Membrane fluidity is the ability of phospholipids and proteins to move freely inside the membrane. It is affected by the phospholipid types in the membrane, the temperature of the surroundings, and the concentration of cholesterol. Saturated phospholipids have straight fatty acid chains with no double bonds, whereas unsaturated phospholipids have a bend in their fatty acid chains with at least one double bond. Saturated phospholipids form a densely packed membrane, causing lower membrane fluidity. Unsaturated phospholipids disrupt the phospholipid packing, leading to higher membrane fluidity. At low temperatures, the membrane lipids are tightly packed with little movement, resulting in a rigid membrane. The lipids show increased movement at high temperatures, making the membrane more fluid. Cholesterol, another membrane component, is inserted between phospholipids, creating a space that increases fluidity during colder temperatures. In warmer temperatures, the steroid rings in cholesterol immobilize the phospholipids, preventing the membrane from becoming too fluid. Organisms maintain optimal membrane fluidity under different conditions by regulating the ratio of saturated to unsaturated phospholipids and cholesterol concentration.