A Level H1 Biology Cells Biomolecules Quiz

A-Level Biology H1 Quiz – Cells Biomolecules: Answer Key

1. Phospholipid arrangement [2]

   • Phospholipids form a bilayer; [1]
   • Hydrophilic phosphate heads face outward towards the aqueous environment (cytoplasm/extracellular fluid); hydrophobic fatty acid tails face inward, away from water. [1]

2. (a) Organelles [2]

   • A: mitochondrion; B: rough endoplasmic reticulum (rough ER).
     (b) Functions in a liver cell [2]
   • Mitochondrion: site of aerobic respiration / synthesis of ATP.
   • Rough ER: synthesis of proteins / transport of proteins.

3. (a) Phase [1] – S phase (synthesis phase).
   (b) Explanation [2]

   • During S phase, DNA replication occurs;
   • radioactive thymidine is incorporated as a precursor of thymine into newly synthesised DNA, so radioactivity concentrates in the nuclei.

4. (a) CO₂ from pyruvate [2]

   • Pyruvate enters the mitochondrial matrix and is converted to acetyl‑CoA, which enters the Krebs cycle;
   • CO₂ is released during the Krebs cycle.
     (b) No CO₂ from glucose [1]
   • Glucose cannot enter the Krebs cycle directly; glycolysis (before CO₂ production) occurs in the cytoplasm, and isolated mitochondria lack the necessary glycolytic enzymes.

5. Water movement [2]

   • Water moves by osmosis / facilitated diffusion through aquaporins;
   • moves down the water potential gradient / from higher water potential to lower water potential.

6. (a) Optimum activity [1]

   • 37°C is the optimum temperature; kinetic energy and molecular collisions are maximal while the enzyme’s tertiary structure is maintained.
     (b) Decrease at 60°C [2]
   • High temperature disrupts hydrogen bonds, ionic bonds and hydrophobic interactions that maintain the enzyme’s specific three‑dimensional shape;
   • the active site denatures, substrate can no longer bind, and activity falls to low levels.

7. Amylose structure–function [4]

   • Amylose is a linear polymer of α‑glucose units linked by α‑1,4 glycosidic bonds;
   • it coils into a helical shape;
   • its compact helical shape allows it to pack tightly, storing large amounts of glucose in a small volume;
   • it is insoluble so does not affect water potential;
   • it can be readily hydrolysed to release glucose for respiration.
     (Any four points)

8. Triglyceride vs phospholipid [4]

   • Triglyceride: one glycerol + three fatty acids; function: long‑term energy storage, insulation, protection.
   • Phospholipid: one glycerol + two fatty acids + a phosphate group; amphipathic; function: major component of cell membranes, forming the bilayer.
     (2 marks structure, 2 marks function, credit comparison)

9. Protein denaturation [3]

   • The protein’s tertiary structure is maintained by hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges;
   • heating provides kinetic energy that breaks these weak bonds, causing the protein to lose its precise conformation (denaturation);
   • the active site or binding site is altered, so the protein can no longer function. (Primary structure remains intact.)

10. (a) Identifications [2]

    • X: deoxyribose sugar; Y: phosphate group; Z: nitrogenous base (any specific base acceptable).
      (b) Bond [1] – Phosphodiester bond.

11. Fluid mosaic model [4]

    • Membrane is composed of a phospholipid bilayer;
    • proteins are scattered throughout, embedded (integral) or on the surface (peripheral);
    • membrane is fluid because phospholipids and some proteins can move laterally;
    • cholesterol stabilises fluidity;
    • carbohydrate chains attach to proteins/lipids on the outer surface (glycocalyx) involved in cell recognition.
      (Any four features)

12. Active transport vs facilitated diffusion [4]

    • Active transport requires metabolic energy (ATP) to move substances against their concentration gradient; e.g., Na⁺/K⁺ pump.
    • Facilitated diffusion moves substances down their concentration gradient through protein channels/carriers without energy; e.g., glucose uptake via GLUT proteins.

13. Water properties [4]

    • High specific heat capacity: water absorbs/releases large amounts of heat with minimal temperature change → stable thermal environment for aquatic organisms, helps maintain constant internal temperature.
    • Cohesion: water molecules stick together via hydrogen bonds → enables water transport in plants (transpiration stream) and surface tension for some organisms.

14. Glycogen vs cellulose [5]

    • Glycogen: branched polymer of α‑glucose with α‑1,4 and α‑1,6 glycosidic bonds; compact; stored in liver/muscle; easily hydrolysed for energy.
    • Cellulose: linear polymer of β‑glucose linked by β‑1,4 glycosidic bonds; parallel chains form microfibrils via hydrogen bonds; main component of plant cell walls; provides structural support; indigestible by most animals.
      (Structure 2, function 2, comparison 1)

15. Competitive vs non‑competitive inhibition [4]

    • Competitive inhibitor: structurally similar to substrate; binds to the active site, preventing substrate binding; effect can be overcome by increasing substrate concentration.
    • Non‑competitive inhibitor: binds to an allosteric site (not active site); alters shape of active site so substrate cannot bind; cannot be overcome by increasing substrate concentration.

16. Endocytosis and exocytosis [4]

    • Endocytosis: cell membrane invaginates, forming a vesicle around large particles (phagocytosis) or fluid (pinocytosis); vesicle pinches off into cytoplasm.
    • Exocytosis: vesicles containing proteins (e.g., secretory vesicles from Golgi) move to and fuse with the cell membrane, releasing contents outside; requires ATP.

17. Nucleus and nucleolus [3]

    • Nucleus: contains most genetic material (DNA), controls cellular activities via gene expression; site of DNA replication and transcription. (2 marks)
    • Nucleolus: synthesises rRNA and assembles ribosomal subunits. (1 mark)

18. Prokaryotic vs plant cell wall [3]

    • Prokaryotic cell wall: composed of peptidoglycan (murein); provides shape and protection.
    • Plant cell wall: composed mainly of cellulose; provides structural support, may contain lignin.
    • Both are external to the cell membrane.

19. Plant cell in hypertonic solution [3]

    • Water moves out of the cell by osmosis (from higher water potential to lower water potential);
    • the vacuole shrinks, the plasma membrane pulls away from the cell wall (plasmolysis);
    • the cell becomes flaccid and loses turgor pressure.

20. DNA vs RNA differences [3]

    • DNA: deoxyribose sugar; RNA: ribose sugar.
    • DNA bases: A, T, C, G; RNA bases: A, U, C, G.
    • DNA is double‑stranded; RNA is usually single‑stranded.
      (Any three clear differences accepted)