O Level Biology Cells Biomolecules Quiz

O-Level Biology Quiz - Cells Biomolecules (Answer Key)

Section A: Multiple Choice Answers

1. C
Mitochondria are found in both plant and animal cells for respiration. Cell walls and chloroplasts are plant-only; large permanent vacuoles are characteristic of plant cells.

2. A
Brick-red with Benedict’s indicates reducing sugar. Purple with Biuret indicates protein. No colour change with Iodine indicates no starch.

3. C
The lock and key hypothesis states that the substrate has a specific shape complementary to the enzyme's active site.

4. B
At 0% change in mass (Solution R), there is no net movement of water, meaning the external concentration is isotonic to the cell sap. Thus, cell sap is approx 0.3 mol/dm³.

5. A
Starch is a polysaccharide made of glucose. Proteins are made of amino acids. Fats (lipids) are made of fatty acids and glycerol.

Section B: Structured Answers

6.
(a) Any two of the following:

1. Cell wall (made of peptidoglycan)
2. Plasmids (circular DNA outside nucleoid)
3. No nucleus / Nucleoid region (DNA not enclosed in a membrane)
4. 70S Ribosomes (smaller than eukaryotic)
   [1 mark for each correct feature, max 2]

(b) Bacteria lack a membrane-bound nucleus (and membrane-bound organelles), whereas animal cells have a true nucleus enclosed by a nuclear envelope.
[1 mark]

7.
(a) An enzyme is a biological catalyst (protein) that speeds up the rate of chemical reactions without being used up.
[1 mark]

(b) As temperature increases from 10°C to 40°C, the kinetic energy of enzyme and substrate molecules increases. This leads to more frequent and successful collisions between the substrate and the active site, forming more enzyme-substrate complexes. Thus, the rate of reaction increases.
[1 mark for kinetic energy/collisions, 1 mark for enzyme-substrate complexes, 1 mark for conclusion]

(c) At 80°C, the enzyme is denatured. The high heat breaks the bonds holding the enzyme's tertiary structure, causing the active site to change shape. The substrate can no longer fit into the active site, so no enzyme-substrate complexes can form.
[1 mark for denaturation/shape change, 1 mark for substrate not fitting]

8.
(a) pH 7
[1 mark]

(b) At pH 3 (acidic), the enzyme amylase is denatured or its activity is significantly reduced because the pH is far from its optimum. The change in pH affects the charges on the amino acids in the active site, altering its shape so the substrate fits less effectively.
[1 mark for denaturation/reduced activity, 1 mark for active site shape/fit]

(c) To keep the temperature constant (control variable), ensuring that only pH affects the rate of reaction.
[1 mark]

9.
(a) To absorb water and mineral ions from the soil.
[1 mark]

(b) It has a long, hair-like projection that increases the surface area to volume ratio for faster absorption. It has a thin cell wall to shorten the diffusion distance.
[1 mark for surface area, 1 mark for thin wall/distance]

(c) Process: Active Transport.
Explanation: Ions are moved against the concentration gradient (from low to high concentration), which requires energy (ATP) produced by respiration.
[1 mark for active transport, 1 mark for against gradient/energy]

10.
(i) Growth and repair (or enzyme/hormone production)
(ii) Biuret solution
(iii) C, H, O
(iv) Energy storage in animals (liver and muscles)
[1 mark for each correct entry]

11.
(a) The volume of liquid inside Tube A will increase.
[1 mark]

(b) The sucrose solution in Tube A has a lower water potential (higher solute concentration) than the distilled water outside. Water molecules move from the region of higher water potential (outside) to the region of lower water potential (inside) through the partially permeable membrane by osmosis.
[1 mark for water potential comparison, 1 mark for direction of flow, 1 mark for mentioning osmosis/membrane]

12.
(a) Any two:

1. Biconcave shape increases surface area for oxygen diffusion.
2. No nucleus allows more space for haemoglobin.
3. Contains haemoglobin to bind oxygen.
4. Thin cell membrane for short diffusion distance.
   [1 mark for each, max 2]

(b) Red blood cells lack a cell wall. In distilled water, water enters by osmosis, causing the cell to swell and burst (lyse). Plant cells have a rigid cell wall that withstands the turgor pressure and prevents bursting.
[1 mark for lack of cell wall in RBC, 1 mark for presence of cell wall in plant]

