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Chap 27 - Prokaryotes.docx

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BIOL 1010
Brent Sellinger

Campbell and Reece Chapter 27 Page ▯ Bacteria and Archaea I. Introduction • It is believed by many biologists that prokaryotes have lived on earth for approximately 3.5 billion years (almost 1.5 billion without eukaryotes) • In terms of impact and numbers, prokaryotes dominate the biosphere • Whitman et al., 1998 (Proc. Natl.Acad. Sci. 95:6578) estimated the number of prokaryotes on Earth to be 4-6 x 10 cells containing350-550 Pg of C(1pg=10^15g) 85- 130 Pg of N 9 - 14 Pg of P • Prokaryotes contain the equivalent of 60 - 100 % of the carbon found in all of the plants on earth and 10 fold more N and P than plants Table 1: The number of prokaryotic cells in different environments on earth (Whitman et al, 1998) Environment Number of Prokaryotic Cells (x 10 )30 Open ocean 0.12 Soil 0.26 Oceanic subsurface (marine sediments below 10 cm) 3.5 Terrestrial subsurface (habitats below 8 m) 0.25 - 2.5 Human lower GIT 1.0x10^14 30 • Prokaryotic cellular production on earth = 1.7 x 10 cells/y • Prokaryotes perform a number of functions including decomposers, pathogens, symbionts • 8,000 species have been described - estimated 400,000 to 4,000,000 species Recall how prokaryotes differ from eukaryotes • Prokaryotes lack membrane bound nucleus and membrane bound organelles • most prokaryotes have cell walls but the structure of the prokaryotic cell wall differs substantially from that found in plants and fungi. What is the structure of the plant and fungal cell wall? • The prokaryotic flagellum differs substantially from the eukaryotic flagellum • The prokaryotic genome is simpler than the eukaryotic genome (Recall: genome = the complete complement of an organisms genetic material). The majority of the prokaryotic genome is typically located in a single circular dsDNA molecule. And the eukaryotic genome is Campbell and Reece ChaPage ▯7 II. Taxonomy/systematics Recall • The 5 kingdom scheme developed by Whittaker (1969) was replaced by the 6 kingdom scheme (Archaebacteria and eubacteria are believed to have diverged early in history). Recall Carl Woese’s work in the late 1970s and early 1980s • The currently accepted classification scheme is the 3 domain system (i.e., Bacteria,Archaea, Eukarya) • Molecular systematics is currently being used to advance our understanding of the origin of life by studying existing forms of life. What is the basis of molecular systematics? • Polyphyletic: group of organisms that evolve from multiple ancestors III. Morphology of Prokaryotic cells • Prokaryotic cells are generally single celled organisms • They may be organized into multicellular groups or complex colonies • There is limited multicellular organization in prokaryotic cells o specialized cells o filaments - Anabaena A. Cell Shape - important step in identification 1. coccus (cocci) or sphere shaped 2. bacillus (bacilli) or rod shaped 3. spirillum (spirilli) or spiral shaped B. Cell Size • diameter generally in the range of 1 to 5 µm • extremes Nanobacterium - 0.1 µm in diameter mycoplasmas – 0.2 µm in diameter Epulopiscium (60 µm in diameter, 0.5 mm in length) Thiomargarita (0.75 mm in diameter) C. Cell surface • All prokaryotes have a plasma membrane similar in basic structure and function to a eukaryotic cell membrane • Prokaryotic cell membranes differ from eukaryotic membranes in a number of ways o Prokaryotic cell membranes generally have higher protein content. Why? o Prokaryotic membranes lack cholesterol but may contain related compounds (hopanoids) Campbell and Reece Chapter 27 Page ▯ • Many prokaryotes have a cell wall external to their plasma membrane. Mycoplasmas lack a cell wall 1. Bacterial Cell wall more protein in prokaryotic membrane then in eukaryotic membrane, important for function Cell wall functions - maintenance of shape - protection - homeostasis • Bacterial cell walls contain a unique polysaccharide called peptidoglycan that is unlike the cellulose and hemicellulose cell walls of plants and chitin cell walls of fungi Peptidoglycan • Composed of linear polysaccharide strands consisting of alternating modified sugars N-acetyl glucosamine (NAG) and N-acetyl muramic acid (NAM) joined by β 1-4 glycosidic linkages; NAM is found only in bacteria • Polysaccharide strands are interconnected by peptide bridges between random NAM residues • peptide bridge composition varies from species