O Level Biology Practice Paper 2

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

Topic: Cells and Biomolecules
Version: 2 of 5

Section A: Multiple Choice Questions

1. D
Reasoning: Cell walls are present in plant cells (made of cellulose) but absent in animal cells. Cell membranes, cytoplasm, and mitochondria are found in both.

2. D
Reasoning: Sperm cells require significant energy for movement, hence many mitochondria. They do not photosynthesize, so no chloroplasts. Root hair cells have mitochondria but are plant cells (would have cell wall/vacuole, though no chloroplasts in roots, sperm is a better fit for "many mitochondria" context in animal biology). Red blood cells have no mitochondria. Palisade cells have chloroplasts.

3. B
Reasoning: The Golgi body modifies, sorts, and packages proteins for secretion or delivery to other organelles. Ribosomes synthesize proteins; mitochondria produce ATP; nucleus stores genetic info.

4. C
Reasoning: Water moves from high water potential to low water potential. If the potato gained mass in X, water entered the potato, meaning X had a higher water potential than the potato cells. If it lost mass in Y, Y had a lower water potential.

5. C
Reasoning: Active transport moves substances against the concentration gradient and requires energy (ATP). Diffusion, osmosis, and facilitated diffusion are passive processes.

6. C
Reasoning: Iodine turns blue-black in the presence of starch. Benedict's staying blue means no reducing sugar. Biuret staying blue means no protein. Ethanol staying clear means no fat.

7. B
Reasoning: All amino acids contain Carbon, Hydrogen, Oxygen, and Nitrogen (CHON). Some contain Sulphur, but not all. Phosphorus is found in nucleic acids and phospholipids, not standard amino acids.

8. B
Reasoning: Enzyme X works at pH 2 (acidic), which is the condition in the stomach (pepsin). Enzyme Y works at pH 8 (alkaline), which is the condition in the small intestine (trypsin/amylase).

9. C
Reasoning: High temperatures break the hydrogen bonds holding the enzyme's tertiary structure, changing the shape of the active site. This is denaturation.

10. B
Reasoning: Enzymes work by lowering the activation energy required for a reaction to proceed. They are not consumed, do not change equilibrium, and do not necessarily increase temperature.

Section B: Structured Questions

11. (a) Ribosome / Rough Endoplasmic Reticulum (RER) [1]
(Note: If A points to dots on membranes, it's RER. If just dots, Ribosome. Given context of "protein synthesis", Ribosome is the fundamental unit, but RER is the organelle. Accept either if diagram is ambiguous, but RER is usually the labelled organelle).

(b) Golgi body / Golgi apparatus [1]

(c)

• Liver cells are metabolically active / perform many chemical reactions (e.g., detoxification, glycogen synthesis). [1]
• These processes require energy (ATP). [1]
• Mitochondria are the site of aerobic respiration/ATP production.

(d) Any one of the following: [1]

• Liver cell has a nucleus; bacterial cell has no nucleus (nucleoid).
• Liver cell has membrane-bound organelles (mitochondria/Golgi); bacterial cell does not.
• Liver cell is larger than a bacterial cell.
• Bacterial cell has a cell wall (peptidoglycan); liver cell does not.

12. (a)

• Change in length = -0.4 cm.
• Percentage change = (Change / Initial) × 100
• (-0.4 / 5.0) × 100 = -8% [2]
  (1 mark for correct calculation, 1 mark for correct answer including negative sign or indication of decrease).

(b)

• The 0.0 mol dm⁻³ solution (pure water) has a higher water potential than the potato cells. [1]
• Water enters the potato cells by osmosis. [1]
• Water moves across the partially permeable cell membrane into the vacuole/cytoplasm. [1]
• The cells become turgid, causing the strip to lengthen.

(c)

• Concentration: 0.4 mol dm⁻³ [1]
• Explanation: At this concentration, there was no change in length (0.0 cm). [1]
• This indicates that the water potential of the solution is equal to the water potential of the potato cells (isotonic), so there is no net movement of water.

13. (a)

• Enzymes are biological catalysts. [1]
• They are proteins that speed up chemical reactions without being used up/changed themselves. [1]

(b)

• The enzyme has an active site with a specific shape. [1]
• The substrate has a complementary shape to the active site. [1]
• The substrate binds to the active site to form an enzyme-substrate complex. [1]
• (Optional: The reaction occurs, and products are released).

(c)

• High temperatures cause the enzyme molecules to vibrate more vigorously. [1]
• This breaks the bonds (hydrogen/ionic) holding the enzyme's structure together. [1]
• The shape of the active site changes (denaturation). [1]
• The substrate can no longer fit into the active site, so the reaction stops/slows significantly.

14. (a)

• Protein Basic Unit: Amino acids [1]
• Protein Function: Growth and repair / Enzymes / Hormones / Antibodies (Any valid function) [1]
• Fat Basic Unit: Fatty acids [1]

(b)

• Reagent: Biuret solution [1]
• Method: Add Biuret solution to the milk sample and shake/mix. [1]
• Positive Result: Colour changes from blue to purple/lilac. [1]

15. (a) Active transport [1]

(b) Any two of the following: [2]

• Villus has a large surface area (due to finger-like projection) for faster absorption.
• Epithelial cells have microvilli (brush border) to further increase surface area.
• Wall of villus is one cell thick (thin epithelium) for short diffusion distance.
• Rich blood supply (capillaries) maintains concentration gradient for glucose/amino acids.
• Lacteal present for absorption of fatty acids/glycerol.

(c) Hepatic portal vein [1]
(Note: Capillaries lead to the hepatic portal vein).

Section C: Free Response Question

16. (a) Comparison of Transport Mechanisms [6]
Award marks for the following points:

Diffusion:

• Movement of particles from high concentration to low concentration (down gradient). [1]
• Does not require energy (passive). [1]
• Does not require membrane proteins (occurs directly through phospholipid bilayer for small/non-polar molecules). [1]

Osmosis:

• Movement of water molecules from high water potential to low water potential (down gradient). [1]
• Does not require energy (passive).
• Occurs across a partially permeable membrane.

Active Transport:

• Movement of particles from low concentration to high concentration (against gradient). [1]
• Requires energy (ATP) from respiration. [1]
• Requires specific carrier proteins in the membrane. [1]

(Note: Students must clearly distinguish the three. 1 mark per distinct correct feature linked to the correct process. Max 6 marks. If they only compare two, max 4 marks.)

(b) Importance of Osmosis in Turgidity [4]

• When plant cells are in a hypotonic solution (higher water potential outside), water enters the cell by osmosis. [1]
• The vacuole fills with water and expands, pushing the cytoplasm against the cell wall. [1]
• The cell wall is strong and rigid (made of cellulose) and resists expansion, creating turgor pressure. [1]
• This turgidity provides structural support to non-woody plants, keeping stems upright and leaves expanded for photosynthesis. [1]
• Without turgidity, cells become flaccid/plasmolysed, and the plant wilts.