A Level H2 Biology Cells Biomolecules Quiz

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

Section A: Molecular Foundations

1. Triglyceride Structure & Energy:

   • Structure: One glycerol molecule bonded to three fatty acids via ester bonds [1].
   • Efficiency: Triglycerides are insoluble/hydrophobic, so they do not affect osmotic balance [1]; they have a higher proportion of C-H bonds per unit mass, providing more energy upon oxidation than glycogen [1].

2. Hydrogen Bonding in Secondary Structure:

   • H-bonds form between the carbonyl oxygen ($\text{C=O}$) and the amino hydrogen ($\text{N-H}$) of the polypeptide backbone [1].
   • This results in regular folding patterns such as $\alpha$-helices [1] or $\beta$-pleated sheets [1].

3. Fatty Acids & Fluidity:

   • Saturated: No double bonds in hydrocarbon chain, straight chain [1]. Unsaturated: One or more $\text{C=C}$ double bonds, creating "kinks" [1].
   • Fluidity: Kinks in unsaturated fats prevent tight packing of phospholipids [1], increasing membrane fluidity at lower temperatures [1].

4. Condensation & Hydrolysis:

   • Condensation: Reaction between amino group and carboxyl group, releasing a water molecule to form a peptide bond [1.5].
   • Hydrolysis: Addition of a water molecule to break the peptide bond, splitting the polypeptide [1.5].

5. Water's Specific Heat Capacity:

   • High specific heat means water requires significant energy to increase in temperature [1].
   • This prevents rapid temperature fluctuations within the cell [1], ensuring enzymes operate within their optimal temperature range [1].

6. $\alpha$-glucose vs $\beta$-glucose:

   • Structure: $\alpha$-glucose has the $-\text{OH}$ group on Carbon 1 below the plane; $\beta$-glucose has it above the plane [2].
   • Function: $\alpha$-glucose is used for energy storage (starch/glycogen); $\beta$-glucose is used for structural support (cellulose) [1].

7. Primary to Tertiary Structure:

   • Primary structure is the unique sequence of amino acids [1].
   • This sequence determines the R-group interactions (ionic, disulfide, hydrophobic, H-bonds) [1], which fold the protein into a specific 3D shape [1].

Section B: Cell Structure & Membrane Transport

8. Prokaryotic vs Eukaryotic:

   • Genetic Material: Prokaryotes have a circular DNA plasmid/nucleoid; Eukaryotes have linear DNA in a membrane-bound nucleus [2].
   • Organelles: Prokaryotes lack membrane-bound organelles; Eukaryotes have mitochondria, chloroplasts, ER, etc. [2].

9. Fluid Mosaic Model:

   • Fluid: Phospholipid bilayer allows lateral movement of molecules [1].
   • Mosaic: Proteins (integral and peripheral) are embedded in the bilayer like a mosaic [1].
   • Phospholipids provide the basic structure/barrier [1]; proteins provide transport, receptors, and cell recognition [1].

10. Facilitated Diffusion:

    • Mechanism: Movement of polar/charged molecules down a concentration gradient via channel or carrier proteins [2].
    • Limit: The number of available transport proteins is finite; once all are occupied, the rate reaches saturation ($V_{max}$) [1].

11. $Na^+/K^+$ Pump:

    • Mechanism: 3 $\text{Na}^+$ ions pumped out, 2 $\text{K}^+$ ions pumped in [1].
    • ATP: ATP hydrolysis provides energy for the conformational change of the pump [1].
    • Potential: Creates a concentration gradient and a net negative charge inside the cell [2].

12. Endocytosis vs Exocytosis:

    • Endocytosis: Invagination of membrane to take in materials (e.g., phagocytosis of bacteria) [2].
    • Exocytosis: Fusion of vesicles with plasma membrane to release contents (e.g., secretion of insulin) [2].

13. Golgi Apparatus:

    • Structure: Stack of flattened cisternae [1].
    • Function: Modifies proteins (e.g., glycosylation), sorts them, and packages them into vesicles for transport [2].

14. Mitochondrial Cristae:

    • Folds increase the surface area of the inner membrane [1].
    • This allows for more Electron Transport Chain complexes and ATP synthase proteins [1], maximizing ATP production via chemiosmosis [1].

Section C: Enzymes & Advanced Cellular Processes

15. Induced Fit vs Lock and Key:

    • Lock and Key: Active site is a rigid, perfect match for the substrate [1].
    • Induced Fit: Active site is flexible; it changes shape slightly upon substrate binding to fit more tightly [1], stressing bonds to lower activation energy [1].

16. Inhibition Types:

    • Competitive: Binds to the active site; increases $K_m$, $V_{max}$ remains unchanged [2].
    • Non-competitive: Binds to an allosteric site; decreases $V_{max}$, $K_m$ usually remains unchanged [2].

17. pH and Denaturation:

    • pH changes alter the charge of R-groups [1].
    • This disrupts ionic and hydrogen bonds that maintain tertiary structure [1], causing the active site to change shape and lose function [1].

18. Nucleolus:

    • Function: Site of rRNA synthesis and ribosome assembly [2].
    • Importance: Ribosomes are essential for translation (protein synthesis) [1].

19. Lysosomes:

    • Function: Contain hydrolytic enzymes to digest cellular waste/foreign material [2].
    • Environment: Use proton pumps to maintain low pH; the membrane prevents these enzymes from leaking and digesting the cytoplasm [2].

20. Cholesterol & Temperature:

    • High Temp: Restricts movement of phospholipids, preventing the membrane from becoming too fluid/leaky [2].
    • Low Temp: Prevents phospholipids from packing too tightly, preventing the membrane from freezing/crystallizing [2].