Why does fluid structure occur?

Since lipids and proteins are maintained together not by covalent bonds, and because of length of fatty acids, unsaturation degree, presence of cholesterol and temperature. Unsaturation increases fluidity, because they donot behave as crystallatice. Cholesterol acts as a bidirectional regulator, which decreases fluidity at high temperature and increases it at low one, via intercalating with phospholipids. Longer CH tail of fatty acids decreases fluidity. All cells must keep suitable fluidity by adjusting each factor, (cells at below 37 C promote activity of unsaturating enzymes to compensate loss of fluidity by temp decrease to maintain fluidity. Plants has unsaturated FA when temp drops at night and winter)