Cell - The Unit of Life

Story: 1665 Robert Hooke saw cork under microscope, saw small boxes like prison cells, called cell. 1838 Schleiden German botanist said all plants made of cells. 1839 Schwann German zoologist said all animals made of cells. Together cell theory: all living things made of cells and products of cells. 1855 Virchow said Omnis cellula e cellula — all cells come from pre-existing cells, no spontaneous generation like maggots don't appear from rotten meat automatically. Modern cell theory adds: cell is basic unit of structure, function, heredity, and all cells have similar chemistry. Unicellular single cell like bacteria does everything, multicellular many cells like you division of labour like company departments.

Prokaryote: Bacteria, PPLO mycoplasma smallest, Blue green algae cyanobacteria. No nuclear membrane, DNA circular naked lies in cytoplasm called nucleoid like books lying open without almirah. No membrane-bound organelles, 70S ribosomes small 50S+30S. Plasmid extra small circular DNA extra book gives antibiotic resistance. Cell wall peptidoglycan like strong wall. Mesosome inward folding of membrane for respiration like pocket, Chromatophores for photosynthesis in BGA, Flagella for movement with basal body 4 rings L P S M in gram negative 2 in gram positive, Pili fimbriae attachment like small hairs holding.

Eukaryote: True nucleus with double envelope with pores like manager cabin with doors. Membrane organelles present like departments with walls. 80S ribosomes big 60S+40S in cytoplasm + 70S in mitochondria plastids because they were bacteria earlier (endosymbiont theory).

Plant vs Animal: Plant has cell wall cellulose like extra outer wall, plastids green, large central vacuole 90% like big water tank pushes nucleus to side. Animal has centriole for division, small vacuoles like small buckets. Cell membrane fluid mosaic Singer Nicolson 1972 phospholipid bilayer with proteins floating like icebergs in sea, cholesterol for stability, proteins for transport.

Nuclear envelope double membrane like double wall with perinuclear space between, with nuclear pores like doors for RNA protein going out in. Outer membrane continuous with ER and has ribosomes like attached. Nucleoplasm jelly inside, Nucleolus dark round rRNA synthesis makes ribosomes like factory making machines.

Chromatin DNA + histone proteins beads-on-string: DNA wraps around histone octamer 8 proteins like thread on spool, then beads called nucleosome. Euchromatin light loose active transcription like open book readable, Heterochromatin dark dense inactive like closed book locked. During division chromatin condenses into chromosomes. Human 46 chromosomes 23 pairs.

ER Endoplasmic Reticulum: Network of cisternae tubes like pipeline. RER rough with ribosomes attached like workers on pipeline doing protein synthesis and modification, extensive in secretory cells like pancreas makes enzymes. SER smooth no ribosomes like smooth pipeline lipid synthesis steroid, detox in liver like cleaning chemicals, tubules. Connected to nuclear envelope.

Golgi Apparatus: Stacks of cisternae like pile of plates, cis forming face receives vesicles from ER like receiving department, trans maturing face ships out like dispatch department. Glycosylation adding sugar tag, packaging like gift wrapping, forms lysosomes. Post office of cell.

Lysosomes: Single membrane bag with acid hydrolases proteases lipases nucleases glycosidases pH 5 acidic like acid bag. Formed by RER Golgi. Types: primary newly formed, secondary with food digesting, residual with undigested leftover, autophagic self eating old organelles. Suicidal bags burst and digest own cell during damage like suicide bomber.

Vacuoles: Single membrane tonoplast like bag cover, cell sap water with anthocyanin pigments blue purple in petals you see colour, Contractile vacuole excretion osmoregulation in amoeba like pump removing water, Food vacuole phagocytosis in protists eating, Central vacuole 90% plant cell provides turgidity rigidity like water filled balloon pushes everything.

Mitochondria powerhouse: Outer smooth, inner infolded cristae like folded fingers with F0-F1 particles ATPase oxysomes like small knobs making ATP, matrix with circular DNA 70S ribosomes, enzymes TCA cycle Krebs, divide by fission maternal inheritance from mother. ATP synthesis aerobic respiration like power station makes electricity ATP. More mitochondria in active cells like muscles.

Plastids: Double membrane also. Leucoplasts colorless storage underground: Amyloplasts starch potato, Elaioplasts oil, Aleuroplasts protein. Chromoplasts colored carotenoids xanthophyll in flowers fruits yellow orange attracts pollinators. Chloroplasts green kitchen: Outer inner, stroma jelly, thylakoids stacked as grana like pile of coins connected by stroma lamella like bridges, contains chlorophyll green pigment, starch granule, circular DNA 70S ribosome, also semi-autonomous. Photosynthesis light reaction in grana coins, dark reaction in stroma jelly. Chloroplast is kitchen, mitochondria is powerhouse, both semi-autonomous with own DNA.

Ribosome rRNA + protein 60% RNA 40% protein. 80S = 60S+40S in eukaryote cytoplasm free floating and bound to RER like free workers vs attached workers, 70S = 50S+30S in prokaryote mitochondria plastids because they were bacteria. S = Svedberg sedimentation unit not size. Two subunits small reads mRNA big makes protein. Polysome polyribosome many ribosomes on one mRNA like many workers on one blueprint makes many proteins fast like factory line. Function protein synthesis translation.

Cytoskeleton like house frame: Microfilaments 6nm actin cell movement amoeboid, Intermediate 10nm support like ropes, Microtubules 25nm tubulin hollow tubes cilia flagella spindle like pillars.

Centrosome MTOC microtubule organising centre with 2 centrioles perpendicular 9+0 triplet microtubules cartwheel structure pericentriolar material forms spindle during division like pulling ropes for chromosomes.

Cilia short many like eyelashes Flagella long few like tail structure 9+2 axoneme 9 doublet microtubules around 2 central singlets with dynein arms motor proteins, basal body 9+0 no central. Microbodies peroxisome detox H2O2, glyoxysome fat to sugar in seeds.