Prokaryotic Cells

Prokaryotic cells are the simpler of the two basic cell types. They have a chemically complex, protective cell envelope, but they lack a well-defined nucleus and lack the membrane-bound cell organelles that eukaryotic cells possess.

Cell envelope

The cell envelope of a prokaryote is a three-layered structure: an outer glycocalyx, a middle cell wall, and an inner plasma membrane. The glycocalyx may be present as a loose sheath (slime layer) or as a tougher capsule. Because bacteria are hard to see clearly without staining, they are usually observed after staining, most commonly by the Gram staining method developed by the Danish bacteriologist Hans Christian Gram. The cell wall is built of peptidoglycan in Gram-positive bacteria and of murein in Gram-negative bacteria, and it gives the cell mechanical strength. The plasma membrane beneath the wall is a phospholipid bilayer. Together, these layers protect the cell and also assist in transport between the cell and its surroundings.

Motility and surface structures

Motile bacteria carry either cilia or flagella, both of which are driven by a rotary movement generated by a basal body that acts like a motor; the other parts of the structure are the filament and the hook. Other surface projections include the tubular pili, which help cells communicate with one another, and fimbriae, which help a cell cling to a surface. The plasma membrane also folds inward at places to form mesosomes, which assist in cell-wall formation, cellular respiration and DNA replication. Some bacteria, especially photosynthetic cyanobacteria, show even longer membrane extensions called chromatophores, which carry the photosynthetic pigments.

Ribosomes and cytoplasm

The cytoplasm of a prokaryote contains dense particles called ribosomes, which carry out protein synthesis; these are described by their sedimentation rate in Svedberg units, and a bacterial ribosome is of the 70S type, made of a larger 50S subunit and a smaller 30S subunit.

Genetic material …