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Secondary 4 Pure Biology Practice Paper 3
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TuitionGoWhere Practice Paper — Answer Key
Pure Biology Secondary 4 | Cells & Biomolecules | Version 3
Section A: Multiple Choice Questions (10 marks)
1. (c) Mitochondrion [1]
Explanation: The mitochondrion is the organelle where aerobic respiration occurs in both plant and animal cells, producing ATP.
2. (d) Carbohydrate [1]
Explanation: Carbohydrates, especially glucose, are the primary and quickest source of energy for cellular processes. Lipids store more energy but are slower to mobilise.
3. (c) Golgi body [1]
Explanation: The Golgi body consists of stacked, flattened membrane sacs (cisternae) with vesicles budding off — a distinctive feature visible under electron microscopy.
4. (b) Provides shape and prevents the cell from bursting in a hypotonic solution [1]
Explanation: The cell wall is a rigid structure made of cellulose that provides mechanical support and prevents osmotic lysis.
5. (c) 37 °C [1]
Explanation: Human enzymes have an optimum temperature around 37 °C (body temperature). At 60 °C, the enzyme would be denatured.
6. (c) Iodine test [1]
Explanation: Iodine solution turns from brown/yellow to blue-black in the presence of starch.
7. (b) It increases the surface area to volume ratio for faster diffusion of oxygen [1]
Explanation: The biconcave shape increases the surface area relative to volume, allowing more efficient uptake and release of oxygen.
8. (d) Chloroplast [1]
Explanation: Chloroplasts are found only in plant cells (and some protists) and are the site of photosynthesis. Cell membrane, nucleus, and mitochondria are present in both.
9. (c) Carbohydrates [1]
Explanation: Glucose is a monosaccharide and maltose is a disaccharide — both are types of carbohydrates.
10. (b) It is selectively permeable, controlling the movement of substances into and out of the cell [1]
Explanation: The cell membrane is made of a phospholipid bilayer with embedded proteins; it allows some substances through while restricting others.
Section B: Structured Questions (20 marks)
11.
(a) Cell A: Plant cell [1]
Cell B: Animal cell [1]
(b) Any two of the following (1 mark each): [2]
- Cell A has a cell wall; Cell B does not.
- Cell A has chloroplasts; Cell B does not.
- Cell A has a large central (permanent) vacuole; Cell B has small or no vacuoles.
- Cell A is rectangular/fixed in shape; Cell B is irregular/rounded in shape.
(c) Example answer: [2]
- Organelle: Mitochondrion
- Function: Carries out aerobic respiration to produce ATP (energy) for the cell.
- Accept also: Nucleus — stores DNA / controls cell activities; Ribosome — site of protein synthesis; Cell membrane — controls entry and exit of substances.
12.
(a) Independent variable: pH [1]
Dependent variable: Time taken for complete digestion (min) [1]
(b) pH 2 [1], because the time taken for complete digestion is the shortest (5 minutes), meaning the enzyme is working fastest at this pH [1].
(c) At pH 5, the pH is too alkaline compared to the optimum pH of pepsin [1]. The enzyme's active site changes shape (the enzyme is denaturing) [1], so the substrate can no longer fit into the active site and the rate of reaction decreases [1].
(d) Any one of the following [1]:
- Temperature
- Volume/concentration of protein solution
- Volume/concentration of pepsin solution
- Same source/type of protein
13.
(a) Any two of the following (1 mark each): [2]
- Contains many chloroplasts to absorb light energy for photosynthesis.
- Tall/columnar shape allows chloroplasts to be arranged to maximise light absorption.
- Large central vacuole pushes chloroplasts to the edges of the cell, closer to light.
- Thin cell wall allows light to penetrate easily.
(b) Palisade mesophyll cells are arranged vertically near the upper surface of the leaf so that they receive maximum sunlight [1]. Being packed with chloroplasts increases the amount of chlorophyll available to absorb light energy [1], making photosynthesis more efficient [1].
14.
(a) Sample P [1] — The iodine test turned blue-black, which is the positive result for starch [1].
(b) Sample Q [1] — Benedict's test produced an orange-red precipitate, which is the positive result for reducing sugar [1].
(c) Sample P [1] — The Biuret test turned purple (positive for protein) AND the ethanol emulsion test turned cloudy white (positive for lipid).
15.
(a) As the sucrose concentration increases from 0.0 to 0.8 mol/dm³, the percentage change in mass of the potato cylinders decreases [1]. The potato cylinders gain mass at low sucrose concentrations and lose mass at high sucrose concentrations [1].
