by assaying each fraction for marker enzymes that are unique to either mitochondria or lysosomes(리소좀. 가수분해  효소를 가진 세포 소기관), the fractions containing these organelles can be identified and the extent of cross contamination can be determined. equilibrium density(or buoyant density)centrifugation is a powerful method for resolving organelles and macromolecules based on density differences(see figure 12A-3)like density gradient centrifugation, this procedure includes a gradient of solute that increases in concentration and density, but in this ease the solute is concentrated so that the density gradient spans the range of densities of the organelles or macromolecules about to be separated. for organelles, a gradient of sucrose is often used, and the density range is 1.10-1.30g/cm(0.75-2.3 m sucrose). this method can also be used to separate different forms of DNA and RNA based on their differing densities. because these macromolecules have higher densities than organelles, a heavy metal salt, such as cesium chloride(CsCl)is commonly used for the density gradient. for a classic experiment in which CsCl density gradients were used to resolve double stranded DNA molecules containing nitrogen isotopes 14N versus 15N, as well as to detect hybrid DNA molecules containing both isotopes, see figure 19-4. figure 12A-7 illustrates the use of equilibrium density centrifugation is first homogenized and subjected to differential centrifugation to obtain a pellet enriched in mitochondria, lysosomes, and peroxisomes. the pellet is then resuspended in a 0.25 M sucrose solution and layered over a gradient of sucrose that spans the densities of the three organelles. during centrifugation, the organelles move into the gradient until each reaches its equilibrium, or buoyant, density-the point in the gradient at which the density of the organelle is exactly equal to the density of the sucrose. at its buoyant density, am organelles has no net force acting on it, so it moves no further. given enough time, all the organelles will reach their characteristic buoyant density positions in the gradient and will reach their character there. when the organelles are at their equilibrium positions, the centrifuge is stopped, the bottom of the tube is punctured, and the fractions are collected. because of their different densities, the three classes of organelles are collected in different fractions. the smooth ER of hepatocytes is also involved in the enzymatic breakdown of stored glycogen, as evidenced by the presence of the glucose -6- phosphatase(포스타파제). glucose -6- phosphatase is one of several prominent membrane bound enzymes that are unique to the ER, and it is often used as a marker to identify and follow the ER during subcellular fractionation. this enzyme catalyzes the removal of phosphate group from glucose -6- phosphate to form free glucose and inorganic phosphate. to understand the importance of this phoshatase, we need to appreciate both the way in which liver cells store glycogen and the reason and mechanism for its subsequent breakdown.