ABSTRACT
Unlike a bacterium, which generally consists of a single intracellular compartment surrounded by a plasma membrane, a eukaryotic cell is elaborately subdivided into functionally distinct, membrane-enclosed compartments. Many vital biochemical processes take place in membranes or on their surfaces. About half the total area of membrane in a eukaryotic cell encloses the labyrinthine spaces of the endoplasmic reticulum (ER). The rough ER has many ribosomes bound to its cytosolic surface. The abundance and shape of membrane-enclosed organelles are regulated to meet the needs of the cell. This is particularly apparent in cells that are highly specialized and therefore disproportionately rely on specific organelles. The evolution of internal membranes evidently went hand-in-hand with the specialization of membrane function. Mitochondria and plastids differ from the other membrane-enclosed organelles because they contain their own genomes. The synthesis of all proteins begins on ribosomes in the cytosol, except for the few that are synthesized on the ribosomes of mitochondria and plastids.
