T5b)
Sugars are hydrophilic and sit on the surface, not in the core. (T3)
Carbohydrates face the non-cytosolic side, never the cytosol. (T3)
25. An integral (transmembrane) protein stays anchored in the membrane because its membrane-spanning region:
Anchoring is via hydrophobic interaction with the core, not covalent bonds to heads. (T3)
A charged stretch would be unstable in the hydrophobic core; TM segments are hydrophobic. (T3)
Membrane anchoring is a hydrophobic-matching phenomenon. (T3)
Answer as lectured A hydrophobic transmembrane segment is energetically stable in the bilayer core and won't spontaneously exit. (T3, T5b)
26. Lipid rafts are best described as:
Rafts are lipid-rich (cholesterol/sphingolipids), by definition. (T3)
They are ordered lipid regions, not holes. (T3)
Answer as lectured Rafts are more ordered, cholesterol/sphingolipid-rich patches that organize membrane proteins. (T3)
Rafts are dynamic lipid domains, not covalent protein grids. (T3)
27. The "mosaic" in the fluid-mosaic model refers to:
The model stresses fluidity, not a rigid fixed pattern. (T3)
Answer as lectured "Mosaic" captures the patchwork of proteins embedded in a fluid lipid sea. (T3)
Membranes contain lipids and proteins, not DNA. (T3)
"Mosaic" refers to interspersed proteins. (T3)
28. Which small molecule crosses a pure lipid bilayer most readily by simple diffusion?
Proteins are far too large/polar to diffuse across a bilayer. (T3)
Answer as lectured Small nonpolar gases dissolve in and cross the hydrophobic core easily. (T3, T4)
Ions are charged and hydrated; they don't cross the hydrophobic core without a channel/transporter. (T4)
Large polar molecules need a transporter to cross efficiently. (T4)
29. New membrane phospholipids are synthesized:
Lipids aren't DNA-templated; they're enzymatically synthesized in the ER. (T3)
Answer as lectured Lipid synthesis occurs on the cytosolic face of the ER; enzymes then move lipids to the other leaflet to grow both. (T3, T5b)
The ER (smooth ER) is the main site of membrane lipid synthesis. (T3, T5a)
Cells synthesize their own membrane lipids internally at the ER. (T3)
30. Membrane asymmetry (different lipids/proteins on the two leaflets) is functionally important because:
Asymmetry doesn't confer ion permeability; channels/transporters do. (T3, T4)
Asymmetry and fluidity are independent properties. (T3)
The leaflets differ; orientation is biologically meaningful. (T3)
Answer as lectured Each leaflet has distinct composition/function, and budding/fusion preserves which face points where. (T3, T5a, T5b)
31. Lowering temperature toward a membrane's transition point tends to:
Temperature changes physical state, not molecular identity. (T3)
Cooling decreases fluidity; heating increases it. (T3)
Answer as lectured Cold slows motion and tightens packing, reducing fluidity (cholesterol buffers this). (T3)
Temperature strongly affects lipid packing and fluidity. (T3)
32. A peripheral membrane protein differs from an integral one in that it:
Peripheral proteins are relatively easily dissociated (e.g. by salt), unlike integral ones. (T3)
Answer as lectured Peripheral proteins bind at the surface via weaker interactions and don't traverse the bilayer. (T3)
Peripheral proteins sit at the surface, not in the core. (T3)
Multipass spanning defines integral proteins, not peripheral ones. (T3)
33. The glycocalyx (carbohydrate layer) on the cell surface primarily functions in:
Unrelated to the surface carbohydrate layer. (T3)
Answer as lectured Surface sugars mediate recognition/adhesion and provide a protective coat. (T3)
ATP synthesis is mitochondrial; the glycocalyx handles recognition/protection. (T3, T5a)
Membrane potential comes from ion gradients/channels, not the glycocalyx. (T3, T4)
34. Why is describing lipid-tail association as a "bond" misleading?
The dominant driver is entropy (water ordering), not enthalpy. (T3)
Answer as lectured The "hydrophobic bond" is a misnomer — it's an entropic effect from releasing ordered water, not a bond. (T3)
There's no covalent bond between tails; the point is that "bond" language misrepresents an entropic effect. (T3)
They associate (aren't repelled); the mechanism is entropic, not a bond. (T3)
