The Prokaryotic Cell
I. Two types of cells are known to exist; the prokaryotic cell and the eukaryotic cell. Bacteria have prokaryotic cells while all other forms of life have eukaryotic cells. The eukaryotic cell differs from the prokaryotic cell in several ways:
-- most obvious is the difference in size, the eukaryotic cell is huge compared to the prokaryotic cell;
-- though some eukaryotic cells have a cell wall (but not animal cells), the cell wall does not contain peptidoglycan. Plant cell walls are made of cellulose which is a polymer of glucose while fungi have either cellulose or a substance known as chitin;
-- Though many organisms exists which are single eukaryotic cells, the eukaryotic cell can also combine to form multicellular plants and animals;
-- Present in the cytoplasm of the eukaryotic cell are internal structures known as organelles and a structure known as the nucleus. Several different organelles have been characterized and will be discussed shortly. Generally, an organelle is enclosed by a phospholipid bilayer. Organelles can be thought of as compartments where specific biochemical reactions occur. In this lecture you will be shown the organelle.
II. As mentioned earlier, the Kingdom of Monera (or Prokaryotae) consists of those organisms that we usually refer to as bacteria. In our discussions we will center on those organisms referred to as Eubacteria as opposed to Archaebacteria. Eubacteria (or true bacteria) lack both a nucleus and any organelles. They have a cytoplasic membrane (cell membrane), a cell wall that usually contains a material known as peptidoglycan and have genetic material that is made of a of DNA which is associated with the inside of the cell membrane. Eubacteria may also have proteinaceous extensions such as fimbriae or flagella. They are surrounded by a loosely associated material referred to as the glycocalyx. From this point on, unless otherwise noted, I will be referring to eubacteria when I refer to a bacterial or prokaryotic cell.
III. Outside the cell wall of the bacterial cell are several types of projections and coverings. These serve several purposes including: movement, attachment to sites in the environment and protection from phagocytosis from other organisms or from macrophages within our bodies.
A. The bacterial flagellum is utilized in movement and is not present on all species of bacteria. A bacteria that has a flagellum (and is thus capable of movement) is said to as being motile. The long strand that projects from the cell is referred to as the filament, it is made up of many subunits of the protein flagellin. At the base of the flagellum is a complex structure that allows the filament to be rotated this structure is referred to as the basil body. Attaching the filament to the basil body is the hook. The basil body looks surprisingly like a mechanical devise similar to that found in the axle of a car. Rotation of the flagellum propels the bacteria forward. This movement requires the input of energy in the form of ATP.
1. Some bacteria have the capacity to move towards or away from a chemical or light source. This capability is referred to as chemotaxis and phototaxis, respectively. It requires the ability to sense levels of the attracting (or repelling) chemical or light source. This allows bacteria to move towards nutrients and/or away from harmful substances.
2. If a cell has one flagellum it said to be monotrichous, if it has a flagellum at each end it is amphitrichous, if it has several flagella at one end it is lophotrichous and if it has flagella distributed all over the cell it is peritrichous.
B. Pili and fimbriae extend from the plasma membrane through the cell wall into the environment.
1. Fimbriae are shorter than pili and serve to allow bacteria to attach to various surfaces and/or to hold bacteria cells together in clumps. They project like bristles from the cell membrane through the cell wall and out into the environment around a bacterial cell. Fimbriae usually consist of proteins or glycoproteins. Since attachment to a site in the body is usually required before that site can be colonized, the chemical makeup of the fimbriae, to what sites they can attach and the strength of that attachment are going to have implications on where in the body a particular species of bacteria can colonize and thus what diseases that species of bacteria can cause.
2. The pili or sex pilus extends from one cell to its neighboring cell. Attachment allows the cells to be drawn close to one another. This results in the formation of a junction connecting the cytoplasm of one cell with the other. At this point small pieces of DNA (genetic material) known as plasmids can be exchanged between bacteria.
IV. Outside the cell membrane of the bacteria cell are found several structures that surround the cell. These are referred to as the cell envelope. It can be broken down into the cell wall and the glycocalyx.
