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Secondary 3 Biology Practice Paper 4
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TuitionGoWhere Practice Paper - Biology Secondary 3
Answer Key and Marking Scheme
Version 4
Section A: Multiple Choice Questions (10 Marks)
| Q | Answer | Explanation |
|---|---|---|
| 1 | D | Plant cells have cell walls, chloroplasts, and large vacuoles. Animal cells do not. Both have cell membranes, cytoplasm, and mitochondria. |
| 2 | B | Rough ER synthesises proteins; mitochondria provide energy. This combination suggests high protein synthesis and secretion (e.g., pancreatic or glandular cells). |
| 3 | C | Proteins always contain C, H, O, N. Many also contain S (in amino acids like cysteine and methionine). |
| 4 | A | High temperatures break bonds holding the enzyme's tertiary structure, changing the shape of the active site (denaturation). This is irreversible. |
| 5 | C | Active transport moves substances against a concentration gradient and requires energy (ATP). Diffusion and osmosis are passive. |
| 6 | A | Water moves from high water potential (distilled water) to low water potential (sugar solution) by osmosis, causing the level in the tubing to rise. |
| 7 | C | Benedict’s test for reducing sugars: Blue → Green → Yellow → Orange → Brick-red precipitate upon heating. |
| 8 | A | The nucleus contains DNA/chromosomes which control cell activities and inheritance. |
| 9 | B | pH 2 is highly acidic, characteristic of the stomach (due to HCl). Pepsin works here. |
| 10 | C | Enzymes are catalysts; they are not used up and can be reused. |
Section B: Structured Questions (30 Marks)
11.
(a)
A: Nucleus [1]
B: Mitochondrion [1]
C: Cell membrane [1]
(b)
- Mitochondria are the site of aerobic respiration. [1]
- They release energy (ATP) required for the metabolic activities of the liver cell. [1]
(c)
- Contains genetic material (DNA) / Controls cell activities / Site of cell division. [1]
(Accept any valid function)
12.
(a)
Rate = 1000 / 60 = 16.7 (accept 16.6–16.7) [1]
(b)
- The rate of reaction increases as temperature increases from 10°C to 40°C. [1]
- The time taken for starch to disappear decreases. [1]
(c)
- At 50°C, the temperature is above the optimum. [1]
- The heat energy breaks the hydrogen bonds holding the enzyme's structure. [1]
- The shape of the active site changes (denaturation). [1]
- The substrate (starch) no longer fits the active site, so no enzyme-substrate complexes form. [1]
(Max 3 marks)
(d)
- pH affects the shape of the active site / enzyme activity. [1]
- To ensure that temperature is the only independent variable affecting the rate (fair test). [1]
13.
(a)
| Feature | Red Blood Cell | Root Hair Cell |
|---|---|---|
| Presence of Nucleus | Absent / No [1] | Present / Yes [1] |
| Presence of Cell Wall | Absent / No [1] | Present / Yes [1] |
| Main Function | Transport oxygen | Absorb water and mineral ions [1] |
| (Note: Marks are awarded for correct entries. Total 3 marks for the whole table as per question structure, but typically 1 per row. Here, 3 marks total for the 3 blank rows requested.) |
(b)
- Biconcave shape increases the surface area to volume ratio. [1]
- This allows for faster diffusion of oxygen in and out of the cell. [1]
(Alternative: No nucleus allows more space for haemoglobin)
(c)
(i) Active transport [1]
(ii) Because ions are moved against the concentration gradient (from low to high concentration). [1]
14.
(a)
(i) Amino acids [1]
(ii) Glucose [1]
(b)
- Add ethanol (alcohol) to the food sample and shake. [1]
- Pour the solution into water. [1]
- A cloudy white emulsion indicates the presence of fat. [1]
(c)
- Glycogen is insoluble in water. [1]
- It does not affect the water potential of the cell / prevents water from entering the cell by osmosis (which would cause it to burst/swell). [1]
(Alternatively: Glycogen is a compact storage form)
15.
(a)
- The net movement of particles [1]
- From a region of higher concentration to a region of lower concentration [1]
- Down a concentration gradient.
(b)
- Single-celled organisms have a large surface area to volume ratio. [1]
- Diffusion distance is short, so diffusion is fast enough to meet metabolic needs. [1]
- Large organisms have a small surface area to volume ratio and cells are deep inside the body. [1]
- Diffusion would be too slow to supply oxygen to internal cells. [1]
(Max 3 marks)
Section C: Free Response Question (10 Marks)
16.
(a)
- The substrate has a specific shape that is complementary to the shape of the enzyme's active site. [1]
- The substrate binds to the active site to form an enzyme-substrate complex. [1]
- This is like a key fitting into a lock. [1]
- The reaction occurs, products are formed, and they are released from the active site. The enzyme remains unchanged. [1]
(b)
(i)
- 80°C is above the optimum temperature for the protease enzyme. [1]
- The enzyme becomes denatured / active site changes shape. [1]
- Therefore, it cannot break down the protein stains. At 40°C, the enzyme is active (near optimum). [1]
(ii)
- Enzymes still work at lower temperatures, but the rate is slower due to lower kinetic energy. [1]
- The manufacturer may have engineered the enzyme to have a lower optimum temperature or be more stable/active at cold temperatures. [1]
- Or, the washing machine cycle is longer, allowing sufficient time for the slower reaction to complete. [1]
(Accept reasonable explanations regarding enzyme adaptation or time compensation)