Secondary 3 Biology Practice Paper 2

TuitionGoWhere Practice Paper — Biology Secondary 3

Answer Key: Cells & Biomolecules (Version 2 of 5)

Section A: Multiple Choice Questions

1. C — Nucleus [1]
Explanation: The nucleus contains DNA, which controls all cellular activities.

2. C — Mitochondrion [1]
Explanation: Mitochondria carry out aerobic respiration, producing ATP in both plant and animal cells.

3. C — Rough endoplasmic reticulum [1]
Explanation: The rough endoplasmic reticulum (RER) is studded with ribosomes on its surface, giving it a "rough" appearance under the electron microscope.

4. C — Carbohydrates [1]
Explanation: Carbohydrates (especially glucose) are the primary and quickest source of energy for cells.

5. B — The enzyme will denature and lose its function [1]
Explanation: A drastic change in pH from 7 to 2 disrupts the enzyme's three-dimensional shape (active site), causing denaturation.

6. B — Controlling the movement of substances into and out of the cell [1]
Explanation: The cell membrane is partially (selectively) permeable and regulates what enters and exits the cell.

7. B — It will swell but not burst due to the cell wall [1]
Explanation: Distilled water is hypotonic relative to the cell. Water enters by osmosis. The rigid cell wall prevents the plant cell from bursting (the cell becomes turgid).

8. C — Protein [1]
Explanation: Proteins are polymers made up of amino acid monomers linked by peptide bonds.

9. C — Chloroplast [1]
Explanation: Chloroplasts are found only in plant cells (and some protists) and are the site of photosynthesis. Animal cells lack chloroplasts.

10. B — Decomposition [1]
Explanation: Catalase breaks down (decomposes) hydrogen peroxide into water and oxygen — a breakdown/decomposition reaction.

Section B: Structured Questions

11.

(a)
A: Nucleus [1]
B: Mitochondrion [1]

(b) The rough endoplasmic reticulum is the site of protein synthesis (because ribosomes attached to it synthesise proteins) [1].
Acceptable alternative: "Transports proteins within the cell" or "folds and modifies proteins."

(c) Structure B (mitochondrion) is called the "powerhouse of the cell" because it is the site of aerobic respiration [1], where glucose is broken down in the presence of oxygen to release energy in the form of ATP (adenosine triphosphate), which powers all cellular activities [1].

12.

(a) As temperature increases from 10 °C to 40 °C, the time taken for starch to disappear decreases [1], meaning amylase activity increases (the reaction rate increases) [1].
Marking note: Students must state both the decrease in time AND the increase in activity/reaction rate for full marks.

(b) At 70 °C, the enzyme amylase is denatured [1]. The high temperature disrupts the bonds that hold the enzyme's three-dimensional shape, altering the active site so that the substrate (starch) can no longer fit, and the enzyme can no longer catalyse the reaction [1].

(c)
Rate of reaction = Amount of starch ÷ Time taken [1]
Rate = 10 units ÷ 2 min = 5 units per minute [1]
Marking note: Award 1 mark for the correct formula/method and 1 mark for the correct answer with units.

13.

(a) Sample X [1] because it has the highest carbohydrate content (75 g per 100 g), and carbohydrates are the main source of quick energy [1].

(b) The Biuret reagent changes from blue to purple/violet [1], indicating the presence of protein.

(c) A balanced diet must include all three biomolecules because:

• Carbohydrates are the main source of quick energy for cellular activities [1].
• Proteins are needed for growth and repair of tissues, and for the synthesis of enzymes and hormones [1].
• Fats provide a concentrated energy reserve, insulate the body, protect organs, and are needed for the absorption of fat-soluble vitamins [1].
  Marking note: Award 1 mark per valid, distinct role. Accept other valid functions (e.g., fats for cell membrane structure, proteins for antibodies).

14.

(a) The rough endoplasmic reticulum (RER) would first show an increase in radioactivity [1] because radioactive amino acids are first incorporated into proteins by ribosomes on the RER [1].
Common mistake: Students may incorrectly say "ribosomes" — while ribosomes do synthesise the proteins, the RER as a structure is the first organelle to show radioactivity because the ribosomes are attached to it.

(b) The pathway is:

1. Radioactive amino acids are assembled into proteins by ribosomes on the rough endoplasmic reticulum [1].
2. The proteins are transported in vesicles to the Golgi body, where they are modified, sorted, and packaged [1].
3. The Golgi body packages the proteins into secretory vesicles, which move to the cell membrane and release the proteins outside the cell by exocytosis [1].

Section C: Data-Based / Application Question

15.

(a) Graph plotting — 3 marks awarded as follows:

• Correct axes (pH on x-axis, time on y-axis) with labels and units [1]
• Appropriate scale that uses at least half the grid in both directions [1]
• All points correctly plotted and a smooth curve or line drawn through the points [1]
  Note: The graph should show a curve with a minimum at pH 2 (time = 5 min), rising steeply from pH 2 to pH 5, and remaining high (no digestion) at pH 5 and pH 7.

(b) The optimum pH for pepsin is approximately pH 2 [1]. This is determined from the graph as the pH at which the time taken is shortest (i.e., the reaction rate is fastest) [1].
Marking note: Accept pH 2–2.5 if the student reads from their graph. The key reasoning is that the shortest digestion time = highest enzyme activity = optimum pH.

(c) At pH 7, the conditions are too alkaline/neutral for pepsin [1]. The enzyme's active site changes shape (the enzyme is denatured or its shape is altered), so the substrate (protein/egg white) can no longer bind to it, and digestion cannot occur [1].
Marking note: "Denatured" is technically more accurate for extreme pH changes; "altered shape" is also acceptable. Award the mark if the student explains that the active site is affected.

(d) Boiling denatures the enzyme pepsin [1]. The high temperature permanently disrupts the enzyme's three-dimensional structure and active site, so even when cooled, the enzyme cannot regain its shape and function, and no digestion occurs [1].
Common mistake: Students may say "the enzyme died" — enzymes are not living, so "denatured" is the correct term. Award the mark if the meaning is clear.

(e)
Accept any one of the following (or other valid digestive enzyme):

Award [1] for a correct enzyme–substrate–product combination.
Marking note: The enzyme must be from the human digestive system. The substrate and product must correctly correspond to the named enzyme.

End of Answer Key