Cell: The Unit of Life

CELL: THE UNIT OF LIFE - QUICK REVISION. CELL THEORY: Leeuwenhoek first described a live cell; Robert Brown discovered the nucleus. Schleiden (1838, plants) and Schwann (1839, animals) formulated the cell theory; Virchow (1855) added that all cells arise from pre-existing cells (Omnis cellula-e cellula). Modern cell theory: (i) all living organisms are composed of cells and products of cells; (ii) all cells arise from pre-existing cells. The cell is the fundamental structural and functional unit of life. PROKARYOTIC vs EUKARYOTIC: Prokaryotes (bacteria, blue-green algae, mycoplasma, PPLO) have no membrane-bound nucleus, no membrane-bound organelles, naked circular DNA, 70S ribosomes; eukaryotes have an organised nucleus with nuclear envelope, membrane-bound organelles, chromosomes and 80S cytoplasmic ribosomes. Mycoplasma (0.3 micrometre) is the smallest cell; ostrich egg is the largest single cell. Bacterial shapes: bacillus, coccus, vibrio, spirillum. CELL ENVELOPE (bacteria): three layers outward-to-inward = glycocalyx (slime layer or capsule), cell wall, plasma membrane. Mesosome = infoldings of plasma membrane (cell wall formation, DNA replication, respiration). Plasmids = extra small circular DNA. Flagellum = filament + hook + basal body; pili and fimbriae are non-motile. Inclusion bodies store reserve material, not membrane bound. PLASMA MEMBRANE: fluid mosaic model (Singer and Nicolson, 1972). Phospholipid bilayer with polar heads outside, hydrophobic tails inside, plus cholesterol and integral/peripheral proteins. Transport: passive (simple diffusion, osmosis = water diffusion, facilitated by carrier proteins) vs active (against gradient, uses ATP, e.g. Na+/K+ pump). CELL WALL: non-living rigid covering of plants and fungi; cellulose, hemicellulose, pectins; middle lamella (calcium pectate) glues cells; plasmodesmata connect cytoplasm. ENDOMEMBRANE SYSTEM (ER + golgi + lysosomes + vacuoles): RER (ribosomes, protein synthesis) and SER (lipid synthesis). Golgi (cis/forming and trans/maturing faces) packages and forms glycoproteins/glycolipids. Lysosomes = hydrolytic enzymes active at acidic pH. Vacuole bound by tonoplast (up to 90 percent of plant cell volume). MITOCHONDRIA: double membrane, inner forms cristae, matrix has 70S ribosomes and circular DNA; site of aerobic respiration producing ATP (power houses); divide by fission. PLASTIDS (plant cells, euglenoids): chloroplasts (photosynthesis; stroma + grana of thylakoids; 70S ribosomes), chromoplasts (carotenoids), leucoplasts (amyloplast-starch, elaioplast-oils, aleuroplast-proteins). RIBOSOMES: RNA + protein, non-membranous (Palade 1953); 80S (60S+40S) eukaryotic, 70S (50S+30S) prokaryotic; S = Svedberg sedimentation coefficient; polysome = many ribosomes on one mRNA. CYTOSKELETON: microtubules + microfilaments + intermediate filaments; support, motility, shape. CILIA and FLAGELLA: 9+2 axoneme of microtubules, covered by plasma membrane, emerge from basal body. CENTROSOME: two centrioles (9 peripheral triplets of tubulin, cartwheel); form basal body and spindle apparatus in cell division. NUCLEUS: described by Robert Brown (1831); nuclear envelope (double membrane + nuclear pores), nucleoplasm, chromatin (DNA + histones, named by Flemming), nucleolus (rRNA synthesis, not membrane bound). Chromosome has centromere with kinetochores; types: metacentric, sub-metacentric, acrocentric, telocentric. MICROBODIES: membrane-bound enzyme-containing vesicles in plant and animal cells.