Practice question

Cells internalize macromolecules too large or polar to cross bilayer via endocytosis involving membrane invagination and vesicle formation consuming energy. Clathrin-mediated endocytosis concentrates receptors such as transferrin receptor, LDL receptor, and EGFR into clathrin-coated pits assembled by adaptor AP2 and accessory proteins EPS15, dynamin GTPase mediates scission producing 100 nm vesicles that shed clathrin via auxilin and Hsc70 and fuse with early endosomes marked by Rab5 and PI3P. Caveolin-dependent uptake forms 60 nm flask-shaped caveolae enriched in cholesterol, regulating lipid homeostasis and signal transduction. Phagocytosis in macrophages uses actin polymerization driven by Rho GTPases to extend pseudopods engulfing bacteria opsonized by IgG. Macropinocytosis non-selectively takes up fluid via ruffling. All require ATP for actin remodeling and dynamin function plus membrane trafficking through endosomal system for sorting, recycling via Rab11 or degradation via Rab7 to lysosomes. Exocytosis is opposite outward fusion, osmosis is water movement, simple diffusion is passive lipid permeation.