(b) Concentration: approximately 0.35 mol/dm³ [1]
Explanation: Incipient plasmolysis is the point at which the cell membrane just begins to pull away from the cell wall [1], which occurs when there is no net movement of water — i.e., the external solution concentration equals the cell sap concentration [1].
(c) The 0.0 mol/dm³ sucrose solution is hypotonic (has a higher water potential) compared to the cell sap of the potato cells [1]. Water molecules move into the potato cells by osmosis, causing the cells to gain mass [1].
16.
(a) To absorb water and mineral ions from the soil [1].
(b) Any two of the following (1 mark each): [2]
- Long/thin extension (root hair) increases surface area for absorption.
- Thin cell wall/epidermis allows easier diffusion of water and mineral ions.
- Contains a large number of mitochondria to provide energy (ATP) for active transport of mineral ions.
- Large surface area to volume ratio for efficient absorption.
(c) Waterlogged soil reduces the oxygen available to root hair cells [1]. Without oxygen, aerobic respiration cannot occur efficiently, so less ATP is produced for active transport of mineral ions [1]. The plant would absorb fewer mineral ions, leading to mineral deficiency and poor growth [1].
Section C: Data-Based / Free Response Question (10 marks)
17.
(a) Rate = 1 ÷ time = 1 ÷ 60 s = 0.0167 s⁻¹ [1] for the calculation step
Rate of reaction = 0.0167 s⁻¹ (or 1/60 s⁻¹) [1]
Accept: 0.017 s⁻¹ to 2 s.f.
(b) As temperature increases from 10 °C to 37 °C, the time taken for starch to be broken down decreases (rate increases) [1]. Above 37 °C, the time taken increases (rate decreases), showing that enzyme activity decreases at higher temperatures [1].
(c) At 37 °C: This is the optimum temperature for amylase [1]. The enzyme molecules have sufficient kinetic energy for frequent successful collisions with substrate molecules [1], and the active site has the correct shape for the substrate to fit [1].
At 70 °C: The enzyme has been denatured [1]. The high temperature breaks the hydrogen bonds and other bonds that maintain the three-dimensional shape of the enzyme [1], so the active site changes shape and the substrate can no longer bind to it [1].
(d) Any two of the following (1 mark each): [2]
- pH of the solution
- Concentration of amylase
- Concentration of starch solution
- Volume of amylase used
- Volume of starch solution used
18.
(a) (i) C — Rough endoplasmic reticulum [1]
(ii) B — Mitochondrion [1]
(iii) A — Nucleus [1]
(b) Enzymes are proteins [1]. The rough endoplasmic reticulum is the site where proteins are synthesised and folded/modified [1]. The Golgi body further modifies, packages, and sorts proteins into vesicles for secretion out of the cell [1]. Cells secreting large amounts of enzymes therefore need more of these organelles to handle the high volume of protein production [1].
(c) Free ribosomes in the cytoplasm make proteins that are used within the cell (intracellular proteins) [1], while ribosomes attached to the rough endoplasmic reticulum make proteins that are destined for secretion out of the cell or for incorporation into membranes (extracellular/membrane proteins) [1].
19.
(a) Solution X: Hypotonic [1]
Solution Y: Isotonic [1]
Solution Z: Hypertonic [1]
(b) Solution X has a higher water potential (lower solute concentration) than the red blood cells [1]. Water molecules moved into the red blood cells by osmosis, causing them to swell and eventually burst (haemolysis) [1].
(c) If the surrounding fluid were hypotonic, red blood cells would take in water by osmosis and burst, reducing the number of cells available to transport oxygen [1]. If the fluid were hypertonic, the cells would lose water and shrink (crenate), also impairing their function [1]. An isotonic environment ensures red blood cells maintain their normal biconcave shape and function efficiently [1].
20.
(a) A biological catalyst is a substance (enzyme) that speeds up the rate of a chemical reaction in a living organism [1] without being used up in the reaction [1].
(b) The enzyme has an active site with a specific shape that is complementary to the shape of its substrate (the lock-and-key fit) [1]. The substrate binds to the active site to form an enzyme-substrate complex [1]. This lowers the activation energy required for the reaction [1], allowing the substrate to be converted into product(s) more quickly. The product(s) are then released, and the enzyme remains unchanged [1].
(c) Cofactors are non-protein molecules (such as metal ions or vitamins) that bind to the enzyme and are necessary for the active site to have the correct shape [1]. Without the cofactor, the enzyme's active site may not be able to bind the substrate properly, so the enzyme cannot catalyse the reaction effectively [1].
End of Answer Key