2024 May/June – Paper 23

Question 1: Virology, Microscopy, and Immunology

1(a) Structure of Viruses

1(a)(i) Capsid Components

• Capsid:
  • Capsomere: The protein subunits that make up the capsid.
  • Protein Coat: The protective protein shell surrounding the viral genome.
• Key Points:
  • The capsid protects the viral genetic material.
  • Composed of multiple capsomeres arranged in a specific symmetry.

1(a)(ii) Viral Genetic Material

• Types of Viral Genomes:
  • RNA Viruses: Typically single-stranded (ssRNA) or double-stranded (dsRNA).
  • DNA Viruses: Can be single-stranded (ssDNA) or double-stranded (dsDNA).
  • Other RNA Types: mRNA, tRNA, rRNA within the viral structure.
• Key Points:
  • The actual genetic material can be RNA or DNA, depending on the virus.
  • Classification based on genome type (e.g., dsRNA, dsDNA).

1(b) Microscopy and Measurement

1(b)(i) Measuring with a Stage Micrometer

• Typical Size Measurement:
  • Example: A structure measuring 50 nm.
• Calculation Formula:
  • A = I / M
    • A: Actual size
    • I: Image size (in nm)
    • M: Magnification (e.g., ×100)
  • Example Calculation:
    • If the image size (I) is 5 mm and magnification (M) is ×1000, then:

• Key Points:
  • Ensure correct use of the formula.
  • Always include units in your measurements.

1(b)(ii) Resolving Power

• Ability to Distinguish Two Points:
  • Known as resolving power.
  • Determines the microscope’s ability to show two points as separate entities.
• Key Point:
  • Higher resolving power allows for better distinction between closely spaced objects.

1(c) Transmission of Malaria

• Vector Identification:
  • Anopheles Mosquitoes: Primary vectors for malaria.
  • Aedes Mosquitoes: Do not transmit malaria.
• Similarities Between Malaria and Other Mosquito-Borne Diseases:
  • Both are transmitted by mosquitoes.
  • Both involve vectors that feed on blood.
• Differences:
  • Anopheles specifically transmits malaria, whereas Aedes transmits diseases like dengue and Zika.
• Key Points:
  • Correct identification of the mosquito species is crucial for disease control.
  • Both diseases are blood-borne and rely on mosquitoes for transmission.

1(d) Immunology: Memory Cells and Herd Immunity

1(d)(i) Immunological Memory and Memory Cells

• Memory Cells:
  • Types: T-lymphocytes (T-cells) and B-lymphocytes (B-cells).
  • Function: Provide long-term immunity by remembering specific antigens.
• Primary Immune Response:
  • Antigens: Proteins are recognized as non-self/foreign.
  • Clonal Selection: Specific lymphocytes are activated and proliferate.
  • Clonal Expansion: Activated cells divide to produce clones.
  • Formation of Memory Cells: Some activated lymphocytes become memory cells.
• Key Points:
  • Memory cells remain in the body for extended periods, enabling a faster and stronger response upon re-exposure to the same pathogen.
  • Essential for artificial active immunity through vaccination.

1(d)(ii) Herd Immunity

• Definition:
  • A large proportion of the population becomes immune to a disease, reducing its spread.
• Mechanism:
  • Immune Population: Limits the chances of non-immune individuals coming into contact with the pathogen.
  • Transmission Cycle: Breaks the cycle, making outbreaks less likely.
• Key Points:
  • High vaccination coverage is necessary, especially if the basic reproduction number (R₀) is high.
  • Protects vulnerable populations who cannot be vaccinated.

Question 2: Collagen Structure and Function

2(a) Collagen Molecules and Fibres

Collagen Molecule Structure

• Polypeptide Chains:
  • Triple Helix: Composed of three helical polypeptide chains.
  • Helical Chains: Tightly wound to form a stable structure.
  • Hydrogen Bonds: Formed between the chains, providing strength.
  • Amino Acid Composition: Typically, every third amino acid is glycine, allowing tight packing.
• Key Points:
  • The unique structure of collagen molecules provides tensile strength.

