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Secondary 3 Biology Practice Paper 3
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TuitionGoWhere Practice Paper — Biology Secondary 3
Answer Key & Marking Scheme
Paper: Practice Paper — Cells & Biomolecules (Version 3 of 5)
Total Marks: 40
Section A: Multiple Choice Questions (10 marks)
1. C — Nucleus [1]
The nucleus contains DNA (genetic material) and controls all cellular activities.
2. C — Mitochondrion [1]
Mitochondria are the sites of aerobic respiration in both plant and animal cells.
3. B — It will swell but not burst due to the cell wall. [1]
Distilled water has a higher water potential than the cell cytoplasm. Water enters by osmosis. The cell wall prevents bursting (turgid).
4. C — Carbohydrates [1]
Carbohydrates (especially glucose) are the main source of quick/ immediate energy for cells.
5. B — The enzyme will be denatured and lose its function. [1]
pH 2 is far from the optimum pH of 7; the enzyme's active site changes shape (denaturation), so it can no longer catalyse the reaction.
6. B — Controlling the movement of substances into and out of the cell [1]
The cell membrane is partially/ selectively permeable and regulates what enters and exits the cell.
7. A — It increases the surface area for faster diffusion of oxygen. [1]
The biconcave shape increases the surface-area-to-volume ratio, allowing faster oxygen diffusion.
8. C — Reducing sugar [1]
Benedict's test: a brick-red precipitate indicates the presence of reducing sugar (e.g., glucose).
9. C — Chloroplast [1]
Chloroplasts are found only in plant cells (for photosynthesis); animal cells lack them.
10. C — Rough endoplasmic reticulum [1]
Amino acids are assembled into proteins on ribosomes attached to the RER, so radioactivity appears there first before moving to the Golgi body and secretory vesicles.
Section B: Structured Questions (20 marks)
11.
(a) Mitochondrion [1]
(b) It is the site of aerobic respiration, where energy (ATP) is released from glucose. [1]
(c) Muscle cells require more energy (ATP) for contraction [1] than skin cells, so they contain more mitochondria to carry out more aerobic respiration. [1]
12.
(a) 40 °C [1] — This is the temperature at which starch disappeared in the shortest time (2 min), meaning amylase activity was highest. [1]
(b) As temperature increases from 40 °C to 60 °C, the enzyme molecules gain excessive kinetic energy, causing the active site to change shape (denaturation) [1]. The substrate can no longer fit into the active site, so the reaction slows down and the time taken increases. [1]
(c) At 70 °C, the enzyme amylase is completely denatured [1]. The high temperature has permanently altered the shape of the active site [1], so the enzyme can no longer catalyse the breakdown of starch.
13.
(a) Any two of the following [1 each, max 2]:
- Contains many chloroplasts
- Has a cell wall
- Has a large central vacuole
- Is rectangular/columnar in shape
(b) The long, narrow shape of the root hair cell increases the surface-area-to-volume ratio [1], which increases the rate of absorption of water and mineral ions from the soil by diffusion/osmosis/active transport. [1]
(c) Both cells need energy (ATP) for life processes such as active transport, cell division, and synthesis of molecules [1], and mitochondria are the organelles that produce ATP through aerobic respiration.
14.
(a)
- Solution X: Starch [1] (iodine turned blue-black)
- Solution Y: Protein [1] (Biuret test turned violet/purple)
- Solution Z: Lipid/Fat [1] (ethanol emulsion test produced a cloudy white emulsion)
(b) Any one of the following [1]:
- Enzymes catalyse chemical reactions in the body.
- Proteins are needed for growth and repair of tissues.
- Antibodies are proteins that help fight infection.
- Proteins can be used as an energy source.
- Haemoglobin (a protein) transports oxygen in red blood cells.
Section C: Data-Based and Extended Response Questions (10 marks)
15.
(a) The red blood cells appear shrunken / crenated / wrinkled [1].
(b) The concentrated salt solution has a lower water potential (more negative) than the cytoplasm of the red blood cells [1]. Water moves out of the red blood cells by osmosis [1], from a region of higher water potential (inside the cell) to a region of lower water potential (the salt solution) across the partially permeable cell membrane [1]. The cells therefore lose water and shrink.
16.
(a) Graph [3]:
- x-axis labelled "pH" (1–5), y-axis labelled "Time taken for egg white to disappear (min)" (0–60 or appropriate scale) [1]
- All 5 points correctly plotted [1]
- A smooth curve drawn through the points, showing a minimum at pH 2 [1]
(b) pH 2 [1]
(c) At pH 5, the pH is too far from the optimum pH of pepsin [1]. The enzyme's active site is denatured (permanently changed in shape), so the substrate (protein) can no longer bind to it and the reaction does not occur. [1]
17.
(a) Any two of the following [1 each, max 2]:
- Long axon (long fibre) to transmit impulses over long distances
- Myelin sheath (insulating layer) to speed up impulse transmission
- Branched dendrites to receive impulses from other neurones
- Synaptic terminals to pass impulses to the next neurone/muscle
(b) Example — Myelin sheath [2]: The myelin sheath acts as an electrical insulator around the axon [1], which speeds up the transmission of electrical impulses by causing the impulse to jump from one node of Ranvier to the next (saltatory conduction). [1]
OR
Example — Long axon [2]: The long axon allows the electrical impulse to be transmitted over long distances within the body [1] without losing signal strength, enabling communication between the brain/spinal cord and distant muscles or glands. [1]
18.
(a) Osmosis [1]
(b) Water moves out of the cell by osmosis [1]. The cell membrane pulls away from the cell wall — the cell becomes plasmolysed [1].
(c) Animal cells do not have a cell wall [1]. Without a rigid cell wall, the animal cell will simply shrink/crenate (shrivel) as water leaves, whereas the plant cell retains its shape due to the cell wall and only the membrane pulls away from the wall. [1]
19.
(a) As temperature increases from 10 °C to 40 °C, the volume of oxygen produced increases (from 3 cm³ to 22 cm³), meaning enzyme activity increases [1].
(b) From 40 °C to 70 °C, the increasing temperature causes the kinetic energy of the enzyme and substrate molecules to become too high [1]. The enzyme's active site becomes permanently changed in shape (denatured) [1], so the substrate (hydrogen peroxide) can no longer fit into the active site, and the rate of reaction decreases until at 70 °C the enzyme is completely inactive (0 cm³ O₂). [1]
(c) Any one of the following [1]:
- Volume/concentration of hydrogen peroxide
- Mass/volume of potato extract (amount of enzyme)
- pH of the solution
- Time period of measurement (2 min)
20.
(a) Protein [1]
(b) The lysosomal enzymes are contained within the lysosome, surrounded by a membrane [1] that separates the enzymes from the cytoplasm, preventing them from digesting the cell's own structures. [1]
(c) The enzymes would digest the cell's own organelles and structures [1], leading to cell death (autolysis).
End of Answer Key
Total: 40 marks