Secondary 3 Biology Semestral Assessment 2 (End of Year) Paper 2

TuitionGoWhere Exam Practice (AI) - Biology Secondary 3

Answer Key & Marking Scheme

Paper: SA2 Practice Paper (Version 2 of 5)
Topic: Cells & Biomolecules

Section A: Multiple Choice Questions

1. C
Reasoning: Animal cells lack cell walls and chloroplasts. White blood cells are animal cells. Palisade, root hair, and xylem are plant cells.

2. B
Reasoning: The Golgi Apparatus modifies, sorts, and packages proteins for secretion. RER synthesizes them; Mitochondria produce energy; Lysosomes digest waste.

3. B
Reasoning: Amino acids are the building blocks of proteins. Protein synthesis occurs at ribosomes. Therefore, radioactivity appears first at the ribosomes (specifically those on the RER for secretory proteins).

4. B
Reasoning: The cell membrane is partially permeable and controls substance movement. A is cell wall; C is mitochondrion; D is nucleus.

5. B
Reasoning: The absence of a nucleus creates more space for haemoglobin, maximizing oxygen-carrying capacity.

Section B: Structured Questions

6.
(a)
(i) Cell Wall [1]
(ii) Vacuole (or Large Central Vacuole) [1]
(iii) Nucleus [1]

(b) Any two of the following:

1. Plant cell has a cell wall; animal cell does not.
2. Plant cell has chloroplasts; animal cell does not.
3. Plant cell has a large central vacuole; animal cell has small/temporary vacuoles (or none).
   [2]

(c) Animal cells do not need rigid support because they are supported by the skeletal system (bones/cartilage) or hydrostatic pressure, and they need flexibility for movement. The cell wall would restrict movement and shape changes. [2]
(1 mark for stating animal cells need flexibility/movement; 1 mark for stating cell wall is rigid/restrictive)

7.
(a) 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, increasing the rate of reaction/enzyme activity. [2]
(1 mark for kinetic energy/collisions; 1 mark for increased rate)

(b) Above 40°C (the optimum temperature), the heat energy breaks the bonds holding the enzyme's tertiary structure together. The enzyme denatures, changing the shape of the active site. The substrate can no longer fit into the active site, so activity decreases rapidly. [3]
(1 mark for denaturation/bonds breaking; 1 mark for active site shape change; 1 mark for substrate no longer fitting)

(c) pH 2 is highly acidic. Enzyme X likely has an optimum pH near neutral (e.g., pH 7). At pH 2, the enzyme denatures due to the disruption of ionic bonds in its structure, rendering it inactive. [2]
(1 mark for denaturation due to pH; 1 mark for inactive/shape change)

8.
(a) Carbon, Hydrogen, Oxygen, Nitrogen. (Sulphur is also acceptable). [1]
(Must include N. C, H, O are assumed from context but N is the distinguishing element for proteins)

(b)
Reagent: Benedict’s Solution [1]
Method: Heat the food sample with Benedict’s solution in a water bath. [1]
Positive Result Colour: Brick-red precipitate (or orange/yellow/green depending on concentration, but brick-red is standard for positive). [1]

(c) Energy storage / Insulation / Protection of vital organs. (Any one) [1]

9.
(a) The root hair cell has a long, thin extension (the root hair). This significantly increases the surface area for absorption of water and mineral ions. The cell wall is thin to allow easy passage of water. [3]
(1 mark for long extension; 1 mark for increased surface area; 1 mark for thin wall/easy passage)

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

(c)
Process: Active Transport [1]
Reason for energy: To move ions against the concentration gradient (from low to high concentration). [1]

10.
(a) SA:Vol Ratio = 24 / 8 = 3:1. [1]

(b) As the surface area to volume ratio decreases (from 6:1 to 2:1), the time taken for the cube to become colourless increases. This is because a smaller SA:Vol ratio means there is less surface area relative to the volume for diffusion to occur, so it takes longer for the acid to diffuse to the center of the cube. [3]
(1 mark for relationship; 1 mark for less surface area relative to volume; 1 mark for longer diffusion distance/time)

(c) Single-celled organisms rely on diffusion across their cell membrane for gas exchange and nutrient uptake. A high SA:Vol ratio ensures that diffusion is rapid enough to meet the metabolic needs of the cell. [2]
(1 mark for reliance on diffusion; 1 mark for meeting metabolic needs)

Section C: Free Response Questions

11.
Red Blood Cell (RBC):

• Function: Transports oxygen from lungs to body tissues.
• Adaptation 1: Biconcave shape increases surface area for faster oxygen diffusion/loading.
• Adaptation 2: No nucleus allows more space for haemoglobin, maximizing oxygen capacity.
• Adaptation 3: Flexible membrane allows it to squeeze through narrow capillaries.

Root Hair Cell:

• Function: Absorbs water and mineral ions from the soil.
• Adaptation 1: Long hair-like projection increases surface area for absorption.
• Adaptation 2: Thin cell wall reduces diffusion distance for water.
• Adaptation 3: Many mitochondria to provide energy (ATP) for active transport of mineral ions.

Comparison:

• Both are specialised for exchange/absorption (RBC for gases, Root Hair for water/ions).
• Both have adaptations to increase surface area (biconcave vs. hair).
• RBC lacks organelles to maximize space; Root Hair has many mitochondria for energy.

[6 marks: 3 for RBC structure/function link, 3 for Root Hair structure/function link. Accept any valid adaptations.]

12.
(a) Enzymes are specific because each enzyme has an active site with a unique shape. Only a substrate with a complementary shape can fit into this active site, like a key fits into a lock. If the shape does not match, the enzyme cannot catalyse the reaction. This ensures that enzymes only catalyse specific reactions. [4]
(1 mark for unique active site shape; 1 mark for complementary substrate; 1 mark for lock and key analogy; 1 mark for specific reaction)

(b) Blood stains contain proteins (haemoglobin/fibrin). Protease enzymes break down proteins into amino acids/peptides, removing the stain. Wool and silk are natural fibres made of protein. If used on these fabrics, the protease would digest the fibres themselves, damaging the clothing. [4]
(1 mark for blood contains protein; 1 mark for protease breaks down protein; 1 mark for wool/silk are protein; 1 mark for enzyme would damage fabric)