A Level H2 Biology Cells Biomolecules Quiz

Answer Key - A-Level Biology H2 Quiz: Cells Biomolecules

Section A

1. Prokaryotes have circular DNA (naked/no histones) located in a nucleoid region; Eukaryotes have linear DNA associated with histones located within a membrane-bound nucleus. [1]
2. Cholesterol acts as a temperature buffer; at high temps, it restricts phospholipid movement (reducing fluidity); at low temps, it prevents tight packing (preventing solidification). [2]
3. Site of ribosomal RNA (rRNA) synthesis and assembly of ribosomal subunits. [2]
4. RER: Studded with ribosomes, involved in synthesis and transport of proteins for secretion or membranes. SER: No ribosomes, involved in lipid/steroid synthesis and detoxification. [2]
5. The potential energy of water per unit volume relative to pure water; measured in kilopascals (kPa) or megapascals (MPa). [2]
6. High SA:Vol ratio means more membrane area available for transport relative to the volume of cytoplasm that needs nutrients, reducing diffusion distance. [2]
7. Peptide bond. [1]

Section B

8. (a) Misfolding exposes hydrophobic R-groups normally buried in the core; these hydrophobic regions interact with each other to avoid the aqueous cytosol, driving aggregation. [2]
9. (b) Patient A is heterozygous (two bands = two different alleles/variants); Patient B is homozygous for the normal allele (one band at HbA position). [2]
10. (a) The operon is normally "off" (repressed) and is only turned "on" (induced) when a specific inducer molecule (e.g., allolactose) binds to the repressor. [2] (b) Prevents waste of energy and amino acids by only synthesizing enzymes when the substrate they act upon is present in the environment. [2]
11. (a) Oxygen consumption is coupled to ATP synthesis; as ADP is converted to ATP, the proton gradient is dissipated, allowing the ETC to move electrons to $O_2$ faster. [3] (b) Oxygen uptake will decrease/stop. Inhibitor blocks the final step of ETC; electrons cannot be transferred to $O_2$, halting the entire chain. [3]
12. DNA: Double-stranded, deoxyribose sugar, Thymine, very long. RNA: Single-stranded, ribose sugar, Uracil, shorter. Both have phosphate backbone and A, C, G bases. [4]
13. Facilitated diffusion: Passive movement of polar/charged molecules via channel/carrier proteins down a concentration gradient. Active transport: Movement against gradient requiring ATP and carrier proteins. [4]
14. Tertiary structure creates a specific 3D shape of the active site; complementary in shape and chemical properties (charge/polarity) to the substrate. [3]
15. ATP stores energy in high-energy phosphoanhydride bonds; hydrolysis of the terminal phosphate releases energy ($\approx 30.5$ kJ/mol) used to power endergonic reactions. [3]

Section C

16. Inner membrane is highly folded into cristae to increase surface area for ETC complexes and ATP synthase. ETC creates a proton gradient in the intermembrane space; ATP synthase uses this gradient (chemiosmosis) to synthesize ATP. [5]
17. Pump uses ATP to move $3\text{Na}^+$ out and $2\text{K}^+$ in against gradients. This creates a concentration gradient and a net loss of positive charge from the interior, contributing to the negative resting potential. [5]
18. Amphipathic nature: Hydrophilic phosphate heads face aqueous environment; hydrophobic fatty acid tails face inward, away from water. This minimizes free energy and forms a stable bilayer. [4]
19. Inducible: Repressor active by default; inducer inactivates repressor (e.g., lac). Logic: Catabolic enzymes made only when substrate present. Repressible: Repressor inactive by default; co-repressor activates it (e.g., trp). Logic: Anabolic enzymes stopped when product is sufficient. [5]
20. Non-polar amino acid in the core is normal; however, if the mutation disrupts a specific interaction or if it was a polar residue essential for a hydrogen bond, it may destabilize the protein. If it causes misfolding, it may expose hydrophobic regions, leading to aggregation and loss of function. [4]