FIVE KINGDOMS OF LIFE:
Monera, Protista, Fungi, Plantae and Animalia Figure 24.9). Of these the only the Monerans are prokaryote, the others are eukaryotes (Chapter 26).
PROKARYOTES:
Prokaryotes are characterized by their small size (microscopic), their 'singleness' and shape (single-celled organisms, rods, cocci and helical [Figure 25.1]), the mode of their replication ( binary fission), the relative simplicity and smallness of their genomes (0.1% DNA of eukaryotes) , the lack of a nucleus and other internal membrane systems (they have a nucleoid; genophore) and the composition of their cell walls and external coats (peptidoglycans, lipopolysaccharides, capsule). There are also differences in the translational machinery, though the gene expression process (transcription and translation) takes place in a fashion similar to eukaryotes . Recent evidence suggests differences in evolutionary relationships among prokaryotes. (Figure 25.10). There are two main branches to the prokaryotic tree- Archaea (archaebacteria) and Bacteria (eubacteria). This has led to new ways of organizing life, as in the case of superkingdom designations known as Domains (Archaea, Bacteria, Eukarya [Figure 25.10]).
CELL SURFACE AND MOVEMENT:
The success of prokaryotes is based on diverse adaptations of form and function. Cell Surface- Protection from hypotonic media by cell wall. Cell wall is complex but involves polymers of carbohydrate and protein ( peptidoglycans, PG) and in some types lipopolysaccharides (See Methods, the Gram Stain). For example a Gram positive bacteria has abundant PG, a Gram negative has little PG. Some bacteria have a capsule- slimy, viscous material- often highly toxic and protective. Some bacteria have pili or stalks, surface appendages that function in adhesion and genetic exchange during bacterial conjugation. Many species of bacteria are motile, that is capable of directed movement. Three mechanisms involved 1. flagella 2. sub-cell wall filaments (spirochetes) and 3. gliding motion. Bacteria are taxic, they respond to physical and/or chemical signals or gradients. For example phototaxis and chemotaxis.
GENETICS:
The prokaryotic genome contains about 1/1000 the amount of DNA as eukaryotes. The genophore (equivalent of a eukaryotic chromosome) is circular and in many cases codes for around 1000 genes. Bacteria also contain plasmids, which are tiny circular packets of DNA that consist of only a few genes, though many of these plasmids code for products that provide antibiotic resistance to the host bacteria. There are three mechanisms of genetic recombination in bacteria, 1. Transformation (genes taken up from the environment) 2. Conjugation (genes taken up from other bacteria) and Transduction ( genes taken up from viruses. Replication takes place by binary fission. Bacillus sp. and others can also form highly resistant spores-even boiling will not kill them- they need to be autoclaved. Ribosomal differences are great enough that certain antibiotics( tetracycline and chloramphenicol) block bacterial ribosome's and not their eukaryotic counterparts.
ENERGY AND CARBON SOURCES:
All major types of nutrition and metabolism evolved in prokaryotes. Photoautotrophs (light,CO2-example Cyanobacteria)/ Chemoautotrophs (inorganic chemicals,CO2-Sulfolobus)/ Photoheterotrophs (light, organic compounds-Lactobacillus, E. coli) /chemoheterotrophs(organic compounds for both energy and carbon source-found widely in monera,protista, fungi and animals). Among chemoheterotrophs also are found saprobes (decomposers) and parasites (obtain nutrients directly from a host).
NITROGEN:
There are prokaryotes that can metabolize nitrogen. For example, ammonia to nitrite (Nitrosomonas), nitrite and nitrate to nitrogen gas (Pseudomonas) and nitrogen gas to ammonia(cyanobacteria). This last process is called nitrogen fixation and is extremely important.
OXYGEN:
Some prokaryotes require oxygen (obligate aerobes), others can use it but don't need it (facultative anaerobes), and still others cannot tolerate oxygen (obligate anaerobes). The origins of glycolysis are also found in the prokaryotes-this involved the use of ATP to breakdown organic compounds and to generate ATP by substrate phosphorylation (think fermentation). The chemiosmotic mechanism of ATP is common to all organisms (Figure 25.7). This couples the transport of protons out of the cell (pH regulation) with the production of a gradient of protons that can be used to reverse the ATPase to form ATP.
PHOTOSYNTHESIS:
The evolution and use of light-absorbing pigments. Transduction of energy forms-light to chemical. Bacteriorhodopsin in halophiles (related chemically to our own eye pigments) absorbs energy of light to transport protons out of the cell( see chemiosmotic mechanism). Other ways to absorb light energy include bacteriochlorophyll (splits hydrogen sulfide instead of water, as does plant chlorophyll a).
Cyanobacteria were the first to use light energy to split water and reduce carbon dioxide to form organic compounds (2.5-3.4 billion years ago).
DOMAINS of organisms (see Table 25.2, 25.3): Archaebacteria and Eubacteria. Differences in DNA sequences (Table 25.2) and other characteristics.
Disease-Bacteria as pathogens-overcoming host defenses. A large number of human diseases are bacterial in origin. How do they affect host? Exotoxins-secreted products, as in the case of C. botulinum(botulism), C. tetani (tetanus) , V. cholerae (cholera) and even E.coli (diarrhea, kidney damage). Endotoxins-components of outer cell wall of Gram- bacteria such as Salmonella sp , not secreted but potent (fever, headaches).
Antibiotics are for the most part isolated from bacteria ( for example Streptomyces) and fungi (for example Penicillium) and in the wild these compounds protect the host from encroaching microbes. Putting microbes to work for us is a big business, from sewage treatment to recombinant gene product formation.
Prokaryotes, Chapter 25 (Knapp)
Monera, Protista, Fungi, Plantae, Animalia Monara = Prokaryotes small size binary fission Cell Wall peptidoglycans binary fission conjugation movement: flagella sub-cell wall filaments gliding motion |
phototaxis chemotaxis (smell) genophore plasmids Transformation Conjugation Transduction antibiotic resistance Photoautotrophs Chemoautotrophs Photheterotrophs Chemoheterotrophs |
saprobes parasites obligate aerobes facultative anaerobes obligate anaerobes ATP Photosynthesis light absorbing pigments Bacteriorhodopsin |