Plasma membrane, fluid-mosaic model, lipid rafts, channels, pumps, active/passive transport.
Cell Membrane And Transport
Plasma membrane
Self-assembled phospholipid bilayer + integral/peripheral proteins separating cell from environment. ~5 nm thick; asymmetric leaflet…
Fluid-mosaic model (Singer-Nicolson)
Singer-Nicolson 1972: plasma membrane is a 2D fluid of laterally-diffusing lipids with proteins embedded like icebergs. Refined by…
Lipid raft
Nanoscale (10-200 nm) membrane domain enriched in sphingolipids + cholesterol + GPI-anchored proteins. Liquid-ordered phase; platforms for…
Ion channel
Integral membrane protein forming a selective aqueous pore for ion diffusion (10⁶-10⁸ ions/s). Gated by voltage, ligand, mechanical force,…
Voltage-gated ion channel
Channel that opens/closes in response to membrane voltage via charged voltage-sensor (S4 segment). Na+, K+, Ca²+, Cl− variants drive action…
Active transport
Transport against electrochemical gradient coupled to energy source. Primary (direct ATP use: P-type ATPases, ABC transporters) vs…
Na+/K+ ATPase
P-type pump exporting 3 Na+ / importing 2 K+ per ATP hydrolysed. Establishes resting membrane potential and Na+ gradient driving secondary…
Endocytosis
Membrane invagination internalising extracellular material. Clathrin-mediated (receptor-mediated, LDL, transferrin), caveolin-mediated,…
Facilitated diffusion
Passive transport via channel or carrier protein down concentration gradient. GLUT family (glucose), aquaporins (water). Saturation…
Membrane potential
Voltage across plasma membrane set by ion gradients + selective permeability. Nernst: E = (RT/zF) ln([out]/[in]); Goldman-Hodgkin-Katz for…