A. The glycocalyx is excreted from the cell and is made up of polysaccharides (long chains of sugars) and/or proteins. The glycocalyx is referred to as either a capsule or a slime layer. The glycocalyx allows some organisms to attach to surfaces, it allows others to attach to each other and form colonies and others to evade phagocytosis.
1. If the glycocalyx is highly organized (cross-linked and rigid) and firmly attached to the cell wall the structure is referred to as a capsule. Bacteria that are capable of making a capsule are not easily identified by our phagocytic cells. Thus an encapsulated bacteria is more dangerous as a pathogen.
2. If the glycocalyx is both loosely organized and attached to the bacterial cell, it is referred to as a slime layer.
B. The cell wall is outside the plasma membrane. Its main function is to support the cytoplasmic membrane so that the cell is protected from rupturing when it is exposed to osmotic shifts. The majority of bacteria can be classified on the basis of how they react when subjected to a procedure referred to as the Gram stain. When identifying bacteria the first step usually is to establish whether they are gram positive or gram negative.
1. The cell wall of both gram positive and gram negative organisms contains the material peptidoglycan in differing amounts. Peptidoglycan is a polymer of polysaccharides and amino acids. The polysaccharides form long chains that are cross-linked with short strings of amino acids.
a. The sugars found in the polysaccharide portion of peptidoglycan are N-acetyl
muramic acid (NAM) and N-acetyl glucosamine (NAG). The
polysaccharide consists of alternating NAM and NAG molecules.
b. Short chains of amino acids extend from the NAM molecules. They cross-link one
polysaccharide with another.
3. Gram positive organisms have thick peptidoglycan layers which also contain other chemicals known as techoic acid and lipotechoic acid. These organisms are susceptible to the enzyme lysozyme (found in tears and some other secretions) and to penicillin.
a. Because the cell wall of these organism is so rigid, growth requires the breakdown
of regions of the peptidoglycan. This is accomplished by autolysins The
action of these autolysins is regulated by techoic acid and lipotechoic acid.
b. The cell wall of gram positive organisms can be damaged or completely stripped away by
exposure to lysozyme. A gram positive bacteria without its cell wall is referred to as a protoplast.
Gram negative organism can have their peptidoglycan layer similarly destroyed; the
resultant cells are referred to as spheroplasts. Both protoplasts and
spheroplasts are susceptible to osmotic lysis if placed in a
hypotonic solution. L-forms are bacteria that are defective in their ability
to produce a functional cell wall. In most cases this is due to a genetic mutation.
4. The peptidoglycan layer of gram negative organisms is much thinner and less rigid than that found in the cell wall of the gram positive organism. Outside of the peptidoglycan layer of a gram negative organism is a phospholipid bilayer referred to as the outer membrane.
a. The outer membrane is similar to the cell membrane
but has several additional types of lipid molecules such as lipopolysaccharides
(LPS) that are not found in the cell membrane. LPS molecules evoke a strong
reaction from the human immune system. Also referred to as endotoxins, these
molecules are, in many cases, a large contributor to the progression of disease caused by
gram negative bacteria.
b. The outer membrane has large openings made by porin proteins which
greatly increase the permeability of the outer membrane by creating relatively large and
nonspecific openings in this membrane. As compared to the cell membrane, the outer
membrane is less selective about what is allowed to diffuse across it but it does inhibit
the movement of some molecules into the space between the cell wall and the cell membrane.
c. Between the thin peptidoglycan layer and the cell membrane is a space known as the periplasmic
space. This space is usually not found in gram positive organisms. Enzymes are
secreted from the cell and are retained in this space thus allowing an area for high
levels of extracellular metabolism to occur.
V. The cell membrane or plasma membrane consists of a phospholipid bilayer with embedded proteins. The middle of this bilayer is where the fatty acid portion of the phospholipids are found. This region is hydrophobic and thus is a barrier to both large molecules (such as glucose) and charged molecules (sodium, potassium, etc.) entering the cell. It is not a barrier to small uncharged molecules (certain steroid hormones, carbon dioxide, oxygen, water, etc.) and thus these molecules can freely pass the membrane unaided. This barrier allows the internal fluid of the cell (cytoplasm or protoplasm) to have a different chemical make up than that found in the fluids of the environment surrounding the cell.
