Secondary 3 Biology Semestral Assessment 2 (End of Year) Paper 4

TuitionGoWhere Practice Paper — Biology Secondary 3

SA2 Practice Paper — Version 4 of 5

Answer Key and Marking Scheme

Section A: Multiple Choice Questions [10 marks]

1. B — Rough endoplasmic reticulum [1]
Explanation: The rough endoplasmic reticulum (RER) has ribosomes attached to its surface and is responsible for synthesising proteins destined for secretion. The smooth ER is involved in lipid synthesis. The Golgi body modifies and packages proteins but does not synthesise them. The mitochondrion is the site of aerobic respiration.

2. C — Golgi body [1]
Explanation: The Golgi body consists of flattened, stacked membrane sacs (cisternae) with vesicles budding off from the trans face. This is a distinctive feature visible under electron microscopy.

3. C — Large central vacuole: Present (Plant), Absent (Animal) [1]
Explanation: Plant cells have a cell wall (A is incorrect — animal cells lack a cell wall). Plant cells have chloroplasts (B is incorrect). Both plant and animal cells have mitochondria (D is incorrect). Plant cells have a large central vacuole, while animal cells have small or no vacuoles.

4. B — Rough endoplasmic reticulum [1]
Explanation: Radioactive amino acids are first incorporated into proteins by ribosomes on the rough endoplasmic reticulum. The proteins then move to the Golgi body for modification, and finally to secretory vesicles. The RER is therefore the first organelle to show increased radioactivity.

5. C — Carbohydrate [1]
Explanation: Carbohydrates (particularly glucose) are the primary and quickest source of energy for cellular activities. Lipids provide more energy per gram but are used for long-term storage. Proteins are primarily structural and functional molecules.

6. B — It will swell and may burst due to water entering the cell. [1]
Explanation: Distilled water is hypotonic relative to the cytoplasm of the red blood cell. Water moves into the cell by osmosis, causing it to swell and potentially burst (haemolysis). Red blood cells lack a cell wall to prevent bursting.

7. B — It controls the movement of substances into and out of the cell. [1]
Explanation: The cell membrane is a partially permeable membrane that regulates the entry and exit of substances. It does not provide rigid support (that is the cell wall in plants), synthesise proteins (ribosomes), or store genetic information (nucleus).

8. B — The cell membrane pulls away from the cell wall — plasmolysis. [1]
Explanation: A concentrated salt solution is hypertonic relative to the cell sap. Water leaves the vacuole by osmosis, causing the cytoplasm and cell membrane to shrink away from the rigid cell wall. This process is called plasmolysis.

9. B — Lysosome [1]
Explanation: Lysosomes are membrane-bound organelles containing hydrolytic (digestive) enzymes. They break down worn-out organelles, engulfed pathogens, and cellular debris through a process called autophagy.

10. C — Amino acid [1]
Explanation: Proteins are polymers made up of amino acid monomers linked by peptide bonds. Fatty acids are components of lipids. Nucleotides are the monomers of nucleic acids. Glucose is a monosaccharide (monomer of carbohydrates).

Section B: Structured Questions [25 marks]

11. Cell Structure and Organisation [6 marks]

(a)

• X: Golgi body (or Golgi apparatus) [1]
• Y: Mitochondrion (plural: mitochondria) [1]

(b) Any one of the following:

• Modifies proteins received from the rough endoplasmic reticulum
• Packages proteins into vesicles for transport
• Produces secretory vesicles
• Adds carbohydrate groups to proteins (glycosylation)
• Sorts and labels proteins for delivery to correct destinations [1]

(c) The mitochondrion is called the "powerhouse of the cell" because:

• It is the site of aerobic respiration (Krebs cycle and oxidative phosphorylation) [1]
• It produces ATP (adenosine triphosphate), the energy currency of the cell, from the breakdown of glucose [1]
  Award 2 marks for a complete explanation linking ATP production to energy release.

(d) Any one of the following:

• Cell wall — provides structural support and protection; maintains cell shape
• Chloroplast — site of photosynthesis; contains chlorophyll to absorb light energy
• Large central vacuole — stores cell sap; maintains turgor pressure to support the cell [1]
  Award 1 mark for correctly identifying a plant-cell-only structure AND stating its function.

12. Cell Specialisation [5 marks]

(a)

• (i): Neuron (or nerve cell) [1]
• (ii): Root hair cell [1]
• (iii): Red blood cell (or erythrocyte) [1]

(b) The biconcave disc shape of a red blood cell:

• Increases the surface area to volume ratio [1]
• Allows more oxygen to diffuse into and out of the cell per unit time, making oxygen transport more efficient [1]
  Alternative acceptable point: The thin centre allows oxygen to diffuse a shorter distance to haemoglobin molecules.
  Award 2 marks for linking the shape to increased surface area and efficient gas exchange.

13. Movement of Substances [5 marks]

(a) A partially permeable membrane allows small molecules (e.g., water, glucose) to pass through but prevents large molecules (e.g., starch) from passing through. [1]
Key point: must mention that it is selective — some substances can pass, others cannot.

(b)

• Starch: Negative (or absent / no starch detected) [1]
• Glucose: Positive (or present / glucose detected) [1]

(c) Glucose molecules are small enough to pass through the pores of the Visking tubing [1]. They move from a region of higher concentration (inside the tubing) to a region of lower concentration (outside in the beaker) by diffusion [1].
Starch molecules are too large to pass through the partially permeable membrane, so they remain inside the tubing.