to species • penicillin and cephalosporin interfere with the formation of the peptide bridges and therefore a functional cell wall Gram negative vs Gram positive cells walls • Christian Gram developed the Gram stain in the late 1800’s. It is a differential staining technique using Crystal Violet and Iodine • This technique is a cornerstone for identifying specific bacteria i) AGram positive cell has a thick layer of peptidoglycan outside the plasma membrane ii)AGram negative cell is surrounded by a cell envelope consisting of: • Asecond outer membrane that is very different from the plasma membrane, more resistant to certain antibiotics and may protect some pathogens from host defenses • Aperiplasmic space found between the plasma membrane and outer membrane • Athin cell wall consisting of a lesser amount of peptidoglycan and found within the periplasmic space Campbell and Reece Chapter 27 Page ▯ In the Gram stain technique the crystal violet – iodine (CV-I) complex is trapped by the thicker peptidoglycan layer in Gram positive cells. The thick cell wall of the Gram positive cell is more able to resist the decolorizing agent’s effect on porosity • polysaccharides and lipopolysaccharides (LPS) may be associated with the cell surface - Gram positive cells walls contain the polysaccharide teichoic acid - Gram negative cells often have LPS associated with the outer membrane - in many pathogens the LPS is toxic (endotoxin) 2. Archaeal cell walls • do not contain peptidoglycan • some contain atypical peptidoglycan called pseudopeptioglycan - NAG plus a different sugar monomer (not NAM) • may be composed of protein or polysaccharide Campbell and Reece Chapter 27 Page ▯ 3. Capsule • gelatinous layer outside the cell wall made of extracellular polysaccharide and protein • functions in attachment and protection (e.g., dehydration and attacks from a host’s immune system) 4. Pili and fimbriae • attachment structures • Pili (singular – pilus) are generally longer and less numerous than Fimbriae (singular – fimbria) • Pili are composed of protein called pilin 5. Movement a. Flagellum • 25 nm in diameter. Can this structure be resolved with the light microscope? Parts of the Prokaryotic Flagellum i. whiplike extension or filament composed of a globular protein called flagellin ii. curved hook (composed of a second protein) iii. basal apparatus (composed of 35 or more different proteins) consisting of a series of rings embedded in the various layers of the cell wall, cell envelope and plasma membrane. o Basal apparatus is a molecular motor that rotates the filament; it is driven by a proton gradient o Filament may rotate at speeds up to 1,200 RPM causing the cells to travel at speeds up to 50 cell length/s. This in relative terms = 3 x as fast as a cheetah can run b. Spirochetes - two or more helical filaments under the outer part of the cell wall - basal motors cause to filaments to rotate. Campbell and Reece Chapter 27 Page ▯ c. Gliding along secreted threads of polysaccharide In a heterogenous environment prokaryotes are capable of movement (taxis) towards or away from stimuli (light, heat, chemicals and electricity) • chemotaxis - chemical stimulus • phototaxis - light stimulus • geotaxis - gravitational stimuli 6. Internal membranes • prokaryotes lack extensive compartmentalization characteristic of eukaryotes • a number of prokaryotes do have specialized membrane structures (arising from the plasma membrane) that function in respiration and photosynthesis (Figure 27.7) D. Genome • The genetic information is found mainly in the nucleoid region • Most of the DNA is found in a single circular double stranded DNA (dsDNA) molecule known as the chromosome (sometimes it is also referred to as the genophore) Exceptions • Some prokaryotes have multiple circular chromosomes (e.g., Rhodobacter sphaeroides has two circular chromosomes) or linear chromosomes (e.g., Borrelia burgdorferi and some Streptomyces spp.) • Some of the prokaryotic genomes may also be in the form of smaller generally circular pieces of dsDNA called plasmids: - Autonomously replicating units that contain only a few genes - The plasmid borne genes are generally not essential but under certain circumstances may be provide the cells with a selective advantage e.g., R-plasmids carry antibiotic resistance genes Metabolic genes – nitrogen fixation and substrate use Toxin production Conjugal transfer IV Reproduction and Adaptation • Proka
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