Cell Biology Basics

Cards (10)

• 1
  Front

  What is the functional significance of the inner mitochondrial membrane being highly folded into cristae?

  Back

  The cristae dramatically increase the surface area available for the electron transport chain and ATP synthase complexes, maximizing the capacity for oxidative phosphorylation. The cristae also create a confined intermembrane space microenvironment that maintains the proton gradient more efficiently.

• 2
  Front

  Why does meiosis II resemble mitosis, and what precise event distinguishes it from meiosis I?

  Back

  Meiosis II separates sister chromatids (like mitosis), whereas meiosis I separates homologous chromosome pairs. The key distinction is that meiosis I is reductional division (halving chromosome number), while meiosis II is equational division. No DNA replication occurs between meiosis I and II.

• 3
  Front

  During which specific stage of mitosis does the spindle assembly checkpoint (SAC) operate, and what does it monitor?

  Back

  The SAC operates at the metaphase-to-anaphase transition. It monitors whether all kinetochores are properly attached to spindle microtubules under tension (biorientation). Unattached kinetochores produce a 'wait' signal by sequestering the APC/C activator Cdc20, preventing premature sister chromatid separation.

• 4
  Front

  What is the difference between the Z scheme in photosynthesis and cyclic electron flow, in terms of products and purpose?

  Back

  The Z scheme (non-cyclic electron flow) moves electrons from water through PSII and PSI to NADP+, producing ATP, NADPH, and O2. Cyclic electron flow involves only PSI, recycling electrons back to the cytochrome b6f complex to generate additional ATP without producing NADPH or O2. Cyclic flow balances the ATP/NADPH ratio for the Calvin cycle.

• 5
  Front

  How does the signal recognition particle (SRP) ensure that membrane and secretory proteins are co-translationally inserted into the ER?

  Back

  SRP recognizes and binds the hydrophobic signal sequence on the nascent polypeptide as it emerges from the ribosome. SRP then pauses translation and escorts the ribosome-mRNA complex to the SRP receptor on the ER membrane. Upon docking, translation resumes and the polypeptide is threaded co-translationally through the Sec61 translocon.

• 6
  Front

  What is the molecular basis for crossing over being confined mainly to specific hotspots during meiosis I prophase?

  Back

  Crossover hotspots are enriched for the histone mark H3K4me3, recognized by PRDM9 (in mammals), which repositions nucleosomes and recruits the recombination machinery. The double-strand break (DSB) is introduced by Spo11. Chromatin accessibility, GC content, and specific DNA motifs also influence hotspot distribution.

• 7
  Front

  Why is the stroma of chloroplasts the site of the Calvin cycle rather than the thylakoid lumen?

  Back

  The stroma contains the soluble enzymes of the Calvin cycle, including RuBisCO, and has direct access to the ATP and NADPH produced at the thylakoid membrane. The thylakoid lumen is highly acidic (due to proton accumulation driving ATP synthesis) and lacks the necessary enzyme complement; the neutral-to-basic pH of the stroma also optimizes RuBisCO activity.

• 8
  Front

  What distinguishes primary active transport from secondary active transport at the mechanistic level?

  Back

  Primary active transport directly uses ATP hydrolysis to move solutes against their electrochemical gradient (e.g., Na+/K+-ATPase). Secondary active transport is driven indirectly by the electrochemical gradient established by primary transport—it couples the downhill movement of one ion (often Na+) to the uphill movement of another solute, using no ATP directly.

• 9
  Front

  What is the precise role of MPF (Maturation Promoting Factor) in driving the G2/M transition, and what are its components?

  Back

  MPF is a heterodimer of Cyclin B and CDK1 (Cdc2). Cyclin B accumulation during G2 activates CDK1; once active, MPF phosphorylates multiple substrates to trigger mitotic entry: condensins (chromosome condensation), nuclear lamins (nuclear envelope breakdown), and the mitotic spindle machinery. MPF also auto-amplifies its own activation through a positive feedback loop involving Cdc25 phosphatase.

• 10
  Front

  How does the fluid mosaic model fail to capture the actual complexity of plasma membrane organization, and what concept better explains lipid raft behavior?

  Back

  The original fluid mosaic model treats the lipid bilayer as a uniform, two-dimensional solvent for freely diffusing proteins. It does not account for lateral heterogeneity. Lipid rafts—small, dynamic, ordered (liquid-ordered phase) microdomains enriched in sphingolipids, cholesterol, and specific GPI-anchored or signaling proteins—arise from preferential packing interactions and are better described by a model of a mosaic of coexisting lipid phases with restricted protein diffusion.