Secondary 4 Combined Science Biology Practice Paper 2

TuitionGoWhere Practice Paper — Answer Key

Subject: Combined Science Biology (Secondary 4)
Topic: Cells & Biomolecules
Version: 2 of 5
Total Marks: 40

Section A — Multiple Choice & Short Answer

1. C — Mitochondrion [1 mark]
Reasoning: The mitochondrion is the organelle where aerobic respiration occurs, producing ATP. Chloroplasts are for photosynthesis (plant cells only), the nucleus stores genetic material, and the vacuole stores substances.

2. The cell membrane controls/regulates the movement of substances into and out of the cell (acts as a selectively permeable barrier). [1 mark]
Marking note: Accept any correct description of selective permeability or control of substance entry/exit. "Protects the cell" alone is insufficient.

3.
(a) Solution X [1 mark]
Reasoning: In a hypotonic solution, water enters the cell by osmosis, causing it to swell and burst (haemolysis).

(b) Solution Y [1 mark]
Reasoning: In an isotonic solution, there is no net movement of water, so the cell remains unchanged.

4. Glucose (a carbohydrate / simple sugar) [1 mark]
Marking note: Accept "carbohydrate" or "glucose." Do not accept "starch" — starch is a storage polysaccharide and must be broken down first.

5.
(a) Iodine solution / iodine in potassium iodide [0.5 mark]
(b) Blue-black [0.5 mark]
(c) Benedict's solution [0.5 mark]
(d) Brick-red precipitate / orange-red precipitate [0.5 mark]
(e) Biuret reagent (sodium hydroxide + copper(II) sulphate) [0.5 mark]
(f) Purple / violet [0.5 mark]
Total: [3 marks]
Marking note: For (d), accept "green/yellow/orange/brick-red" as a range indicating a positive result, but "brick-red" is the clearest positive.

6. Enzymes are specific to their substrate due to the shape of their active site. The active site of amylase is complementary in shape to the glucose-glucose (α-1,4 glycosidic) bonds in starch, but not to the glucose-glucose (β-1,4 glycosidic) bonds in cellulose. Therefore, amylase cannot bind to or break down cellulose. [2 marks]
Marking note: Award 1 mark for mentioning enzyme specificity / active site shape, and 1 mark for linking it to the structural difference between starch and cellulose.

7. Any two of the following: [1 mark each, total 2 marks]

1. Diffusion occurs down a concentration gradient, whereas active transport occurs against a concentration gradient.
2. Diffusion does not require energy (ATP), whereas active transport requires energy (ATP).
3. Diffusion does not require carrier proteins (simple diffusion), whereas active transport requires carrier/transport proteins.
   Marking note: Answers must compare the same feature (e.g., direction, energy, proteins) to earn the mark. Vague statements like "they are different" are not accepted.

8. Iodine molecules are small enough to pass through the pores of the dialysis tubing into the bag, where they react with starch to produce a blue-black colour. However, the observation states the water outside turned blue-black — this means iodine moved into the bag and reacted with starch inside. The solution inside should have turned blue-black. [Correction: Re-read the question.]

The water outside turned blue-black, meaning iodine moved into the bag and reacted with starch. The solution inside remaining colourless is contradictory — however, if the question states the outside turned blue-black, then iodine entered the bag and reacted. Starch molecules are too large to pass through the dialysis tubing, so starch remained inside. [2 marks]
Marking note: Award 1 mark for stating that iodine (small molecules) can pass through the tubing, and 1 mark for stating that starch (large molecules) cannot pass through. The key concept is that dialysis tubing is selectively permeable based on molecular size.

Clarified answer: Iodine molecules are small enough to diffuse through the selectively permeable dialysis tubing into the bag, where they reacted with starch to give a blue-black colour. Starch molecules are too large to pass through the tubing, so they remained inside the bag. [2 marks]

Section B — Structured Response

9.
(a) The rough endoplasmic reticulum transports proteins / is involved in protein synthesis (due to attached ribosomes). [1 mark]
Marking note: Accept "protein synthesis" or "transport of proteins."

(b) The Golgi apparatus modifies, sorts, and packages proteins (including enzymes) into vesicles for secretion out of the cell. Cells that secrete large amounts of enzymes need a well-developed Golgi apparatus to process and package these enzymes efficiently for export. [2 marks]
Marking note: Award 1 mark for the function of the Golgi apparatus and 1 mark for linking it to enzyme secretion.