35. Which best explains why a bilayer is a barrier to ions but not to O₂?
Answer as lectured Crossing the core requires shedding hydration and passing charge through a nonpolar zone — easy for O₂, prohibitive for ions. (T3, T4)
The core is hydrophobic, which is precisely why ions are excluded. (T3)
Ions are tiny; the barrier is about charge/hydration, not size relative to proteins. (T4)
O₂ is nonpolar/neutral; ions are charged — that's why O₂ crosses and ions don't. (T4)
36. A membrane protein embedded in the ER membrane with its glycosylated domain in the ER lumen will, after reaching the plasma membrane, present that glycosylated domain:
Lumenal ≡ extracellular; the glycosylated domain faces out, never the cytosol. (T5a)
Plasma-membrane proteins don't route to the nucleus; the domain faces the extracellular space. (T5a)
Answer as lectured The lumenal face becomes the extracellular face (topology is preserved through trafficking), so the sugars end up outside. (T3, T5a, T5b)
A hydrophilic glycosylated domain sits on a surface, not in the core. (T3)
Topic 4 — Transport of Small Molecules (Q37–54)
37. The single best predictor of whether a transport process needs metabolic energy is:
Answer as lectured Moving a solute uphill (against its gradient) requires energy input; downhill movement is passive. (T4)
Ions can move passively (down gradient) or actively; direction relative to gradient is what matters. (T4)
Facilitated diffusion uses proteins but is passive; energy need depends on gradient direction. (T4)
Fluidity doesn't determine energy requirement; gradient direction does. (T4)
38. Ion channels differ from transporters (carriers) in that channels:
Channels are far faster than carriers. (T4)
Channels are passive (down-gradient); active pumping is a carrier/pump function. (T4)
Answer as lectured Channels conduct ions quickly through an open pore; carriers bind and undergo conformational changes, moving far fewer solutes per second. (T4)
Channels don't use ATP; they conduct down the gradient. (T4)
39. The resting membrane potential of a typical cell is set primarily by:
The pump is electrogenic but contributes <10% of the resting potential directly; K⁺ leak channels dominate. (T4)
Answer as lectured At rest the membrane is most permeable to K⁺ through leak channels, so Vm sits near EK; the pump contributes only a small direct amount. (T4)
K⁺ permeability through leak channels dominates the resting potential. (T4)
Glucose transport doesn't set membrane potential; K⁺ leak channels do. (T4)
40. The K⁺ channel selectivity filter discriminates K⁺ from the smaller Na⁺ by:
Selectivity is a passive structural property of the filter, not ATP-driven. (T4)
If it were just a size sieve, the smaller Na⁺ would pass more easily — the opposite of reality. Selectivity is by carbonyl geometry. (T4)
Answer as lectured The carbonyls substitute for water around K⁺ at exactly the right geometry; Na⁺ is too small to contact them properly, so it's excluded despite being smaller. (T4)
The filter coordinates (attracts/stabilizes) K⁺ via carbonyls; it doesn't repel it. (T4)
41. The Na⁺/K⁺ pump moves, per ATP hydrolyzed:
Answer as lectured The pump exports 3 Na⁺ and imports 2 K⁺ per ATP, making it electrogenic (net positive charge out). (T4)
It's 3 Na⁺ out, 2 K⁺ in — not the other way. (T4)
Na⁺ goes out and K⁺ comes in (3:2). (T4)
The ratio is 3:2, not 1:1. (T4)
42. Secondary active transport (e.g. Na⁺-glucose symport) is powered by:
It's powered by an ion gradient, not light. (T4)
Answer as lectured The downhill movement of Na⁺ drives glucose uphill; the Na⁺ gradient was built by the ATP-driven pump. (T4)
Glucose is moving against its gradient here — that's not simple diffusion. (T4)
Secondary transporters don't hydrolyze ATP directly; they use a pre-existing ion gradient. (T4)
43. An electrochemical gradient combines:
The concentration term matters too. (T4)
It's concentration plus voltage, not temperature/pressure. (T4)
For ions, the electrical term also matters. (T4)
Answer as lectured For a charged solute, both its concentration difference and the membrane voltage contribute to the driving force. (T4)
44. A ligand-gated ion channel opens in response to:
Voltage sensing defines voltage-gated channels, not ligand-gated. (T4)
Gating here is by ligand binding, not ATP. (T4)
Answer as lectured Ligand-gated channels open when their specific ligand binds — the basis of fast chemical synaptic transmission. (T4)
Stretch opens mechanically-gated channels. (T4)
45. Aquaporins increase the rate of:
Aquaporins move water; ATP synthesis is unrelated. (T4, T5a)
Aquaporins conduct water, not Na⁺. (T4)
Answer as lectured Aquaporins are selective water channels that greatly speed osmosis while excluding ions. (T4)
Glucose uses GLUT/SGLT, not aquaporins. (T4)
46. The Na⁺/K⁺ pump is described as "primary" active transport because it:
Using a pre-existing gradient defines secondary transport. (T4)
Answer as lectured Primary active transporters use ATP directly; the gradients they build then power secondary transport. (T4)
The pump moves ions against their gradients, using ATP. (T4)
It's an active pump, not a passive channel. (T4)
47. Osmosis is the movement of:
Answer as lectured Water moves to equalize solute concentration — toward the more concentrated side. (T4)
Osmosis is passive water movement, not ion pumping. (T4)
Osmosis specifically refers to water movement. (T4)
Water moves toward higher solute concentration. (T4)
48. If the Na⁺/K⁺ pump were suddenly inhibited, the most immediate consequence would be:
The gradients depend on the pump; losing it has major downstream effects. (T4)
Osmotic problems can develop, but the immediate effect is gradient run-down, not instant lysis. (T4)
The pump's direct contribution to Vm is small; the main effect is slow gradient dissipation, not an instant flip. (T4)
Answer as lectured Without the pump maintaining gradients, they run down; secondary transporters lose their power source and the resting potential decays over time. (T4)
49. Facilitated diffusion through a carrier protein is still "passive" because:
Answer as lectured No energy is added — the carrier just accelerates downhill movement of an otherwise poorly permeant solute. (T4)
Facilitated diffusion uses no ATP; that would make it active. (T4)
It doesn't require energy or voltage input; it follows the gradient. (T4)
Answer as lectured Channels pass millions of ions per second; carriers/pumps cycle far more slowly due to conformational changes. (T4)
Topic 5a — Intracellular Compartments (Q55–72)
