O Level Biology Practice Paper 3

TuitionGoWhere Exam Practice (AI) — Biology O-Level

Practice Paper: Cells & Biomolecules (Version 3 of 5) — Answer Key

Total Marks: 60

Section A: Recall and Short Answer (14 marks)

1. [1] Chloroplast.
Teaching note: Chloroplasts contain chlorophyll and are the site of photosynthesis in plant cells. Only plant cells (and some algae) have them.

2. [1] Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N); sometimes Sulphur (S).
Teaching note: Proteins are made of amino acids which contain these elements. Carbohydrates and fats lack nitrogen.

3. [1] Any one: maintains cell turgor / stores water and dissolved substances / supports the cell.
Teaching note: The large central vacuole in plant cells is filled with cell sap and helps keep the cell rigid.

4. [1] Iodine solution.
Teaching note: Iodine turns blue-black in the presence of starch; this is a standard food test.

5. [2] Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration.
Marking: 1 mark for "high to low concentration"; 1 mark for "net movement of particles".
Common mistake: Writing only "movement" without direction or concentration reference.

6. [1] The active site is the region on the enzyme where the substrate binds.
Teaching note: It has a specific shape complementary to the substrate.

7. [1] Active transport.
Teaching note: Active transport uses energy (ATP) to move substances against a concentration gradient.

8. [1] Reducing sugar.
Teaching note: Benedict's solution gives brick-red with reducing sugars (e.g., glucose) after heating.

Section B: Structured Response and Interpretation (26 marks)

9. [2]
(a) Bacterial cells have a cell wall made of peptidoglycan; animal cells have no cell wall (or cellulose wall in plants, but absent in animals). [1]
(b) Bacterial cells contain plasmids; animal cells do not. [1]
Alternative acceptable: Bacteria have 70S ribosomes / nucleoid region without membrane-bound nucleus.
Teaching note: Bacteria are prokaryotes; animal cells are eukaryotes.

10. [3]

• Increased protein intake → more amino acids → deamination in liver → more urea produced. [1]
• More urea in blood increases solute concentration. [1]
• Kidney reabsorbs more water by osmosis to expel urea, but urine becomes more concentrated (or volume adjusts via ADH). [1]
  Marking: Process link (1) + osmotic principle (1) + kidney regulation (1).
  Common mistake: Confusing urine volume with concentration.

11. [4]

• Enzymes are biological catalysts that speed up digestion without being used up. [1]
• Amylase breaks down starch to maltose in the mouth/salivary gland. [1]
• Pepsin (stomach) breaks down proteins to peptides in acidic pH. [1]
• Lipase (pancreas/duodenum) breaks down fats to fatty acids and glycerol. [1]
  Marking: Catalyst role (1) + two named enzyme–substrate–product pairs (2) + location (1).
  Teaching note: Each enzyme is specific to its substrate.

12. [3]

• Air stone pumps oxygen into solution. [1]
• Root cells use O₂ for aerobic respiration to produce ATP. [1]
• ATP provides energy for active transport of mineral ions into roots. [1]
  Common mistake: Stating "plants need air" without linking to ATP and ion uptake.

13. [2]
(a) Cell wall. [1]
(b) Chloroplast (or large vacuole). [1]
Based on image: Plant cell shows cell wall and chloroplasts; animal cell lacks both.

14. [3]
(a) [1] 40°C.
(b) [2] Above 40°C, enzyme denatures; tertiary structure breaks, active site loses shape, substrate cannot bind, rate falls. [2]
Marking: Denaturation (1) + loss of active site function (1).

15. [3]

• Both are polysaccharides of glucose. [1]
• Starch is branched/helical and used for energy storage. [1]
• Cellulose is straight-chain with β-glucose, forms cell wall for support. [1]
  Teaching note: Different glycosidic bonds give different functions.

Section C: Extended Application (20 marks)

16. [6]

1. Zygote divides by mitosis to form embryo. [1]
2. Embryo travels down fallopian tube. [1]
3. Blastocyst implants into uterine wall. [1]
4. Placenta develops for nutrient/oxygen exchange. [1]
5. Umbilical cord connects fetus to placenta. [1]
6. Amniotic fluid protects fetus. [1]
   Common mistake: Using meiosis instead of mitosis.

17. [4]
(a) [1] Osmosis.
(b) [3] Animal cell: swells and may burst (lysis). [1] Plant cell: becomes turgid, cell wall prevents bursting. [2]
Teaching note: Water potential higher outside → water enters by osmosis.

18. [4]
A: Starch [1]
B: Reducing sugar [1]
C: Protein [1]
D: Fat/Lipid [1]
Teaching note: Match test colours to biomolecules.

19. [4]

• Enzyme has active site with specific shape. [1]
• Substrate fits like key into lock. [1]
• Enzyme-substrate complex forms. [1]
• Reaction occurs, products released, enzyme unchanged. [1]
  Teaching note: Explains enzyme specificity.

20. [3]

• Fats are not polymers of glucose like carbs. [1]
• Fats are made of glycerol + fatty acids, not monosaccharides. [1]
• Fats have different ratio/structure and function (long-term energy). [1]
  Common mistake: Assuming same elements = same class.

End of Answer Key