All eucaryotic cells are interspersed with extensive membranous systems that structure the cell's interior into numerous compartments. The membranes are of uniform thickness and consist of a lipid bilayer . The ER forms a continuous system with the nuclear envelope . The latter is composed of two lipid bilayers perforated by the nuclear pores .
Organelles are membrane-bordered functional unities of the cell.
Both mitochondria and plastids have an inner and an outer membrane. Plastids have an additional internal membrane system, the thylakoid system. Mitochondria occur in animal as well as in plant cells, plastids only in plants. Microbodies are surrounded by only one membrane. They harbour specific metabolic pathways and are, dependent on these, classified as peroxysomes or glyoxysomes .