The Prokaryotic Cell

There were several other contributors to the germ theory of disease. Louis Pasteur studied fermentation and developed the swan-neck flask experiment, which proved that some organisms causing bacterial contamination were airborne. Joseph Lister studied postoperative infections and insisted on strict handwashing during surgery. He also developed the concept of using antiseptics for surgical procedures.

Robert Koch developed Koch’s postulates, which attributed certain diseases to a specific pathogen. Several infectious diseases, such as cholera, anthrax, and tuberculosis. This was the first incidence of the idea of “one microorganism, one disease”. This completely did away with the miasma theory and supported the germ theory of disease.

THE PROKARYOTIC CELL

There are two different types of cells, which are very different from one another. All cells will have a cell membrane, regardless of the type. All will have cytoplasm, which is the watery component of the cell; all will have some type of nucleic acid or genetic material and all will have ribosomes. These components will be there whether the cell is prokaryotic or eukaryotic. Beyond this, however, prokaryotic cells and eukaryotic cells are very different from one another.

In general, eukaryotic cells are larger than prokaryotic cells. Prokaryotic cells, compared to eukaryotic cells, are relatively simple and they do not have membrane-bound organelles. There are some inclusions that help to compartmentalize the cells. Figure 11 depicts what a typical prokaryotic cell looks like:

Figure 11.

We have already talked about bacterial shapes but not about their arrangement. Remember, there are cocci that are round, bacilli that are rod-shaped, Vibrio that are curved, and spirochetes and spirillum that are spiral. Cocci can be single or can occur in pairs, called a Diplococcus. Tetrads involve cells in a square shape and Streptococcus involves a chain of cocci. Staphylococcus will occur in a cluster and Streptobacillus, which is a chain of bacilli.

Most of the prokaryotes will have a cell wall, which helps to make up the structure of the cell. The advantage of the cell wall is that it protects the cell from any change in osmotic pressure. This becomes important, too, in the structure of plant cells. Osmotic pressure involves the concentration of solutes in a given body of water. If there is a high concentration of solutes or dissolved substances, water will preferentially enter that space in order to dilute it out more. The reverse is true in very dilute solutions separated by a membrane with another space that is more concentrated. Water will leave the dilute solution and enter the concentrated space.

The terms isotonic, hypertonic, and hypotonic refer to the relative concentrations of solutes in solution. A cell inside a hypotonic medium is in a dilute medium and the cell may burst. A cell inside a hypertonic medium is in a concentrated medium and the cell will shrink. A cell in an isotonic medium will neither burst or shrink. The cell wall helps

plant cells and bacterial or prokaryotic cells so they are not destroyed by changes in the tonicity of the surrounding environment.

Prokaryotic cells have nucleoids rather than nuclei. There is usually one circular chromosome in the prokaryotic cell and it is circular and haploid, meaning it does not have a pair. There is no nucleus but there are nucleoid-associated proteins or NAPs that help to package and organize the chromosome. NAPs are like the histone proteins in eukaryotes that organize the genetic material.

Plasmids are common in prokaryotic cells. These are considered extrachromosomal DNA arranged in small circles. Some cells have several hundreds of plasmids in one cell. Plasmids are not completely unique to bacteria and can be seen in both archaea and eukaryotic species. Many will be crucial to cell survival because they confer some type of advantage, such as improved antibiotic resistance.

As mentioned, all cells have ribosomes to help make proteins. The ribosomes of each domain of life are unique to the domain. Ribosomes are largely made from ribosomal RNA and, in prokaryote, they are found in the cytoplasm. They are smaller than the ribosomes found in eukaryotic cells. There are two subunits, called the small subunit and the large subunit.

Prokaryotic cells have inclusions, the purpose of which is to store excess nutrients. Because they are condensed in an inclusion, the nutrients in these structures do not contribute to the osmotic pressure of the cell. Glycogen and starches, which are clusters of glucose molecules, are often found in inclusions. Volutin granules also can be found in inclusions and contain in organic phosphate in some bacteria. A few bacterial species store elemental sulfur molecules for use in metabolic processes. Gas vacuoles are inclusions that assist in buoyancy. One species has iron oxide in the inclusions.

Some bacterial species can form endospores, which will protect the bacterial DNA during periods of dormancy when there are no nutrients available or a satisfactory environment. Bacterial cells can survive for a long time as just an endospore. Endospores can be resistant to radiation and temperature changes and are dehydrated. They do not exhibit metabolic activity and require specialized endospore staining to be

seen microscopically. Spores form when the cell divides and buds off a spore that has been surrounded by a thick cortex.