55. Glycolysis occurs in which cellular location?
Glycolytic enzymes are in the cytosol, not the nucleus. (T5a)
Glycolysis happens in the cytosol; the matrix hosts the citric acid cycle, not glycolysis. (T5a)
The inner membrane hosts the electron transport chain; glycolysis is cytosolic. (T5a)
Answer as lectured Glycolysis is cytosolic; only the downstream steps (pyruvate oxidation, citric acid cycle, oxidative phosphorylation) are mitochondrial. (T5a)
56. A chloroplast has how many distinct membrane systems?
The count is three: outer, inner, thylakoid. (T5a)
Chloroplasts have three membrane systems, not one. (T5a)
Chloroplasts add a third, the thylakoid membrane, where the light reactions occur. (T5a)
Answer as lectured Beyond the outer and inner envelope membranes, chloroplasts have a separate internal thylakoid membrane system — three in total. (T5a)
57. The proton-motive force that drives ATP synthesis is generated by:
The force is a proton gradient built by the ETC, not ATP diffusion. (T5a)
Answer as lectured ETC pumping builds an H⁺ gradient across the inner membrane; its energy is the proton-motive force. (T5a)
Glycolysis is cytosolic and doesn't pump protons. (T5a)
The ETC builds the gradient; ATP synthase lets H⁺ flow back to make ATP. (T5a)
58. ATP synthase produces ATP by:
ATP synthase uses the proton gradient; glucose breakdown is upstream. (T5a)
Answer as lectured Protons flowing through ATP synthase drive rotational catalysis that makes ATP from ADP + Pi. (T5a)
Mitochondrial ATP synthase uses a proton gradient, not light. (T5a)
In synthesis mode, protons flow inward and ATP is made. (T5a)
59. Which mitochondrial membrane is highly impermeable and folded into cristae?
Thylakoids are in chloroplasts; cristae are inner-mitochondrial folds. (T5a)
Cristae are a mitochondrial inner-membrane feature. (T5a)
The outer membrane is permeable (porins) and smooth; the inner one forms cristae. (T5a)
Answer as lectured The inner membrane is impermeable (forcing controlled transport), folded into cristae, and houses the ETC and ATP synthase. (T5a)
60. Mitochondrial DNA is inherited:
Answer as lectured mtDNA is passed through the maternal line via the egg's cytoplasm. (T5a)
Paternal mitochondria generally don't contribute; inheritance is maternal. (T5a)
Unlike nuclear DNA, mtDNA is maternally inherited. (T5a)
mtDNA is inherited (maternally), not synthesized de novo each generation. (T5a)
61. Some antibiotics can affect mitochondria as a side effect because mitochondrial ribosomes:
Answer as lectured Antibiotics that target bacterial ribosomes can also hit mitochondrial ones because both descend from a bacterial ancestor. (T5a)
Mitochondrial ribosomes are in the matrix, and their bacterial nature explains antibiotic sensitivity. (T5a)
Ribosomes are RNA-protein complexes; the point is their bacterial resemblance. (T5a)
Mitochondrial ribosomes are bacterial-like, not identical to cytosolic ones — hence the selectivity issue. (T5a)
62. The nuclear envelope is:
The outer nuclear membrane is continuous with the ER. (T5a)
It's a lipid bilayer envelope, not a bacterial cell wall. (T5a)
Answer as lectured The nuclear envelope is a double membrane contiguous with the ER, with NPCs gating transport. (T5a)
It's a double membrane with pores (NPCs). (T5a)
63. Topologically, the interior (matrix) of a mitochondrion is equivalent to:
The Golgi lumen is on the vesicular pathway; the matrix is not. (T5a, T5b)
Answer as lectured The matrix is reached by direct translocation (TOM/TIM), not vesicle fusion; it isn't topologically continuous with the ER/Golgi lumen. (T5a, T5b)
The matrix is not continuous with the outside; it's reached by translocation. (T5a)
ER lumen ≡ cell exterior, but the mitochondrial matrix is a separate topological space reached by translocation. (T5a)
64. The nucleolus is the site of:
Degradation is proteasomal/lysosomal; the nucleolus assembles ribosomes. (T5a, T6)
Answer as lectured The nucleolus is where rRNA is made and ribosomal subunits are assembled. (T5a)
Lipids are made in the ER; the nucleolus makes ribosomal components. (T5a, T3)
ATP is made in mitochondria; the nucleolus builds ribosomes. (T5a)
65. A nuclear localization signal (NLS) versus a nuclear export signal (NES):
Nuclear transport is signal-directed and highly regulated. (T5a)
Import = basic; export = hydrophobic. (T5b)
Answer as lectured Import is directed by a basic cluster; export by a hydrophobic motif — opposite chemistries. (T5a, T5b)
They are amino acid signals, not sugars, and differ in chemistry. (T5b)
