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Secondary 3 Biology Practice Paper 1
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Secondary 3 Biology Quiz - Cells Biomolecules (Answer Key)
Total Marks: 40
Section A: Multiple Choice Answers
1. D
Reasoning: Cell walls are present in plant cells but absent in animal cells. Cell membranes, cytoplasm, and mitochondria are found in both.
2. B
Reasoning: Rough Endoplasmic Reticulum (RER) is involved in protein synthesis, and mitochondria provide the energy required for synthesis and secretion.
3. B
Reasoning: Oxygen moves by diffusion down its concentration gradient. Glucose absorption in the villi often involves active transport to move against the concentration gradient into the blood.
4. A
Reasoning: High temperatures break the bonds holding the enzyme's structure, changing the shape of the active site (denaturation). This is irreversible.
5. C
Reasoning: Biuret solution turns purple/violet in the presence of protein. Benedict’s is for reducing sugars, Iodine for starch, Ethanol for fats.
Section B: Structured Answers
6.
(a) Red Blood Cell (Erythrocyte) [1]
(b) Any two of: [2]
- Biconcave shape: Increases surface area for faster diffusion of oxygen.
- No nucleus: Provides more space for haemoglobin to carry oxygen.
- Thin cell membrane: Shortens diffusion distance for oxygen.
(c) It lacks a nucleus (and therefore DNA/chromosomes), which is required for cell division/mitosis. [1]
7.
(a) An enzyme is a biological catalyst [1] made of protein [1] that speeds up chemical reactions without being used up. [2]
(b) As temperature increases, kinetic energy of enzyme and substrate molecules increases [1]. This leads to more frequent and successful collisions between the substrate and the active site [1]. [2]
(c) The high temperature breaks the hydrogen bonds holding the enzyme's tertiary structure [1]. The active site changes shape and is no longer complementary to the substrate [1]. The enzyme is denatured [1]. [3]
(Note: Accept 2 marks for clear explanation of denaturation)
8.
(a) Osmosis is the net movement of water molecules [1] from a region of higher water potential to a region of lower water potential [1] through a partially permeable membrane. [2]
(b) The sugar solution inside the tubing has a lower water potential than the distilled water outside [1]. Water molecules move into the tubing by osmosis [1]. This increases the volume/pressure inside the tubing, causing the liquid level to rise [1]. [3]
(c) The liquid level would remain unchanged (stay the same) [1]. There is no water potential gradient (concentrations are equal), so there is no net movement of water [1]. [2]
9.
(a) Both involve the movement of substances across a cell membrane. [1]
(b) Any two differences: [2]
- Diffusion does not require energy (passive); Active transport requires energy (ATP).
- Diffusion moves substances down a concentration gradient; Active transport moves substances against a concentration gradient.
- Diffusion involves small molecules/ions; Active transport often involves ions or larger molecules.
10.
(a) [3]
- Protein Basic Unit: Amino Acids
- Fat Basic Unit: Fatty Acids
- Protein Elements: (Already given as C,H,O,N - correct)
(Note: Student must fill in "Amino Acids" and "Fatty Acids")
(b) Add Benedict’s solution to the milk sample [1]. Heat the mixture in a water bath (boil) [1]. A positive result is a colour change from blue to green/yellow/orange/brick-red precipitate [1]. [3]
11.
(a) A: Nucleus, B: Chloroplast, C: Cell Membrane [3]
(b) Chloroplasts contain chlorophyll and are the site of photosynthesis, where light energy is converted to chemical energy (glucose). [1]
12.
(a) The long hair-like projection increases the surface area to volume ratio [1], allowing for a faster rate of water absorption by osmosis [1]. [2]
(b) The concentration of ions in the soil water is often lower than inside the root hair cell [1]. Therefore, ions must be moved against the concentration gradient, which requires energy (active transport) [1]. [2]
13.
(a) The substrate has a specific shape that fits into the enzyme's active site [1], like a key fits into a lock [1]. This forms an enzyme-substrate complex [1], leading to the reaction. [3]
(b) It ensures that specific reactions occur at the right time and place, preventing unwanted side reactions and maintaining metabolic order. [1]
14.
(a) Mix the food sample with ethanol (alcohol) and shake [1]. Pour the solution into water [1]. [2]
(b) Formation of a cloudy white emulsion. [1]
15.
(a) The curve should peak at pH 2 and drop to zero (or very low) activity by pH 7. [2]
(b) Pepsin is adapted to work in acidic conditions. At pH 8 (alkaline), the enzyme will be denatured [1] because the change in pH affects the bonds holding the active site shape [1]. [2]
Section C: Free Response Answers
16.
Muscle cells require large amounts of energy (ATP) for contraction [1]. Mitochondria are the site of aerobic respiration, which releases energy [1]. Skin cells have lower energy demands, so they have fewer mitochondria. [2]
17.
Organic molecules contain carbon-hydrogen (C-H) bonds and are usually complex (e.g., Glucose, Protein) [1]. Inorganic molecules generally do not contain C-H bonds and are simpler (e.g., Water, Carbon Dioxide, Salts) [1]. [2]
18.
Water leaves the cell by osmosis because the external solution has a lower water potential [1]. The vacuole shrinks [1]. The cytoplasm pulls away from the cell wall [1]. This process is called plasmolysis. The cell wall remains rigid, but the cell becomes flaccid. [3]
19.
Enzymes are catalysts [1]. They lower the activation energy but are not chemically changed or consumed in the reaction [1]. They can be reused multiple times. [2]
20.
Lysosomes contain digestive enzymes [1]. These enzymes break down waste materials, old organelles, and ingested particles [1]. Without functional lysosomes, these materials accumulate and cannot be recycled or removed, potentially damaging the cell [1]. [2]