Collagen Fibre Structure

• Fibres:
  • Cross-Linking: Covalent bonds link collagen molecules together.
  • Parallel Arrangement: Collagen molecules are arranged in parallel within fibres.
  • Staggered Alignment: Molecules are offset to enhance strength.
• Key Points:
  • Collagen fibres are highly organized and provide structural support to tissues like skin, bones, and tendons.

2(b) Enzyme Function in Collagen Synthesis

Laccase and Monolignols in Collagen

• Laccase Enzyme:
  • Function: Catalyzes the oxidation of monolignols to form lignin.
  • Active Site: Undergoes induced fit to bind monolignols.
  • Mechanism:
    • Binding: Monolignols bind to the active site, causing a conformational change.
    • Catalysis: Lowers the activation energy, facilitating the reaction.
    • Product Release: Laccase returns to its original shape after the reaction.
  • Key Points:
    • Laccase requires cofactors like copper ions.
    • Essential for the formation of strong lignin structures in plant cell walls.

Question 3: Blood Vessels and Gas Exchange

3(a) Features of Arteries, Capillaries, and Veins

• Key Points:
  • Arteries: Thick walls with smooth muscle and tunica media for maintaining blood pressure.
  • Capillaries: Thin walls composed solely of endothelium, facilitating gas exchange.
  • Veins: Contain smooth muscle and tunica media but have thinner walls than arteries.

3(b) Gas Exchange in Lungs

3(b)(i) Gas Movement Arrows

• Oxygen (O₂): Moves from the alveolar space into the capillary lumen.
• Carbon Dioxide (CO₂): Moves from the capillary lumen into the alveolar space.
• Key Points:
  • Diffusion occurs due to partial pressure gradients.

3(b)(ii) Factors Enhancing Gas Exchange

• Deoxygenated Blood:
  • Low partial pressure of O₂ in alveolar capillaries.
• Oxygen Uptake:
  • O₂ binds to hemoglobin, forming oxyhemoglobin.
• Continuous Removal:
  • Oxygenated blood is transported away via pulmonary veins.
• Capillary Network:
  • Extensive capillary networks increase surface area for gas exchange.
• Ventilation:
  • Maintains a high concentration gradient between alveolar air and blood.
• Key Points:
  • Efficient gas exchange relies on constant blood flow and ventilation.

3(c) Types of Blood Vessels

• Pulmonary Vein:
  • One of the two types of veins in the body.
  • Characteristics:
    • Semilunar Valves: Similar to aortic valves.
    • Function: Carries oxygenated blood from the lungs to the heart.
• Key Points:
  • Semilunar Valves: Prevent backflow of blood into the pulmonary artery.

Question 4: Plant Tissues and Biochemical Pathways

4(a) Suberin and Lignin in Plant Tissues

Suberin

• Function:
  • Hydrophobic barrier found in the Casparian strip of endodermal cells.
  • Prevents water from moving through the apoplast, forcing it to pass through the symplast.
• Key Points:
  • Controls the movement of solutes into the xylem.
  • Protects against toxins and pathogens entering the vascular system.

Lignin

• Function:
  • Provides structural support in cell walls of xylem vessel elements.
  • Prevents collapse of xylem vessels under negative pressure during water transport.
• Key Points:
  • Lignin makes cell walls rigid and impermeable to water loss.
  • Essential for maintaining the integrity of water transport systems in plants.

4(b) Enzyme Mechanism: Laccase Action on Monolignols

• Laccase Enzyme:
  • Active Site: Undergoes induced fit to accommodate monolignols.
  • Binding: Monolignols bind to the active site, causing a conformational change.
  • Reaction:
    • Laccase catalyzes the oxidation of monolignols, leading to lignin polymerization.
    • Lowers activation energy, facilitating the reaction.
  • Product Release: Enzyme returns to its original shape for reuse.
• Key Points:
  • Laccase requires cofactors like copper ions for activity.
  • Critical for the synthesis of lignin, contributing to plant structural integrity.