66. Peroxisomes are distinguished from mitochondria in that peroxisomes:
Peroxisomes are single-membrane and DNA-free. (T5a)
Oxidative phosphorylation is mitochondrial; peroxisomes do oxidative/detox chemistry. (T5a)
Answer as lectured Peroxisomes are single-membrane, genome-less organelles that import folded proteins — unlike double-membrane, DNA-containing mitochondria that import unfolded proteins. (T5a, T5b)
67. Heteroplasmy and a threshold effect in mitochondrial disease mean that:
The threshold effect means one mutant copy among many is usually tolerated. (T5a)
Answer as lectured Because cells have many mtDNA copies, disease manifests only when the mutant fraction exceeds a functional threshold. (T5a)
Heteroplasmy specifically means a mix of genotypes. (T5a)
mtDNA mutates; heteroplasmy describes the resulting mixture. (T5a)
68. The primary functional rationale for compartmentalizing a eukaryotic cell into organelles is:
Compartmentalization is associated with larger, more complex cells, not smaller ones. (T5a)
Answer as lectured Membranes let each compartment maintain its own pH, ion, and enzyme set, enabling otherwise incompatible reactions to run efficiently. (T5a)
It organizes enzymes into optimal environments, not eliminates them. (T5a)
Compartments still exchange material via regulated transport; the point is controlled separation. (T5a)
69. Where in the mitochondrion is the electron transport chain located?
The ETC is in the inner mitochondrial membrane, not the cytosol. (T5a)
The matrix holds the citric acid cycle; the ETC is membrane-embedded in the inner membrane. (T5a)
The outer membrane is permeable and doesn't house the ETC. (T5a)
Answer as lectured ETC complexes and ATP synthase sit in the inner membrane, pumping H⁺ into the intermembrane space. (T5a)
70. Why can proteins destined for the nucleus be imported while fully folded, unlike those entering mitochondria?
Answer as lectured The NPC accommodates folded cargo, whereas TOM/TIM require the protein threaded through as an extended chain. (T5a, T5b)
Mitochondria import by translocation, not vesicles; the folding difference is the point. (T5a, T5b)
The NPC is a gate — but one wide enough for folded proteins. (T5a)
They carry an NLS; the difference is the pore vs translocator mechanism. (T5a)
71. Which sequence of ATP-yielding metabolism is correctly localized?
Glycolysis is cytosolic and the ETC is membrane-bound, not all matrix. (T5a)
Only glycolysis is cytosolic; the others are mitochondrial. (T5a)
Answer as lectured The pathway starts in the cytosol and moves into the mitochondrion for the cycle and the ETC/ATP synthase. (T5a)
Glycolysis is cytosolic, the cycle is in the matrix, and the ETC is in the inner membrane. (T5a)
72. The lumen of the ER, Golgi, endosomes, lysosomes, and transport vesicles are all topologically equivalent to:
The nuclear interior is continuous with the cytosol, not with the endomembrane lumens. (T5a)
The matrix is a separate topological space reached by translocation. (T5a)
Answer as lectured These lumens are continuous (via vesicle budding/fusion) with the outside of the cell — the core topological rule of the endomembrane system. (T5a, T5b)
The cytosol is equivalent to the nuclear interior, not these lumens; the lumens are equivalent to the exterior. (T5a)
Topic 5b — Protein Sorting (Q73–90)
73. The three fundamental routes a protein can take from the cytosol to its destination are:
Answer as lectured These three mechanisms (gated pores, transmembrane translocation, vesicles) cover all protein sorting. (T5b)
Sorting is signal-directed via three specific routes, not passive diffusion. (T5b)
Nuclear pores are one route; translocators and vesicles are the others. (T5b)
Secretion (vesicular) is one path among three. (T5b)
74. A protein synthesized on an ER-bound ribosome is most likely destined for:
Answer as lectured ER-bound ribosomes make proteins that enter the secretory pathway (secreted, membrane, or organelle-lumen proteins). (T5b)
Mitochondrial proteins are made on free ribosomes and imported post-translationally. (T5b)
Cytosolic proteins are made on free ribosomes; ER-bound ones enter the secretory path. (T5b)
Nuclear proteins are made on free ribosomes and imported through pores. (T5b)
75. Mitochondrial targeting sequences are recognized primarily by their:
Mitochondrial signals vary in sequence; recognition is by amphipathic-helix shape. (T5b)
Ubiquitin marks proteins for degradation, not mitochondrial import. (T5b, T6)