(c) Muscle cells require large amounts of energy (ATP) for contraction. Mitochondria are the sites of aerobic respiration where ATP is produced. Therefore, muscle cells need more mitochondria to meet their higher energy demands compared to skin cells. [2 marks]
Marking note: Award 1 mark for linking muscle cells to high energy demand, and 1 mark for linking mitochondria to ATP production.

10.
(a) As temperature increases from 10 °C to 40 °C, the volume of oxygen collected increases, meaning enzyme activity increases. [1 mark]
Marking note: Accept "activity increases" or "rate of reaction increases."

(b) At 50 °C, the enzyme (catalase) has been partially denatured. The shape of the active site has changed due to excessive kinetic energy breaking the bonds that maintain the enzyme's tertiary structure. Fewer enzyme-substrate complexes can form, so less oxygen is produced. [2 marks]
Marking note: Award 1 mark for mentioning denaturation and 1 mark for explaining the effect on the active site / enzyme-substrate complex formation.

(c) Predicted volume: approximately 0–1 cm³ (very little or no oxygen). [1 mark]
Reason: At 70 °C, the enzyme would be completely denatured, so it would no longer function as a catalyst. The reaction would essentially stop. [1 mark]
Marking note: Accept any value close to 0. The reason must refer to complete denaturation.

11. Three structural adaptations of red blood cells for oxygen transport: [1 mark each, total 3 marks]

1. Biconcave disc shape — increases the surface area-to-volume ratio, allowing faster diffusion of oxygen into and out of the cell.
2. No nucleus — creates more space inside the cell for haemoglobin, increasing oxygen-carrying capacity.
3. Contains haemoglobin — a protein that binds reversibly to oxygen, enabling efficient oxygen transport.
   Marking note: Accept any three valid adaptations with correct explanations. "Small size" or "flexible membrane" are also acceptable if linked to function (e.g., passing through narrow capillaries).

12.
(a) The cell membrane would pull away from the cell wall (the cell contents shrink), and the cell would appear plasmolysed. [1 mark]
Marking note: Accept "plasmolysis" or a clear description of the cell membrane detaching from the cell wall.

(b) Osmosis [1 mark]
Reasoning: Water moved out of the cell by osmosis (from a region of higher water potential inside the cell to a region of lower water potential in the concentrated salt solution).

(c) Animal cells do not have a cell wall. When placed in a concentrated salt solution, water leaves the animal cell by osmosis, causing the cell to shrink and become crenated (wrinkled). In contrast, plant cells have a rigid cell wall that maintains the cell's shape even as the cell membrane pulls away from it during plasmolysis. [2 marks]
Marking note: Award 1 mark for describing what happens to the animal cell (crenation/shrinking) and 1 mark for explaining the role of the cell wall in plant cells.

Section C — Data & Application

13.
(a) [1 mark each, total 3 marks]

• Sodium ions: into the cell (higher outside → lower inside)
• Potassium ions: out of the cell (higher inside → lower outside)
• Glucose: into the cell (higher outside → lower inside)

(b) The process is active transport. [1 mark] This process requires energy in the form of ATP (produced by aerobic respiration in mitochondria). [1 mark]
Marking note: Both the process name and the energy requirement must be stated for full marks.

(c) The cell maintains a high internal potassium ion concentration because potassium ions are essential for many cellular processes, including enzyme function, protein synthesis, and maintaining the resting potential in nerve and muscle cells. The sodium-potassium pump actively transports K⁺ into the cell against its gradient to maintain this concentration. [2 marks]
Marking note: Award 1 mark for a valid reason why K⁺ is needed inside the cell, and 1 mark for mentioning active transport / the sodium-potassium pump.

14.
(a) Sample P contains starch. [1 mark]
Evidence: Sample P turned blue-black with iodine solution, which is the positive result for starch. [1 mark]

(b) Sample R contains protein but no reducing sugar. [1 mark]
Evidence: Sample R turned purple with the Biuret test (positive for protein) and remained blue with Benedict's test (negative for reducing sugar). [1 mark]

(c) Sample P contains: starch, protein, and fat (lipid). [1 mark]
Reasoning: Blue-black with iodine = starch; purple with Biuret = protein; milky white with ethanol emulsion test = fat.
Marking note: All three biomolecules must be named for the mark.

End of Answer Key
Total: 40 marks