A. Hydrophilic interactions between the highly charged phosphate and nitrogen group of the phospholipid and the water molecules in the extracellular environment and the cytoplasm stabilize the bilayer shape. The fatty acid chains are hydrophobic and interact with each other in the central region of the bilayer.
B. If membranes consisted of only phospholipids cells would be unable to internalize those substances that cannot freely cross the membrane (some of which are necessary for the survival of the cell.) The proteins embedded in the bilayer allow the cell to selectively import or export molecules across the cell membrane. Most of these proteins extend from the cell's internal (cytoplasmic) surface to the cell's external (extracellular) surface and thus are referred to as transmembrane proteins. These proteins render membranes selectively permeable.
C. Movement of substances into or out of the prokaryotic cell across the cell membrane is accomplished by several routes including simple diffusion, facilitated diffusion and active transport.
1. A molecule existing at a higher concentration on one side of the cell's membrane than the other will naturally move down its gradient or go from the side of highest concentration to the side of lowest concentration. If the membrane does not present a barrier to the movement of this molecule, it will diffuse into or out of the cell so that the concentration in the cytoplasm is equal to the concentration in the extracellular environment. This type of movement is referred to as simple diffusion.
2. If the molecule is charged or large, the membrane will limit its movement "down" the gradient. Specific proteins that span the membrane (transmembrane protein) form channels that allow specific molecules or ions to move across the membrane down their concentration gradient. This type of movement is known as facilitated diffusion.
3. If the a molecule moves "up" its concentration gradient, a specific transmembrane protein and energy must be utilized. This is referred to as active transport. Adenosine triphosphate (ATP) usually supplies the energy to transmembrane protein that transports ions or molecules up their concentration gradient either into or out of the cell.D. Selective permeability is central to the establishment of concentration gradients of particular molecules across the cell membrane. The natural tendency of molecules is to spread out until they are evenly distributed. Through the aforementioned means, cells are capable of concentrating certain molecules, atoms or ions in the cytoplasm and eliminating other molecules, atoms or ions from the cytoplasm. Establishment of such a gradient requires input of energy. Once established, these gradients can serve as a source of energy in some biological processes.
E. The cell membrane of the prokaryotic cell is responsible for several functions that our cell membranes are not. The cell membrane often has folds that form small pockets. These folds are referred to as mesosomes. Their purpose is still a matter of debate, but it is believed that by increasing the area of the cell membrane, mesosomes allow greater numbers of transmembrane proteins and other substances that are found embedded in the membrane to be present. The formation of the pocket may also be significant in the process of oxygen dependent ATP generation.
1. For those bacteria able to utilize oxygen in the same fashion we do, the cell membrane contains the cytochromes and other enzymes needed to electron transport and chemosmotic ATP production. In our cells the cytochromes and ATPases are found embedded in the inner membrane of the mitochondria.
2. The DNA of bacteria associate with the cell membrane. This association is essential for proper allocation of the genetic material during cell division.
VI. As mentioned earlier, the cytoplasm or protoplasm of the prokaryotic cell lacks a true nucleus and any membrane bound organelles. Enzymes, nutrients, ribosomes, RNA and DNA all are suspended in the gel-like protoplasm.
A. Though no membrane bound nucleus is evident in bacterial cells, the DNA is localized in an area referred to as the nucleoid. The DNA is not free but associated with many proteins. Whereas the DNA of eucaryotic cells is arranged in long strands, DNA of bacteria is circular and is referred to as chromatin body or bacterial chromosome,. Along with the main bacteria chromosome, often there are smaller circular pieces of DNA that are referred to as plasmids.
1. The bacterial chromosome can have thousands of genes. The plasmid carries only a handful of genes (sometimes only one), but the genes for antibiotic resistance are often found on plasmids. This allows for rapid spread of these genes among bacteria.
