Cell

The basic structure of all cells, whether prokaryote and eukaryote, is the same. All cells have a plasma membrane through which substances pass into and out of the cell. With the exception of a few minor differences, plasma membranes are the same in prokaryotes and eukaryotes. The interior of both kinds of cells is called the cytoplasm. Within the cytoplasm of eukaryotes are embedded the cellular organelles; the cytoplasm of prokaryotes contains no organelles. Finally, both types of cells contain small structures called ribosomes that function in protein synthesis. Composed of two protein subunits, ribosomes are not bounded by membranes; therefore, they are not considered organelles. In eukaryotes, ribosomes are either bound to an organelle, the endoplasmic reticulum, or exist as "free" ribosomes in the cytoplasm. Prokaryotes contain only free ribosomes.