Answer as lectured The import machinery reads the amphipathic-helix shape, which is why diverse sequences work as long as they fold this way. (T5b)
The signal is an amino acid amphipathic helix, not a sugar. (T5b)
76. Mitochondrial protein import is:
Mitochondrial import is post-translational; ribosomes don't dock on mitochondria. (T5b)
Answer as lectured Chaperones keep the completed protein unfolded so it can pass through the outer (TOM) and inner (TIM) translocators. (T5b)
Mitochondria import by translocation, not vesicles. (T5b)
Proteins must be unfolded to be imported into mitochondria. (T5b)
77. In co-translational ER import, the ER signal sequence is first recognized by:
Rabs direct vesicle tethering, not initial signal recognition. (T5b)
Ubiquitin ligases tag proteins for degradation; SRP handles ER targeting. (T5b, T6)
Answer as lectured SRP binds the emerging hydrophobic signal, halts elongation, and docks the complex on the SRP receptor at the ER. (T5b)
M6P sorting happens later (Golgi→lysosome); ER entry starts with SRP. (T5b)
78. After an ER signal peptide is cleaved by signal peptidase, it:
The cleaved peptide remains in the translocon rather than diffusing off. (T5b)
Answer as lectured The cleaved signal peptide stays associated with the translocon and is disposed of there — it doesn't simply diffuse away. (T5b)
It's a removed targeting tag, not part of the functional protein. (T5b)
It's retained at the translocon and degraded, not secreted. (T5b)
79. Membrane-protein topology in the ER is established by:
M6P is a lysosomal sorting tag, unrelated to membrane topology. (T5b)
Answer as lectured Alternating start/stop-transfer sequences set which segments cross and which stay, defining single- or multi-pass topology. (T5b)
Ubiquitin is for degradation; topology is set by transfer signals. (T5b, T6)
Topology is precisely signal-determined, not random. (T5b)
80. N-linked glycosylation in the ER adds a sugar tree to which residue?
Cysteine forms disulfide bonds; N-glycans go on asparagine. (T5b)
Answer as lectured N-linked glycans are attached en bloc to asparagine side chains in the ER lumen. (T5b)
N-linked glycosylation targets asparagine, not glycine. (T5b)
Lysine is the ubiquitin attachment site; N-glycosylation is on asparagine. (T5b, T6)
81. The calnexin/calreticulin cycle in the ER functions in:
It promotes folding; terminally misfolded proteins are handled separately by ERAD. (T5b)
Lipid synthesis is separate; calnexin does glycoprotein QC. (T5b, T3)
It's a folding-QC system, not a proton pump. (T5b)
Answer as lectured These lectin chaperones (with UGGT sensing) hold immature glycoproteins until properly folded. (T5b)
82. Terminally misfolded ER proteins are cleared by ERAD, which:
Terminally misfolded proteins are degraded, not repaired. (T5b)
ERAD routes them to the cytosolic proteasome, not the nucleus. (T5b, T6)
Misfolded proteins are degraded, not secreted. (T5b)
Answer as lectured ERAD exports misfolded proteins back to the cytosol, where they are ubiquitinated and degraded by the proteasome — directly linking sorting QC to Topic 6. (T5b, T6)
83. Vesicles budding from the ER to the Golgi (anterograde) are coated with:
COPI coats retrograde Golgi→ER vesicles, not anterograde ER→Golgi. (T5b)
Clathrin coats TGN→endosome and plasma-membrane endocytic vesicles — not ER→Golgi (that's COPII). (T5b)
Answer as lectured COPII coats ER→Golgi (anterograde) vesicles. This is the correction to watch: it is not clathrin. (T5b)
Anterograde ER→Golgi transport uses COPII. (T5b)
84. COPI-coated vesicles mediate:
Answer as lectured COPI runs the return path (Golgi→ER), recovering escaped ER residents. (T5b)
Nuclear import is via pores, not COPI vesicles. (T5b, T5a)
Plasma-membrane endocytosis uses clathrin, not COPI. (T5b)
Anterograde ER→Golgi is COPII; COPI is retrograde. (T5b)
85. Clathrin coats are used for:
That's COPI, not clathrin. (T5b)
That's COPII, not clathrin. (T5b)
Mitochondrial import uses translocators, not clathrin vesicles. (T5b, T5a)
Answer as lectured Clathrin operates at the TGN→endosome step and in plasma-membrane endocytosis. (T5b)
86. The specificity of vesicle docking (which vesicle fuses with which target) is provided by:
Answer as lectured Rabs tether vesicles to the right target, and complementary SNAREs on vesicle and target drive specific fusion. (T5b)
Targeting is highly specific via Rab/SNARE recognition, not random. (T5b)
M6P sorts lysosomal enzymes specifically; general docking specificity is Rab/SNARE. (T5b)
Ubiquitin is for degradation; docking specificity is Rab/SNARE. (T5b, T6)
