describe the mode of action of phagocytes (macrophages and neutrophils)

Cambridge International AS & A‑Level Biology (9700) – Complete Syllabus Notes

How to Use These Notes

• Each sub‑section lists the **Learning Outcomes (LO)** from the official syllabus and the **Assessment Objectives (AO)** they address:
  • AO1 – recall of factual knowledge.
  • AO2 – application, explanation and interpretation of concepts.
  • AO3 – analysis, evaluation and design of experiments.
• **Practical Boxes** (Practical Box) summarise the investigations required for Papers 3 & 5, with key steps, data to record and the AO(s) assessed.
• **Exam‑style Question** panels give examples of the type of answer expected at each AO level.
• Use the **AO‑Mapping Tables** at the end of the AS and A‑Level sections to check which objectives each topic covers.

1. AS‑Level Topics (1 – 11)

1.1 Cell Structure

• Learning Outcomes
  • Explain the cell theory and differentiate prokaryotic from eukaryotic cells (AO1, AO2).
  • Identify major organelles and describe their functions (AO1, AO2).
  • Describe the detailed structure of the plasma membrane: phospholipid bilayer, cholesterol, glycolipids, glycoproteins, integral and peripheral proteins (AO1, AO2).
  • Discuss the role of the cell wall in plants, fungi and bacteria (peptidoglycan, pseudo‑peptidoglycan, chitin) (AO1, AO2).
• Practical Box – Light Microscopy
  • Prepare onion epidermis and human cheek‑cell slides, stain with methylene blue, record magnification, draw labelled diagrams (AO3).
• Exam‑style Question (AO2)

  “Explain how the presence of cholesterol in the plasma membrane influences membrane fluidity at low temperature.”

1.2 Biomolecules

• Learning Outcomes
  • Describe the structure–function relationships of carbohydrates, proteins, lipids and nucleic acids (AO1, AO2).
  • Explain the properties of water that are biologically important: polarity, cohesion, surface tension, high specific heat, high latent heat of vaporisation (AO1, AO2).
  • Outline the role of enzymes as biological catalysts (AO1, AO2).
• Practical Box – Qualitative Tests for Biomolecules
  • Perform Benedict’s test, iodine test, biuret test and Sudan III test; record colour change and interpret (AO3).

1.3 Enzymes

• Learning Outcomes
  • Explain enzyme–substrate interaction, induced‑fit model and the role of the active site (AO1, AO2).
  • Interpret Michaelis–Menten kinetics; define V_max, K_m and their biological significance (AO2).
  • Describe the effect of temperature (including denaturation curves), pH and substrate concentration on enzyme activity (AO2).
  • Distinguish between competitive, non‑competitive and allosteric inhibition and illustrate each with a diagram (AO2).
• Practical Box – Enzyme Kinetics (Amylase)
  • Vary substrate concentration, measure rate of starch breakdown (iodine assay), plot Michaelis–Menten and Lineweaver‑Burk graphs; calculate K_m and V_max (AO3).

1.4 Membranes & Transport

• Learning Outcomes
  • Describe the fluid‑mosaic model, including the role of cholesterol, glycolipids and glycoproteins (AO1, AO2).
  • Explain passive diffusion, facilitated diffusion, active transport (primary & secondary) and the role of ATPases (AO1, AO2).
  • Compare carrier proteins with channel proteins, giving an example of each (AO2).
  • Describe bulk‑phase transport: phagocytosis, pinocytosis and exocytosis (AO2).
  • Explain the principles of osmosis and the effect of solute concentration on water movement (AO1, AO2).
• Practical Box – Osmosis
  • Use potato discs in sucrose solutions (0 %–30 %); calculate % water loss and plot against solute concentration (AO3).

1.5 Cell Cycle & Mitosis

• Learning Outcomes
  • Outline the phases of the cell cycle (G₁, S, G₂, M) and the purpose of each (AO1, AO2).
  • Describe the major checkpoints (G₁/S, G₂/M, spindle‑assembly) and the role of cyclins and cyclin‑dependent kinases (CDKs) in regulating progression (AO2).
  • Compare mitosis and meiosis, highlighting the significance for growth, repair and genetic diversity (AO2).
• Practical Box – Observation of Mitosis
  • Stain onion root tip cells with acetocarmine; identify prophase, metaphase, anaphase, telophase and calculate mitotic index (AO3).

