Secondary 4 Pure Biology Practice Paper 2

TuitionGoWhere Practice Paper - Pure Biology Secondary 4 (Answers)

Version 2 of 5 — Answer Key

Section A: Cell Structure and Organisation (18 marks)

1. [1] Ribosome(s).
Teaching note: Ribosomes are the organelles where amino acids are joined to form proteins. They may be free in the cytoplasm or attached to rough endoplasmic reticulum.

2. [1] Biconcave (disc) shape / no nucleus.
Teaching note: The biconcave shape increases surface area for oxygen diffusion; lack of nucleus also allows more space for haemoglobin.

3. [1] E (mitochondrion).
Teaching note: A = cell wall, B = vacuole, C = chloroplast (photosynthesis), D = nucleus (control), E = mitochondrion (aerobic respiration / ATP production).

4. [3]

• Root hair cell has a long hair-like projection increasing surface area [1]
• This increases rate of water absorption by osmosis [1]
• Thin cell wall / large vacuole aids water uptake [1]
  Teaching note: Adaptation links structure to function. More surface area = faster osmosis from soil.

5. [2] Any two:

• Plant cell has cell wall, animal cell does not [1]
• Plant cell has chloroplasts, animal cell does not [1]
• Plant cell has large central vacuole, animal cell has small/no vacuole [1]
  (Max 2 marks)

6. [2]

• Muscle cells need large amounts of energy (ATP) for contraction [1]
• Mitochondria produce ATP via aerobic respiration, so many mitochondria meet energy demand [1]

Section B: Movement of Substances (18 marks)

7. [2] Osmosis is the diffusion of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane.
Marking: 1 mark for water movement, 1 mark for partially permeable membrane / water potential gradient.

8. [1] R.
Teaching note: R shows water leaving the cell (outward arrows) = hypertonic external solution.

9. [2]

• The sugar solution around R has lower water potential than the cell sap [1]
• Water moves out of the cell by osmosis down the water potential gradient [1]

10. [3]

• Diffusion is passive (no energy) movement down concentration gradient [1]
• Active transport requires energy (ATP) [1]
• Active transport moves substances against concentration gradient [1]

11. [3]

• Water leaves the red blood cells by osmosis [1]
• Because salt solution has lower water potential than cell cytoplasm [1]
• Cells shrink / crenate and may be damaged [1]

12. [2] Example: Glucose absorption in villi of small intestine (active transport) [1]; or ion uptake by root hair cells [1]. Substance: glucose / mineral ions.

Section C: Biological Molecules and Enzymes (24 marks)

13. [1] Amino acid.

14. [2] Iodine solution [1]; blue-black colour with starch [1].

15. [2] Protein [1] and fat [1].
Teaching note: Biuret = protein; ethanol emulsion = fat. Milk contains both.

16. [3]

• Enzyme has a specific active site [1]
• Substrate fits into active site like a key into a lock [1]
• Enzyme-substrate complex forms, reaction occurs, products released [1]

17. [1+2+3 = 6 marks]
(a) [1] pH 2.
(b) [2] Rate increases from 40% at pH 1 to 100% at pH 2, then decreases steadily to 0% at pH 7 [2].
(c) [3] Above pH 3, the enzyme's active site changes shape (denatures) [1] because H+ ion concentration drops altering ionic bonds [1]; substrate can no longer bind, so activity falls [1].

18. [1+3+3 = 7 marks]
(a) [1] Temperature.
(b) [3] Higher temperature increases kinetic energy [1]; more enzyme-substrate collisions [1]; faster reaction until optimum ~45°C [1].
(c) [3] At 55°C amylase denatures [1]; active site lost [1]; starch not digested / rate near zero [1].

19. [1] Carbon, hydrogen, oxygen (C, H, O).

20. [3]

• Lactase breaks down lactose into glucose and galactose [1]
• Without lactase, lactose remains undigested in milk [1]
• Causing bloating / diarrhoea as bacteria ferment lactose in gut [1]

Total Marks: 60