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B2: Cell Structure, Transport and Immunology

Year 1 / ASYear 2 / A-Level All Boards (AQA, Edexcel, OCR, WJEC, CCEA) AQA

Ultrastructure of eukaryotic and prokaryotic cells, fluid mosaic membranes, co-transport, cell-mediated and humoral immunity.

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📋 Key Definitions and Core Concepts

Fluid Mosaic Model: Phospholipid bilayer with lateral mobility of lipids and embedded mosaic of transport/receptor proteins.
Antigen: A cell-surface glycoprotein or macromolecule that triggers an immune response.

🔍 Key Principles & Specification Requirements

  • Transport: simple diffusion (small/non-polar), facilitated diffusion (channels/carriers), active transport (ATP against gradient), co-transport (Na⁺-glucose in ileum).
  • Humoral immunity: B-cells differentiate into plasma cells producing specific monoclonal antibodies.
  • Cell-mediated immunity: cytotoxic T-cells destroy virus-infected and cancerous cells via perforins.

💡 Worked Example Question

Exam-Style Question

Question:

Explain the co-transport of glucose in the mammalian ileum epithelium.

Model Solution & Mark Scheme:

1. Na⁺/K⁺ ATPase actively pumps Na⁺ out of epithelial cell into bloodstream, lowering intracellular [Na⁺].
2. Na⁺ diffuses down gradient from lumen into cell via sodium-glucose co-transporters, dragging glucose against its gradient.
3. Glucose exits into capillary blood via facilitated diffusion.

❓ Practice Questions & Mark Schemes

Q1: Compare TEM and SEM electron microscopes.

Show Model Answer

Answer: TEM: transmits electrons through ultra-thin section (2D internal ultrastructure, 0.1 nm resolution). SEM: scans electron beam across surface (3D topography, 20 nm resolution).

Q2: Describe the role of memory B-cells in secondary immune response.

Show Model Answer

Answer: Memory B-cells circulate in blood; upon re-exposure to antigen, they rapidly divide into plasma cells producing high antibody titers before symptoms develop.

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