Secondary 3 Biology Practice Paper 5

Secondary 3 Biology Quiz - Cells Biomolecules (Answer Key)

Total Marks: 40

Section A: Multiple Choice Questions

1. D
Explanation: Cell walls, chloroplasts, and large permanent vacuoles are found in plant cells but not animal cells. Cell membranes, cytoplasm, and mitochondria are found in both. [1]

2. B
Explanation: Rough Endoplasmic Reticulum (RER) is involved in protein synthesis, and mitochondria provide the energy required for synthesis and secretion. This suggests a secretory cell (e.g., pancreatic cell). [1]

3. B
Explanation: The Golgi apparatus modifies, sorts, and packages proteins for secretion or delivery to other organelles. RER synthesizes proteins; Mitochondria produce ATP; Nucleus controls cell activities. [1]

4. C
Explanation: In a hypertonic (concentrated) solution, water leaves the red blood cell by osmosis. The cell shrinks and the membrane becomes wrinkled (crenation). Animal cells do not have cell walls, so they do not become plasmolysed in the same way plant cells do, nor do they burst (which happens in hypotonic solutions). [1]

5. C
Explanation: Proteins are polymers made of amino acid monomers. Starch and glycogen are polysaccharides made of glucose. Cellulose is a polysaccharide made of glucose. [1]

Section B: Structured Questions

6.
(a) Root hair cell. [1]
(b) Any two from:

1. Long hair-like extension / large surface area.
2. Thin cell wall.
3. Large vacuole.
4. Many mitochondria (though not always visible in simple diagrams, often implied).
   Award 1 mark for each correct feature. [2]
   (c) Explanation for feature 1 (Long extension):
   Increases the surface area to volume ratio [1], which increases the rate of absorption of water and mineral ions from the soil [1].
   OR
   Explanation for feature 2 (Thin wall):
   Shortens the diffusion distance [1] for water/mineral ions to enter the cell [1]. [2]

7.
(a) An enzyme is a biological catalyst [1] made of protein that speeds up chemical reactions without being used up [1]. [2]
(b) Lock and Key Hypothesis:

1. The enzyme has a specific shape called the active site [1].
2. The substrate has a complementary shape to the active site [1].
3. The substrate fits into the active site like a key into a lock, forming an enzyme-substrate complex [1]. [3]

8.
(a) Rate = 1 / 30 = 0.033 s⁻¹. [1]
(b) At 60°C, the high temperature breaks the hydrogen bonds holding the enzyme's tertiary structure together [1]. This causes the active site to change shape / denature [1]. The substrate can no longer fit into the active site, so no enzyme-substrate complexes are formed [1]. [3]

9.
(a) Diffusion (or Facilitated Diffusion). [1]
(b) Active Transport. [1]
(c) Active transport moves substances against the concentration gradient (from low to high concentration) [1]. This requires energy (ATP) to pump the molecules across the membrane [1]. [2]

10.
(a) Cell wall / Plasmid / Circular DNA / Flagellum (if present). Accept Cell Wall. [1]
(b) Nucleus (with nuclear membrane) / Membrane-bound organelles (e.g., mitochondria). Accept Nucleus. [1]

11.
(a) Reagent: Benedict’s solution [1].
Procedure: Heat the mixture in a water bath (boil) [1].
Positive Result: Colour changes from blue to green/yellow/orange/brick-red precipitate [1]. [3]
(b) Solution turns from blue to purple/violet [1]. [1]

12.
(a) It allows some substances (small, non-polar) to pass through freely [1] while preventing others (large, polar, charged) from passing [1]. [2]
(b) The interior of the phospholipid bilayer is hydrophobic (non-polar) [1]. Large polar molecules like glucose are hydrophilic and cannot dissolve in/pass through the hydrophobic fatty acid tails [1]. [2]

13.
(a) Aerobic respiration. [1]
(b) Muscle cells require a large amount of energy (ATP) for contraction [1]. Mitochondria are the site of aerobic respiration, which releases energy [1]. [2]

14.
(a) Adenine (A), Thymine (T), Cytosine (C), Guanine (G). [2] (1 mark for 2-3 correct, 2 marks for all 4).
(b) Adenine pairs with Thymine (A-T) [1]; Cytosine pairs with Guanine (C-G) [1]. [2]

15.
(a) Osmosis is the net movement [1] of water molecules [1] from a region of higher water potential to a region of lower water potential [1], through a partially permeable membrane. [3]

Section C: Free Response Questions

16. Structure of Plant Cell:

• Cell wall, cell membrane, cytoplasm, nucleus, vacuole, chloroplasts, mitochondria.
  Functions (Any 3):

1. Cell Wall: Made of cellulose; provides structural support and prevents bursting under turgor pressure.
2. Chloroplast: Contains chlorophyll; site of photosynthesis to make food (glucose).
3. Vacuole: Contains cell sap; maintains turgor pressure and stores substances.
4. Nucleus: Contains DNA; controls cell activities and division.
5. Mitochondria: Site of aerobic respiration; releases energy.
   Marks: 1 mark for identifying structure, 1 mark for correct function. Max 6 marks. [6]

17. Red Blood Cell Adaptations:

1. Biconcave shape: Increases surface area to volume ratio for faster diffusion of oxygen [1].
2. No nucleus: Provides more space for haemoglobin to carry oxygen [1].
3. Contains haemoglobin: Binds reversibly with oxygen to transport it [1].
4. Thin cell membrane: Shortens diffusion distance for oxygen entry/exit [1].
   Any 4 points well-explained. [4]

18. Effects of Temperature and pH:
Temperature:

• As temperature increases, kinetic energy increases, leading to more frequent collisions between enzyme and substrate, increasing rate [1].
• At optimum temperature, rate is maximum [1].
• Above optimum, heat breaks bonds in the enzyme, changing the shape of the active site (denaturation) [1]. Substrate no longer fits, rate drops to zero [1].
  pH:
• Each enzyme has an optimum pH [1].
• Deviations from optimum pH change the charge of amino acids in the active site, altering its shape [1].
• Extreme pH causes denaturation, stopping the reaction [1].
  Max 6 marks for clear explanation of both factors and denaturation. [6]

19.
(a) The concentrated sugar solution has a lower water potential than the potato cell sap [1]. Water moves out of the potato cells by osmosis [1] through the partially permeable cell membrane [1]. The cells lose turgor pressure and become flaccid/plasmolysed, causing a decrease in mass [1]. [4]
(b) The mass would increase [1]. Water would enter the cells by osmosis because distilled water has a higher water potential than the cell sap [1]. [2]

20.
(a) Carbon, Hydrogen, Oxygen, Nitrogen (C, H, O, N). Sulphur is also acceptable. [1]
(b) Starch: Polymer of glucose (monosaccharide); contains only C, H, O; forms helical/granular structures [1].
Protein: Polymer of amino acids; contains C, H, O, N (and sometimes S); forms complex 3D shapes (primary, secondary, tertiary structures) held by peptide bonds [1].
Comparison: Different monomers, different elements, different bonding/structure [1]. [3]
(c) Proteins are needed for growth and repair of tissues [1]. They are also needed to make enzymes, hormones, and antibodies [1]. Humans cannot store amino acids, so they must be consumed regularly. [2]