2024 May/June – Paper 22

1. Respiratory System and Bronchioles

1(a) Function of Bronchioles

• Contraction and Relaxation:
  • Contraction decreases the diameter (lumen) of bronchioles, causing constriction.
  • Relaxation increases the diameter, causing dilation or widening.
  • Relevance:
    • Bronchioles can stretch, expand, or recoil.
    • In the context of the bronchus or trachea, refer to extracellular fluid (ECF) in subsequent material processes (mps).
• Vasoconstriction and Vasodilation:
  • Vasoconstriction: Narrowing of blood vessels.
  • Vasodilation: Widening of blood vessels.

1(b) Microscopy and Measurement

1(b)(i) Stage Micrometer

• Understand the use of a stage micrometer and its scale for accurate measurements under a microscope.

1(b)(ii) Converting Measurements

• Example Conversion:
  • 0.2 mm = 200 µm
  • Ensure all measurements include units and numerical values.
  • Use standard form where applicable (e.g., 2 × 10⁻⁴ m vs. 2.5 × 10⁻⁴ m).

1(c) Capillary Structure and Function

1(c)(i) Structure of Capillaries

• Thin Walls:
  • Capillaries have walls that are one cell thick, primarily composed of endothelial cells.
• Red Blood Cells:
  • Transported in single file due to their diameter being similar to capillary lumen size (~7 µm).
  • Capillaries lack smooth muscle and elastic layers, allowing flexibility for blood flow.

1(c)(ii) Function of Capillaries

• Exchange of Substances:
  • Allow passage of tissue fluid, blood plasma, oxygen (O₂), and carbon dioxide (CO₂).
• Efficient Gas Exchange:
  • Small diameter ensures short diffusion distances, facilitating efficient exchange of respiratory gases.

1(d) Gene Mutations and Protein Function

1(d)(i) Gene Mutations

• Definition:
  • Changes in the sequence of base pairs in a DNA molecule.
  • Can alter the sequence of amino acids in a polypeptide, affecting protein structure.

1(d)(ii) RNA Splicing and Alternative Splicing

• Alternative Splicing:
  • Differences in RNA splicing can lead to variations in the mRNA and the resulting proteins.
  • Involves removal of introns (non-coding sequences) and joining of exons (coding sequences) in different combinations.

1(d)(iii) Impact on Protein Function

• Binding Sites:
  • Mutations can alter the shape or number of binding sites on a protein without affecting the overall tertiary structure.
  • Changes may affect the enzyme’s ability to bind substrates effectively.

2. Cell Structures and Functions

2(a) Cell Organelles and Their Functions

2(a)(i) Key Organelles:

• Nucleolus:
  • Manufactures ribosomal subunits from proteins and ribosomal RNA.
• Smooth Endoplasmic Reticulum (SER):
  • Synthesizes triglycerides and other lipids.
• Centrioles (Centrosomes):
  • Organize microtubules of the cell cytoskeleton.

2(a)(ii) Diagram Labeling:

• Ensure all structures are correctly drawn and labeled:
  • Nucleolus: Spherical organelle within the nucleus.
  • Smooth ER: Single membrane-bound tubular sacs without ribosomes.
  • Centrioles: Two short lines or cylinders, typically near the nucleus.

2(b) Phagocytosis in Macrophages

2(b)(i) Process of Phagocytosis:

• Engulfing Bacteria:
  • Macrophage cell membrane surrounds and engulfs bacterial cells using pseudopodia.
  • Involves chemotaxis where bacteria bind to macrophage receptors, possibly aided by opsonization (antibody binding).

2(b)(ii) Lysosomes:

• Function:
  • Lysosomes fuse with the phagosome to form a phagolysosome, where enzymes digest the engulfed bacteria.

2(c) Immune Response to Vaccines

• Live Vaccines:
  • Replication: Cells replicate, leading to high levels of foreign antigens.
  • Immune Response: Strong primary immune response with formation of memory cells.
  • Active Immunity: Provides long-lasting immunity without causing the disease.
  • Benefits:
    • Effective against specific pathogens (e.g., M. tuberculosis).
    • Minimal side effects and potential for herd immunity.

3. Enzymes and Antibodies

3(a) Enzyme-Substrate Interaction (Lysozyme Example)

• Lock and Key Model:
  • Active Site: Specific shape complementary to the substrate (peptidoglycan).
  • Enzyme-Substrate Complex: Formation leads to the hydrolysis of peptidoglycan into NAM and NAG.
  • Products: NAM (N-acetylmuramic acid) and NAG (N-acetylglucosamine), often with water as a reactant.

