ch25

Chapter 25

PROKARYOTES: DOMAINS BACTERIA AND ARCHAEA ARCHAEA

All organisms are classified in three Domains, the highest taxon: Eukaryotes, Bacteria and Archaea. here are t!o "ranches of prokaryote e#olution: Archaea and Bacteria $Eu"acteria%.

he Domain Bacteria includes the "acteria and the cyano"acteria, !hich !ere kno!n as "luegreen algae. he Archaea li#e in extreme en#ironments such as hot pools, acidic ha" itats and anoxic en#ironments. &rokaryotes are e#ery!here. hey ha#e a tremendous impact on life on Earth. '.5 "illion years ago ((((( prokaryotic type cells. ).5 "illion years ago ((((( eukaryotic cells called acritarchs.

STRUCTURE OF THE PORKARYOTIC CELL &rokaryote Characteristics •

•

• • • •

• • • • •

• •

•

&rokaryote cells are smaller than those of eukaryotes* they usually are "et!een ).+ and +.5 m. -ycoplasmas are a"out +.2 m. Almost all ha#e !alls that gi#e them one of three "asic forms: rods $"acilli%, spheres $cocci% or coils $spirilla%. Cyano"acteria ha#e mucilage sheaths to hold doens of cells together. Cyano"acteria can produce specialied cells called heterocysts  and ak!etes . hey lack mem"rane "ound organelles. heir chromosomes are circular D/A molecules lacking protein and are located in an area called the !"c#eo$ . 0E1 lack split genes $no introns%. &rokaryotes ha#e ri"osomes called +3 ri"osomes. hey di#ide "y "inary fission. hey lack mitosis and meiosis. 4ccasionally undergo genetic recom"ination through conugation $exchange of D/A% "y means of pili $sing. pilus%. hey often ha#e slimy capsules and a flagellum, !hich allo!s limited motility. -any are colonial forming filaments "ut the cells remain independe nt !ithout any cytoplasmic connections.  /utrition is "y the a"sorption of food in solution solution through their cell !all and plasma mem"rane* some o"tain energy through chemical reactions in#ol#ing inorganic su"stances $e.g. 023 or 32+'2( thiosulfate ion% and others are photosynthetic.

CELL %ALL

Eu"acteria: peptidogly peptidoglycan can !ith muramic acid cell !all. &ra'()ost*e ce##s hick !all, )5 6 7+nmlayer of peptidoglycan. peptidoglycan. • &eptidoglycan is a polysaccharide made of /(acetylgucosamine and /(acetylmuramic acid. • he molecules peptide "ridges form crosslinks and the !all "ecomes one giant molecule. •

 For more details about the the structure of the gram-positive gram-positive cell wall see: http:88student.cc"cmd.edu8courses8 http:88stud ent.cc"cmd.edu8courses8"io)9)8lecguid "io)9)8lecguide8unit)8prostru e8unit)8prostruct8c!. ct8c!.html html &ra'(!e+at*e ce##s he peptidoglycan layer is thin, a"out )+nm. • A layer of lipopolysaccharide $&3% co#ers the peptidoglycan layer. • he &3 does not contri"ute to the strength of the !all "ut is semipermea"le in nature and pre#ents • large proteins from approaching the cell mem"rane. &3 is extremely toxic to animals. •

 For more details about the the structure of the gram-negative gram-negative cell wall see: http:88student.cc"cmd.edu8courses8 http:88stud ent.cc"cmd.edu8courses8"io)9)8lecguid "io)9)8lecguide8unit)8prostru e8unit)8prostruct8gnc! ct8gnc!.html .html Archae"acteria: cell !all incorporates other su"stances "ut no muramic acid.

FLA&ELLA

-any "acteria ha#e flagella Cyano"acteria and archaeans lack flagella. he flagellum is hollo! and is anchored to the cell "y a set of rings. 4ne ring is attached in or near the cell mem"rane and the other in the peptidoglycan layer. ;n gram(negati#e "acteria, a third set of rings lies near the &3 layer. ;3;4/ Bacteria are haploid and they lack meiosis. heir reproduction is "y -!ary .sso!.

D/A replicates replicates and a trans#erse !all is formed "y the ingro!th of the plasma mem"rane and cell !all. A "acterium may ha#e se#eral circular chromosomes "efore the cell di#ides. E?C0A/@E 4< @E/E;C -AE=;A Tra!s.or'ato! : hen a "acterium dies and falls apart, its D/A "reaks into short chains containing containing a fe! genes* other "acteria cells a"sor" these small pieces and some may "e inserted into the cells D/A thus thus  "ecoming part of the cell cell genome. he foreign genes genes can no! "e passed to daughter daughter cells. Tra!s$"cto!: a #irus in#ades the "acterium and reproduces. 3mall pieces of the "acterium D/A can "e incorporated into the #irus. hen a #irus attacks a ne! "acterium, the D/A from the first "acterium is released and then is incorporated into the D/A of the second "acterium. Bacterium enymes usually destroy the foreign D/A though. Co!/"+ato!: ;t occurs !hen t!o cells of different mating types come together and genetic material is transferred from one cell to the other "y means of a se0 )#"s. he pilus is made of proteins.

@enetic exchange has ne#er "een found in cyano"acteria.

METABOLISM &rokaryotes ha#e alternate methods for many meta"olic processes like photosynthesis, cellular respiration and sources of energy. -any "acteria utiliing a fe! simple sugars and essential minerals synthesie all their o!n #itamins, lipids, amino acids and other macromolecules. ;n many plant cells, many amino acids and lipids are synthesied in plastids, !hich are prokaryote(deri#ed organelles, and not in the cytosol. &04431/0E3;3 See Mauseth pages 240 and 241 .

Cyano"acteria produce car"ohydrates using the C ', Cal#in Cycle, of photosynthesis. photosynthesis. his process is the same as the one used "y land plants. his similarity is not surprising "ecause chloroplasts in all pro"a"ility arose from the endosym"iosis of cyano"acteria. c yano"acteria. • •

• • •

Cyano"acteria ha#e chlorophyll a as the photosynthetic pigment "ut lack chlorophyll b. Accessory pigments are phyco"ilin, !hich a"sor"s light most strongly "et!een 2+ and 9+ nm, yello! one of the spectrum. And phycoerythrin, !ith maximum a"sorption of light at 55+ nm, "lue one of the spectrum. hey posses a &3 ; and &3 ;; $photosystems%. $photosystems%. 4xygen is a "yproduct of photosynthesis, oxygenic photosynthesis.

&urple and green "acteria do not ha#e chlorophyll chlorophyll a, "ut instead ha#e "acteriochlorophylls. •

• •

•

•

Bacteriochlorophylls Bacteriochlorophylls are made of tetrapyrroles similar to chlorophyll "ut differ in some of the side chains and functional groups. here are carotenoids present that act as accessory pigments. here is only a &3 ; present* &3 ;; is a"sent and electrons cannot "e put "ack onto the  "acteriochlorophyll.  "acteriochlorophyll. he original electron electron comes "ack through a series of carriers, so the electron electron flo! is cyclic. uinone carriers pump electrons from the cytoplasm into the periplasm creating an electrochemical gradient that is used to generated A&. his is a chemiosmotic process.  /o oxygen is formed, formed, anoxygenic photosynthesis. photosynthesis.

Bacterial photosynthesis is a proton pumping process.  /;=4@E/