Chapter 27 ProkaryotesLecture Outline Overview They re Almost Everywhere Prokaryotes were the earliest organisms on Earth Today they still dominate the biosphere More prokaryotes inhabit a handful of fertile soil or the mouth or skin of a human than the total Their collective biomass outweighs all eukaryotes combined at least tenfold number of people who have ever lived Prokaryotes are wherever there is life They thrive in habitats that are too cold too hot too salty too acidic or too alkaline for any eukaryote Prokaryotes have even been discovered in rocks two miles below the surface of the Earth Why have these organisms dominated the biosphere since the origin of life on Earth Prokaryotes display diverse adaptations that allow them to inhabit many environments They have great genetic diversity Prokaryotes are classified into two domains Bacteria and Archaea which differ in structure physiology and biochemistry Concept 27 1 Structural functional and genetic adaptations contribute to prokaryotic success Most prokaryotes are unicellular although some species aggregate or permanently in colonies Most prokaryotes have diameters in the range of 1 5 m compared to 10 100 m for most The largest prokaryote discovered so far has a diameter of 750 m eukaryotic cells The three most common shapes among prokaryotes are spheres cocci rods bacilli and helices Cell Surface Structures One of the most important features in all prokaryotes is their cell wall that maintains cell shape provides physical protection and prevents the cell from bursting in a hypotonic environment In a hypertonic environment most prokaryotes lose water and plasmolyze like other walled cells The cell walls of prokaryotes differ in molecular composition and construction from those of eukaryotes Most bacterial cell walls contain peptidoglycan a network of modified sugar polymers cross Severe water loss inhibits the reproduction of prokaryotes used to preserve foods Eukaryotic cell walls are usually made of cellulose or chitin linked by short polypeptides This molecular fabric encloses the entire bacterium and anchors other molecules that extend from its surface Archaeal cell walls contain a variety of polysaccharides and proteins but lack peptidoglycan The Gram stain is a valuable tool for identifying specific bacteria based on differences in their cell walls Gram positive bacteria have simple cell walls with relatively large amounts of peptidoglycans Gram negative bacteria have less peptidoglycan and are structurally more complex with an outer membrane that contains lipopolysaccharides carbohydrates bonded to lipids Gram staining is a particularly valuable identification tool in medicine Among pathogenic bacteria gram negative species are generally more deadly than gram positive species The lipopolysaccharides on the walls of gram negative bacteria are often toxic and the outer membrane protects the pathogens from the defenses of their hosts Gram negative bacteria are commonly more resistant than gram positive species to antibiotics because the outer membrane impedes entry of the drugs Many antibiotics including penicillin inhibit the synthesis of cross links in peptidoglycans preventing the formation of a functional wall especially in gram positive species These drugs cripple many species of bacteria without affecting human and other eukaryote cells that do not synthesize peptidoglycans The cell wall of many prokaryotes is covered by a capsule a sticky protective layer of polysaccharide or protein 1 Capsules allow cells to adhere to their substrate or to other ind in a colony Capsules can also shield pathogenic prokaryotes form attacks by their host s immune systems Some prokaryotes stick to one another or to their substrate is by hair like appendages called fimbriae and pili Fimbriae are usually more numerous and shorter than pili These structures can fasten pathogenic bacteria to the mucous membranes of the host Sex pili are specialized for holding two prokaryote cells together long enough to transfer DNA during conjugation About half of all prokaryotes are capable of directional movement Motility The flagella of prokaryotes differ in structure and function from those of eukaryotes Some species can move at speeds exceeding 50 m sec about 100 times their body length per second The most common structure the enable prokaryotes to move is the flagella which may be scattered over the entire surface or concentrated at one or both ends of the cell In a heterogeneous environment many prokaryotes exhibit taxis movement toward or away from a stimulus Prokaryotes that exhibit chemotaxis respond to chemicals by changing their movement patterns Solitary E coli may exhibit positive chemotaxis toward other members of their species enabling the formation of colonies Internal and Genomic Organization The cells of prok are simpler than those of eukaryotes in both internal structure and genomic organization Prokaryotic cells lack the complex compartmentalization found in eukaryotic cells Instead prokaryotes use specialized infolded regions of the plasma membrane to perform many metabolic functions including cellular respiration and photosynthesis Prokaryotes have smaller simpler genomes than eukaryotes On average a prokaryote has only about one thousandth as much DNA as a eukaryote In the majority of prokaryotes the genome consists of a ring of DNA with few associated proteins The ring is called the prokaryotic chromosome and is located in the nucleoid region a part of the cytoplasm that appears lighter than the surrounding cytoplasm in electron micrographs Prokaryotes may also have smaller rings of DNA called plasmids which consist of only a few genes Plasmid genes provide resistance to antibiotics direct metabolism of unusual nutrients and other special contingency functions Prokaryotes can survive in most environments without their plasmids because their chromosomes program all essential functions Plasmids replicate independently of the chromosome and can be transferred between partners during conjugation Although the general processes for DNA replication and translation of mRNA into proteins are fundamentally alike in eukaryotes and prokaryotes some of the details differ For example prokaryotic ribosomes are slightly smaller than the eukaryotic version and differ in protein and RNA content These differences are great enough that selective antibiotics including tetracycline and erythromycin bind to prokaryotic ribosomes to block