Plasma membrane composed of a phospholipid bilayer in which relatively disperse membrane proteins could freely diffuse. Hydrophobic effect: tendency of polar molecules like water to exclude hydrophobic molecules. Membrane fluidity is dependent on fatty acid composition and temperature. All fatty acids initially synthesized as saturated molecule. Desaturates remove 2 hydrogen atoms from neighbouring carbon atoms introduce double bond. Bacteria, archaea, protists, and plants can thrive at temperatures at which the plasma membrane would solidify. Able to regulate the proportion of unsaturated fatty acids desaturase transcript. Decrease temperature increase desaturase mrna increase unsaturated fatty acids. Act as membrane buffers: at high temperatures: help restrain movement of lipids, at low temperatures: disrupt fatty acid dissociation. Integral membrane proteins (transmembrane proteins: traverse lipid bilayer, distinct domains: extracellular, transmembrane (tm), intracellular (cytoplamic, tm domain. Form an alpha helix: peripheral membrane proteins, positioned on membrane surface do not interact with hydrophobic core, held to membrane by interacting with integral membrane proteins or lipids.