archaea, bacteria, and viruses chapter 19. prokaryotes and eukaryotes terms introduced by edouard...
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Prokaryotes Carl Woese –Late 1970s –Proposed using the rRNA gene to create universal tree of life rRNA critical for proteins synthesis Useful in determining evolutionary relationshipsTRANSCRIPT
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Archaea, Bacteria, and Viruses
Chapter 19
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Prokaryotes and Eukaryotes
• Terms introduced by Edouard Chatton in 1920s
• Based on microscopic observations
Prokaryotes Eukaryotes
“primitive nucleus” “true nucleus”
Lack clear nucleus and other inclusions
Clear nucleus and other inclusions
All organisms with cells that lack a nucleus
All organisms with cells that have a nucleus
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Prokaryotes
• Carl Woese– Late 1970s– Proposed using the rRNA gene to create
universal tree of life• rRNA critical for proteins synthesis• Useful in determining evolutionary relationships
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Three Domains
Domain Cell Type Description
Eukarya Eukaryotic Membrane bounded organelles, linear chromosomes
Archaea ProkaryoticFound in extreme environments, cell structure differs from members of Domain Bacteria
Bacteria Prokaryotic Ordinary bacteria, found in every habitat on earth, play major role as decomposers
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Why Should A Botanist Study Prokaryotes?
• Reasons for studying prokaryotes– Many of the biochemical compounds,
enzymes, and metabolic pathways of plants also are found in prokaryotes.
– The evolutionary ancestors of plants were prokaryotes.
– Plants form ecological associations with prokaryotes.
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Viruses
• Consist of– Either DNA or RNA– Protein coat
• Not prokaryotes• Noncellular cannot live independently• Discoveries obtained studying viruses can
be used to guide plant research
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Prokaryotic Cell Structure
• Lacks internal membrane-enclosed organelles
• Surrounded by plasma membrane– Bacteria plasma membrane lipids similar to
those of eukaryotes– Archaeal plasma membrane lipids very
different• Held together by stronger bonds
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Categories of Bacterial Cells
• Divided on basis of differential staining technique developed by Gram– Gram-negative cells– Gram-positive cells
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Gram-positive Cells
• Capsule– Waxy polysaccharide– Protects some human pathogens from being
engulfed by immune system cells• Penicillin very effective against Gram-
positive cells– Inhibits formation of cell wall– Causes lysis of cells in hypotonic solutions
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Comparison of Gram-positive and Gram-negative Cells
Gram-positive Gram-negative
Thick peptidoglycan cell wall Thin peptidoglycan cell wall
No second membrane – some have waxy polysaccharide capsule
Second membrane outside cell wall – lipopolysaccharide layer composed of phospholipids, polysaccharide, and protein
No periplasmic spacePeriplasmic space between cell wall and lipopolysaccharide layer
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Shapes of Bacteria• Determined by cell wall
– Also functions to keep cell from bursting in hypotonic solution
– Composed of peptidoglycan• Shapes
– Cocci small, round cells– Bacilli rods– Vibrios bent or hooked rods– Spirilla helical forms– Stalked forms
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Archaea
• Most have paracrystalline surface layer (S layer)– Composed of protein or glycoprotein– Sensitive to proteases and surfactants
• Some have outer covering of pseudopeptidoglycan
• Some have thick walls of polysaccharide• Typically lack an outer membrane
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Bacterial DNA
• Typically a single, circular chromosome– Size in Escherichia coli
• 1.4 mm in length• Contains 4.6 million nucleotide pairs
• Not surrounded by nuclear envelope• Complexed with specific structural proteins
that organize it into loops• Localized in area called nucleoid
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Archaeal DNA
• Chromosome complexed with histone proteins, similar to chromosomes of eukaryotes
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Plasmids
• Accessory genes• Small circles of DNA approximately 2,000
to 200,000 nucleotide pairs in length• Can replicate independently of main
chromosome
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Plasmids
