A Level H1 Biology Cells Biomolecules Quiz

A-Level Biology H1 Quiz – Answers & Marking Guide

Cells & Biomolecules (Total 50 marks)

Section A: Short Answer & Structured Questions (20 marks)

1. Properties of water

• High specific heat capacity → buffers temperature changes, stabilising internal body temperature / aquatic habitats.
• High latent heat of vaporisation → effective cooling by evaporation / sweating.
• Cohesion / surface tension → supports water transport in xylem / habitat for surface‑dwelling organisms.
• Solvent properties → medium for metabolic reactions / transport of dissolved substances.
  (Award 1 mark per property + appropriate significance; maximum 2 marks.)

2. Nucleus and its double membrane
(a) The nucleus (1 mark).
(b) The double membrane (nuclear envelope) separates transcription (nucleus) from translation (cytoplasm) allowing post‑transcriptional modification of mRNA before it leaves; nuclear pores regulate exchange of materials, maintaining internal environment for DNA replication/repair (1 mark).
(Accept any valid explanation linking structure to function.)

3. Reducing sugar test
(a) Benedict’s reagent (copper(II) sulfate in alkaline solution) (1 mark).
(b) Blue solution → green/yellow/orange/brick‑red precipitate upon heating; the colour depends on concentration of reducing sugar (1 mark).
(Accept any correct description of colour change.)

4. Glycogen vs cellulose
(a) Glycogen: highly branched, coiled; Cellulose: straight, unbranched chains that form parallel bundles linked by hydrogen bonds. (1 mark)
(b) Glycogen: energy storage in animals/fungi, can be rapidly mobilised; Cellulose: structural component of plant cell walls, provides tensile strength. (1 mark)

5. Fluid‑mosaic model
(a) Correctly label a phospholipid (head as circle, tails as wavy lines) – diagram may be sketch; accept verbal description: “the molecule with a hydrophilic head and two hydrophobic tails” (1 mark).
(b) Fluid: phospholipids and some proteins can move laterally within the bilayer. Mosaic: proteins are embedded in/on the bilayer in a scattered, patchy arrangement (1 mark for each aspect).
(Up to 2 marks)

6. Mitochondria in small intestine cells
The cells absorb nutrients (e.g. glucose, amino acids) by active transport, which requires ATP. Active transport is energy‑demanding; the large number of mitochondria ensures sufficient ATP synthesis to power these processes (2 marks).
(1 mark for active transport/energy demand, 1 mark for link to ATP/mitochondria.)

7. pH and amylase activity
(a) pH 7 (or near neutral) (1 mark).
(b) At extreme pH, the ionic interactions (hydrogen bonds, ionic bonds) that maintain the enzyme’s tertiary structure are disrupted, causing denaturation. The active site changes shape and can no longer bind the substrate, so activity falls sharply (2 marks).
(Accept reference to denaturation + loss of active site shape.)

8. Plasmolysis
(a) Plasmolysis (1 mark).
(b) The concentrated sucrose solution has a lower (more negative) water potential than the cell sap. Water moves out of the cell by osmosis, across the partially permeable membrane, causing the vacuole to shrink and the cytoplasm to pull away from the cell wall (2 marks).
(1 mark for water potential gradient, 1 mark for consequent movement/loss of turgor.)

9. DNA monomers and bonds

• Deoxyribonucleotides (deoxyribose sugar, phosphate, nitrogenous base). Adjacent nucleotides are linked by phosophodiester bonds (formed between the phosphate group of one nucleotide and the 3’ OH of the deoxyribose sugar of the next).
  (Award 1 mark for naming nucleotide type, 1 mark for phosphodiester bond. Accept detailed description of components.)

10. NAD in glycolysis
NAD acts as a hydrogen (electron + proton) carrier. In glycolysis, NAD accepts two hydrogen atoms (is reduced) during the oxidation of triose phosphate to pyruvate, forming reduced NAD (NADH). This NADH then transports hydrogen to the electron transport chain for ATP production later.
(1 mark for role as hydrogen carrier, 1 mark for specific step in glycolysis.)

Section B: Diagram / Data Interpretation (15 marks)

11. Organelle X and secretory pathway
(a) Rough endoplasmic reticulum (rough ER) / rER (1 mark).
(b) Protein synthesised on ribosomes of rER → transported inside rER cisternae for folding → vesicle pinches off and moves to Golgi apparatus → Golgi modifies, packages protein into secretory vesicle → vesicle moves to cell membrane → exocytosis (fusion with membrane and release) (3 marks).
(1 mark for each two correct stages, up to 3.)

12. Glucose and sodium transport
(a) Facilitated diffusion (or secondary active transport / cotransport) (1 mark).
(b) Sodium ions are actively transported out of the filtrate to maintain a low intracellular sodium concentration. The sodium‑potassium pump (Na⁺‑K⁺ ATPase) on the basal membrane pumps Na⁺ out against its concentration gradient, using ATP. Even when blood concentration is slightly lower, the pump actively maintains the gradient essential for cotransport of glucose and other substances (2 marks).
(1 mark for “maintaining gradient/active process”, 1 mark for link to cotransport or specific mechanism.)

