Key Concepts of Section 4.4

• Microscopy has revealed a common set of organelles that are present in eukaryotic cells.

• Disruption of cells by vigorous homogenization, sonication, or other techniques releases their organelles. Swelling of cells in a hypotonic solution weakens the plasma membrane, making it easier to rupture.

• Sequential differential centrifugation of a cell homogenate yields fractions of partly purified organelles that differ in mass and density.

• Equilibrium density-gradient centrifugation, which separates cellular components according to their densities, can further purify cell fractions obtained by differential centrifugation.

• Immunological techniques using antibodies against organelle-specific membrane proteins are particularly useful in purifying organelles and vesicles of similar sizes and densities.

• Proteomic analysis can identify all the protein components in a preparation of a purified organelle.