11.2Characteristic Structures in Prokaryotes
Prokaryotes (eubacteria and archaea) lack any structure that could be called a nucleus enveloped by a biological membrane and have circular DNA within an irregularly shaped region called nucleoid. In bacterial cells, transcription, translation, and other metabolic processes are not separated but occur in the same place (i.e., the cytoplasm). The outer side of the cell membrane is often surrounded by a cell wall (see Fig. 11-1, Fig. 11-7).
Gram staining*1 is a well-known method for classifying eubacteria.
This method is used to classify bacteria as gram positive and gram negative according to the different structures of their cell wall. The outer surface of the cell membrane in gram-positive bacteria is covered by a thick cell wall composed of peptidoglycans*2 and teichoic acid (Fig. 11-7B). On the other hand, gram-negative bacteria have a thin peptidoglycan layer on the exterior of the cell wall, and this layer is covered by a separate membrane (outer membrane) primarily composed of proteins, lipoproteins, and lipopolysaccharides. The hydrophilic space formed between the cell membrane and outer membrane is called the periplasm.
In addition to these structural differences in cells, large differences exist between prokaryotes and eukaryotes. For example, prokaryotes proliferate by binary fission without undergoing mitosis, and the structure of RNA polymerase and ribosomes in prokaryotes is largely different from that in eukaryotes (see Selection 1 of Chapter 7, Table 7-1). Differences can also be observed in the structures of the lipids in their cell membrane. For example, thermophilic archaea have a type of lipid that is formed when hydrocarbon chains form ether bonds instead of ester bonds with glycerol. This allows archaea to inhabit even harsh, high temperature environments while maintaining the structures and functions of the cell membrane. The highly heat-resistant ether bonds are believed to be critical for this characteristic (see Column Selection 3 of Chapter 6).
Some prokaryotes have a flagellum, which is used as an apparatus for movement. This flagellum has a completely different structure from that of the flagellum found in eukaryotes. Prokaryotic flagella are composed of a protein called flagellin and have a tube-shaped structure approximately 0.02 μm in diameter and tens of micrometers long. The flagellum is directly connected to molecular motors embedded in the cell membrane. This motor also has a special driving mechanism, and it rotates at a high speed using energy from an intra- and extracellular H+ concentration gradient (Column Chapter 17).
In addition to the flagellum, prokaryotes possess fibrous structures called pili. Pili are tubular structures composed of proteins and extend far from the cell membrane. These structures are often observed in gram-negative bacteria, but few gram-positive bacteria also possess them. These pili are used by bacteria when attaching to proteins or saccharide chains on the surface of a targeted bacterial or eukaryotic cell.
*1 A simple method for classifying bacteria. It was named after Hans Gram, who developed this method. This staining method is used to broadly classify bacteria into two types: gram-positive bacteria, which are stained violet, and gram-negative bacteria, which are stained pink/red.
*2 Glycan (saccharide chains are formed when two types of saccharides, N-acetylglucosamine and N-acetylmuramic acid, repeatedly overlap) structures cross-linked by short peptides (generally composed of four amino acids).
Fig. 11-7 Cell wall in plant and prokaryotic cells
(A) Cell wall of a plant cell. The right is a schematic showing the components of the cell wall. (B) Cell wall of a prokaryotic cell (Staphylococcus aureus, a type of gram-positive bacterium)