Overview
This lecture covers all of AQA A-level Biology Topic 2, focusing on cell structure, microscopy, the cell cycle and mitosis, membrane transport, cell recognition, immunity, HIV, and monoclonal antibodies.
Eukaryotic & Prokaryotic Cell Structure
- Eukaryotic cells contain nucleus, rough/smooth endoplasmic reticulum, Golgi apparatus, lysosomes, ribosomes, mitochondria, permanent vacuole (plants), chloroplasts (plants), and cellulose cell wall (plants).
- The nucleus contains linear DNA, nucleoplasm, nucleolus (makes RNA and ribosomes), and is the site of DNA replication and transcription.
- Rough ER synthesizes proteins; smooth ER synthesizes lipids and carbohydrates.
- Golgi apparatus modifies, stores, and transports proteins and lipids; forms lysosomes.
- Lysosomes digest pathogens, worn out organelles, and dead cells.
- Mitochondria are the site of aerobic respiration, producing ATP; contain 70s ribosomes and looped DNA.
- Vacuoles (plants) provide support and store substances; chloroplasts carry out photosynthesis.
- Prokaryotic cells (bacteria) are much smaller, lack membrane-bound organelles, have circular DNA, 70s ribosomes, murein cell wall, possible plasmids, capsule, and flagella.
- Viruses are non-living, acellular, containing genetic material (DNA/RNA), capsid, and attachment proteins.
Microscopy & Cell Fractionation
- Magnification = image size ÷ actual size; ensure units match (mm to μm).
- Resolution is the minimum distance to distinguish two points, limited by wavelength (light: lower, electrons: higher).
- Optical microscopes: color images, lower resolution/magnification, view living samples.
- Electron microscopes: black and white, higher resolution/magnification, require vacuum (dead samples).
- Transmission EM gives 2D images; scanning EM gives 3D images.
- Eyepiece graticule is calibrated with stage micrometer for measuring specimens.
- Cell fractionation: homogenization (breaking cells in cold, isotonic, buffered solution), filtration, ultracentrifugation (nuclei → chloroplast/mitochondria → lysosomes/ER → ribosomes).
Cell Cycle & Mitosis
- Cell cycle: interphase (G1, S, G2), mitosis, cytokinesis.
- Prokaryotes divide by binary fission; viruses replicate only inside host cells.
- Mitosis: prophase (chromosomes condense), metaphase (line up at equator), anaphase (chromatids separated), telophase (chromosomes decondense).
- Mitotic index = cells in mitosis ÷ total cells.
- Uncontrolled mitosis leads to tumors (cancer).
Membrane Structure & Transport
- Plasma membrane described by fluid mosaic model: phospholipid bilayer, proteins, cholesterol, glycoproteins, glycolipids.
- Simple diffusion: passive, small/lipid-soluble molecules down concentration gradient.
- Facilitated diffusion: passive, uses protein/channel/carrier for large/polar molecules.
- Osmosis: passive, water moves from high to low water potential (towards more negative), across partially permeable membrane.
- Active transport: requires ATP, moves substances against gradient via carrier proteins.
- Co-transport: sodium ions actively transported to create gradient; glucose/amino acids move into cell with sodium ions via co-transporter.
Cell Recognition & Immune Response
- Cells display proteins (antigens) for recognition as "self" or "non-self".
- Pathogens, transplanted cells, abnormal cells, and toxins can act as non-self, triggering response.
- Antigen variability (e.g., mutations in influenza) reduces immunity effectiveness.
- Phagocytosis: phagocytes engulf pathogens, lysosome fuses to digest, antigens presented on surface.
- T lymphocytes (T cells): cell-mediated immunity, include helper T cells (activate B cells, phagocytes, cytotoxic T cells), cytotoxic T cells (destroy infected body cells with perforin protein).
- B lymphocytes: humoral immunity, activated by antigen and helper T cells, undergo clonal expansion and differentiate into plasma cells (produce antibodies) and memory cells.
- Primary response: slower, lower antibody production; secondary response: faster, higher antibody production.
Antibodies & Immunity Types
- Antibodies: proteins with variable regions for antigen binding, cause agglutination to aid phagocytosis.
- Passive immunity: antibodies received, no exposure to pathogen/antigen, no memory cells (e.g. breastfeeding).
- Active immunity: memory cells produced after pathogen exposure or vaccination.
- Herd immunity: widespread vaccination protects unvaccinated individuals.
HIV & AIDS
- HIV is a retrovirus targeting and destroying helper T cells, weakening immune response and causing AIDS.
- HIV contains RNA, reverse transcriptase, a capsid, and a host-derived lipid envelope.
- Destroys immune system, making individuals vulnerable to opportunistic infections.
Monoclonal Antibodies & Medical Use
- Monoclonal antibodies are identical antibodies with specificity for one antigen.
- Used for targeted cancer therapies (direct/indirect), pregnancy and disease tests (e.g., ELISA), and COVID-19 testing.
- Production raises ethical issues due to animal use.
Key Terms & Definitions
- Prokaryotic cell — cell lacking a nucleus and membrane-bound organelles.
- Eukaryotic cell — cell with a nucleus and membrane-bound organelles.
- Murein — glycoprotein forming bacterial cell walls.
- Magnification — number of times larger the image is compared to the object.
- Resolution — the ability to distinguish two points as separate.
- Isotonic — solution with the same water potential as the cell.
- Antigen — foreign protein on cell surfaces that triggers an immune response.
- Osmosis — movement of water from higher to lower water potential.
- Passive immunity — immunity gained without exposure to antigen (no memory cells formed).
- Active immunity — immunity developed after exposure to antigen/pathogen (memory cells formed).
Action Items / Next Steps
- Review each organelle’s structure and function.
- Practice magnification and calibration calculations.
- Memorize the stages and roles in the cell cycle and mitosis.
- Study the fluid mosaic model details and all types of membrane transport.
- Learn immune response steps and antibody structure.
- Revise key differences between prokaryotes, eukaryotes, and viruses.
- Complete any assigned workbook exercises or required readings referenced in your course materials.