1.6 Nucleic Acids

• Learning Outcomes
  • Describe DNA structure, base‑pairing rules and antiparallel orientation (AO1, AO2).
  • Explain replication, transcription and translation, including the role of RNA polymerase, ribosomes and tRNA (AO2).
  • Classify mutations (point, frameshift) and discuss their possible effects on protein function (AO2).
• Practical Box – DNA Extraction from Strawberries
  • Extract DNA, precipitate with ethanol, visualise a white filament; discuss yield and purity (AO3).

1.7 Transport in Organisms

• Learning Outcomes
  • Explain diffusion of gases in lungs, gills and plant leaves; relate to partial pressure and surface area (AO2).
  • Describe the human circulatory system (heart, blood vessels) and the principles of bulk transport of nutrients and gases (AO2).
  • Outline the structure and function of plant xylem and phloem, including the cohesion‑tension theory and pressure‑flow hypothesis (AO2).
• Practical Box – Respiration Rate of Yeast
  • Measure CO₂ production in glucose solution at different temperatures; plot rate vs. temperature (AO3).

1.8 Gas Exchange

• Learning Outcomes
  • Describe the structure of the human lung (bronchi, bronchioles, alveoli) and the diffusion barrier (AO1, AO2).
  • Explain the role of partial pressure gradients, surface area and membrane thickness in gas exchange (AO2).
  • Compare gas‑exchange adaptations in insects (tracheae) and plants (stomata) (AO2).

1.9 Infectious Disease

• Learning Outcomes
  • Classify pathogen types (bacteria, viruses, fungi, protozoa) and describe routes of transmission (AO1, AO2).
  • Define incubation period, infectious dose and latent period (AO1, AO2).
  • Outline innate defence mechanisms (physical barriers, phagocytes, inflammation, complement) and adaptive immunity (antibodies, cell‑mediated) (AO2).
  • Explain the principles of vaccination (attenuated, inactivated, sub‑unit, conjugate) and herd immunity (AO2).
• Practical Box – Antimicrobial Testing (Disc Diffusion)
  • Inoculate *E. coli* lawn, place antibiotic discs, measure zones of inhibition; interpret susceptibility (AO3).

1.10 Immunity (Focus of this Note)

• Learning Outcomes
  • Distinguish innate from adaptive immunity (AO1, AO2).
  • Identify the main cells of the immune system (phagocytes, lymphocytes, mast cells, dendritic cells) and their functions (AO1, AO2).
  • Describe key molecules: antibodies (Ig classes), cytokines, complement proteins (AO1, AO2).
  • Explain antigen presentation, major histocompatibility complex (MHC I & II) and the activation of T‑cells (AO2).

1.11 Summary of AO Mapping – AS Topics

2. A‑Level Extension Topics (12 – 19)

12. Energy & Respiration

• Learning Outcomes
  • Describe the steps of glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation (AO1, AO2).
  • Calculate the net ATP yield from aerobic respiration of glucose (AO2).
  • Explain anaerobic pathways in muscle and yeast and their physiological significance (AO2).
  • Analyse the effect of temperature on enzyme rate using Q₁₀ values (AO3).
• Practical Box – Respirometry
  • Measure O₂ consumption of germinating beans at 10 °C, 20 °C and 30 °C; plot rate vs. temperature and calculate Q₁₀ (AO3).

13. Photosynthesis

• Learning Outcomes
  • Explain the light‑dependent reactions: photophosphorylation, water splitting, production of O₂, NADPH and ATP (AO1, AO2).
  • Describe the Calvin‑Benson cycle, the role of Rubisco and the differences between C₃, C₄ and CAM pathways (AO2).
  • Interpret chlorophyll fluorescence data to assess photosynthetic efficiency (AO3).
• Practical Box – Pigment Separation
  • Thin‑layer chromatography of spinach extract; identify chlorophyll a, chlorophyll b and carotenoids (AO3).

