Section 1: Cell Theory
The Cell Theory — proposed by Schleiden and Schwann (1838–39) — states that all organisms are composed of cells and that the cell is the fundamental unit of life. Virchow added the third principle: "Omnis cellula e cellula" (every cell from a pre-existing cell, 1855). Robert Hooke (1665) first coined the term "cell" observing cork. Leeuwenhoek was the first to observe living cells. Cell Theory has exceptions: viruses (not made of cells) and mature mammalian RBCs (no nucleus).
Section 2: Prokaryotic Cells
Prokaryotes (bacteria, cyanobacteria) are 1–10 µm with no membrane-bound nucleus — genetic material lies as naked circular DNA in the nucleoid region. The cell wall is peptidoglycan (murein). Ribosomes are 70S (50S + 30S). Mesosomes are plasma membrane infoldings assisting respiration and division. Plasmids are extrachromosomal circular DNA carrying antibiotic resistance genes. Flagella are made of flagellin (no 9+2 arrangement). Prokaryotes divide by binary fission (no spindle).
Section 3: Eukaryotic Cell — Nucleus and Endomembrane System
Eukaryotic cells (10–100 µm) have a true nucleus with a double-membraned nuclear envelope and nuclear pores. The nucleolus synthesizes rRNA. Cytoplasmic ribosomes are 80S (60S + 40S). The endomembrane system — ER (rough: protein synthesis; smooth: lipid synthesis, detox) → Golgi apparatus (cis face receives from ER; trans face releases) → lysosomes (hydrolytic enzymes, "suicide bags") → vacuoles — handles protein and lipid processing ("Every Girl Loves Vacations").
Section 4: Semi-Autonomous Organelles
Mitochondria and chloroplasts have their own circular DNA and 70S ribosomes, making them semi-autonomous. This supports the endosymbiotic theory (Lynn Margulis, 1967). Mitochondria: double membrane; inner membrane folded into cristae for ATP synthesis; matrix contains Krebs cycle enzymes. Chloroplasts: double membrane; internal thylakoids stacked into grana (light reactions); stroma for Calvin cycle (dark reactions).
Section 5: Plasma Membrane and Cell Wall
The plasma membrane follows the Fluid Mosaic Model (Singer & Nicolson, 1972): phospholipid bilayer with integral and peripheral proteins, glycoproteins (glycocalyx), and cholesterol (animal cells). Plant cells have a cellulose cell wall with middle lamella (calcium pectate) cementing adjacent cells. Fungi have chitin walls. Animal cells have no cell wall.
Section 6: Cytoskeleton, Cilia, Flagella, Centrioles
The cytoskeleton comprises microtubules (tubulin), microfilaments (actin), and intermediate filaments. Eukaryotic cilia and flagella have the 9+2 microtubule arrangement; dynein drives movement. Centrioles (absent in most plants) organize the mitotic spindle. Prokaryotic flagella use flagellin and rotate via proton gradient.
Section 7: Plant vs Animal Cell
Plant cells: cell wall (cellulose), plastids (chloroplast/chromoplast/leucoplast), large central vacuole (tonoplast), plasmodesmata, cell plate cytokinesis (centrifugal), no centrioles, starch storage. Animal cells: centrioles, lysosomes, cholesterol in membrane, cleavage furrow cytokinesis (centripetal), glycogen storage, no plastids, no cell wall. Key mnemonic: "Plants Can't Walk, Animals Carry Luggage."