LIFE 120 Lecture Notes - Lecture 5: Lipid Bilayer, Facilitated Diffusion, Passive Transport
Document Summary
The plasma membrane separates the living cell from is surroundings. The plasma membrane exhibits selective permeability, allowing some substances to cross is more easily than others. Phospholipids are the major constituents of membranes: exists as a stable boundary between two aqueous compartments, selective permeability, controls passage. The hydrophobic/hydrophilic interactions between water and phospholipids create a bi-layer of lipid molecules that separate inside from outside. Membranes are not static sheets of molecules. Most of the lipids and some proteins in a membrane can shift about laterally. The lateral movement of phospholipids is rapid; proteins move more slowly. How quickly molecules move within and across membranes is a function of temperature and the structure of the hydrocarbon tails in the bilayer. Short chain fatty acids and unsaturated fatty acids in phospholipids increase fluidity. Cholesterol reduces membrane fluidity at moderate temperatures, but at low temperatures hinders solidification: warm: restrains movement of phospholipids, cold: maintains fluidity by preventing tight packing.
