Cellular Processes and Organelles Flashcards

A cell biologist treats cells with oligomycin, a drug that blocks the F₀ subunit of ATP synthase. Which of the following would be the MOST likely immediate consequence in a mitochondrion?

Answer: The proton gradient across the inner mitochondrial membrane would increase

ATP synthase (Complex V) uses the proton gradient to drive ATP synthesis. When its F₀ subunit is blocked, protons can no longer flow back into the matrix through ATP synthase. Since the electron transport chain (Complexes I–IV) continues pumping protons into the intermembrane space but those protons cannot return via ATP synthase, the proton gradient (Δψ) builds up. This backpressure eventually inhibits the ETC, but the immediate consequence is a rise in the proton gradient.

In a cell undergoing active endocytosis, a vesicle fuses with an early endosome. Which organelle does this cargo most likely encounter NEXT as the endosome matures, and what is the primary change in that compartment?

Answer: Late endosome; the lumen becomes progressively more acidic due to V-ATPase activity

After fusion with an early endosome (pH ~6.0–6.5), the maturing endosome transitions to a late endosome (multivesicular body) with a progressively lower pH (~5.5), driven by vacuolar H⁺-ATPase (V-ATPase) pumping protons in. This acidification begins the denaturation of receptor–ligand complexes. The lysosome is the final destination (pH ~4.5–5.0), not the next step. Cargo does not return to the Golgi or ER at this stage.

A mutation eliminates signal recognition particle (SRP) function in a cell. Which of the following proteins would be MOST directly affected?

Answer: A transmembrane receptor destined for the plasma membrane

SRP (signal recognition particle) recognizes the N-terminal signal sequence on nascent polypeptides and directs ribosomes to the rough ER for co-translational translocation. Proteins destined for the secretory pathway — including transmembrane receptors headed for the plasma membrane — depend on SRP. Histones are nuclear proteins translated on free ribosomes; cytochrome c uses a different import pathway into mitochondria; pyruvate kinase is a cytosolic protein with no signal sequence.

During oxidative phosphorylation, ubiquinone (coenzyme Q) is described as a 'mobile electron carrier.' What structural property allows ubiquinone — but NOT cytochrome c — to diffuse laterally within the inner mitochondrial membrane?

Answer: Ubiquinone is a small, hydrophobic isoprenoid molecule dissolved in the lipid bilayer, while cytochrome c is a water-soluble protein on the membrane surface

Ubiquinone (CoQ) is a small, lipid-soluble quinone with a long isoprenoid tail that embeds it within the hydrophobic core of the lipid bilayer, allowing free lateral diffusion to shuttle electrons between Complexes I/II and Complex III. Cytochrome c, by contrast, is a small water-soluble heme protein that diffuses along the outer face of the inner membrane (in the intermembrane space). Cytochrome c is NOT embedded in the bilayer and does not have iron–sulfur clusters anchoring it to the membrane.

A researcher uses a fluorescent dye to label the plasma membrane of a living cell, then follows the dye over 30 minutes. The dye is eventually detected inside the cell in a compartment with a pH of approximately 4.8 and high concentrations of cathepsins. Which process and organelle best explain this observation?

Answer: Receptor-mediated endocytosis followed by lysosomal delivery

The observations — internalization of plasma membrane material, pH ~4.8, and presence of cathepsins (lysosomal cysteine proteases) — are hallmarks of the endolysosomal pathway. Membrane-bound dye is taken into endocytic vesicles (including via pinocytosis or receptor-mediated endocytosis), travels through early and late endosomes, and ultimately reaches the lysosome (pH 4.5–5.0) where hydrolytic enzymes including cathepsins degrade cargo. Autophagy internalizes cytoplasmic contents (not plasma membrane patches). Peroxisomes do not have cathepsins or that pH. Exocytosis moves material outward.

In eukaryotic cells, the protein p97 (VCP) is an AAA-ATPase involved in ER-associated degradation (ERAD). Which of the following best describes p97's role in this pathway?

Answer: It uses ATP hydrolysis to retrotranslocate ubiquitinated misfolded proteins from the ER membrane into the cytosol for proteasomal degradation

ERAD is the quality-control pathway that disposes of misfolded ER proteins. After misfolded proteins are recognized by ER chaperones (e.g., BiP/GRP78) and ubiquitinated by ER-resident E3 ubiquitin ligases, p97/VCP uses the energy of ATP hydrolysis to mechanically extract (retrotranslocate) these ubiquitinated substrates from the ER membrane (or lumen) back into the cytosol. There, they are handed off to the 26S proteasome for degradation. p97 is a cytosolic/membrane-associated ATPase, not a luminal chaperone, and it does not remove ubiquitin chains.