Concepts: Prokaryotic cells

2.1 The prokaryotic plan

Bacteria, blue-green algae, mycoplasma and PPLO are prokaryotes. They are generally smaller than eukaryotic cells and multiply faster. A bacterial cell may be rod-like (bacillus), spherical (coccus), comma-shaped (vibrio) or spiral (spirillum). In spite of that variety, the plan inside is the same in every prokaryote.

A cell wall surrounds the cell membrane, except in mycoplasma, where the wall is missing. Cytoplasm is the semi-fluid filling of the cell. No well-defined nucleus exists, and no nuclear membrane wraps the genetic material, which lies bare as one circular DNA molecule, the genomic DNA. Many bacteria also carry small circular DNA outside the genomic DNA, and these are called plasmids. Plasmids give special characters to the bacterium, and antibiotic resistance is the standard example. Plasmids are also the tools of genetic engineering, which is why the chapter mentions them. No membrane-bound organelle of the eukaryotic kind appears in a prokaryote; the ribosome is the only organelle found there. Inclusion bodies store reserve material in the cytoplasm and are not enclosed by any membrane, and the mesosome is a special infolding of the cell membrane.

2.1.1 The cell envelope

The envelope of a bacterium is a three-layered structure. The outermost layer is the glycocalyx, below it is the cell wall, and below the wall is the plasma membrane. Each layer has its own job, but the three act as one protective unit. Glycocalyx composition and thickness differ between bacteria: sometimes it forms a loose slime sheath, sometimes a thick, tough capsule. Gram staining divides bacteria into Gram-positive and Gram-negative groups on the basis of how this envelope takes up the stain.

The cell wall gives the bacterium its shape and gives it strong structural support, so that the cell does not burst or collapse. The plasma membrane is selectively permeable and is the surface through which the cell deals with the outside world; structurally it resembles the eukaryotic membrane. A mesosome is formed when the plasma membrane extends into the cell as vesicles, tubules or lamellae. It increases the surface area of the membrane, and it takes part in cell wall formation, DNA replication and the distribution of DNA to daughter cells. In cyanobacteria there are additional membranous extensions called chromatophores, which carry pigments.

2.1.2 Movement and surface structures

A bacterium may be motile or non-motile. A motile bacterium has thin filamentous extensions of the cell wall called flagella, and their number and arrangement vary. A bacterial flagellum has three parts: the filament, the hook and the basal body. Filament is the longest piece and projects outwards from the cell surface; the hook links it to the basal body, and the basal body anchors everything in the envelope.

Two other surface structures, pili and fimbriae, play no part in motility. Pili are long tubes of a special protein; fimbriae are short bristle-like fibres growing out of the cell. In some bacteria fimbriae help attach the cell to rocks in streams and to host tissues.

2.1.3 Ribosomes and inclusion bodies

Ribosomes of a prokaryote sit on the plasma membrane. Each measures roughly 15 by 20 nanometres and consists of a 50S and a 30S subunit; the pair forms the 70S ribosome. Protein synthesis happens on ribosomes. A single mRNA may carry a row of ribosomes, the polyribosome or polysome, and that row translates the message into protein. Reserve material is stored in the cytoplasm as inclusion bodies, which are not bound by any membrane. Phosphate granules, cyanophycean granules and glycogen granules are examples, and gas vacuoles appear in blue-green, purple and green photosynthetic bacteria.

2.1.4 Why the prokaryotic plan works

The prokaryotic cell succeeds without a nucleus or membrane-bound organelles because it keeps its operations small and fast. Its circular DNA sits directly in the cytoplasm, so a copy can begin while the previous round of division is still finishing, and the cell can divide in a short time. Its ribosomes work while attached to the mRNA, so protein synthesis and growth run at the same time. The envelope, the mesosome and the inclusion bodies pack several functions into a small space, and plasmids let a bacterium pick up useful genes such as antibiotic resistance without changing its main chromosome. That combination of a small genome, a fast division rate and a flexible set of extra genes explains why bacteria live almost everywhere. Plasmids are not essential for ordinary growth, but they can give a bacterium an advantage when the environment changes, which is why they persist in natural populations.