87. Lysosomal hydrolases are sorted to lysosomes by which tag, added in the Golgi?
Answer as lectured M6P is added in the cis-Golgi and recognized by the M6P receptor in the TGN, routing enzymes to lysosomes. (T5b)
Ubiquitin marks proteins for proteasomal degradation, not lysosomal sorting. (T5b, T6)
The ER signal gets them into the ER; M6P then routes them to lysosomes. (T5b)
88. A secreted protein that carries no special sorting signal beyond ER entry will, by default, be:
Secretory-pathway proteins aren't nuclear; the default is constitutive secretion. (T5b)
Lysosomal targeting requires the M6P tag; without it the default is secretion. (T5b)
Answer as lectured The default (unregulated) fate for a secretory protein at the TGN is constitutive secretion; diversion requires a specific signal. (T5b)
Regulated (stored) secretion requires specific packaging signals; the default is constitutive. (T5b)
89. The Golgi's cisternal maturation model proposes that:
Secretory proteins pass through and are processed by the Golgi. (T5b)
Maturation has the cisternae progressing and enzymes recycling backward. (T5b)
Answer as lectured Cargo stays in maturing cisternae that move cis→trans, while Golgi enzymes are retrieved retrograde (COPI). (T5b)
The Golgi extensively modifies/sorts proteins as they traverse it. (T5b)
90. Which correctly matches the coat protein to its transport step?
Answer as lectured Anterograde ER→Golgi is COPII, retrograde Golgi→ER is COPI, and clathrin serves the TGN and endocytosis. (T5b)
COPII is anterograde; COPI is retrograde. (T5b)
Each step uses a distinct coat (COPII/COPI/clathrin). (T5b)
ER→Golgi is COPII (not clathrin); endocytosis is clathrin (not COPII). (T5b)
Topic 6 — Recycling of Proteins and Other Molecules (Q91–108)
91. The three degradation systems differ on two axes. Which mapping is correct?
Ubiquitin-tagged degradation is proteasomal; peroxisomes do oxidative chemistry. (T6)
The proteasome has no membrane; it's a protein complex like a ribosome. (T6)
Answer as lectured Two organelles (bulk digestion, oxidative breakdown) versus a non-membrane protein complex doing selective, tagged degradation. (T6)
92. Lysosomal enzymes are active at ~pH 5.5. The functional payoff of this is:
They specifically don't work outside the acidic lysosome — the safeguard. (T6)
Answer as lectured The pH-dependence is a safety mechanism beyond the membrane: escaped hydrolases can't function at cytosolic pH. (T6)
The benefit is enzyme inactivity at neutral pH, independent of membrane permeability. (T6)
Lysosomal pH is about safe digestion, not ATP. (T6, T5a)
93. A worn-out organelle is degraded by autophagy through:
Organelle turnover is lysosomal (autophagy), not export. (T6)
Worn organelles are digested internally. (T6)
The proteasome degrades individual proteins, not organelles. (T6)
Answer as lectured The cell packages the organelle into an autophagosome, which delivers it to the lysosome for digestion. (T6)
94. Peroxisomal β-oxidation and mitochondrial β-oxidation divide labor by:
Answer as lectured Very-long-chain fatty acids are shortened in peroxisomes before mitochondria finish the job. (T6)
Both do β-oxidation, split by chain length. (T6)
β-oxidation acts on fatty acids; peroxisomes take the longer ones. (T6)
Peroxisomes take the longer fatty acids. (T6)
95. Catalase, abundant in peroxisomes, protects the cell by:
Catalase acts on H₂O₂; ubiquitination is a separate pathway. (T6)
Catalase is an H₂O₂-decomposing enzyme, not a pump. (T6)
Answer as lectured Peroxisomal oxidations produce H₂O₂; catalase breaks it down before it can damage the cell. (T6)
ATP synthesis is mitochondrial; catalase handles H₂O₂. (T6, T5a)
96. The ubiquitin–proteasome pathway is essential to the immune system because:
Answer as lectured The peptides the proteasome produces feed antigen presentation, enabling adaptive immunity. (T6)
Antigen presentation is a central UPP function. (T6)
Its immune role is peptide/antigen production, not membrane building. (T6)
Antibodies come from B/plasma cells; the proteasome supplies antigenic peptides. (T6)
97. The direct products of proteasomal degradation are:
The proteasome cuts to peptides, not straight to amino acids. (T6)
Protein degradation yields peptides, not nucleotides. (T6)
It destroys proteins into peptides; it doesn't refold them. (T6)
Answer as lectured The proteasome yields peptides; separate peptidases finish them, and some peptides go to antigen presentation. (T6)
98. The 26S proteasome consists of:
That's the ribosome; the proteasome is core + cap. (T6)
The proteasome is a protein barrel, not a bilayer sac. (T6)
Answer as lectured The 20S barrel holds the proteolytic sites internally; the 19S cap recognizes ubiquitin and unfolds substrates. (T6)
The proteasome is a non-membrane protein complex, not a vesicle. (T6)
99. Human and yeast ubiquitin are 96% identical over its 76 residues. This means approximately:
Answer as lectured 96% × 76 ≈ 73 identical, so ~3 differ. A single difference would be ~99% identity, not 96%. (T6)
24 differences would be ~68% identity, far below 96%. (T6)
96% is not 100%; ~3 residues differ. (T6)
One difference in 76 residues is ~99% identity; 96% implies ~3 differences. (T6)
100. Ubiquitin's C-terminal Gly76 attaches to its target via:
The linkage is covalent (isopeptide), not a hydrogen bond. (T6)
Answer as lectured The Gly76 carboxyl links to a lysine side chain — an isopeptide bond that leaves the target backbone intact. (T6)
It uses the lysine side chain, making it an isopeptide bond, not a backbone peptide bond. (T6)
It's a C–N isopeptide linkage, not a disulfide. (T6)
101. Match the ubiquitin chain type to its signal:
A single ubiquitin signals endocytosis-type events, not degradation (which needs a Lys48 chain). (T6)
Lys48 = degradation; Lys63 = DNA repair. (T6)
Only Lys48-linked chains signal proteasomal degradation; others are non-degradative. (T6)
Answer as lectured The linkage encodes the message — the same tag molecule means different things depending on chain type. (T6)
