Secondary 4 Combined Science Biology Practice Paper 3

TuitionGoWhere Practice Paper — Combined Science Biology Secondary 4 (Version 3) Answer Key

Total Marks: 40

Section A (8 marks)

1. Diffusion [1]
Teaching note: Oxygen moves from alveolus (high concentration) to muscle cell (low concentration) by diffusion, a passive process needing no energy.

2. Mitochondrion (plural: mitochondria) [1]
Teaching note: Mitochondria are the sites of aerobic respiration where ATP is produced.

3. Cell wall is made of cellulose and is rigid; cell membrane is a phospholipid bilayer and is flexible / selectively permeable. [1]
Accept any one clear difference.

4. Red blood cell (erythrocyte) [1]
Teaching note: Mature mammalian red blood cells lack a nucleus and are biconcave to increase surface area for oxygen transport.

5. Osmosis [1]
Teaching note: Osmosis is diffusion of water across a partially permeable membrane.

6. Protein [1]
Teaching note: Proteins are polymers of amino acids; they include enzymes and structural molecules.

7. Facilitated diffusion [1]
Teaching note: Glucose uses a carrier protein but moves down its gradient without energy.

8. Ribosome [1]
Teaching note: Ribosomes link amino acids to form proteins.

Section B (16 marks)

9. [2]

• Skeletal muscle cells need more ATP for contraction (1 mark).
• Therefore they have more mitochondria to carry out respiration and release energy (1 mark).
  Common mistake: Stating "they are bigger" without linking to energy need.

10. [3]

• Glucose moves by facilitated diffusion through carrier proteins down its concentration gradient (1 mark).
• It does not require energy from ATP (1 mark).
• Rate is affected by concentration gradient steepness; steeper gradient = faster rate (1 mark).
  Teaching note: Facilitated diffusion is passive; carriers are specific.

11. [2]
Any two from: cell wall, chloroplasts, large central vacuole (1 mark each, max 2).
Based on Q11-fig1: Plant cell shows wall, chloroplasts, vacuole absent in animal cell.

12. [2]

• Diffusion is passive movement down a concentration gradient; no energy needed (1 mark).
• Active transport uses energy (ATP) to move substances against a gradient (1 mark).
  Marking: 1 mark per correct contrast.

13. [2]

• Water enters by osmosis; cell swells and may burst (lysis) (1 mark).
• Because outside has higher water potential; water moves in across membrane (1 mark).

14. [2]

• Fat cells store energy as lipids (1 mark).
• Liver cells need proteins for metabolism; fat cells specialize in storage so more lipid (1 mark).

15. [2]
Any two: controls entry/exit of substances; receptor site; separates cell from environment; maintains shape (1 mark each, max 2).

Section C (16 marks)

16. [4]
Mark breakdown:

• Oxygen diffuses from alveolus to blood capillary (1)
• Binds haemoglobin, pumped via pulmonary vein, left heart, aorta to coronary artery (1)
• Diffuses from capillary to heart muscle tissue fluid (1)
• Diffuses into mitochondrion of heart muscle cell (1)
  Structures named: alveolus, capillary, red blood cell, heart (left atrium/ventricle), aorta, coronary artery, muscle cell membrane, mitochondrion.

17. [4 total: 1+2+1]
(a) Active transport [1]
(b) Higher temperature increases kinetic energy and protein carrier motion; faster movement until 30 °C [2]
(c) Enzymes/proteins denature or already near max rate [1]

18. [3]
(a) Bacteria lack nucleus (DNA free in cytoplasm); animal has nucleus [1+1]
(b) Bacteria have peptidoglycan wall; animal cells have no wall (membrane only) [1]
Marking: 1 mark per valid contrast point.

19. [3]

• High temperature breaks H-bonds in protein [1]
• Enzyme loses 3D shape = denaturation [1]
• Active site changes, substrate no longer fits, no catalysis [1]

20. [4]

• Disagree [1]
• Diffusion moves any small molecules (e.g. O₂) down gradient; osmosis is only water [1]
• Osmosis needs partially permeable membrane; diffusion may not [1]
• Osmosis is water-specific, diffusion is general [1]
  Marking descriptors: clear comparison = 4; partial = 2–3.