Prokaryotes: no nucleus (nucleoid region), circular DNA without histones, 70S ribosomes, peptidoglycan cell wall.
Eukaryotes: membrane-bound nucleus and organelles, linear DNA wrapped on histones, 80S ribosomes.
Organelles and Protein Trafficking
Rough ER synthesizes secretory proteins; smooth ER makes lipids and detoxifies; the Golgi modifies and sorts (cis receives, trans ships); lysosomes digest at pH about 5; peroxisomes degrade hydrogen peroxide via catalase.
Functions of organelles and cell compartmentalization
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→
trans-Golgi→
secretory vesicle→
plasma membrane
An N-terminal signal peptide plus the signal recognition particle (SRP) dock the ribosome on the RER; a mannose-6-phosphate tag routes proteins to lysosomes.
Endosymbiosis and the Cytoskeleton
Mitochondria have their own circular DNA, 70S ribosomes, and double membranes, and they divide by binary fission — evidence they descend from free-living bacteria; inheritance is maternal.
Microfilaments (actin, 7 nm) drive motility and the cleavage furrow; intermediate filaments (10 nm) provide support; microtubules (tubulin, 25 nm) build the mitotic spindle, cilia, and flagella.
Key Takeaways
70S ribosomes and peptidoglycan mark prokaryotes; 80S and membrane-bound organelles mark eukaryotes.
Secretory proteins travel RER to Golgi to plasma membrane, guided by signal sequences.
Mitochondrial DNA, ribosomes, and fission behavior support endosymbiotic theory.
Actin, intermediate filaments, and microtubules divide the labor of shape, support, and transport.