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Secondary 4 Pure Biology Preliminary Examination Paper 5
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TuitionGoWhere Practice Paper - Pure Biology Secondary 4
Answer Key & Marking Scheme (Version 5)
Subject: Pure Biology (6093)
Level: Secondary 4
Paper: Prelim Practice Paper
Section A: Multiple Choice & Short Structured Questions
1. C
Reasoning: The cell has a tail (flagellum) for movement and many mitochondria to provide energy for this movement. This is characteristic of a sperm cell. [1]
2. A
Reasoning: At 60°C, the high temperature breaks the bonds holding the enzyme's tertiary structure, denaturing it. The active site changes shape, and the substrate can no longer bind. [1]
3.
(a) Osmosis [1]
(b) Plasmolysed / Plasmolysis [1]
4.
- Reducing Sugar: Brick-red / Orange-red [1]
- Protein: Biuret solution [1]
- Starch: Blue-black / Black [1]
5.
(a) Any two of the following:
- Thin wall / one cell thick: Short diffusion distance. [1]
- Large surface area (many villi/microvilli): Increases rate of absorption. [1]
- Rich blood supply/capillaries: Maintains concentration gradient. [1]
(Note: Must link structure to function for full marks)
(b) Active transport [1]
Reasoning: Movement is against the concentration gradient (low to high), which requires energy.
6.
(a) The region on the enzyme [1] where the substrate binds [1].
(b)
- High temperature causes the enzyme to denature. [1]
- The bonds holding the enzyme structure break. [1]
- The shape of the active site changes. [1]
- Substrate no longer fits the active site / No enzyme-substrate complexes formed. [1]
(Max 3 marks)
7.
(a) 0.3 mol/dm³ [1]
Reasoning: This is the point where there is no net change in mass (isotonic).
(b)
- The water potential of the 0.1 mol/dm³ solution is higher than the potato cell sap. [1]
- Water moves into the potato cells by osmosis. [1]
- Through the partially permeable cell membrane. [1]
8. B
Reasoning: Proteins contain C, H, O, N, and often S. Carbohydrates contain only C, H, O. [1]
9.
(a) Plant cell [1]
(b) Site of photosynthesis / Contains chlorophyll to trap light energy. [1]
10.
(a) Enzyme: The larger molecule with the indentation. Substrate: The smaller molecule fitting into the indentation. [2]
(b)
- The active site has a specific shape. [1]
- Only a substrate with a complementary shape can fit/bind. [1]
Section B: Structured & Data Response Questions
11.
(a) Graph Marks:
- Axes labelled correctly with units (pH, Volume/cm³). [1]
- Scale suitable and uniform. [1]
- Points plotted correctly. [1]
- Points joined with a smooth curve or straight lines between points. [1]
(b) pH 7 [1]
(c) - At pH 3, the enzyme activity is very low (2 cm³). [1]
- At pH 7, the enzyme activity is highest (48 cm³). [1]
- pH 3 is acidic; the H+ ions interfere with the bonds in the enzyme. [1]
- This causes the enzyme to denature / change the shape of the active site. [1]
- At pH 7, the enzyme is at its optimum shape for substrate binding. [1]
(Max 4 marks)
12.
(a) Any two:
- RBC has no nucleus; WBC has a nucleus. [1]
- RBC is biconcave; WBC is irregular/spherical. [1]
- RBC contains haemoglobin; WBC does not. [1]
(b) - Biconcave shape increases surface area to volume ratio. [1]
- Allows faster diffusion of oxygen in and out. [1]
- Lack of nucleus provides more space for haemoglobin. [1]
- Haemoglobin binds to oxygen. [1]
(Max 3 marks)
(c) - Phagocyte engulfs the bacteria by extending cytoplasm (phagocytosis). [1]
- Forms a vacuole/phagosome around the bacteria. [1]
- Lysosomes fuse with the vacuole. [1]
- Digestive enzymes break down/digest the bacteria. [1]
(Max 3 marks)
13.
(a) Mass increases. [1]
(b)
- Distilled water has a higher water potential than the concentrated sugar solution. [1]
- Water molecules move from the beaker (high water potential) into the tubing (low water potential). [1]
- Through the partially permeable visking membrane. [1]
- By osmosis. [1]
(c) Any two: - Active transport requires energy (ATP); Osmosis does not. [1]
- Active transport moves substances against concentration gradient; Osmosis moves water down water potential gradient. [1]
- Active transport requires carrier proteins; Osmosis occurs through the membrane/pores. [1]
14.
(a) Peptide bond [1]
(b)
- Add Biuret solution to the food sample. [1]
- Safety: Wear safety goggles / Handle with care (Biuret contains copper sulphate/alkali). [1]
- Positive result: Colour changes to purple/violet. [1]
(c) - High temperature causes enzymes to denature. [1]
- Active site changes shape. [1]
- Metabolic reactions slow down or stop because substrates cannot bind. [1]
15.
