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Cell Biology
The World of Cells
Prokaryotic Cells
Prokaryotic Cells

Prokaryotic cells are the simplest systems that exhibit all of the signs of life. They are the smallest types of cell, averaging 2-5 Ám in length, which makes them just visible under the light microscope.

Despite their small size, inside each cell there is the complete chemical and biochemical machinery necessary for growth, reproduction and the acquisition and utilization of energy.

Prokaryotes have a large array of abilities. Some of them live in the absence of oxygen, some live in extreme conditions of heat or cold, others at the bottom of oceans where the only source of energy is hot hydrogen sulfide bubbling up from the core of the earth.

Common Features

Energy comes in many forms, and various types of prokaryotic cells are adept at using almost all of them. In large measure the actual structure of one of these cells reflects the way in which it acquires energy, but, despite their diversity, all prokaryotic cells have the following features in common.
A Cell Wall.

Prokaryotic cells walls give structural integrity and shape to the cell and serve to anchor the whip-like flagellae (see below).

A Plasma Membrane.

Just inside the cell wall, the plasma membrane is a selective barrier which regulates the passage of materials to from the cell. It is through this membrane that a cell must exchange food molecules, gases and other vital ingredients. Composed of phospholipid and protein membranes form thin, flexible, self-sealing, highly selective barriers between the inside of the cell and the outside world.

Lack of Compartmentalization.

Within the plasma membrane is a viscous solution of proteins and other soluble materials. This is the cytoplasm, a complex semifluid material that holds all the molecules, large and small, necessary for carrying out the major metabolic reactions. The cytoplasm is not subdivided by membranes into organelles, a lack of compartmentalization that is most evident when we look at the organization of the genetic material. This is an important distinguishing feature of these types of cells.

Structure of the Genetic Material.

Genetic information in prokaryotic cells is carried on a single circular piece of DNA which is attached to the cell membrane and in direct contact with the cytoplasm. There is no enclosing membrane, so there is no true nucleus, but simply a concentration of DNA known as a nucleoid. There are no special proteins associated with this DNA molecule.

Plasmids. Some prokaryotes also carry smaller circles of DNA called plasmids. The genetic information on the plasmids is transferrable between cells, allowing prokaryotes to share such abilities as antibiotic resistance. Humans have discovered that prokaryotic plasmids can be genetically engineered. Today, they are isolated, changed to carry other interesting information and then reintroduced into new cells. In this way unique and usefull little bacterial factories can be designed, created and put to work.

Structure of Flagella.

Some prokaryotes propel themselves by means of whip-like filaments called flagella. These are strands of protein that pass though the outer surface of the cell body either either singly or in tufts. Energy provided by the plasma membrane rotates the flagellum by means of a unique rotating 'joint' and this in turn moves the bacterium through its liquid world. Prokaryotic flagella are very different from similar looking structures used by eukaryotic cells.

Other Components.

Prokaryotic cells also contain ribosomes, small complexes of RNA and protein, on which new proteins are assembled. Photosynthetic prokaryotes also contain pigments used in the capture of light energy; marine bacteria contain gas bubbles which help them keep afloat; still others have tiny deposites of iron in their cytoplasm which act like magnets and help orientate them with the earth's magnetic field. There seems to be no limit to the diveristy of these ancient organisms.

Binary Fission.

Reproduction in prokaryotic cells is by binary fission; a process of growth, enlargment and division. The DNA molecule of the cell is accurately duplicated and the two copies separated form each other by movement of the cell membrane to which they are attached. The cell then divides into two smaller but identical cells and each begins its own independent existence.

Family Groups.

Most prokaryotes are eubacteria ("true bacteria"), but one branch of this family, the cyanobacteria, has evolved the ability to carry out photosynthesis (trapping light energy), which they do in a very similar way to algae and other green plants. Although physically similar in appearance, the archaebacteria ("ancient bacteria") are very different to the eubacteria. Among other things the archaebacteria and eubacteria differ in membrane chemistry and the structure of the cell wall. Archebacteria still inhabit the extremes of the earth's environments, tolerating elevated temperatures and high concentrations of acid or salt.

© 2001, Professor John Blamire