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Secondary 4 Combined Science Biology Practice Paper 3
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TuitionGoWhere Practice Paper - Combined Science Biology Secondary 4
Answer Key & Marking Scheme (Version 3)
Section A: Multiple Choice Questions
| Q | Answer | Explanation |
|---|---|---|
| 1 | D | Plant cells have cell walls, chloroplasts, and large vacuoles; animal cells do not. Both have membranes, cytoplasm, and mitochondria. |
| 2 | B | Palisade mesophyll cells contain chloroplasts for photosynthesis and a large vacuole. Root hair cells lack chloroplasts. Animal cells lack both. |
| 3 | B | Ribosomes are the site of protein synthesis. |
| 4 | B | Diffusion is the net movement of particles from high to low concentration without energy. |
| 5 | B | Small, non-polar molecules like oxygen and carbon dioxide diffuse directly through the phospholipid bilayer. Glucose requires facilitated diffusion. Starch and proteins are too large. |
| 6 | B | Water leaves the cell by osmosis into the hypertonic solution. As water leaves, the solute concentration inside increases, lowering the water potential. |
| 7 | B | Enzymes are biological catalysts made of proteins. |
| 8 | C | Extreme pH (like pH 2 for a neutral enzyme) causes permanent denaturation. The active site is permanently distorted. Returning to pH 7 will not restore activity. |
| 9 | C | Proteins are polymers of amino acids. |
| 10 | B | Benedict’s solution tests for reducing sugars (blue to brick-red precipitate upon heating). Iodine tests for starch. Biuret tests for protein. Ethanol tests for fats. |
Section B: Structured Questions
11.
(a)
A: Nucleus [1]
B: Mitochondrion [1]
C: Cell membrane [1]
(b)
- Site of aerobic respiration [1]
- Releases energy (ATP) for cellular activities [1]
(c)
- The nucleus contains DNA/chromosomes [1]
- DNA carries the genetic information/instructions for cell division/mitosis [1]
12.
(a)
Rate = 1 / 60 = 0.017 s⁻¹ [1]
(b)
- At 40°C, the rate is faster (time is shorter) than at 20°C [1]
- Higher temperature increases kinetic energy of enzyme and substrate molecules [1]
- This leads to more frequent successful collisions between enzyme and substrate [1]
(c)
- At 60°C, the enzyme is denatured [1]
- The high heat breaks bonds holding the tertiary structure, changing the shape of the active site [1]
- The substrate can no longer fit into the active site, so no reaction occurs [1] (Note: Only 2 marks available, so any two of these points)
13.
(a)
- Biconcave shape OR No nucleus OR Contains haemoglobin [1] (Accept any one)
(b)
- If biconcave: Increases surface area for faster diffusion of oxygen [1]
- If no nucleus: More space for haemoglobin to carry oxygen [1]
- If haemoglobin: Binds reversibly with oxygen to form oxyhaemoglobin [1] (Must match part a)
(c)
- Root hair cells are located underground/in the soil [1]
- There is no light available for photosynthesis, so chloroplasts are not needed [1]
14.
(a)
- The level of liquid in the capillary tube rises [1]
(b)
- The sugar solution has a lower water potential than the distilled water [1]
- Water molecules move from the beaker (high water potential) into the visking tubing (low water potential) by osmosis [1]
- This increases the volume of liquid in the tubing, pushing it up the capillary tube [1]
(c)
- The level would remain the same / no change [1] (Because water potential is equal on both sides)
15.
(a)
- Amino acids [1]
(b)
- Reagent: Biuret solution [1]
- Positive Result: Colour change from blue to purple/violet [1]
(c)
- Structural support (e.g., collagen, keratin) OR Transport (e.g., haemoglobin) OR Movement (e.g., actin/myosin) OR Immunity (e.g., antibodies) [1]
Section C: Free Response Questions
16. Comparison of Diffusion and Active Transport [6]
Marking Guide: 1 mark for each distinct correct point. Max 6 marks.
- Direction: Diffusion moves substances down the concentration gradient (high to low) [1]. Active transport moves substances against the concentration gradient (low to high) [1].
- Energy: Diffusion is a passive process and does not require energy (ATP) [1]. Active transport is an active process and requires energy (ATP) from respiration [1].
- Proteins: Simple diffusion does not require membrane proteins [1]. Active transport requires specific carrier proteins (pumps) in the membrane [1].
- (Alternative point): Facilitated diffusion uses channel/carrier proteins but is still passive, whereas active transport specifically uses carrier proteins powered by ATP.
17. Digestion of Starch [8]
Marking Guide:
- Mouth: Digestion begins in the mouth. Salivary glands secrete salivary amylase [1]. Starch is broken down into maltose [1].
- Stomach: Food moves to the stomach. The acidic pH denatures salivary amylase, so starch digestion stops temporarily [1].
- Small Intestine (Duodenum): Pancreas secretes pancreatic amylase into the duodenum [1]. Pancreatic amylase continues to break down remaining starch into maltose [1].
- Small Intestine (Ileum): The lining of the ileum secretes maltase [1]. Maltase breaks down maltose into glucose [1].
- Absorption: Glucose is absorbed into the blood capillaries in the villi by active transport (and facilitated diffusion) [1].
18. Adaptations of the Small Intestine [6]
Marking Guide:
- Villi/Microvilli: The inner wall has finger-like projections called villi, which have microvilli on the epithelial cells [1]. This greatly increases the surface area for faster absorption of nutrients [1].
- Epithelium: The villi are covered by a layer of epithelial cells that is one cell thick [1]. This provides a short diffusion distance for nutrients to enter the blood [1].
- Blood Supply: Each villus contains a dense network of blood capillaries [1]. The blood flow maintains a concentration gradient by carrying away absorbed nutrients (like glucose and amino acids), ensuring continuous diffusion/transport [1].
- (Note: Lacteals for fat absorption can be mentioned but the question asks generally about digested food, focusing on the main three adaptations above is sufficient for 6 marks).
19. Importance of Water [6]
Marking Guide: 1 mark for role, 1 mark for explanation. Max 3 roles.
- Solvent: Water is a universal solvent [1]. It allows chemical reactions to occur in solution and helps transport substances (e.g., nutrients, waste) in the blood and phloem [1].
- Temperature Regulation: Water has a high specific heat capacity [1]. It absorbs a lot of heat without a large rise in temperature, helping organisms maintain a stable internal body temperature [1].
- Reactant: Water is a reactant in hydrolysis reactions (digestion) and photosynthesis [1]. It is essential for breaking down large molecules into smaller ones [1].
(Alternative: Turgor pressure in plants for support; Lubricant in joints/pleural fluid).
20. Enzyme Specificity and Lock and Key Hypothesis [6]
Marking Guide:
- Specificity: Each enzyme can only catalyse one specific reaction or act on one specific substrate [1].
- Active Site: The enzyme has a region called the active site [1]. The active site has a specific 3D shape [1].
- Lock and Key: The substrate has a complementary shape to the active site, like a key fits into a lock [1].
- Complex: The substrate binds to the active site to form an enzyme-substrate complex [1].
- Result: This lowers the activation energy, allowing the reaction to proceed. If the shape does not match, the substrate cannot bind, and no reaction occurs [1].