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Secondary 4 Pure Biology Practice Paper 1
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TuitionGoWhere Practice Paper — Answer Key
Pure Biology Secondary 4 — Cells & Biomolecules (Version 1 of 5)
Section A: Multiple Choice Questions
1. C — Mitochondrion [1]
- Marking note: Chloroplasts are only in plant cells. The Golgi body and ER are not the site of aerobic respiration.
2. C — Golgi body [1]
- Marking note: Stacked membrane sacs with budding vesicles are characteristic of the Golgi body. RER has ribosomes on its surface.
3. B — Provides shape and prevents the cell from bursting in a hypotonic solution [1]
- Marking note: The cell membrane (not cell wall) controls entry/exit of substances. Chloroplasts carry out photosynthesis.
4. C — have a large surface area due to their long, narrow extension [1]
- Marking note: Root hair cells do not have chloroplasts (they are underground). The large vacuole is a feature but not the primary adaptation for absorption.
5. C — Carbohydrates [1]
- Marking note: Lipids are for long-term energy storage. Proteins have structural and catalytic roles. Nucleic acids store genetic information.
6. A — The enzyme was denatured and lost its active site shape [1]
- Marking note: At 80 °C, the enzyme's tertiary structure is disrupted (denaturation), so the substrate can no longer fit the active site.
7. B — Blue solution turns brick-red precipitate [1]
- Marking note: Purple is the Biuret test (proteins). Blue-black is the iodine test (starch).
8. B — Water moved into the cell by osmosis, causing it to swell and burst [1]
- Marking note: Distilled water is hypotonic relative to the cell contents, so water enters by osmosis. Animal cells lack a cell wall and burst (haemolysis).
9. D — Chloroplast [1]
- Marking note: Cell membrane, nucleus, and mitochondria are present in both plant and animal cells.
10. C — Forming a major component of cell membranes [1]
- Marking note: Enzymes (proteins) catalyse reactions. DNA (nucleic acids) stores genetic information. Haemoglobin (protein) transports oxygen.
Section B: Structured Questions
11.
(a)
- B: Mitochondrion [1]
- D: Golgi body (or Golgi apparatus) [1]
(b) Any one of the following:
- Carries out aerobic respiration / produces ATP [1]
- Releases energy for cell activities
(c) Ribosomes are the site of protein synthesis [1]. Digestive enzymes are proteins, so cells that secrete large amounts of digestive enzymes need many ribosomes to synthesise these proteins [1]. [2]
(d) The nucleus contains DNA / genetic material [1], which controls cell activities / directs protein synthesis / determines cell function [1]. [2]
[Total: 7 marks]
12.
(a)
- Independent: pH [1]
- Dependent: Time taken for complete digestion (or rate of reaction) [1]
(b) Any one of the following:
- Temperature
- Volume/concentration of protein solution
- Volume/concentration of pepsin solution
- Same source/type of protein [1]
(c) As pH increases from 1 to 5, the time taken for digestion increases [1], meaning enzyme activity decreases as pH increases / pepsin works best at low pH (pH 2) [1]. [2]
(d) At pH 5, the pH is far from the optimum pH of pepsin (pH 2) [1]. The enzyme's active site changes shape / the enzyme is partially denatured, so fewer enzyme-substrate complexes are formed [1]. [2]
(e) Prediction: The digestion time would be greater than 40 minutes (or the protein would not be digested) [1]. Explanation: At pH 6, the enzyme would be further from its optimum and likely fully denatured, so it would have little or no activity [1]. [2]
[Total: 9 marks]
13.
(a) Osmosis (or plasmolysis) [1]
- Marking note: Accept "plasmolysis" as the name of the process/result. "Osmosis" alone is also acceptable as the underlying process.
(b) The concentrated salt solution has a lower water potential than the cell sap [1]. Water molecules moved out of the cell (from high water potential to low water potential) through the partially permeable cell membrane by osmosis [1]. As water left the cell, the cell membrane shrank away from the rigid cell wall [1]. [2]
(c) Any one of the following:
- The cell wall is fully permeable, while the cell membrane is partially/selectively permeable.
- The cell wall is made of cellulose, while the cell membrane is made of phospholipids and proteins.
- The cell wall is rigid, while the cell membrane is flexible.
- The cell wall is present only in plant cells, while the cell membrane is present in all cells. [1]
[Total: 4 marks]
Section C: Free Response Question
14.
(a) A biological catalyst is a substance (enzyme) that speeds up the rate of a chemical reaction [1] without being used up / without being changed at the end of the reaction [1]. [2]
(b) Enzymes speed up reactions by lowering the activation energy [1]. According to the lock-and-key hypothesis, the substrate fits into the active site of the enzyme like a key fits into a lock [1]. The shape of the active site is complementary to the shape of the substrate [1]. When the substrate binds to the active site, an enzyme-substrate complex is formed, and the reaction takes place more easily [1]. [3]
(c)(i) As temperature increases from 0 °C to 37 °C, the rate of reaction increases [1]. Above 37 °C, the rate of reaction decreases sharply, reaching near zero at around 70 °C [1]. The optimum temperature is 37 °C [1 — bonus, already covered by the two marks above]. [2]
(c)(ii) Above 37 °C, the enzyme molecules gain excessive kinetic energy [1]. The bonds holding the enzyme's tertiary structure (e.g., hydrogen bonds, ionic bonds) break [1]. The shape of the active site changes (denaturation), so the substrate can no longer fit into the active site, and the enzyme can no longer function [1]. [3]
[Total: 10 marks]
Paper Total: 40 marks