Cell Structure & Organelles

Prokaryotes vs. Eukaryotes

• 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.
• Secretory pathway: $\text{RER} \to \text{transport vesicle} \to \text{cis-Golgi} \to \text{trans-Golgi} \to \text{secretory vesicle} \to \text{plasma membrane}$
• 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.