In order to study the properties of membrane proteins in cell-free systems it is necessary to disrupt the cell structure and then separate the resulting intracellular organelles and membrane vesicles by differential centrifugation and density gradient centrifugation. (Broken membranes reseal into small bubble-like structures called vesicles or micro-somes.) Fractions enriched in a certain type of membrane or intracellular organelle may then be solubilized in a detergent and a membrane protein purified in the presence of detergent by appropriate chromatographic methods (see Chapter 22).