O Level Biology Practice Paper 3

TuitionGoWhere Practice Paper - Biology O-Level (Answer Key)

Version: 3 of 5
Subject: Biology (6093)
Topic: Cells & Biomolecules

Section A: Multiple Choice & Short Structured Questions

1. B
[1]
Reasoning: Prokaryotes (bacteria) have cell walls (peptidoglycan) but lack a true nucleus and membrane-bound organelles like mitochondria.

2. D
[1]
Reasoning: The large permanent vacuole fills with water, pushing the cytoplasm against the cell wall, creating turgor pressure.

3. B
[1]
Reasoning: Palisade mesophyll cells are located near the upper surface to receive maximum light and are packed with chloroplasts.

4. C
[1]
Reasoning: Haemoglobin is a protein. Proteins are polymers of amino acids. Cellulose, glycogen, and starch are carbohydrates (polymers of glucose).

5. A
[1]
Reasoning: Iodine stays orange-brown (no starch). Biuret stays blue (no protein). Benedict’s turns brick-red (reducing sugar present).

6.

• Enzymes have an active site with a specific shape. [1]
• Only a substrate with a complementary shape can fit into the active site to form an enzyme-substrate complex. [1]
  [2]

7.
(a) Stomach [1]
Reasoning: Enzyme P works best at pH 2 (acidic), which is the condition in the stomach (pepsin).
(b)

• The low pH (high acidity) breaks the bonds holding the enzyme's structure. [1]
• The active site changes shape (denaturation), so the substrate can no longer bind. [1]
  [2]

8.

• Modifies, sorts, and packages proteins (received from RER). [1]
• Transports them in vesicles to the cell membrane for secretion. [1]
  [2]

9.

• Active transport requires energy (ATP); diffusion does not. [1]
• Active transport moves substances against the concentration gradient; diffusion moves down the concentration gradient. [1]
  [2]

10.
(a) Crenated (shrivelled/shrunken). [1]
(b)

• The salt solution has a lower water potential than the cytoplasm of the red blood cell. [1]
• Water moves out of the cell by osmosis across the partially permeable membrane. [1]
  [2]

Section B: Structured Questions

11.
(a) Golgi apparatus (or Golgi body). [1]
(b)

• RER is the site of protein synthesis. [1]
• Pancreatic cells secrete digestive enzymes (which are proteins), so they need extensive RER to synthesise them. [1]
  [2]
  (c)
• Mitochondria are the site of aerobic respiration. [1]
• They provide ATP/energy required for the synthesis and active transport/secretion of enzymes. [1]
  [2]

12.
(a) Graph Requirements: [4]

• X-axis labelled "Temperature (°C)" with linear scale. [1]
• Y-axis labelled "Volume of Oxygen (cm³)" with linear scale. [1]
• Points plotted correctly. [1]
• Smooth curve drawn (bell-shaped). [1]

(b)

• As temperature increases, kinetic energy of enzyme and substrate molecules increases. [1]
• This leads to more frequent successful collisions between enzyme and substrate. [1]
  [2]

(c)

• The high temperature (60°C) caused the enzyme to denature. [1]
• The active site changed shape and is no longer complementary to the substrate, so no reaction occurs. [1]
  [2]

13.
(a) Glucose. [1]
(b)

• Cellulose Function: Provides structural support / makes cell walls rigid. [1]
• Cellulose Location: Plant cell walls. [1]
• Glycogen Function: Energy storage. [1]
• Glycogen Location: Liver and muscle cells (in animals). [1]
  [4]

14.
(a) Long hair-like projection increases surface area for absorption. [1]
(b)
(i) Active transport. [1]
(ii)

• Active transport requires energy. [1]
• Mitochondria produce ATP (via respiration) to provide this energy. [1]
  [2]

Section C: Free Response Questions

15.
(a)

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

(b)

• Protease breaks down protein (egg white) into amino acids/peptides. [1]
• Amino acids/peptides are soluble, causing the cloudy suspension to become clear. [1]
  [2]

16.

• Water is a polar molecule, allowing it to dissolve many ionic and polar substances (metabolites, ions, gases). [1]
• This allows chemical reactions to occur in solution and facilitates transport of substances within organisms (e.g., blood plasma, cytoplasm). [1]
  [2]

17.

• Microvilli: Finger-like projections on epithelial cells increase surface area for absorption. [1]
• Thin Wall: The villus wall is one cell thick, providing a short diffusion distance for nutrients. [1]
• Rich Blood Supply: Capillaries maintain a concentration gradient by carrying away absorbed nutrients (glucose/amino acids). [1]
• Lacteal: Absorbs fatty acids and glycerol (fats) into the lymphatic system. [1]
  [4]

18.

• Osmosis is the movement of water molecules only; Diffusion is the movement of any molecules/ions. [1]
• Osmosis occurs across a partially permeable membrane; Diffusion does not require a membrane (though it can occur across one). [1]
• Osmosis is specifically from high water potential to low water potential; Diffusion is from high concentration to low concentration. [1]
  [3]

19.

• Starch Test: Add iodine solution. [1] Positive result: Colour changes from orange-brown to blue-black. [1]
• Protein Test: Add Biuret solution (or sodium hydroxide and copper sulfate). [1] Positive result: Colour changes from blue to purple/violet. [1]
  [4]

20.

• Catalysts speed up the rate of chemical reactions without being used up. [1]
• Body temperature (37°C) is relatively low, so reactions would be too slow to sustain life without enzymes. [1]
• Enzymes lower the activation energy required for reactions to proceed rapidly at body temperature. [1]
  [3]