Cell membrane structure
The fluid mosaic model: a phospholipid bilayer (hydrophilic heads out, hydrophobic tails in) about 7 nm thick, in which proteins float (fluid) in a scattered pattern (mosaic). Channel and carrier proteins transport polar molecules and ions. Cholesterol sits between phospholipids, regulating fluidity and reducing permeability to water and ions. Glycoproteins and glycolipids act as receptors and antigens for cell recognition. In cell signalling, a hormone binds a complementary receptor on the membrane, triggering a cascade inside (often via a second messenger such as cyclic AMP). Higher temperatures increase permeability (phospholipids move more; proteins denature), shown by pigment leaking from beetroot discs.
≈ 40 minEukaryotic and prokaryotic cell structure
Eukaryotic cells have a nucleus (double envelope with pores, chromatin, nucleolus that makes ribosomes) and membrane-bound organelles: rough ER (ribosomes; makes and transports proteins), smooth ER (lipids, steroids), Golgi body (modifies, packages proteins into vesicles), mitochondria (aerobic respiration, ATP; folded cristae), lysosomes (digestive enzymes), 80S ribosomes, centrioles and microtubules. Plant cells also have a cellulose cell wall, chloroplasts and a large vacuole. Prokaryotic cells are smaller, with no nucleus (circular DNA, plasmids), 70S ribosomes, a peptidoglycan (murein) wall and sometimes a capsule and flagellum. Viruses are non-living: nucleic acid in a protein capsid, sometimes an envelope.
≈ 45 minMicroscopy and magnification
Magnification ; convert to the same unit first (1 mm = 1000 µm; 1 µm = 1000 nm). Resolution is the ability to distinguish two close points: light microscopes resolve about 200 nm; electron microscopes about 0.5 nm because electrons have a much shorter wavelength. TEM gives 2D sections with internal detail; SEM gives 3D surface images; both need a vacuum, so specimens are dead. An eyepiece graticule is calibrated against a stage micrometer (divisions of known size, often 10 µm) at each objective. Cell fractionation: homogenise in cold, isotonic, buffered solution, then ultracentrifuge: nuclei sediment first, then mitochondria, then ribosomes.
≈ 40 minTransport across membranes
Diffusion: net movement down a concentration gradient (passive). Facilitated diffusion: polar molecules and ions through channel or carrier proteins (passive). Osmosis: net movement of water from higher to lower water potential (ψ) across a partially permeable membrane; pure water ψ = 0, solutions negative. Plant cells: ; in concentrated solutions they become plasmolysed; in water, turgid. Animal cells burst (lysis) or shrink (crenation). Active transport moves substances against a gradient using ATP and carrier proteins; co-transport (e.g. Na⁺ and glucose in the ileum) uses a gradient set up by active transport. Endocytosis/exocytosis move bulk material in vesicles. Rate of diffusion increases with surface area, concentration gradient and temperature, and decreases with distance.
≈ 50 minImmune response and vaccination
Phagocytes (neutrophils, macrophages) engulf pathogens; macrophages become antigen-presenting cells. T-helper cells with complementary receptors are activated and release cytokines. B cells with complementary receptors undergo clonal selection and expansion, forming plasma cells (secrete antibodies) and memory cells. Cytotoxic T cells kill infected cells. Antibodies: four polypeptide chains (2 heavy, 2 light), variable regions with two antigen-binding sites, held by disulfide bonds; cause agglutination, neutralisation and opsonisation. The secondary response is faster and larger because of memory cells. Active immunity (own antibodies; natural infection or vaccination) is long-lasting; passive (given antibodies; placenta, milk, injection) is short-term. Herd immunity protects the unvaccinated. Monoclonal antibodies target specific antigens for diagnosis and treatment.
≈ 50 minMitosis and the cell cycle
The cell cycle: interphase (G1 growth and protein synthesis; S phase DNA replication; G2 growth and checks), mitosis (nuclear division) and cytokinesis. In mitosis: prophase (chromosomes condense into two sister chromatids joined at the centromere; nuclear envelope breaks down; spindle forms), metaphase (chromosomes line up at the equator, attached to spindle fibres), anaphase (centromeres divide; chromatids pulled to opposite poles), telophase (nuclear envelopes reform). Result: two genetically identical diploid cells, for growth, repair and asexual reproduction. Mitotic index = cells in mitosis ÷ total cells. Telomeres protect chromosome ends; stem cells keep dividing; cancer results from mutations in genes controlling the cycle.
≈ 45 min