14. Biomolecules — Enzymes [5 marks]

(a) As temperature increases from 0°C to 40°C, the rate of reaction increases (or increases steadily / increases rapidly). [1]
Accept: "The rate of reaction increases with increasing temperature" or "directly proportional" (within this range).

(b) At 40°C, the enzyme is at its optimum temperature [1]. The enzyme and substrate molecules have the maximum kinetic energy, leading to the highest frequency of successful collisions and the most enzyme-substrate complexes being formed per unit time [1].
Award 2 marks for identifying optimum temperature AND explaining in terms of kinetic energy and collisions.

(c) At temperatures above 50°C, the enzyme denatures [1]. The high temperature breaks the hydrogen bonds and other bonds that maintain the enzyme's tertiary (3D) shape. The active site changes shape so that the substrate can no longer fit into it, and the enzyme can no longer function [1].
Award 2 marks for identifying denaturation AND explaining the change in active site shape.

15. Biomolecules — Food Tests [4 marks]

(a) Sample C contains starch [1].
Explanation: The iodine test turned blue-black, which is the positive result for starch. (Brown/yellow indicates absence of starch.)

(b) Sample B contains reducing sugar [1].
Explanation: Benedict's test turned brick-red (or orange/red precipitate), which is the positive result for reducing sugar. (Blue indicates absence of reducing sugar.)

(c) Sample A contains both protein and lipid [2].
Explanation:

• The Biuret test turned purple, indicating the presence of protein [1].
• The ethanol emulsion test turned cloudy white, indicating the presence of lipid [1].
  Note: Sample D also contains lipid (cloudy white in ethanol emulsion test) but tested negative for protein (Biuret remained blue).

Section C: Data-Based and Application Questions [15 marks]

16. Osmosis in Plant Cells [5 marks]

(a) 0.4 mol/dm³ [1]. At this concentration, the percentage change in mass is 0.0%, meaning there is no net movement of water into or out of the potato cells. The solution is isotonic with the cell sap of the potato [1].

(b) The 0.0 mol/dm³ sucrose solution is hypotonic (or has a higher water potential) compared to the cell sap of the potato [1]. Water molecules move into the potato cells by osmosis (from a region of higher water potential to a region of lower water potential), causing the cells to gain mass [1].

(c) A percentage change in mass of less than −9.3% (e.g., −12%, −14%, or any value more negative than −9.3%) [1].
Explanation: At 0.8 mol/dm³, the change is −9.3%. At a higher concentration (1.0 mol/dm³), more water will leave the cells, resulting in a greater percentage decrease in mass.*

17. Cell Organelle Pathway [4 marks]

(a) Correct sequence:
Ribosome → Rough endoplasmic reticulum → Golgi body → Secretory vesicle → Cell membrane [2]
Award 2 marks for all five in correct order. Award 1 mark for at least three consecutive organelles in the correct order. Award 0 marks if the sequence is largely incorrect.

(b) The Golgi body:

• Receives proteins from the rough endoplasmic reticulum in vesicles [1]
• Modifies the proteins (e.g., adds carbohydrate groups to form glycoproteins)
• Packages the modified proteins into secretory vesicles for transport to the cell membrane
• Sorts and labels proteins for delivery to their correct destinations [1]
  Award 2 marks for any two valid roles of the Golgi body in the secretion pathway.

18. Enzyme Specificity [3 marks]

(a)

• Starch: Broken down (or positive result — starch is digested into maltose) [1]
• Protein: Not broken down (or no change / negative result) [1]
• Lipid: Not broken down (or no change / negative result) [1]
  Explanation: Amylase is specific to starch due to the complementary shape of its active site. It cannot bind to protein or lipid substrates, so no reaction occurs with those substrates.*

19. Cell Membrane Structure [2 marks]

(a) Fluid Mosaic Model [1]

(b) Any one of the following:

• The phospholipid molecules can move laterally within the bilayer, giving the membrane a fluid consistency
• The protein molecules are scattered/embedded throughout the bilayer like tiles in a mosaic, and some can move
• The membrane is not rigid — the components are free to move within the plane of the membrane [1]

20. Application — Food Preservation [1 mark]

Heating to 80°C denatures the enzymes responsible for browning [1]. The high temperature disrupts the enzyme's tertiary structure and alters the shape of the active site, so the enzyme can no longer catalyse the browning reaction.

Mark Summary

Common Mistakes and Marking Notes

• Q4: Students often select Golgi body instead of RER. Emphasise that radioactive amino acids are first incorporated at ribosomes on the RER — the Golgi body receives proteins after they are made.
• Q11(c): Students may state "produces energy" without mentioning ATP. Award full marks only if ATP or "energy currency" is referenced.
• Q13(c): Students may describe osmosis instead of diffusion. Glucose moves by simple diffusion (not osmosis, which refers specifically to water).
• Q14(c): Students may say "the enzyme dies" — this is incorrect terminology. Enzymes are not living; they denature.
• Q16(c): Accept any reasonable prediction more negative than −9.3%. Students should demonstrate understanding of the trend in the data.
• Q18: Students may not connect enzyme specificity to the lock-and-key hypothesis. Encourage use of terms like "complementary shape" and "active site."