A Level H1 Biology Practice Paper 2

TuitionGoWhere Exam Practice (AI) — Biology H1 A-Level Practice Paper (Version 2) Answer Key

Total Marks: 60
Section A: 45 marks
Section B: 15 marks

Section A Answers

1. (a) [2] Structure B = rough endoplasmic reticulum (1). Role: site of protein synthesis (ribosomes attached) and initial folding/modification of proteins (1).
(b) [1] Presence of membrane-bound nucleus / mitochondria / Golgi body (any one eukaryotic feature visible).
Teaching note: Eukaryotic cells have membrane-bound organelles; bacterial cells lack them. Rough ER is identified by ribosomes on surface.

2. [2] Phospholipids form a bilayer (1). Hydrophilic heads face outward to aqueous environments; hydrophobic tails face inward away from water (1).
Marking: 1 for bilayer, 1 for orientation of heads/tails.

3. [2] Glucose binds to a carrier protein in the membrane (1). The protein changes shape and releases glucose into the cell down its concentration gradient, no ATP needed (1).
Reference to Fig. 2: carrier protein shown spanning bilayer; glucose higher outside.

4. [3] Pyruvate enters mitochondria and is converted to acetyl-CoA then enters Krebs cycle, releasing CO₂ (1). Glucose cannot be directly metabolised in mitochondria because glycolysis occurs in cytoplasm (1). Therefore no CO₂ from glucose in isolated mitochondria (1).

5. [2] Bacterial cell has no membrane-bound nucleus (1); has circular DNA and 70S ribosomes / peptidoglycan wall (1). Liver cell has nucleus, organelles.

6. (a) [1] 40°C.
(b) [2] Above 40°C, enzyme denatures (1); tertiary structure breaks, active site lost, rate falls (1).

7. [2] Glycosidic bond (1); condensation (dehydration) reaction (1).

8. [1] Reduces fluidity at high temp / prevents crystallisation at low temp / stabilises membrane.

9. [2] Phase Q (S phase) (1). Thymidine is incorporated into DNA during replication in S phase (1).

10. [3] Cellulose is a polymer of β-glucose with straight chains (1). Chains linked by hydrogen bonds form microfibrils (1). Provides tensile strength to cell wall (1).

11. [2] Hypertonic solution (1). Water leaves cell by osmosis, cell shrinks/crenates (1).

12. [3] Lock-and-key: substrate fits rigid active site (1). Induced-fit: active site changes shape on binding substrate (1). Induced-fit explains broader specificity and better catalysis (1).

13. (a) [1] Saturation of transport / carriers fully occupied.
(b) [2] Facilitated diffusion (1) because rate plateaus (saturation) not linear as in simple diffusion (1).

14. [2] Pluripotency = can differentiate into all body cell types (1). Embryonic stem cells (1).

15. [3] Quaternary = 4 polypeptide subunits (2 α, 2 β) (1). Allows cooperative binding of O₂ (1). Small change in one subunit affects others, efficient loading/unloading (1).

16. [2] Bile salts emulsify triglyceride (1) increasing surface area for lipase action (1).

17. [2] Net water moves left to right (1). Lower water potential on right (higher sucrose) draws water by osmosis (1).

18. [2] Hydrogen bonds, ionic bonds, disulfide bridges, hydrophobic interactions (any two).

19. [2] Carboxyl group of one amino acid reacts with amino group of another (1). Water removed, peptide bond formed (condensation) (1).

20. [2] Cell recognition, receptor sites, cell signalling, adhesion (any two).

Section B Answers (choose 1)

21. [15] Fluid mosaic model: membrane is phospholipid bilayer with proteins embedded/attached (3). Phospholipids give fluidity (2). Integral proteins transport/channels (3). Glycoproteins/glycolipids for recognition (3). Cholesterol stabilises (2). Role in compartmentalisation, signalling, transport (2).
Descriptors: 15 marks = breadth (all components) + function links.

22. [15] Substrate conc: rate increases then plateaus (saturation) (4). pH: optimum, denaturation away from optimum (4). Regulation: controls metabolic flux, prevents waste (4). Examples e.g. digestive enzymes (3).