The cells which occur in animals (from Protozoa to Mammalia) and plants (from algae to angiosperms) and contain true nuclei are called eukaryotic cells.
They are very much larger than prokaryotic cells (the cells which lack a distinct nucleus enclosed by a nuclear envelope, the nuclear material being scattered in the cytoplasm).
The eukaryotic cells are essentially two envelope systems and secondary membranes envelope the nucleus and other cellular organellas and to a great extent they pervade the cytoplasm.
These cells may vary in shape, size, and physiology but all the cells have a typical structure with little variations in number and location of cellular organelles.
The classical model of a generalized cell, based on the observations of the light microscope, shows that the cell possesses an outer limiting membrane, called the plasma membrane or cell membrane; a mass of cytoplasm which seemed to possess a vague internal organization or cytoskeleton and which contains a membrane-bounded nucleus and various cellular organelles, such as the mitochondria, Golgi complex, centrioles and various other ill-defined structures of granular and vacuolar nature.
The introduction of electron microscope in 1939-40 made possible detailed examination of cellular organelles and other subcellular entities and it altered the model of the generalized cell.
Cellular organelles such as the mitochondria, which in the light microscope appear as rod-shaped bodies, in the electron microscope are seen to be membranous structure of highly organized nature.
The matrix of the cell, originally called the protoplasm by von Mohl (1846), which in the light microscope appears as a homogeneous, clear, colloidal suspension, in the elelectron microscope is shown to consist of a multiphased complex system, containing numerous membranous elements.