AP Biology Unit 2 Essentials

Dividing the cell into membrane-bound organelles is the eukaryotic superpower. It lets incompatible reactions happen simultaneously in different rooms (lysosomes can be acidic without acidifying the cytoplasm), concentrates enzymes where they're needed, and creates specialized environments for things like ATP production (mitochondria) and photosynthesis (chloroplasts). The biggest piece of evidence for how this happened is endosymbiotic theory: mitochondria and chloroplasts were once free-living prokaryotes that took up residence inside a larger cell.

The fluid mosaic model is more than memorization — it explains everything about what gets in and out of the cell. The hydrophobic interior of the phospholipid bilayer lets small nonpolar molecules slip through but blocks everything charged or large. To move those, the cell relies on embedded proteins: channels and carriers for facilitated diffusion, pumps for active transport, and vesicles for bulk transport. The structure of the membrane IS its function.

Diffusion and osmosis happen on their own — they're entropy doing its thing. But cells often need to keep things uneven: more K⁺ inside than outside, more Ca²⁺ in the ER than in the cytoplasm, a pH gradient across the mitochondrial membrane. Maintaining these gradients requires energy (ATP), and the gradients themselves are then used to do work — most importantly, to make more ATP in cellular respiration and photosynthesis. This sets up Unit 3.