13.
(a) At low light intensity, light is the limiting factor for both plants, so the rate is determined by the available light, not the plant type.
[1 mark]

(b) Plant X (sun plant) is adapted to higher light intensities. It may have more chloroplasts, more enzymes for the light-independent reaction, or thicker leaves, allowing it to utilise high light intensity more efficiently than Plant Y, which is adapted to shade and saturates at lower light levels.
[1 mark for adaptation detail, 1 mark for linking to higher rate]

14.
(a) Mechanical digestion physically breaks down large food pieces into smaller pieces (increasing surface area) without changing chemical structure. Chemical digestion uses enzymes to break down large insoluble molecules into small soluble molecules.
[1 mark for each definition]

(b) Enzyme: Salivary Amylase.
Substrate: Starch.
[1 mark for each]

15.
(a) Palisade mesophyll.
[1 mark]

(b) Stomata allow for the exchange of gases (carbon dioxide enters for photosynthesis; oxygen and water vapour exit). They can open and close to regulate water loss (transpiration).
[1 mark for gas exchange, 1 mark for regulation]

Section C: Free Response Answers

16.
Marking Guide:

• Role: Enzymes act as biological catalysts to speed up the breakdown of large, insoluble food molecules into small, soluble molecules for absorption. [1]
• Example 1: Amylase (in saliva/pancreas) breaks down starch into maltose. [1]
• Example 2: Protease (e.g., pepsin in stomach or trypsin in small intestine) breaks down proteins into peptides/amino acids. [1]
• Example 3: Lipase (in pancreas/small intestine) breaks down fats (lipids) into fatty acids and glycerol. [1]
  (Note: Must include enzyme name, substrate, and product for full marks.)

17.
Marking Guide:

• Villi: The inner wall of the ileum is folded into finger-like projections called villi, which greatly increase the surface area for absorption. [1]
• Microvilli: The epithelial cells on the villi have microvilli (brush border), further increasing surface area. [1]
• Thin Wall: The villus wall is one cell thick, providing a short diffusion distance for nutrients to enter the blood. [1]
• Blood Capillaries: A dense network of capillaries absorbs amino acids and glucose, maintaining a concentration gradient for diffusion. [1]
• Lacteal: A central lacteal absorbs fatty acids and glycerol. [1]
  (Any 4 points well-explained.)

18.
Marking Guide:

• Diffusion: Movement of particles from high to low concentration (down gradient). Passive (no energy required). Example: Oxygen entering cells, CO2 leaving leaves. [2 marks for description + example]
• Active Transport: Movement of particles from low to high concentration (against gradient). Active (requires energy/ATP). Example: Mineral ion uptake by root hairs, glucose uptake in villi. [2 marks for description + example]
  (Comparison must highlight direction of gradient and energy requirement.)

19.
Marking Guide:

• Stomach: Food enters the stomach. Gastric glands secrete gastric juice containing hydrochloric acid (kills bacteria, provides optimum pH for pepsin) and pepsin. [1]
• Pepsin breaks down proteins into peptides. [1]
• Small Intestine: Partially digested food enters the duodenum. Bile (from liver) neutralises acid and emulsifies fats. [1]
• Pancreatic juice contains trypsin (breaks peptides to amino acids), amylase (starch to maltose), and lipase (fats to fatty acids/glycerol). [1]
• Final digestion occurs on the villi surface (e.g., maltase breaks maltose to glucose).
  (Focus on protein digestion as per question prompt, but general flow is accepted if protein specific steps are clear.)

20.
Marking Guide:

• Specificity: Each enzyme has a unique active site shape that only fits a specific substrate (lock and key). [1]
• Temperature: Low temp reduces kinetic energy (slow rate). Optimum temp gives max rate. High temp denatures enzyme (changes active site shape), stopping reaction. [1]
• pH: Each enzyme has an optimum pH. Deviating from this pH changes the charge/shape of the active site, reducing activity or causing denaturation. [1]
• Link: Both temperature and pH affect the 3D structure of the enzyme, specifically the active site, thereby affecting its ability to bind to the specific substrate. [1]