When conditions improve, the endospore germinates and reenters a vegetative state. Metabolic activity begins and the cell becomes vegetative again. Only some bacteria will form endospores. Most commonly, Bacillus and Clostridium species will form spores, leading to diseases like tetanus, anthrax, pseudomembranous colitis, botulism, and gas gangrene. The spores are hard to kill but can be killed with extreme sterilization.

All cells are characterized by the presence of a plasma membrane. These membranes are selectively permeable, meaning they allow some substances but not others to pass through them. According to the fluid mosaic model, the cell membrane components are not rigid but “swim” in relation to one anther like rafts on a lake. Most of the membrane is a phospholipid bilayer, which is water-loving on the outside but water-hating on the inside. Figure 12 shows a typical plasma membrane:

Figure 12.

Archaea of unique cell membranes. The phospholipids are connected with ether bonds, rather than the ester bonds found in eukaryotes and bacterial cells. There are branched chains in archaea but straight chains in the other cell types. Some archaea have a lipid bilayer, while others have a lipid monolayer.

Proteins are crucial to the function of the cell membrane. They are important in cell-cell communications, the ability to sense the environment, and help to confer pathogenic virulence. Sugars and proteins together are called glycoproteins that help identify the cell. The different cell types will have very different glycoproteins.

You should know a little bit about membrane transport. Remember, the cell membranes to not just allow every molecule to get through. Osmosis involves just the transport of water across the membrane. Simple diffusion happens with small lipid-soluble or gaseous molecules and goes from an area of high concentration to an area of low concentration. Facilitated diffusion makes use of a carrier molecule to get a molecule through the membrane but no energy is required. Active transport involves the input of cellular energy in order to move things from an area of low concentration to an area of high concentration. ATP or adenosine triphosphate is the main energy currency of the cell.

While there are no true membrane structures in prokaryotes, the plasma membrane will infold in order to enclose the necessary photosynthetic pigments so that the cell can participate in photosynthesis. These can be called thylakoids in cyanobacteria but in other photosynthetic bacteria, these are referred to as lamellae, chromatophores, or chlorosomes.

Bacteria, archaea, fungi, and plants have cell walls but animal cells do not. The cell wall contains peptidoglycan in bacteria, which helps to define them. The cell walls of gramnegative cell walls are different from those in gram positive cell walls. Many antibiotics are directed at peptidoglycan because they are specific to the bacterial species. Peptidoglycan can also be recognized by the human immune system in order to define them as being pathogenic.

Gram positive organisms have thick layers of teichoic acid as part of the cell wall, while this is a thin layer in gram-negative organisms. Gram-negative cell walls have an outer

membrane above the teichoic acid layer. This membrane contains lipoproteins, lipopolysaccharides, and porins. Acid-fast bacteria contain waxy mycolic acids as part of the cell wall. The outer cell membrane of gram-negative bacteria is part of the toxic nature of these bacterial species.

Some bacteria have a glycocalyx or sugar-coating. There are two types. One is a capsule, which is more organized and made from polysaccharides or sugars and protein. The other is a slime layer, which is less organized but is loosely attached to the underlying cell wall. Washing can remove the slime layer. The glycocalyx helps in cell adherence to surfaces and help to make biofilms. Biofilms protect the cells by forming a protective colony that resists disinfection. Capsules can prevent the uptake of the bacteria by the immune cells.

Bacteria are noted for their filamentous appendages that contribute to several functions in the cell. The three appendages are fimbriae, pili, and flagella. Fimbriae and pili are so similar that they are often terms that are not distinguished from one another. There are hundreds of fimbriae that are relatively short. The aid in the attachment of cells to other cells and to surfaces. This is importance in the virulence process.

Pili, on the other hand, are not as numerous and are longer than fimbriae. The F pilus or sex pilus is crucial in the transfer of cellular DNA between two bacterial cells. It does not mean that bacterial cells engage in reproduction or sex. It is just the exchange of genetic information.

Flagella are used in cell movement. They move like propellers that allow the motility of the cell. There is a basal body at the base of the flagellum that motorizes the whip-like flagella. Flagella can be arranged in four different ways. Monotrichous arrangement is a singular flagellum. Amphitrichous flagella have one flagellum at one or both ends of the cell. Lophotrichous flagella involves a tuft of flagella at each end. Peritrichous arrangement has flagella surrounding them.