• Examples of information carried by plasmid genes– Antibiotic resistance– Enzymes and structural proteins that transfer
copies of plasmid to Bacteria that do not have any
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Plasmids
– F plasmid• Can incorporate itself into main chromosome• Contains genes for making tube called F pilus
– Connects its cell with another that lacks F plasmid– Transfers plasmid or chromosomal DNA from donor (cell
with pilus) to receiver cell – Transfer of DNA is called conjugation
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Prokaryotic Ribosomes
• General composition and structure similar to those of eukaryotes– Two subunits made of RNA and protein
• Smaller than eukaryotic ribosomes• rRNAs of Archaea
– More similar to those of eukaryotes than Bacteria
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Binary Fission
• Method of reproduction in prokaryotic cells• Differs from mitosis
– Prokaryotes lack microtubules therefore do not have spindle apparatus
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Flagella• Used by many Bacterial and some Archaeal
cells for swimming• Formed of subunits of protein flagellin• Parts of flagellum
– Filament– Hook– Basal body
• Powered by basal body• In some instances can reverse swimming
direction
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Pilus
• Extracellular organelle• Thin, hollow, nonmotile projection from cell• Proteins at ends of structure attach cell to
solid surfaces or to receptors on other cells
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Cisternae
• Cisternae or thylakoid membranes• Found in some prokaryotic cells• Consist of flattened bladders than enclose
separate compartments within cytoplasm• Function
– in light reactions of photosynthesis in photosynthetic prokaryotes
– Energy storage
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Endospores
• Means of survival for some prokaryotic cells
• Small, desiccated cells in condition of suspended animation
• Contain complete genome and needed chemicals for germination and growth when conditions improve
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Endospores
• Resistant to many things such as boiling, oxidizing agents, antibiotic compounds
• Formation involves activation of special genes– In Bacteria such as Clostridium tetani
• Nucleoid and ribosomes surrounded by spore wall• Rest of cell degenerates
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Endospores
– Actinobacteria• Form spores on vertical stalk• Spores blown to new sites by air currents
– Myxobacteria• Form sacs of endospores• Spores released when sac is hydrated
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Nutritional Requirements of Prokaryotes
• Methods of obtaining carbon– Autotroph (“self-feeding) incorporate carbon
into organic molecules from inorganic sources– Heterotroph (“other feeding”) derive carbon
from breakdown of organic compounds
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Nutritional Requirements of Prokaryotes
• Methods of deriving energy– Chemotroph (“chemical feeding”) obtain
energy from catalyzing inorganic reactions– Phototroph (“light feeding”) obtain energy
by absorbing light photons
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Nutritional Requirements of Prokaryotes
Carbon source Energy source
Chemoautotroph Carbon from inorganic source incorporated into organic molecules
Catalyze inorganic reactions
Chemoheterotroph Carbon from breakdown of organic compounds
Catalyze inorganic reactions
Photoautotroph Carbon from inorganic source incorporated into organic molecules
Derive energy by absorbing light photons
Photoheterotroph Carbon from breakdown of organic compounds
Derive energy by absorbing light photons
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Groups of Archaea
• Methanogens• Halophiles• Thermoacidophiles
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Groups of Archaea
• Methanogens – Chemoautotrophs– Require anoxic environment to obtain energy– Produce methane
Reaction used by methanogens to derive energy:
CO2 + 4H2 CH4 + 2H2O
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Groups of Archaea
• Halophiles– Live in saturated salt solutions– Some have little or no cell wall
• Will burst if moved from its normal environment– Example: Halobacterium halobium
• Unique type of photosynthesis• Photoreceptor – bacteriorhodopsin• No electron transport chain• Cannot make carbohydrates by reducing CO2 • Photoheterotroph
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Groups of Archaea
• Thermoacidophiles– Live in hot, acidic environments– Optimum temperature is 70 to 75ºC with
maximum of 88ºC– Optimum pH is 2-3 (minimum pH 0.9)
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Chemoheterotrophs
