Structure of the Prokaryotic Cell

A generalised prokaryotic cell, though structurally far simpler than a eukaryotic cell, is not a formless blob -- it has a definite, genetically determined shape, maintained largely by its cell wall, and a characteristic internal organisation of its own. Bacterial cells occur in a limited number of basic shapes: spherical cells are called cocci, rod-shaped cells are called bacilli, short comma-shaped cells are called vibrios, and spiral or corkscrew-shaped cells are called spirilla. Bacterial cell size typically falls in the range of roughly 0.2 to a few micrometres in width and up to around 10 micrometres in length, making individual bacteria visible only under the magnification of a light or electron microscope.

Almost every prokaryotic cell -- with the notable exception of mycoplasma, which lack one entirely -- is surrounded by a rigid cell wall lying just outside the plasma membrane. This cell wall is what confers the cell's characteristic shape and, just as importantly, gives it the mechanical strength to resist bursting when water moves into the cell by osmosis, or collapsing when the surrounding medium is comparatively concentrated. Immediately beneath the cell wall lies the plasma membrane, which encloses the region of the cell classically referred to as the protoplasm -- the entire living content of the cell.

Within this protoplasm, the most important structure is the cell's genetic material: a single, long, circular, double-stranded DNA molecule that is not membrane-bound but is instead folded and negatively supercoiled into a compact, irregularly shaped mass held in place with the help of associated proteins. This structure, in direct contact with the surrounding cytoplasm, is called the nucleoid. In addition to this main chromosome, many bacterial cells also carry one or more much smaller, circular pieces of extra-chromosomal DNA called plasmids. Plasmids are not usually essential for the bacterium's everyday survival, but they often carry genes that confer special, situation-specific advantages -- most famously, genes for resistance to particular antibiotics. Because plasmids can be transferred between bacterial cells and can be engineered to carry genes of interest, they are also one of the most important tools used in genetic engineering and recombinant DNA technology.

Scattered through the cytoplasm of many prokaryotic cells are inclusion bodies -- deposits of reserve material that, unlike the organelles of a eukaryotic cell, are not bound by any membrane of their own. These may include granules of glycogen, droplets of lipid, granules of a specialised polymer characteristic of cyanobacteria called cyanophycean granules, and granules of stored polyphosphate. Because they lack a surrounding membrane, these deposits lie directly within the cytoplasm rather than in a separate membrane-bound compartment. …