Secondary 4 Pure Biology Practice Paper 5

TuitionGoWhere Practice Paper — Pure Biology Secondary 4

Answer Key & Marking Scheme

Version 5 of 5

Section A: Multiple Choice Questions [10 marks]

1. C — Mitochondrion
Marking note: The mitochondrion is the site of aerobic respiration (Krebs cycle and electron transport chain) in both plant and animal cells. Chloroplasts are found only in plant cells and are the site of photosynthesis.

2. B — Controlling the movement of substances into and out of the cell
Marking note: The cell membrane is partially (selectively) permeable and regulates what enters and leaves the cell. It does not provide rigid support (that is the cell wall in plants), store genetic information (nucleus), or carry out photosynthesis (chloroplast).

3. B — Rough endoplasmic reticulum
Marking note: Ribosomes attached to the endoplasmic reticulum give it a "rough" appearance. The rough ER is involved in protein synthesis and transport.

4. C — Carbohydrate
Marking note: Carbohydrates (especially glucose) are the primary and quickest source of energy for cellular activities. Lipids provide more energy per gram but are slower to metabolise. Proteins are primarily structural and functional molecules.

5. C — Enzymes lower the activation energy of reactions.
Marking note: Enzymes are catalysts that speed up reactions by lowering the activation energy required. They are not consumed, do not change the equilibrium, and do not increase activation energy.

6. C — It denatures and loses its three-dimensional shape.
Marking note: High temperatures break the hydrogen bonds and other weak interactions that maintain the enzyme's tertiary structure. This changes the shape of the active site, so the substrate can no longer bind — the enzyme is denatured.

7. C — Maltose
Marking note: Maltose is a disaccharide composed of two glucose molecules. Glucose and fructose are monosaccharides. Starch is a polysaccharide.

8. B — Biconcave disc shape increasing surface area-to-volume ratio
Marking note: The biconcave shape maximises the surface area-to-volume ratio, allowing efficient oxygen diffusion. Red blood cells also lack a nucleus to make more room for haemoglobin.

9. B — Water from a region of high water potential to low water potential.
Marking note: Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution) across a partially permeable membrane.

10. B — Iodine test
Marking note: Iodine solution turns blue-black in the presence of starch. Benedict's test detects reducing sugars, Biuret test detects proteins, and the ethanol emulsion test detects lipids.

Section B: Structured Questions [30 marks]

11.

(a) A: Mitochondrion; B: Golgi body [2]
Marking: 1 mark each. Accept "mitochondria" (plural).

(b) Any one of the following: [1]

• Site of aerobic respiration / produces ATP / releases energy from glucose.
  Marking note: Must refer to energy/ATP/respiration. Simply stating "produces energy" is acceptable.

(c) The Golgi body receives proteins from the rough endoplasmic reticulum, modifies them (e.g., adds carbohydrate groups to form glycoproteins), packages them into vesicles, and transports them to the cell membrane for secretion. [2]
Marking: 1 mark for modification/packaging; 1 mark for transport/secretion to cell membrane.

(d) Any two of the following: [2]

• Plant cells have a cell wall; animal cells do not.
• Plant cells have chloroplasts; animal cells do not.
• Plant cells have a large central vacuole; animal cells have small or no vacuoles.
  Marking: 1 mark each. Must be a valid structural difference.

12.

(a) As pH increases from 1 to 6, the time taken for complete digestion increases (or the rate of digestion decreases), meaning enzyme activity decreases. Pepsin is most active at pH 2 (shortest digestion time of 5 minutes). [2]
Marking: 1 mark for describing the trend (activity decreases as pH increases); 1 mark for identifying the optimum pH or noting that pH 2 gives the fastest digestion.

(b) At pH 2, the active site of pepsin has the correct shape and charge to bind to the substrate (protein) most effectively. The enzyme's tertiary structure is stable at this pH, allowing the highest rate of enzyme-substrate complex formation. [2]
Marking: 1 mark for correct shape/charge of active site; 1 mark for effective binding / maximum enzyme-substrate complex formation.

(c) At pH 6, the enzyme pepsin has denatured — the active site has changed shape due to the alteration in pH, so the substrate can no longer bind to it. [1]
Marking note: "Denatured" or "active site changed shape" must be stated. Simply saying "it doesn't work" is insufficient.

(d) Any two of the following: [2]

• Temperature
• Volume/concentration of pepsin
• Volume/concentration of protein solution
• Volume of each solution used
  Marking: 1 mark each. Must be a valid controlled variable.

(e) Prediction: Between 5 and 12 minutes (accept any value in this range, e.g., 8–10 minutes). [1]
Reasoning: pH 2.5 is between pH 2 (5 min) and pH 3 (12 min), so the digestion time should be between these two values. [1]
Marking: 1 mark for a reasonable prediction within the range; 1 mark for correct reasoning based on interpolation.

13.

(a)

• Solution X: Hypotonic
• Solution Y: Isotonic
• Solution Z: Hypertonic
  [3]
  Marking: 1 mark each. Hypotonic = lower solute concentration outside cell, so water enters; Isotonic = equal concentration, no net movement; Hypertonic = higher solute concentration outside, water leaves.

(b) Solution X is hypotonic relative to the red blood cell cytoplasm. This means the solution has a higher water potential than the cell cytoplasm. Water molecules move by osmosis from the region of higher water potential (the solution) into the region of lower water potential (the cell). The cell swells and eventually bursts (haemolysis) because the cell membrane cannot withstand the increasing internal pressure. [3]
Marking: 1 mark for identifying higher water potential outside; 1 mark for water moving into the cell by osmosis; 1 mark for cell swelling/bursting due to water influx.