14. Homeostasis

• Learning Outcomes
  • Explain regulation of blood glucose (insulin, glucagon) and calcium (PTH, calcitonin) (AO1, AO2).
  • Describe thermoregulation in mammals (sweating, shivering, vasoconstriction/dilation) (AO2).
  • Analyse a data set showing blood glucose changes after a carbohydrate load and evaluate the experimental design (AO3).

15. Control & Coordination

• Learning Outcomes
  • Describe the generation and propagation of an action potential, synaptic transmission and the role of neurotransmitters (AO1, AO2).
  • Explain hormone synthesis, transport in the bloodstream, receptor interaction and signal transduction pathways (AO2).
  • Discuss the integration of nervous and endocrine responses during a stress reaction (AO2).
  • Design an experiment to compare the speed of neural vs. hormonal signalling (AO3).

16. Inheritance

• Learning Outcomes
  • Apply Mendelian principles, linkage and chromosome theory to predict genetic ratios (AO1, AO2).
  • Explain the use of PCR, gel electrophoresis and DNA sequencing in genetic analysis (AO2).
  • Evaluate the reliability of a pedigree analysis for a recessive disorder (AO3).
• Practical Box – DNA Extraction & Gel Electrophoresis
  • Extract DNA from strawberries, run on 1 % agarose gel, visualise with ethidium bromide; interpret band pattern (AO3).

17. Selection & Evolution

• Learning Outcomes
  • Describe natural selection, genetic drift, gene flow and speciation mechanisms (AO1, AO2).
  • Interpret evidence from fossils, comparative anatomy and molecular phylogenetics (AO2).
  • Critically evaluate a data set showing allele frequency change in a laboratory population over 10 generations (AO3).

18. Classification & Conservation

• Learning Outcomes
  • Explain the hierarchical system of taxonomy and the rules of binomial nomenclature (AO1, AO2).
  • Construct and interpret phylogenetic trees using morphological and molecular data (AO2).
  • Assess the effectiveness of different conservation strategies (protected areas, captive breeding, biodiversity hotspots) (AO3).

19. Genetic Technology

• Learning Outcomes
  • Describe recombinant DNA techniques, gene cloning and the CRISPR‑Cas9 system (AO1, AO2).
  • Discuss the ethical, social and environmental implications of GMOs and gene therapy (AO3).
  • Design a CRISPR experiment to knock‑out a specific plant gene and outline controls (AO3).

2.1 Practical Skills Required for Papers 3 & 5 (A‑Level)

2.2 AO‑Mapping – A‑Level Topics

3. In‑Depth: Mode of Action of Phagocytes (Neutrophils & Macrophages)

3.1 Learning Objective

Describe the complete mode of action of phagocytes (macrophages and neutrophils) – AO1 (knowledge) and AO2 (explain the mechanisms). Include an example of data interpretation to illustrate AO3.

3.2 Overview of Phagocytosis

1. Chemotaxis – migration toward chemotactic factors (formyl‑Met‑Leu‑Phe, C5a, IL‑8). Receptors are G‑protein‑coupled (GPCR) which trigger actin polymerisation and pseudopod formation.
2. Adherence (Recognition)
   • Opsonisation enhances binding:
     • Fcγ receptors bind the Fc region of IgG‑coated microbes.
     • Complement receptors (CR1, CR3) bind C3b‑opsonised particles.
   • Pattern‑recognition receptors (PRRs) such as Toll‑like receptors (TLRs) recognise pathogen‑associated molecular patterns (PAMPs) (e.g., LPS).
3. Engulfment – actin‑driven extension of pseudopods encloses the particle in a membrane‑bound phagosome.
4. Phagosome‑Lysosome Fusion – the phagosome matures, fusing with lysosomes to form a phagolysosome (pH ≈ 4.5). Hydrolytic enzymes (acid phosphatase, lysozyme, cathepsins) become active.
5. Killing Mechanisms
   • Oxidative (respiratory) burst: NADPH oxidase transfers electrons from NADPH to O₂ → superoxide (O₂⁻). Superoxide dismutates → H₂O₂; myeloperoxidase (MPO) uses H₂O₂ to generate hypochlorous acid (HOCl) and other ROS.

