AQA Biology A-level Topic 2: Cells

Methods of Studying Cells

• Microscopes: Key tool for studying cells.
  • Light Microscopes:
    • Use convex glass lenses.
    • Resolve images 0.2 μm apart, limited by light wavelength.
    • Magnification = size of image / size of real object.
  • Electron Microscopes:
    • Higher resolution than light microscopes.
    • Types: Transmission Electron Microscopes (TEM) and Scanning Electron Microscopes (SEM).
    • Use a beam of electrons focused by electromagnets.
    • TEM: Electron beam through specimen; dark images where electrons absorbed.
    • SEM: Electron beam scatters creating 3D image.
    • Limitations: Requires vacuum, complex staining, specimens must be thin (TEM), SEM lower resolution than TEM.

Cell Fractionation and Ultracentrifugation

• Cell Fractionation: Separates cell organelles for detailed study.
  • Differential Centrifugation:
    • Cells blended into homogenate.
    • Centrifuge separates organelles by weight.
    • Sequentially spins at higher speeds to separate lighter organelles.
    • Use a cold, buffered solution to protect organelles.

Cell Structure

• Eukaryotic Cells:
  • Contain nucleus and membrane-bound organelles.
  • Nucleus: Double membrane with pores, contains chromatin and nucleolus.
  • Endoplasmic Reticulum:
    • Rough (RER): Protein processing.
    • Smooth (SER): Lipid processing.
  • Golgi Apparatus: Processes and packages proteins, produces lysosomes.
  • Mitochondria: Site of respiration, double membrane with cristae.
  • Centrioles: Produce spindle fibers for cell division.
  • Ribosomes: Protein production.
  • Lysosomes: Digestive enzymes.
• Prokaryotic Cells (e.g., Bacteria):
  • No membrane-bound organelles, have cell wall, capsule, plasmid, flagellum, pili, ribosomes, mesosomes.
• Viruses: Non-living; nucleic acid within protein coat.

Cell Division - Mitosis

• Mitosis: Produces identical daughter cells for growth and repair.
  • Stages: Prophase, Metaphase, Anaphase, Telophase.
  • Interphase: Cell growth and preparation.
  • Cytokinesis: Cytoplasm divides.
• Binary Fission: Prokaryotic cell division.
• Viruses: Do not undergo cell division.

Biological Membranes

• Structure: Phospholipid bilayer with proteins.
  • Functions: Substance control, receptors, cell adhesion.
• Membrane Components:
  • Proteins: Intrinsic/extrinsic, aid movement, support, receptors.
  • Cholesterol: Increases rigidity, reduces leakage.
  • Glycolipids/Glycoproteins: Receptors, cell recognition and adhesion.
• Movement Across Membranes:
  • Diffusion: Passive, high to low concentration.
  • Facilitated Diffusion: Channel proteins transport polar molecules.
  • Osmosis: Water movement through membrane.
  • Active Transport: Requires ATP, low to high concentration.
  • Exocytosis/Endocytosis: Large particles transported via vesicles.
  • Co-Transport: Ions move substances in cells, e.g., via sodium-potassium pump.

Bacteria and Viruses

• Bacteria: Prokaryotic, independent, larger than viruses.
• Viruses: Require host, smaller, composed of nucleic acid and protein coat.
• Bacterial Disease Example: Tuberculosis (Mycobacterium tuberculosis).
• Viral Disease Example: HIV/AIDS.

Immune Response

• Non-specific: Physical barriers, inflammation, lysozyme, interferon, phagocytosis.
• Specific: Antigen-specific, involves B cells (humoral response) and T cells (cell-mediated response).
• Antibodies: Produced by plasma cells, Y-shaped, bind to specific antigens.

Immunity

• Active Immunity: From own immune response.
• Passive Immunity: From another's antibodies.
• Natural vs. Artificial:
  • Natural active: Disease exposure.
  • Natural passive: Maternal antibodies.
  • Artificial active: Vaccination.
  • Artificial passive: Antibody injection.

Vaccines

• Introduce pathogen to stimulate immune response.
• Success depends on cost, side effects, ease of production, herd immunity.

Monoclonal Antibodies

• Produce clones of a single antibody type.
• Uses: Direct/indirect therapy, diagnosis, pregnancy testing.

HIV

• Structure: Lipid envelope, protein capsid, RNA, reverse transcriptase.
• Infection Mechanism: Binds to CD4 on T-Helper cells.
• ELISA: Detects HIV antigens.
• Antibiotics ineffective; treated with antiretroviral drugs.