102. In the E1–E2–E3 cascade, which enzyme determines substrate specificity?
Answer as lectured With ~500 E3s (vs one E1), E3 diversity is what selects which protein gets ubiquitinated. (T6)
Specificity is set at tagging (E3), before the proteasome. (T6)
One E1 acts generically; specificity comes from E3. (T6)
Ubiquitin is the tag; E3 chooses the target. (T6)
103. The role of ATP in the ubiquitin cascade is to:
ATP charges ubiquitin here; lysosomal acidification is a separate proton-pump process. (T6)
M6P is a lysosomal sorting tag, unrelated to ubiquitin activation. (T6, T5b)
Answer as lectured ATP hydrolysis drives E1's activation of ubiquitin, initiating the transfer relay (and later powers proteasomal unfolding). (T6)
Cleavage occurs in the proteasome; ATP here activates ubiquitin. (T6)
104. I-cell disease is caused by:
I-cell is a lysosomal sorting defect, not proteasomal. (T6)
The defect is in M6P tagging, not ubiquitination. (T6)
Answer as lectured Without M6P tagging, hydrolases follow the default secretory route out; lysosomes accumulate undigested material as inclusion bodies. (T6, T5b)
I-cell is a sorting defect affecting many enzymes at once. (T6)
105. Tay-Sachs disease differs from I-cell disease in that Tay-Sachs is:
It's a lysosomal enzyme deficiency, unrelated to the proteasome. (T6)
Tay-Sachs is a single enzyme deficiency, not a sorting defect. (T6)
Answer as lectured Tay-Sachs = a single missing enzyme; I-cell = a tagging/sorting failure affecting many enzymes. (T6)
It's a lysosomal storage disease, not a peroxisomal catalase issue. (T6)
106. During proteasomal degradation, the ubiquitin tag is:
Ubiquitin is recycled as a tag, not turned into ATP. (T6)
Answer as lectured A deubiquitinating step frees ubiquitin for reuse before the substrate is threaded into the core. (T6)
Ubiquitin is recycled, not degraded with the target. (T6)
It's cleaved off early for reuse. (T6)
107. RING and HECT E3 ligases differ in that:
Both are ligases transferring ubiquitin; neither degrades or synthesizes proteins directly. (T6)
The distinction is mechanistic, not by location. (T6)
Answer as lectured RING stimulates direct transfer; HECT forms a ubiquitin–E3 intermediate before transferring to the substrate. (T6)
Both recognize substrates; they differ in transfer chemistry. (T6)
108. Peroxisomes resemble mitochondria in one respect but differ in another. Which is correct?
Both are self-replicating by division. (T6, T5a)
Peroxisomes have no DNA; only mitochondria carry a genome. (T6, T5a)
Answer as lectured Both grow and divide, yet peroxisomes lack DNA and have only one membrane (and import folded proteins). (T6, T5a)
Peroxisomes are single-membrane and import folded proteins. (T6, T5a)
§ Integration — Cross-Topic Synthesis (Q109–120)
109.Chaining three topics: Why can an antibiotic that targets bacterial ribosomes produce mitochondrial side effects in human cells?
If they matched cytosolic ribosomes there'd be no selective mitochondrial hit; they're bacterial-like. (T5a)
Mitochondria have their own (bacterial-like) ribosomes — that's the point. (T5a)
The relevant target is the bacterial-type mitochondrial ribosome, not the nucleus. (T5a)
Answer as lectured Endosymbiotic origin (T2/T5a) explains why mitochondrial ribosomes resemble bacterial ones — the shared target is the side-effect mechanism. (T2, T5a)
110.Same motif, opposite fate: The ER translocon (T5b) and the proteasome (T6) share a mechanical logic. What is it, and how do outcomes differ?
M6P tagging is a Golgi function, unrelated to either channel. (T5b)
Answer as lectured Identical "recognize–unfold–thread" motif, opposite biological meaning: delivery to function vs destruction. (T5b, T6)
Neither synthesizes protein; that's the ribosome. (T5b, T6)
The proteasome is a non-membrane complex with no genome. (T6)
111.Tag failure → disease: Trace how a single enzyme defect produces I-cell disease, starting from N-glycosylation.