3(b) Comparison of Lysozyme and Penicillin

Similarities:

• Function: Both cause bacterial cell lysis by targeting the cell wall.
• Outcome: Weakening or destruction of the peptidoglycan layer, leading to cell bursting.

Differences:

• Mechanism:
  • Lysozyme: Enzyme that hydrolyzes glycosidic bonds in peptidoglycan.
  • Penicillin: Antibiotic that inhibits transpeptidase enzymes, preventing cross-linking of peptidoglycan.
• Effectiveness:
  • Lysozyme: Effective at all stages of bacterial growth.
  • Penicillin: Only effective during cell wall synthesis.
• Molecular Structure:
  • Lysozyme: Larger, globular protein with multiple binding sites.
  • Penicillin: Smaller molecule containing a β-lactam ring.

4. Transport Processes in Plants

4(a) Structure of Polysaccharides (e.g., Cellulose)

• Monomers:
  • Composed of β-glucose units.
• Glycosidic Bonds:
  • Linked by 1,4 glycosidic bonds (and sometimes 1,6 for branching).
• Structure:
  • Linear, unbranched chains that are straight and can form tight, compact structures through hydrogen bonding.

4(b) Enzyme Regulation in Nitrate Reduction

4(b)(i) Regulation Mechanism:

• Nitrate Reductase Synthesis:
  • Increased nitrate levels can upregulate the synthesis of nitrate reductase, enhancing the rate of nitrate reduction.

4(b)(ii) Accuracy in Measurement:

• Spectrophotometry:
  • Provides quantitative results by measuring absorbance related to nitrite concentration.
  • Ensures accuracy through calibration curves and detection of low concentrations.

4(c) Proton Transport and Amino Acid Uptake

4(c)(i) Proton Gradient Mechanism:

• Active Transport:
  • Protons (H⁺) are actively pumped out of companion cells using ATP.
• Proton Gradient:
  • Builds up in the apoplast, creating a high concentration outside the cell.
• Facilitated Diffusion:
  • Protons move back into companion cells down the gradient, often cotransporting amino acids against their concentration gradient.

4(c)(ii) Plasmodesmata Function:

• Movement:
  • Transport of substances occurs down the concentration gradient through plasmodesmata, connecting companion cells and sieve tubes.

4(d) Enzyme Activity and Reaction Rates

• Concentration Effects:
  • Lower substrate concentrations (e.g., 0.2 mmol dm⁻³) result in slower reaction rates compared to higher concentrations (e.g., 5.0 mmol dm⁻³).
  • Reaction rate graphs show steeper slopes for higher substrate concentrations.

4(e) Active vs. Facilitated Transport Experiments

• Active Transport Characteristics:
  • Requires energy (ATP) and involves protein carriers.
  • Experiments show decreased uptake under low oxygen, low temperature, or when protein synthesis is inhibited.
• Facilitated Diffusion Characteristics:
  • Does not require energy and relies on transport proteins to move substances down their concentration gradient.

5. Genetics and Chromosome Structure

5(a) DNA Structure and Packaging

• Coiling and Supercoiling:
  • DNA coils and supercoils to become more compact, facilitating packaging within the nucleus.

5(b) Homologous Chromosomes

• Characteristics:
  • Pairs of chromosomes that are homologous have two sister chromatids each.
  • Connected by a centromere and associated with histone proteins.
  • Telomeres protect the ends of chromatids.
  • Highly condensed during certain cell cycle stages.

6. Virology and Immune Response

6(a) Human Immunodeficiency Virus (HIV)

• ART (Antiretroviral Therapy) Benefits:
  • Increases and maintains the number of T-lymphocytes (T-helper cells).
  • Strengthens the immune system, improving response to infections.
  • Decreases viral load, reducing the risk of opportunistic diseases.

6(b) ART Mechanism and Benefits

• Function:
  • Prevents death of T-lymphocytes.
  • Enhances immune response, increasing chances of recovery from diseases.

6(c) HIV-Related Vascular Changes

6(c)(i) Coronary Artery Positioning:

• Understand the anatomical placement of coronary arteries in relation to the heart structure.

6(c)(ii) Blood Vessel Adaptations:

• Structural Changes:
  • Blood vessels may develop thicker walls with more smooth muscle and elastic fibers to withstand higher blood pressure.
  • Enhanced collagen in the tunica externa/adventitia contributes to vessel durability.
• Functional Implications:
  • Prevents vessel collapse and ensures efficient blood flow.

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.