(a) Palisade mesophyll [1]
(b)
- Cells are packed tightly together. [1]
- Contain many chloroplasts. [1]
- Located near the upper surface to receive maximum light. [1]
- Columnar shape allows efficient packing and light absorption. [1]
(Max 3 marks)
(c) - Photosynthesis does not occur in the dark (no light). [1]
- Stomata close to prevent unnecessary water loss (transpiration) when CO2 is not needed for photosynthesis. [1]
Section C: Free Response Question
16.
(a) Compare and contrast plant and animal cells [6]
Award 1 mark for each valid point, max 6. Must include both similarities and differences.
Similarities:
- Both have a cell membrane, cytoplasm, nucleus, mitochondria, ribosomes.
- Both are eukaryotic.
Differences: - Plant cells have a cell wall; animal cells do not.
- Plant cells have chloroplasts; animal cells do not.
- Plant cells have a large permanent vacuole; animal cells have small/temporary vacuoles.
- Plant cells store carbohydrates as starch; animal cells store as glycogen.
- Plant cells are generally regular/fixed shape; animal cells are irregular.
(b) Importance of enzymes [4]
- Enzymes are biological catalysts that speed up chemical reactions. [1]
- They lower the activation energy required for reactions. [1]
- In digestion: Break down large insoluble food molecules into small soluble ones (e.g., amylase breaks down starch). [1]
- In metabolism: Control cellular processes like respiration, DNA replication, and protein synthesis at body temperature. [1]
17.
(a) Define diffusion [2]
- The net movement [1]
- Of particles/molecules from a region of higher concentration to a region of lower concentration [1]
- Down a concentration gradient. [1]
(Max 2 marks)
(b) Experiment: Temperature and Diffusion [6]
- Use agar cubes containing an indicator (e.g., phenolphthalein/alkali). [1]
- Place cubes in acid solutions at different temperatures (e.g., 20°C, 30°C, 40°C). [1]
- Keep volume of acid and size of agar cubes constant (control variables). [1]
- Measure the time taken for the agar cube to change colour completely (or become colourless). [1]
- Repeat the experiment at each temperature to calculate an average. [1]
- Plot a graph of temperature against rate of diffusion (1/time). [1]
(c) Why large organisms cannot rely on diffusion [2]
- Large organisms have a small surface area to volume ratio. [1]
- Diffusion is too slow to supply oxygen/remove waste to cells deep inside the body. [1]
18.
(a) Lock-and-key hypothesis [4]
- Enzyme has an active site with a specific shape. [1]
- Substrate has a complementary shape. [1]
- Substrate fits into the active site like a key into a lock. [1]
- Forms an enzyme-substrate complex. [1]
(b) Competitive inhibitors [4]
- Competitive inhibitors have a similar shape to the substrate. [1]
- They bind to the active site of the enzyme. [1]
- This blocks the substrate from binding. [1]
- Reduces the rate of reaction because fewer enzyme-substrate complexes are formed. [1]
(c) Inactive enzymes [2]
- To prevent the enzyme from digesting the cell's own proteins/structures. [1]
- Ensures the enzyme only acts when/where it is needed (e.g., in the stomach lumen, not in the cell producing it). [1]
19.
(a) Osmosis in hypertonic solution [4]
- Hypertonic solution has lower water potential than cell sap. [1]
- Water moves out of the cell by osmosis. [1]
- Through the partially permeable cell membrane. [1]
- Cytoplasm shrinks and pulls away from the cell wall (plasmolysis). [1]
(b) Importance of turgor pressure [4]
- Turgor pressure is the pressure of the cytoplasm against the cell wall. [1]
- It provides support to non-woody plants/herbaceous stems. [1]
- Keeps leaves expanded to maximize light absorption for photosynthesis. [1]
- Without turgor, plants wilt. [1]
(c) Root hair cell adaptations [2]
- Long hair-like projection increases surface area for water/ion uptake. [1]
- Thin cell wall for short diffusion distance. [1]
- Many mitochondria to provide energy for active transport of ions. [1]
(Max 2 marks)
20.
(a) Chemical elements [3]
- Carbohydrates: C, H, O [1]
- Fats (Lipids): C, H, O [1]
- Proteins: C, H, O, N (and S) [1]
(b) Functions of lipids [4]
- Long-term energy storage (more energy per gram than carbohydrates). [1]
- Insulation (thermal insulation under skin). [1]
- Protection/cushioning of vital organs. [1]
- Component of cell membranes (phospholipids). [1]
- Waterproofing (e.g., waxy cuticle in plants, sebum in skin). [1]
(Max 4 marks)
(c) Water as a solvent [3]
- Water is a polar molecule. [1]
- It dissolves many substances (ions, polar molecules like glucose, amino acids). [1]
- Allows metabolic reactions to occur in solution / Transport of nutrients and waste in blood/sap. [1]