Viscosity is an important concept from Fluid Mechanics that should be kept in mind in the study of cell biology. Viscosity refers to the resistance of a fluid to flow, or more properly, the resistance of fluid to a high rate of deformation under shearing stresses. Another way to think of viscosity is as the internal friction of the fluid.
In cell biology, viscosity is an important property of cell membranes. The viscosity of a bilayer membrane in large part depends on whether the fatty acid chains are stacked in a rigid state or exist in a relatively disordered, fluid state. The two characteristics of the fatty acid chains that promote the rigid state are length and the degree of saturation. Long, straight, saturated hydrocarbon chains maximize Van der Waals interactions, increasing membrane viscosity (also increasing the melting point).
In animal cells, cholesterol plays a major roll in moderating membrane fluidity. Cholesterol decreases membrane viscosity at low temperatures, but increases viscosity at high temperature. Fitting between the fatty acid chains, cholesterol prevents their crystallization. However, cholesterol also blocks large motions of the fatty acid chains, which, conversely makes the membrane less fluid at higher temperatures. Cholesterol thus acts like a fluidity buffer for membranes. It keeps the viscosity of cell membranes within an acceptable range.