01Why compartments help
Membranes let a eukaryotic cell run incompatible reactions at once: digestion in lysosomes, protein folding in the ER, ATP production in mitochondria. Internal membranes also add surface area for enzymes.
Prokaryotes are limited to their plasma membrane, one reason they stay small.
02Endosymbiotic theory
Mitochondria and chloroplasts descend from free-living prokaryotes engulfed by an ancestral cell about 1.5–2 billion years ago.
Evidence: both have a double membrane, their own circular DNA, 70S ribosomes like bacteria, and they divide by binary fission. Their inner membranes have transport proteins similar to bacterial ones.
03Notebook box
Endosymbiosis evidence (memorize 4): double membrane, circular DNA, bacterial-type ribosomes, binary fission.
Worked example: Antibiotics that block bacterial ribosomes sometimes harm human cells. Which organelle is likely affected? → Mitochondria, whose ribosomes resemble bacterial ribosomes.