Hydrolases traffic through the secretory pathway to lysosomes, not into mitochondria. (T5b)
Answer as lectured The whole chain (T5b→T6): losing the M6P-adding enzyme breaks lysosomal delivery, so hydrolases follow the default secretory route and inclusion bodies accumulate. (T5b, T6)
The defect is enzyme sorting (M6P), not membrane formation. (T6)
I-cell involves M6P/lysosomal sorting, not ubiquitin/proteasome. (T6)
112.One physical principle, three uses: Which set of processes all harness a transmembrane proton gradient?
None of these is driven by a transmembrane proton gradient. (T5a)
These don't use proton gradients. (T3, T5b)
Answer as lectured All three exploit an H⁺ gradient — ATP synthase (T5a), the lysosome's proton pumps (T6), and proton-coupled transporters (T4). (T4, T5a, T6)
None is proton-gradient-driven. (T5a, T5b)
113.Why membranes force the problem transporters solve: The property of the lipid bilayer that necessitates channels and transporters is:
Membranes are fluid; the barrier is the hydrophobic core's chemistry. (T3)
The bilayer is impermeable to ions — that's why transporters are needed. (T3, T4)
Answer as lectured Membrane structure (T3) creates the barrier; transport proteins (T4) exist precisely to move ions and polar molecules the core excludes. (T3, T4)
The glycocalyx isn't why transport proteins exist; the hydrophobic core is. (T3)
114.Same force, two scales: The hydrophobic effect (T3) explains both bilayer self-assembly and:
Code universality reflects common ancestry, not the hydrophobic effect. (T2)
Maternal inheritance isn't a hydrophobic-effect phenomenon. (T5a)
Ubiquitin attachment is an enzymatic isopeptide bond, not the hydrophobic effect. (T6)
Answer as lectured The same entropic driver that buries lipid tails keeps a protein's hydrophobic TM segment in the core (relevant to start/stop-transfer topology, T5b). (T3, T5b)
115.Recognition strategies compared: Mitochondrial targeting signals and nuclear localization signals are recognized differently. Which contrast is correct?
Both are amino acid signals, not sugars, and are recognized differently. (T5b)
Answer as lectured Import machinery recognizes the mitochondrial signal's 3-D amphipathic character, while nuclear import reads a defined basic-residue motif. (T5a, T5b)
Neither requires ubiquitin; that's a degradation tag. (T6)
116.Topology preserved end-to-end: A sugar added to a protein's asparagine in the ER lumen ends up on the cell's outer surface because:
N-glycans are added in the ER lumen (non-cytosolic face). (T5b, T3)
Answer as lectured The topological rule (T5a) plus orientation-preserving vesicle traffic (T5b) means lumenal glycans (T5b) display extracellularly (T3). (T3, T5a, T5b)
The ER lumen is equivalent to the exterior, not the cytosol. (T5a)
No flipping is needed; the lumenal face becomes the outer face by the topological rule. (T5a)
117.Choosing the disposal route: A cell must eliminate (i) a single misfolded cytosolic protein, (ii) a worn mitochondrion, and (iii) excess very-long-chain fatty acids. The correct systems are:
The proteasome handles individual proteins only, not organelles or fatty acids. (T6)
Answer as lectured Selective single-protein → proteasome; organelle → lysosomal autophagy; long-chain fatty acids → peroxisomal β-oxidation. The triage is the point. (T6)
A single cytosolic protein is proteasomal; fatty acids are peroxisomal. (T6)
A single cytosolic protein → proteasome; an organelle → lysosome; fatty acids → peroxisome. (T6)
118.Quality control hands off: A glycoprotein that repeatedly fails folding in the ER is ultimately destroyed by:
ERAD routes to the cytosolic proteasome, not mitochondria. (T6)
It's degraded via ERAD/proteasome, not stored. (T5b)
Misfolded proteins are degraded, not secreted. (T5b)
Answer as lectured ER folding QC (T5b) that fails routes the protein out for ubiquitin–proteasome destruction (T6) — a direct sorting-to-degradation handoff. (T5b, T6)
119.Two "3-vs-2" facts, don't confuse them: Which pairing is correct?
The pump is 3:2 and chloroplasts have three membranes, not two. (T4, T5a)
Pump is 3 out/2 in; chloroplasts have three membrane systems. (T4, T5a)
Answer as lectured Pump stoichiometry (T4) is 3:2; chloroplasts have three membranes (outer/inner/thylakoid) vs the mitochondrion's two (T5a). (T4, T5a)
Na⁺ goes out and K⁺ in; chloroplasts have three membranes. (T4, T5a)
120.The corrections, as one theme: Several course corrections share a logic: the intuitive-sounding claim is wrong because it ignores a mechanism. Which pairing of "intuitive claim → actual mechanism" is correct?
They're COPII-coated; clathrin is the misconception here. (T5b)
The proteasome makes peptides, not amino acids — the claim is the misconception being corrected. (T6)
Glycolysis is cytosolic, not mitochondrial — that's the correction, not a confirmation. (T5a)
Answer as lectured Both replace an intuitive but wrong model with the real mechanism: K⁺ selectivity by carbonyl geometry (T4), and the entropy-driven hydrophobic effect (T3). (T3, T4)