Bacteria, since they are prokaryotes, lack a nuclear membrane and membrane-bound organelles. Biochemical processes that normally occur in a choloroplast or mitochondrion of eukaryotes will take place in the cytoplasm of prokaryotes. Bacterial DNA is circular and arrayed in a region of the cell known as the nucleoid, shown in Figure 3. Scattered within bacterial cytoplasm are numerous small loops of DNA known as plasmids. Bacterial genes are organized in by gene systems known as operons. The cytoplasm also contains numerous ribosomes, the structures where proteins are assembled.

As a group, bacteria are nutritionally quite diverse. Some bacteria are photosynthetic autotrophs, while others are heterotrophs. Bacteria play important ecological roles as decomposers, as well as important elements of phytoplantonic organisms at the base of many food chains.

Plasmids are small DNA fragments known from almost all bacterial cells. These plasmids may carry between two and thirty genes. Some plasmids seem to have the ability to move in and out of the bacterial chromosome. As such they are important tools to the biotechnology arsenal.

Bacteria have flagella, although the bacterial flagellum has a different microtubule structure than the flagella of eukaryotes. Cell walls of bacteria contain the peptidoglycan instead of the cellulose found in cell walls of plants and some algae. Ribosomes are the structures in cells where proteins are assembled. Bacterial ribosomes have different sized ribosomal subunits than do eukaryotes.

Bacteria typically have one of three shapes: rods (bacilli), spheres (cocci) or spiral (spirilla). Unicellular, they often stick together forming clumps or filaments.