13. Temperature and diffusion
(a) As temperature increases, the distance travelled by the dye increases (1 mark).
(b) At higher temperature, particles have greater kinetic energy, so they move faster. The dye molecules diffuse more rapidly through the agar gel because both dye and water molecules are moving more quickly, increasing the rate of diffusion (2 marks).
(1 mark for kinetic theory/particle motion, 1 mark for linking to rate of diffusion.)

14. Catalase activity at two temperatures
(a) At 35 °C, oxygen is produced rapidly and reaches a high final volume; the curve rises steeply. At 60 °C, initial rate is slower, final volume is lower, and the curve flattens earlier (2 marks).
(Award for comparing rate and final volume.)
(b) At 60 °C, catalase begins to denature. The enzyme’s tertiary structure is disrupted by breaking hydrogen/ionic bonds, altering the active site shape. Fewer functional enzyme molecules remain, so the reaction slows and stops before all substrate is used, resulting in a lower final volume of oxygen (2 marks).
(1 mark for denaturation, 1 mark for explaining lower final yield due to inactive enzyme.)

15. Translation and mutation
(a) Using standard genetic code: AUG – Methionine (start codon), CCG – Proline, UAC – Tyrosine, UGA – Stop (translation terminates) → peptide: Met–Pro–Tyr (2 marks).
(b) Substitution of UAC (Tyr) to UAG (stop codon) would cause premature termination of translation, resulting in a truncated, likely non‑functional protein (1 mark).

Section C: Extended Response (15 marks)

16. Membrane structure and transport (4 marks)
Phospholipid bilayer: hydrophobic core prevents free passage of ions and large polar molecules; forms a selective barrier.
Transport proteins: channel proteins provide hydrophilic pores for ions/water; carrier proteins undergo conformational change to transport specific molecules (facilitated diffusion or active transport).
Selective permeability: the membrane allows lipid‑soluble (small non‑polar) molecules to diffuse directly, while ions and polar solutes require transport proteins, enabling the cell to control internal composition.
(1 mark for bilayer, 1 mark for transport proteins, 1 mark for selective permeability, 1 mark for coherent linkage. Allow up to 4 marks overall.)

17. DNA vs RNA (4 marks)
DNA: double‑stranded helix; deoxyribose sugar; bases A, T, G, C; stores genetic information; self‑replicates; located mainly in nucleus.
RNA: single‑stranded (usually); ribose sugar; bases A, U, G, C; involved in protein synthesis (mRNA, tRNA, rRNA); short‑lived; can act as an enzyme (ribozyme).
(For full marks, at least two structural and two functional differences must be given. No mark for a list; comparison required.)

18. Enzyme specificity and induced‑fit (4 marks)
The active site is a three‑dimensional cleft/crevice formed by the folding of the polypeptide chain (tertiary structure). Its shape is complementary to a specific substrate. According to the induced‑fit model, when the substrate enters, the active site undergoes a conformational change to mould around the substrate, enhancing the fit. This precise fit lowers the activation energy by stressing substrate bonds and orienting reactive groups, leading to high specificity – only the correct substrate can trigger the conformational change and be catalysed.
(1 mark – description of active site, 1 mark – induced‑fit concept, 1 mark – link to lowering activation energy, 1 mark – specificity reason.)

19. Hydrogen bonds in biomolecules (3 marks)

• DNA: hydrogen bonds between complementary bases (A–T, C–G) hold the two polynucleotide strands together, enabling the double helix and facilitating replication/transcription.
• Proteins: hydrogen bonds help stabilise the secondary structure (α‑helix, β‑pleated sheet) and also contribute to tertiary structure, maintaining the specific shape required for function (e.g. enzyme active site).
• Water: hydrogen bonds between water molecules give water its cohesive and thermal properties vital for life.
  (Choose any two; 1.5 marks per valid example – award 1 for molecule, 0.5 for clear explanation. Max 3 marks.)

20. Xylem and eosin uptake (4 marks)
Eosin is a dye that travels with water in the xylem. Water movement in the xylem is driven by transpiration pull – cohesion between water molecules (hydrogen bonds) and adhesion to the vessel walls create a continuous column. The dye is carried as a tracer, showing that water (and dissolved solutes) moved up through the xylem vessels. Surrounding cells (phloem, cortex) do not stain because water is transported primarily in the xylem conduits, and the dye does not leak out due to lignified walls and the direction of water flow.
(1 mark – identification of xylem as water‑conducting tissue, 1 mark – transpiration pull/cohesion‑adhesion, 1 mark – dye as tracer, 1 mark – explanation why only xylem stained.)

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