Cellular Processes and Organelles Flashcards

A cell biologist treats cells with brefeldin A, a drug that disrupts COPI vesicle formation. Which cellular process would be most directly impaired?

Answer: Retrograde transport from the Golgi back to the endoplasmic reticulum

COPI (coat protein complex I) vesicles mediate retrograde transport — moving proteins and lipids back from the Golgi apparatus to the endoplasmic reticulum (ER). Brefeldin A inhibits the ARF GTPase required for COPI coat assembly, specifically blocking this backward retrieval pathway. Anterograde ER-to-Golgi transport uses COPII vesicles, endocytosis uses clathrin, and mitochondrial import relies on TOM/TIM complexes — none of which involve COPI.

During oxidative phosphorylation, uncoupling proteins (UCPs) in the inner mitochondrial membrane allow protons to leak back into the matrix without passing through ATP synthase. What is the immediate thermodynamic consequence?

Answer: The proton-motive force is dissipated as heat rather than used to synthesize ATP

The proton-motive force (PMF) across the inner mitochondrial membrane stores energy as both a pH gradient and a membrane potential. When UCPs provide an alternative channel for protons to re-enter the matrix, this electrochemical potential energy is released as heat (thermogenesis) rather than captured as ATP. The ETC does not halt — it actually increases its rate to maintain the gradient — and ATP synthase does not reverse under these conditions. NADH is consumed faster, not accumulated.

A mutation eliminates the mannose-6-phosphate (M6P) tagging enzyme in a human cell line. Which organelle would most likely fail to receive its complement of newly synthesized hydrolytic enzymes?

Answer: Lysosome

Mannose-6-phosphate is the molecular zip code that directs soluble hydrolytic enzymes (acid hydrolases) from the trans-Golgi network to lysosomes. M6P receptors in the Golgi recognize this tag and package the enzymes into vesicles destined for late endosomes/lysosomes. Without M6P tagging, the enzymes are secreted extracellularly by default instead of being delivered to lysosomes. Peroxisomal enzymes are imported via PTS1/PTS2 signals; sER and the nucleolus have different protein-targeting mechanisms entirely.

In a eukaryotic cell undergoing rapid protein secretion, ribosomes translating a secretory protein pause briefly after synthesizing approximately 70 amino acids of the nascent polypeptide. What molecular event causes this pause?

Answer: The signal recognition particle (SRP) binds the signal sequence and arrests translation until the ribosome docks with the ER membrane

As the N-terminal signal sequence of a secretory protein emerges from the ribosome exit tunnel (~70 aa synthesized), the Signal Recognition Particle (SRP) recognizes and binds it. This binding causes elongation arrest — a deliberate translational pause — that persists until the SRP-ribosome complex docks with the SRP receptor on the rough ER membrane. Only then does translation resume, co-translationally threading the protein into the ER lumen. Signal peptidase cleaves the signal sequence after insertion but does not cause the initial arrest; BiP acts in the lumen after translocation.

A researcher uses fluorescence microscopy to track a mitochondrion in a living cell and observes it fuse with another mitochondrion and then undergo fission 20 minutes later. Which proteins are primarily responsible for mediating the outer mitochondrial membrane fusion and fission steps, respectively?

Answer: Mitofusins (MFN1/MFN2) for fusion; dynamin-related protein 1 (DRP1) for fission

Mitochondrial outer membrane fusion is driven by the GTPases Mitofusin 1 and Mitofusin 2 (MFN1/MFN2), which tether and fuse adjacent outer membranes. Inner membrane fusion is separately mediated by OPA1. Fission of the outer membrane is orchestrated by DRP1 (Dynamin-Related Protein 1), a cytosolic GTPase recruited to the outer membrane where it forms helical rings that constrict and sever the membrane. BAX/BAK are pro-apoptotic proteins involved in cytochrome c release, not routine fission.

Plant cells possess a large central vacuole that can occupy up to 90% of cell volume. If the vacuolar membrane (tonoplast) suddenly became freely permeable to all solutes, what would be the most likely immediate effect on the cell?

Answer: The cell would lose turgor pressure as the osmotic gradient driving water into the vacuole collapses

The central vacuole maintains high solute concentration (ions, organic acids, sugars) that generates a water potential gradient drawing water in by osmosis, creating turgor pressure against the rigid cell wall — the force that keeps plant cells firm. If the tonoplast became freely permeable, the solute concentration difference between the vacuole and cytoplasm would equilibrate, eliminating the osmotic driving force for water accumulation in the vacuole. Turgor pressure would drop and the cell would become flaccid (plasmolysis-like). While some cytoplasmic disruption would occur, the cell wall prevents bursting, and the vacuolar pH (~5) would not reach the apoplast to dissolve the wall.