(c) Root hair cells have a long, thin extension (root hair) that greatly increases the surface area for water absorption. They also have a large vacuole that helps maintain a water potential gradient, and a thin cell wall and cell membrane that allow rapid water uptake. [2]
Marking: 1 mark for increased surface area; 1 mark for any other valid adaptation (thin cell wall, large vacuole, water potential gradient).

14.

(a)

• Sample P: Starch
• Sample Q: Lipid and reducing sugar (glucose)
• Sample R: Protein
  [3]
  Marking: 1 mark each. Sample P: iodine test positive = starch. Sample Q: Benedict's positive = reducing sugar; ethanol emulsion positive = lipid. Sample R: Biuret positive = protein.

(b) The Biuret reagent consists of sodium hydroxide (NaOH) and copper(II) sulfate (CuSO₄). If protein is present, the solution changes from blue to purple/violet. [2]
Marking: 1 mark for naming the reagents (NaOH and CuSO₄); 1 mark for the colour change (blue → purple/violet).

(c) Heating provides the activation energy needed for the redox reaction between the reducing sugar and copper(II) ions in Benedict's reagent to occur. Without heating, the reaction would be too slow to observe a visible colour change. [1]
Marking note: Must mention activation energy or speeding up the reaction.

Section C: Free Response Questions [20 marks]

15.

(a) As substrate concentration increases, the rate of reaction increases rapidly at first, then gradually levels off until it reaches a maximum rate (Vmax), beyond which further increases in substrate concentration have no effect on the rate. [2]
Marking: 1 mark for initial increase; 1 mark for levelling off / reaching maximum.

(b) At high substrate concentrations, all the active sites of the enzyme molecules are occupied by substrate molecules at any given moment. The enzyme is working at its maximum capacity (saturated), so adding more substrate cannot increase the rate of reaction because there are no free active sites available. [3]
Marking: 1 mark for all active sites occupied; 1 mark for enzyme saturation; 1 mark for no further increase possible.

(c) The new curve should start at the same origin, rise more slowly than the original curve, and reach the same Vmax at a higher substrate concentration. [2]
Marking: 1 mark for same Vmax; 1 mark for curve shifted to the right (higher substrate concentration needed to reach Vmax).

(d) A competitive inhibitor has a similar shape to the substrate and competes with the substrate for binding to the active site of the enzyme. When the inhibitor occupies the active site, the substrate cannot bind, reducing the rate of reaction. However, this inhibition can be overcome by increasing the substrate concentration, as the substrate outcompetes the inhibitor. [2]
Marking: 1 mark for competing for the active site; 1 mark for effect can be overcome by increasing substrate concentration.

16.

(a) Any three of the following: [3]

• Cell wall
• Chloroplast
• Large central vacuole
  Marking: 1 mark each. These are structures found in plant cells but not animal cells.

(b) The large central vacuole is filled with cell sap (a solution of sugars, salts, and other substances). This creates a high solute concentration inside the vacuole, which lowers the water potential inside the cell. Water enters the cell by osmosis from the surrounding cytoplasm and from outside the cell. The influx of water creates turgor pressure against the cell wall, keeping the cell firm and turgid, which supports the plant. [3]
Marking: 1 mark for cell sap / solute concentration in vacuole; 1 mark for water entering by osmosis; 1 mark for turgor pressure supporting the cell/plant.

(c) When a plant cell is placed in a concentrated salt solution, the external solution has a lower water potential than the cell sap. Water molecules move out of the cell by osmosis, from the region of higher water potential (inside the cell) to the region of lower water potential (the salt solution). As water leaves the cell, the vacuole shrinks and the cytoplasm pulls away from the cell wall. This process is called plasmolysis. The cell becomes flaccid and the plant wilts. [4]
Marking: 1 mark for lower water potential outside the cell; 1 mark for water moving out by osmosis; 1 mark for plasmolysis / cytoplasm shrinking; 1 mark for cell becoming flaccid / plant wilting.

17.

(a) A biological catalyst is a substance (enzyme) that speeds up the rate of a chemical reaction in a living organism without being consumed or permanently changed in the process. [2]
Marking: 1 mark for speeding up the reaction; 1 mark for not being consumed / unchanged.

(b) According to the lock and key hypothesis, the enzyme's active site has a specific three-dimensional shape that is complementary to the shape of the substrate molecule. The substrate fits into the active site like a key fits into a lock. Once bound, an enzyme-substrate complex is formed, the reaction takes place, and the products are released. The enzyme remains unchanged and can be reused. [3]
Marking: 1 mark for complementary shape of active site and substrate; 1 mark for enzyme-substrate complex formation; 1 mark for products released and enzyme unchanged.

(c) A competitive inhibitor binds to the active site of the enzyme, competing directly with the substrate. Its effect can be overcome by increasing substrate concentration. A non-competitive inhibitor binds to a different part of the enzyme (an allosteric site), causing a change in the shape of the active site so the substrate can no longer bind. Its effect cannot be overcome by increasing substrate concentration because the inhibitor does not compete with the substrate for the active site. [3]
Marking: 1 mark for competitive inhibitor binding to active site; 1 mark for non-competitive inhibitor binding to allosteric site and changing active site shape; 1 mark for the key difference — competitive inhibition can be overcome by more substrate, non-competitive cannot.

End of Answer Key

This answer key was generated by TuitionGoWhere AI. Mark allocations are estimates based on syllabus expectations and may vary from actual examination marking schemes.