Secondary 4 Pure Biology Preliminary Examination Paper 3

TuitionGoWhere Practice Paper - Pure Biology Secondary 4

Answer Key & Marking Scheme Topic: Cells and Biomolecules (Version 3)

Section A: Multiple Choice & Structured Questions

1. B [1]

• Reasoning: Starch is a carbohydrate (C, H, O). Insulin is a protein (C, H, O, N, and often S due to disulphide bridges). Option B is the most accurate standard answer for O-Level biology regarding protein composition vs carbohydrate.

2. C [1]

• Reasoning: High numbers of mitochondria (energy for synthesis/secretion), rough ER (protein synthesis), and Golgi (packaging/modifying proteins) indicate a cell specialised for secreting proteins, such as pancreatic enzymes.

3. B [1]

• Reasoning: Channel/carrier proteins (Y) are involved in facilitated diffusion and active transport of ions/large molecules. X (heads) are hydrophilic but do not allow free passage of all water-soluble molecules (selectively permeable). Z (tails) are hydrophobic.

4. B [1]

• Reasoning: The isotonic point (no net change in mass) lies between the concentration where mass increases (+5.0% at 0.2) and where it decreases (-5.0% at 0.4). Therefore, the water potential of the cell sap is equivalent to a concentration between 0.2 and 0.4 mol/dm³.

5. A [1]

• Reasoning: Active transport requires carrier proteins and ATP. Uptake of glucose by villi against a concentration gradient is active transport. Oxygen uptake is diffusion. Water loss is osmosis/diffusion. Glucose entering liver cells after a meal is usually facilitated diffusion (down gradient) or active transport depending on context, but A is the definitive example of active transport in the syllabus.

6. (a) pH 7 [1] (b)

• At pH 10, the enzyme is denatured. [1]
• The high pH alters the shape of the active site / breaks bonds holding the tertiary structure, so the substrate no longer fits. [1] (c) Salivary amylase (or Amylase) [1]

7. (a) Sample C [1]

• Reasoning: Biuret is purple (protein present). Benedict's is blue (no reducing sugar). Iodine is orange-brown (no starch). (b)
• Add ethanol to the sample and shake. [1]
• Pour the solution into water. A cloudy white emulsion indicates the presence of fat/lipid. [1] (Note: The question asks how to confirm if ethanol test was NOT performed, implying an alternative method or describing the standard test properly. Since ethanol emulsion IS the standard test, the question implies describing the positive result confirmation or potentially the Sudan III test if known, but at O-Level, describing the emulsion test steps is the expected answer for "how to test". If the prompt implies the test in the table wasn't done, the student must describe the procedure.)

8. (a) Crenation [1] (b)

• The solution outside the cell has a lower water potential (higher solute concentration) than the cytoplasm of the red blood cell. [1]
• Water moves out of the cell by osmosis. [1]
• The cell loses water and shrinks/shrivels. [1] (c)
• Plant cells have a rigid cell wall (made of cellulose). [1]
• The cell wall withstands the turgor pressure / prevents the cell from bursting when water enters. Animal cells lack a cell wall. [1]

Section B: Data Interpretation & Application

9. (a) Graph Description: [4]

• X-axis: Temperature (°C), Y-axis: Volume of Oxygen (cm³). [1]
• Correct scale and labels. [1]
• Points plotted correctly. [1]
• Smooth curve drawn (bell-shaped), peaking at 40°C and dropping to 0 at 60°C. [1]

(b)

• As temperature increases, kinetic energy of enzyme and substrate molecules increases. [1]
• This leads to more frequent collisions between enzyme and substrate. [1]
• More enzyme-substrate complexes are formed per unit time, increasing the rate of reaction. [1]

(c)

• At 60°C, the enzyme is denatured. [1]
• The heat breaks the bonds maintaining the enzyme's structure, changing the shape of the active site. [1]
• The substrate (hydrogen peroxide) can no longer fit into the active site, so no reaction occurs. [1]

(d)

• The volume of oxygen produced would increase. [1]
• At 40°C (optimum), the enzyme is not saturated. Increasing substrate concentration provides more collisions until the enzyme becomes saturated (Vmax). [1]

10. (a) Any two of: [2]

• Finger-like projection / long structure.
• Microvilli on the epithelial cells.
• Thin wall (one cell thick). (Note: Thin wall aids diffusion distance, not strictly surface area, but often accepted in broad "adaptation for absorption" questions. Strictly for SA: Microvilli and folding).

(b) (i) Active Transport [1] (ii)

• It allows glucose to be absorbed against the concentration gradient. [1]
• This ensures that all available glucose is absorbed from the intestine into the blood, maximizing energy uptake. [1]

Section C: Extended Response

11. (a) Comparison: [4]

• Structure 1: Cell Wall. [1]
  • Function: Provides structural support and protection / maintains cell shape / prevents bursting. [1]
• Structure 2: Chloroplast (or Large Permanent Vacuole). [1]
  • Function (Chloroplast): Contains chlorophyll for photosynthesis. [1]
  • (Alternative: Vacuole: Contains cell sap to maintain turgor pressure / stores nutrients/waste).

(b) (i) Root Hair Cell: [3]

• Has a long, hair-like extension which greatly increases the surface area for absorption. [1]
• Thin cell wall / permeable to water and ions. [1]
• Many mitochondria to provide energy (ATP) for active transport of mineral ions. [1]

(ii) Red Blood Cell: [3]

• Biconcave shape increases surface area to volume ratio for faster diffusion of oxygen. [1]
• Contains haemoglobin which binds to oxygen. [1]
• No nucleus provides more space for haemoglobin / allows the cell to be flexible to squeeze through capillaries. [1]

(c) Lock and Key Hypothesis: [5]

• The enzyme has a specific 3D shape with a region called the active site. [1]
• The substrate has a complementary shape to the active site. [1]
• The substrate fits into the active site like a key fits into a lock. [1]
• This forms an enzyme-substrate complex. [1]
• The reaction occurs (bonds broken/formed), and products are released, leaving the enzyme unchanged and ready to react again. [1]

END OF MARKING SCHEME