     O₂ + NADPH → O₂⁻ → H₂O₂ → HOCl

   • Reactive nitrogen species (RNS): inducible nitric‑oxide synthase (iNOS) in macrophages produces NO, which combines with superoxide to form peroxynitrite (ONOO⁻).
   • Non‑oxidative: lysozyme, defensins, lactoferrin (iron‑sequestering), proteases (cathepsins).
6. Exocytosis – indigestible debris is expelled from the cell.

3.3 Neutrophils (Polymorphonuclear Leukocytes – PMNs)

• Most abundant circulating leukocyte; first responders to acute bacterial and fungal infection.
• Life span in tissue: 6–8 h; undergo programmed apoptosis after phagocytosis.
• Granule Types
  • Primary (azurophilic) granules: myeloperoxidase, defensins, lysozyme, cathepsin G.
  • Secondary (specific) granules: lactoferrin, collagenase, NADPH oxidase components.
• Oxidative burst – rapid, high‑output production of ROS; kills most engulfed microbes within minutes.
• Limited antigen‑presentation capacity; role is primarily innate killing.
• Release of cytokines (IL‑1β, TNF‑α) that amplify inflammation.

3.4 Macrophages (Mononuclear Phagocytes)

• Derived from circulating monocytes that differentiate in tissues (e.g., Kupffer cells, alveolar macrophages, microglia, osteoclasts).
• Life span: weeks to months; many can self‑renew locally.
• Granules & Enzymes: abundant lysosomal enzymes, iNOS, arginase, metalloproteinases.
• Oxidative burst – moderate, sustained; often combined with nitric‑oxide production, especially against intracellular pathogens such as *Mycobacterium*.
• Antigen presentation – process phagolysosomal peptides, load onto MHC II, migrate to cell surface to activate CD4⁺ helper T‑cells (critical link to adaptive immunity).
• Secrete a wide range of cytokines (IL‑1, IL‑6, TNF‑α, IL‑12) that recruit additional immune cells and shape the adaptive response.
• Involved in tissue repair: release growth factors (PDGF, TGF‑β) and promote angiogenesis.

3.5 Comparison of Neutrophils and Macrophages

3.6 Example AO3 – Data Interpretation

Investigation: Quantify the oxidative burst of neutrophils and macrophages using a luminol‑based chemiluminescence assay after exposure to opsonised *E. coli*.

Interpretation (AO3)

1. Both cell types show a low baseline (100 RLU) before stimulation, confirming the assay background is comparable.
2. Neutrophils exhibit a rapid, high‑amplitude burst, reaching a plateau by 5 min (≈ 12‑fold increase). This reflects the strong, short‑lived oxidative burst typical of PMNs.
3. Macrophages display a slower, lower‑amplitude increase that continues to rise up to 15 min (≈ 6‑fold increase). This matches the moderate, sustained ROS production described for macrophages.
4. Statistical analysis (e.g., two‑sample t‑test) shows the difference at 5 min is highly significant (p < 0.01), supporting the hypothesis that neutrophils generate a more vigorous early oxidative response.
5. Potential sources of error: variation in cell number, incomplete opsonisation, or luminol quenching by extracellular antioxidants. Controlling cell counts (e.g., 1 × 10⁶ cells mL⁻¹) and confirming opsonisation with a complement assay would improve reliability.

3.7 Exam‑style Question Panels

4. Quick Reference Tables

4.1 Summary of Key Terms (AO1)

4.2 Common Cytokines Produced by Phagocytes (AO2)

5. Final Checklist for Students

• Can you list the steps of phagocytosis and the key molecules involved at each step? (AO1)
• Can you explain why neutrophils are more effective at killing extracellular bacteria, whereas macrophages are essential for antigen presentation? (AO2)
• Can you design a realistic experiment (including controls) to test the effect of a new anti‑inflammatory drug on the oxidative burst of neutrophils? (AO3)