Question 5: Signal Transduction and Cell Cycle Regulation

5(a) Ligand Binding and Signal Transduction

• Ligand Binding:
  • Secretion: Cells secrete ligands or transport ligands through the circulatory system.
  • Receptor Binding: Ligands bind to specific receptors on target cells.
• Signal Transduction:
  • Triggering Reactions: Binding activates secondary messengers (e.g., cyclic AMP).
  • Enzyme Cascade: Initiates a series of enzymatic reactions within the target cell.
• Key Points:
  • Specificity of ligand-receptor interactions ensures precise cellular responses.
  • Secondary messengers amplify the signal within the cell.

5(b) Non-Competitive Inhibition

• Mechanism:
  • Binding Site: Non-competitive inhibitors bind to an allosteric site, not the active site.
  • Shape Change: Binding induces a conformational change in the enzyme.
  • Effect: Active site becomes less complementary to the substrate, reducing enzyme activity.
• Key Points:
  • Non-competitive inhibition cannot be overcome by increasing substrate concentration.
  • Alters the enzyme’s functionality without blocking substrate binding directly.

5(c) Cell Cycle Inhibitors

5(c)(i) Palbociclib/p21Cip1

• Function:
  • Role in Cell Cycle: Inhibits cyclin-dependent kinases (CDKs) necessary for DNA replication.
  • Effect: Stops DNA replication during the S phase, preventing cell division.
• Key Points:
  • Useful in cancer treatment by halting uncontrolled cell proliferation.

5(c)(ii) RO-3306

• Function:
  • Cell Cycle Phase: Stops the cell cycle in the G₂ phase.
  • Effect: Prevents mitosis, maintaining high numbers of mitochondria and ensuring DNA replication has occurred.
• Key Points:
  • Inhibits cell division by blocking progression into mitosis.
  • Helps in studying cell cycle dynamics and controlling cell proliferation.

5(d) Control of the Cell Cycle

• Mechanism:
  • Stopping Before Mitosis/Cytokinesis:
    • Prevents uncontrolled cell division, which can lead to tumor growth.
• Effect:
  • Tumor Suppression: Reduces the risk of increasing tumor size by regulating cell division.
• Key Points:
  • Critical for maintaining normal cell growth and preventing cancer.

Question 6: Molecular Biology and Genetics

6(a) Differences Between mRNA and DNA

• Key Points:
  • mRNA: Transcribed from DNA, carries genetic information to the ribosome for protein synthesis.
  • DNA: Stores genetic information, composed of two complementary strands forming a double helix.

6(b) Base Pairing and Hydrogen Bonds

6(b)(i) Base Pairing: P and B

• P and B Bases:
  • Incorrect Pairing: P and B are artificial or incorrect base pairs.
  • Key Points:
    • In natural DNA, adenine pairs with thymine, and cytosine pairs with guanine.

6(b)(ii) C and G Bonding

• Cytosine (C) and Guanine (G):
  • Hydrogen Bonds: Held together by three hydrogen bonds.
  • Key Points:
    • Stronger bond compared to adenine-thymine pairing.
    • Contributes to the stability of the DNA double helix.

Question 7: Miscellaneous Topics

7(a) DNA Replication and Repair

7(b) Photosynthesis and Cellular Respiration

General Exam Tips

• Diagram Labeling:
  • Ensure all diagrams are accurately labeled with key structures.
  • Include annotations where necessary to clarify functions and interactions.
• Units and Conversions:
  • Always include appropriate units in measurements.
  • Be comfortable converting between units (e.g., mm to µm).
• Understanding Processes:
  • Focus on understanding the mechanisms behind physiological and biochemical processes rather than just memorizing facts.
• Application of Knowledge:
  • Be prepared to apply concepts to different scenarios, such as how mutations affect protein function or how transport mechanisms respond to environmental changes.
• Answer Structuring:
  • Structure answers clearly, addressing each part of the question.
  • Use scientific terminology accurately and concisely.
• Time Management:
  • Allocate time based on the marks available for each question.
  • Ensure you answer all parts of multi-part questions.
• Practice Past Papers:
  • Familiarize yourself with the exam format and question styles.
  • Time yourself while practicing to improve speed and accuracy.