Cellular Processes and Organelles Flashcards

During oxidative phosphorylation, the electron transport chain creates a proton gradient across the inner mitochondrial membrane. If a researcher adds a protonophore (an agent that makes membranes permeable to H⁺), which of the following outcomes would be expected?

Answer: ATP synthesis halts because the proton gradient collapses, but O₂ consumption continues or increases

Protonophores uncouple the electron transport chain from ATP synthase by dissipating the proton gradient. Without the gradient, ATP synthase cannot drive phosphorylation of ADP. However, the electron transport chain continues (and may even speed up due to loss of back-pressure), consuming O₂ without productive ATP output. This uncoupling effect is why some compounds like DNP were dangerously used as weight-loss agents — energy is released as heat rather than stored as ATP.

A cell biologist treats cells with brefeldin A, a drug that disrupts the Golgi apparatus by causing it to fuse back into the endoplasmic reticulum. Which of the following cellular functions would be MOST directly impaired?

Answer: Post-translational glycosylation and sorting of secretory proteins

The Golgi apparatus is the central hub for post-translational modification (including complex glycosylation), sorting, and packaging of proteins destined for secretion, the plasma membrane, or lysosomes. Disrupting the Golgi with brefeldin A would prevent these modifications and misdirect or stall vesicular trafficking. Free ribosome translation, mitochondrial DNA replication, and nuclear import are independent of Golgi function.

In a cell undergoing apoptosis, cytochrome c is released from the mitochondrial intermembrane space into the cytosol. What is the direct consequence of this release?

Answer: Cytochrome c associates with Apaf-1 and procaspase-9 to form the apoptosome, activating executioner caspases

When released into the cytosol, cytochrome c binds to Apaf-1 (apoptotic protease-activating factor 1), forming a complex that recruits and activates procaspase-9. This multiprotein complex — the apoptosome — then activates downstream executioner caspases (e.g., caspase-3), committing the cell to apoptosis. Cytochrome c does not directly fragment DNA, activate the proteasome, or trigger the unfolded protein response.

A mutation eliminates the mannose-6-phosphate (M6P) tagging system in a human cell. Which of the following would be the most likely outcome?

Answer: Lysosomes fail to receive hydrolytic enzymes, causing accumulation of undigested substrates inside lysosomes

Mannose-6-phosphate is the molecular 'zip code' that targets lysosomal hydrolases from the Golgi to lysosomes. Without M6P tagging, these enzymes are secreted extracellularly instead of being sorted to lysosomes. Lysosomes then lack the enzymes needed to break down macromolecules, causing the substrate accumulation characteristic of lysosomal storage diseases (e.g., I-cell disease). Peroxisomal, mitochondrial, and ER folding pathways use entirely different targeting signals.

Gap junctions between adjacent animal cells are composed of connexin proteins forming connexons. Which of the following scenarios BEST illustrates a situation where gap junction closure would be physiologically beneficial?

Answer: A damaged cell in a tissue closing its gap junctions to prevent the spread of apoptotic signals to healthy neighbors

While gap junctions are vital for coordinating responses across cell populations (as in heart muscle and neural networks), closure of gap junctions in a dying or damaged cell protects neighboring healthy cells by preventing the passage of pro-apoptotic signals, calcium waves, or toxic molecules. This 'bystander protection' is a documented physiological role of gap junction closure. The other options all describe contexts where open gap junctions are beneficial.

The signal recognition particle (SRP) cycle is essential for co-translational insertion of proteins into the endoplasmic reticulum membrane. If the GTPase activity of both the SRP and its receptor (SR) were permanently inactivated, what would be the primary consequence?

Answer: The ribosome-SRP complex would dock at the ER but fail to release, blocking the translocon and halting cotranslational insertion

SRP recognizes the signal peptide and pauses translation, then docks the ribosome-SRP complex at the SR on the ER membrane. GTP hydrolysis by both SRP and SR is required for the dissociation step — releasing SRP from the ribosome so translation can resume through the translocon. If GTPase activity is blocked, the SRP-SR complex remains locked together on the translocon, preventing release and recycling of SRP and blocking the channel for new polypeptide translocation. Signal peptide recognition itself (step 1) would still occur.