2. Transfer of plasmids from one bacteria to another can occur in several ways. Usually this entails the activity of the pili and a process referred to as conjugation.B. The ribosome of bacteria is very similar to that of the eucaryotic cell both in structure (two subunits) and in function. The bacterial ribosome is slightly smaller than the eucaryotic ribosome. Genetic material is utilized in bacteria in a manner that is nearly identical to how it is utilized in the eucaryotic cell.
C. When stained in certain ways, the protoplasm of a bacterial cell will show areas that are obviously different in their staining properties. These areas are referred to as inclusions or granules.
1. In most cases inclusions are deposits of energy rich materials such as the glucose polymer, glycogen. These deposits are surrounded by a thin membrane.
2. Granules usually are usually deposits of inorganic salts or other compounds. They usually are not enclosed by a membrane.
VII. It should be stressed that bacteria are all prokaryotic and very small (1-10 microns) compared with an average eucaryotic cell (10-100 microns). The shape of a bacterial cell is determined by the shape of its cell wall which, in turn, is determined by the composition (proteins and other materials) of the cell wall.
A. The shape of the cell wall is, in most cases, a genetically determined trait. Environment conditions limiting the availability of certain nutrients needed for proper formation of a cell wall can alter the shape of a bacteria. Pleomorphic bacteria have the genetic capacity to take on different shapes.
B. The arrangement of bacteria refers to the configuration of bacteria that remain associated with one another after binary fission. These shapes are a function of the symmetry of the planes of division.
1. Cocci are usually spherically shaped cells. In some cases they may be oval or slight deformed.
a. Bacterial cells that remain associated only in pairs are referred to as diplococci.
b. Long chains of cocci form if bacterial cells remain associated after multiple divisions and always divide along the same plane. These chains are referred to as streptococci.
c. Bacteria that divide in two planes which are at right angles (perpendicular planes) to each other produce structures known as tetrads. This grouping of cells resembles a four-square court.
d. Bacterial cells capable of division in three perpendicular planes form cubes of cells. This structure is known as sarcinae.
e. If the bacteria divide in planes with random association to the other divisional planes, structures that appear like bunches of grapes will form. This structure is referred to as a staphylococci.
2. Bacillus refers to a rod shaped bacteria. In some cases the rod will be round almost to the point of an oval. Such bacteria are referred to as coccobacillus. Bacilli only divide in one plane across the short axis of the bacteria. It should be noted that when the term Bacillus is in italics it is referring to a particular genus of bacteria (ex. Bacillus cereus)
a. Diplobacilli are those bacillus that associate in pairs only.
b. Streptobacilli are those rods that associate in long chains. They fail to disassociate after fission.
c. Organisms of the genus Corynebacterium retain a small region of attachment after division. Consequently they tend to fold into arrangements known as palisades.
3. Spiral bacteria are bent rods. The amount of spiral differs for different species. Those with little spiral and thus appear as a comma are referred to as vibrios. Those that have a little more spiral and appear as an "S" or like a corkscrew and are fairly rigid with a polar flagellum are known as spirillum. Those that are also extremely spiraled but are more flexible are known as spirochetes. Though they lack a an external flagellum, they are still motile. Movement is accomplished by alteration of the shape of the cell. Neither spirilla nor spirochetes form multicellular arrangements.
VIII. There are several groups of bacteria that have unique structural characteristics that need to be pointed out at this time. In most cases these differences center around the structure of the cells, cell wall or their lifecycle
A. To be able to grow rickettsias must invade eucaryotic cells to survive and thus are referred to as obligate intracellular parasites. They are tiny, fairly typical gram negative bacteria. They lack the capacity to make ATP and must rely upon the ATP generated by the host cell. They alternate hosts between humans and insects. They are responsible for causing diseases such as Rocky mountain spotted fever, Q fever and epidemic typhus.
B. Chlamydia are also very tiny, obligate intracellular parasites but do not rely upon insects for part of there lifecycle. They are similar to the rickettsias in that they lack the capacity to generate ATP. One of the most common venereal diseases is caused by Chlamydia trachomatis.
C. The genus Mycoplasma is made up of tiny bacteria that do not have a cell wall. Differences in their plasma membrane may protect them from osmotic lysis. Though some mycoplasmas live freely, others live within eucaryotic cells where the osmotic conditions are constant.
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