Units 2-3 cover cell structure, membrane transport, cell communication, and the cell cycle — the fundamental biology of how cells function, interact, and reproduce.
Prokaryotes (bacteria): no nucleus, no membrane-bound organelles, circular DNA, ribosomes (70S). Eukaryotes: nucleus with nuclear envelope, membrane-bound organelles. Key organelles: nucleus (DNA), mitochondria (cellular respiration), chloroplasts (photosynthesis, plants), ER (rough = protein synthesis, smooth = lipid synthesis), Golgi (modification, packaging), lysosomes (digestion), vacuoles. Endosymbiotic theory: mitochondria and chloroplasts were once independent prokaryotes.
Fluid mosaic model: phospholipid bilayer with embedded proteins. Functions: selective permeability, cell recognition, signalling. Passive: diffusion (high → low concentration), osmosis (water through semipermeable membrane), facilitated diffusion (channel/carrier proteins). Active: against gradient, requires ATP. Bulk transport: endocytosis (in), exocytosis (out). Tonicity: hypertonic (cell shrinks), hypotonic (cell swells), isotonic.
Cell signalling: reception (ligand binds receptor) → transduction (signal relay, often phosphorylation cascade) → response (gene expression, enzyme activity). Types: autocrine, paracrine, endocrine. Cell cycle: G1 → S (DNA replication) → G2 → M (mitosis + cytokinesis). Mitosis: PMAT (prophase, metaphase, anaphase, telophase). Checkpoints: G1, G2, M — ensure DNA integrity. Cancer: uncontrolled cell division due to mutations in proto-oncogenes or tumor suppressors.
Multiple lines of evidence: (1) Mitochondria and chloroplasts have their own circular DNA, similar to bacteria. (2) They have double membranes — inner membrane from the original prokaryote, outer from the engulfing cell. (3) They have their own ribosomes (70S, like bacteria, not 80S like eukaryotes). (4) They reproduce by binary fission, like bacteria. (5) Their size is similar to bacteria. (6) Phylogenetic analysis shows their DNA is more closely related to certain bacterial groups than to the nuclear DNA of the host cell.
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