BIOL 112 Midterm Review: Cell Structure, Membranes, Gene Expression and Energy Metabolism

Terms in this set (30)

1. Prokaryotic vs eukaryotic cellsProkaryotes (bacteria, archaea) lack a nucleus and membrane-bound organelles and are usually smaller. Eukaryotes have a nucleus, an endomembrane system and organelles such as mitochondria.
2. Why do phospholipids spontaneously form a bilayer in water?They are amphipathic: hydrophilic phosphate heads face the water on both sides while hydrophobic fatty acid tails cluster together inside, away from water.
3. Simple diffusion vs facilitated diffusion vs active transportSimple diffusion: small nonpolar molecules cross the bilayer down their gradient. Facilitated diffusion: channels or carriers move solutes down their gradient. Active transport uses energy (often ATP) to move solutes against their gradient.
4. What happens to an animal cell placed in a hypertonic solution?Water leaves the cell by osmosis toward the higher solute concentration outside, so the cell shrinks (crenates). In a hypotonic solution it swells and may lyse.
5. What does the Na+/K+ ATPase do?Using one ATP per cycle, it pumps 3 Na+ out and 2 K+ in, building gradients that power secondary active transport and contribute to the membrane potential.
6. Trace the path of a secreted protein from synthesis to export.Translation begins on a free ribosome, which docks on the rough ER; the protein enters the ER lumen, travels in vesicles to the Golgi for processing, then leaves in secretory vesicles by exocytosis.
7. How is a protein targeted to the ER?An N-terminal signal sequence is recognized by the signal recognition particle (SRP), which pauses translation and brings the ribosome to an SRP receptor and translocon on the ER membrane.
8. Give evidence for the endosymbiotic origin of mitochondria and chloroplasts.They have double membranes, their own circular DNA, bacteria-like 70S ribosomes, and divide by a fission-like process.
9. Key structural features of DNATwo antiparallel strands in a double helix with a sugar-phosphate backbone. A pairs with T (2 hydrogen bonds) and G pairs with C (3 hydrogen bonds).
10. What did the Meselson-Stahl experiment show?Using heavy (15N) and light (14N) nitrogen, they showed DNA replication is semiconservative: each new double helix has one parental strand and one new strand.
11. Leading strand vs lagging strand synthesisDNA polymerase only adds nucleotides 5' to 3'. The leading strand is made continuously; the lagging strand is made in short Okazaki fragments, each starting from an RNA primer, later joined by DNA ligase.
12. State the central dogma of molecular biology.Genetic information flows from DNA to RNA (transcription) to protein (translation). Reverse transcription in retroviruses is a notable exception to the DNA-first direction.
13. How does transcription start in bacteria, and which way is RNA made?Sigma factor helps RNA polymerase bind the promoter. The polymerase reads the template strand 3' to 5' and builds RNA 5' to 3', with no primer needed.
14. Name three processing steps for eukaryotic pre-mRNA.Addition of a 5' cap, addition of a 3' poly-A tail, and splicing by the spliceosome to remove introns and join exons.
15. What happens at the A, P and E sites of the ribosome?A site: incoming aminoacyl-tRNA binds. P site: holds the tRNA with the growing polypeptide. E site: the empty tRNA exits.
16. What are the start and stop codons?Start: AUG, which codes for methionine. Stop: UAA, UAG and UGA, which are recognized by release factors rather than tRNAs.
17. Silent vs missense vs nonsense vs frameshift mutationsSilent: same amino acid. Missense: a different amino acid. Nonsense: creates a premature stop codon. Frameshift: an insertion or deletion not divisible by 3 shifts the reading frame.
18. How is the lac operon regulated when lactose is absent vs present?Lactose absent: the lac repressor binds the operator and blocks transcription. Lactose present: allolactose binds the repressor and releases it from the operator, so the genes can be transcribed.
19. What role do cAMP and CAP play in the lac operon?When glucose is low, cAMP rises and binds CAP. CAP-cAMP binds near the promoter and boosts RNA polymerase binding, a form of positive control.
20. How do enzymes speed up reactions?They lower the activation energy by stabilizing the transition state. They do not change the overall ΔG or the equilibrium position, and they are not used up.
21. Competitive vs noncompetitive (allosteric) inhibitionA competitive inhibitor binds the active site and can be overcome with more substrate. A noncompetitive inhibitor binds elsewhere and changes the enzyme's shape, lowering Vmax regardless of substrate level.
22. How does ATP drive unfavourable reactions?ATP hydrolysis is strongly exergonic. Cells couple it to endergonic reactions, often by transferring a phosphate to a reactant, so the combined reaction has a negative ΔG.
23. Where does each stage of cellular respiration occur in a eukaryotic cell?Glycolysis: cytosol. Pyruvate oxidation and the citric acid cycle: mitochondrial matrix. Electron transport chain and ATP synthase: inner mitochondrial membrane.
24. Explain chemiosmosis in mitochondria.The electron transport chain pumps H+ into the intermembrane space. H+ flows back into the matrix through ATP synthase, which uses that energy to make ATP. O2 is the final electron acceptor, forming water.
25. What is the purpose of fermentation?To regenerate NAD+ from NADH so glycolysis can keep producing ATP when oxygen is absent. Lactic acid or ethanol is produced as a by-product.
26. Light reactions vs the Calvin cycleLight reactions (thylakoid membranes) use light to split water, release O2 and make ATP and NADPH. The Calvin cycle (stroma) uses that ATP and NADPH to fix CO2 into sugar, with rubisco catalyzing carbon fixation.
27. Name the three cytoskeleton filaments and a main role of each.Microtubules (tubulin): tracks for transport, spindle, cilia and flagella. Actin microfilaments: cell shape, crawling, muscle contraction. Intermediate filaments: mechanical strength.
28. How does a G protein-coupled receptor pathway amplify a signal?One ligand-bound receptor activates many G proteins, which activate adenylyl cyclase to make many cAMP molecules; cAMP activates protein kinase A, which phosphorylates many targets.
29. Gram-positive vs Gram-negative bacterial cell wallsGram-positive: thick peptidoglycan layer that retains crystal violet (stains purple). Gram-negative: thin peptidoglycan between two membranes, with an outer membrane containing lipopolysaccharide (stains pink).
30. Describe the four levels of protein structure.Primary: amino acid sequence. Secondary: alpha helices and beta sheets held by backbone hydrogen bonds. Tertiary: overall 3D fold from side-chain interactions. Quaternary: assembly of multiple subunits.