What Are Cells Made Of - Prokaryotic Cell Structure (Cell Envelope)

Welcome back to the final part of our forray into the microscopic world of Prokaryotes. We have looked at how prokaryotes survive with naked DNA and have glanced at the difference between prokaryotic and eukaryotic ribosomes. We also looked at how the differences between prokaryotes and eukaryotes are capitalised upon in the fight against pathogens.

Continuing along this theme we shall have a closer look at the outside of the prokaryotic cell. Here we shall get an overview of the structures that make up the Cell envelope

The capsule is a protective layer possessed by some Bacteria that enhances their pathogenicity. This surface layer is made up of long strings of polysaccharides (long chains of sugar). Depending on how well this layer is stuck to the membrane it is called either a capsule or, if not well adhered, a slime layer. This layer enhances pathogenicity by acting as an invisibility cloak - it hides the cell surface antigens that white blood cells recognise.

So important is this capsule to the virulence of certain bacteria, that those strands without a capsule do not cause disease - they are avirulent. Examples of such bacteria are E. coli and S. pneumoniae

The Prokaryotic Cell Wall is made of a substance called peptidoglycan - a sugar-protein molecule. The precise make up of this varies hugely from species to species, and forms the basis of prokaryotic species identificationThis organelle provides structural support, protection from phagocytosis and dessication and comes in two categories: Gram Positive and Gram Negative.

Gram Positive cells retain the purple gram stain because their cell wall structure is thick and complex enough to trap the stain. Gram Negative cells lose this stain because the wall if much more thin. A diagramatic representation of each type of cell wall is given opposite.

All living things react to their environment, and bacteria are no different. To those in possession of flagella (sing. flagellum), they are employed to move the cell towards or away from different stimuli, such as light, food sources or poisons (such as antibiotics). The motors that propel such structures are marvels of evolution - far more efficient than anything humanity has created.

Contrary to common belief, these locomotive structures can be found all over the surface of a bacterium, not just at the end. This structure is often pointed to by creationists as proof of intelligent design, arguing that half a flagellum is no use to anything (an extension of what good is half an eye) I point such sceptics towards any book by Richard Dawkins, who deals with such matters far better than I.

Pili (sing. Pilus) are smaller, hairlike projections that sprout over the surface of most bacteria. These often act as anchors, securing the bacterium to a rock, intestinal tract, tooth or skin. Without such structures, the cell loses virulence (its' ability to infect) as it cannot hold on to the host structures.

Unlike cancer therapy, the treatment of pathogens is usually well targetted. Antibiotics attack proteins or structures (such as the capsule or pili) that have no eukaryotic counterpart. Due to this, the antibiotic can kill prokaryotes whilst leaving the eukaryotic cells of the animal or human intact.

There are several classes of antibiotics, classified according to how they work:

1. Cephalosporins: first discovered in 1948 - they prevent proper production of a bacterial cell wall.
2. Penicillins: the first class of antibiotic discovered in 1896 then rediscovered by Flemming in 1928. Florey and Chain isolated the active ingredient from the penicillium mould in the 1940s. Prevent proper production of bacterial cell walls
3. Tetracyclins: interfere with bacterial ribosomes, preventing protein synthesis. Due to more pronounced side-effects, this is not often used with common bacterial infections. Discovered in the 1940s
4. Macrolides: another protein synthesis inhibitor. Erythromycin, the first of its class, was discovered in the 1950s
5. Glycopeptides: prevent polymerisation of the cell wall
6. Quinolones: interefere with important enzymes involved with DNA replication in prokaryotes. Due to this they have very few side-effects
7. Aminoglycosides: Streptomycin, which was also developed in the 1940s, was the first to be discovered in this class. They bind to the smaller bacterial ribosome subunit, thus preventing protein synthesis. These do not work well against anaerobic bacteria.