• Live on organic compounds of living or dead tissue or on excretions of other organisms
• Roles– May be harmful parasites
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Chemoheterotrophs
– Can be beneficial• Compete for niches with potential pathogens• Gut chemoheterotrophs provide humans with
vitamin K• In dead tissue and on excretions, play role of
recycling carbon, nitrogen, and other elements
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Chemoheterotrophs
• Some undergo fermentation– Lactobacillus
• Extensively studied bacterium, E. coli, is chemoheterotroph– Group Proteobacteria– Family Enterobacteriaceae
• Members live in soil and in intestines of animals• Often called enteric or coliform Bacteria
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Chemoheterotrophs
– Presence of coliform Bacteria in water supplies indicates contamination with sewage
• Humans sewage carries pathogenic Bacteria and viruses
– Some strains of E. coli are not harmful, others produce toxins that cause severe infections
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Examples of Chemoheterotrophs Genus Phylum Comment
Rhizobium Proteobacteria N2-fixing plant symbionts
Frankia Actinobacteria N2-fixing plant symbionts
Erwinia, Agrobacterium, Pseudomonas syringae Proteobacteria Plant pathogens
Desulfovibrio, Desulfomonas Proteobacteria Sulfate-reducing bacteria
Stigmatella, Chondromyces Proteobacteria Myxobacteria; colonial spore formers
Streptomyces Actinobacteria Antibiotic producers
Spirochaeta, Treponema Spirochaetes Long, thin, spiral-shaped; some pathogenic
Bacillus Firmicutes Aerobic endospore-formers
Escherichia Proteobacteria Enteric; model organism
**Rhodopseudomonas, Chromatium Proteobacteria Anaerobic phototrophs; purple nonsulfur
and purple sulfur groups
**Can be photoautotroph or chemoheterotroph
All of the above examples are in the Domain Bacteria.
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Chemoautotrophs
• Examples– Lithotrophs
• Specialize in oxidation of inorganic compounds• Recycle nitrogen and sulfur
– Nitrogen and methane oxidizers– Methanogenic Archaea– Thermophilic and thermoacidophilic Archaea
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Examples of Chemoautotrophs
Domain Genus Phylum Comment
Archaea Methanococcus, Methanospirillum Euryarchaeota
MethanogenicCO2 + 4H2 CH4 + 2H2O
ArchaeaThermoplasma
Euryarchaeota Thermoacidophile; grows at pH1-4 and 33-67ºC
ArchaeaPyrolobus Crenarchaeota Extreme thermophile; grows
up113ºC
BacteriaNitrosomonas, Nitrobacter, Methylomonas
Proteobacteria Nitrogen, methane oxidizers
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Photoautotrophs
• Includes – Green sulfur Bacteria– Purple nonsulfur Bacteria– Cyanobacteria
• Light absorbing pigments– Bacteriochlorophyll
• Anaerobic phototrophs
– Chlorophyll• Cyanobacteria
• All reduce carbon to CO2
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Examples of Photoautotrophs
Domain Genus Phylum Comment
Bacteria **Rhodopseudomonas, Chromatium Proteobacteria
Anaerobic phototrophs; purple nonsulfur and purple sulfur groups
Bacteria Chlorobium Chlorobi Anaerobic phototrophs; green sulfur group
Bacteria Anabaena, Nostoc, Prochloron Cyanobacteria Oxygen producers
**Can be either photoautotroph or chemoheterotroph
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Photoautotrophs and Endosymbiosis
• Primitive cyanobacteria and chloroxybacteria thought to be evolutionary precursors of plastids of photosynthetic eukaryotes
• Strong evidence on similarities between light-harvesting complexes of– Cyanobacteria and red algae– Chloroxybacteria and green algae
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Symbiotic Relationships Between Prokaryotes and Plants
• Rhizobium lives in soil– Synthesizes enzyme nitrogenase
• Converts N2 to ammonium (NH4+)
• Forms close mutualistic relationship with legumes– Plant contributes high energy carbohydrates and a
protected environment– Bacterium contributes nitrogenase and other enzymes– Both partners benefit from supply of fixed nitrogen
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Symbiotic Relationships Between Prokaryotes and Plants
• Association occurs in special organs called root nodules
• Sequence of events in establishment of relationship– Root secretes attractive chemical– Chemical induces Rhizobium in vicinity to
swim toward root and begins induction of nitrogen fixation genes in Rhizobium
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Symbiotic Relationships Between Prokaryotes and Plants
– Rhizobium enters at a root hair and moves inward through infection thread
– Rhizobium loses its cell wall and begins synthesizing nitrogen-fixing enzymes as it moves inward
– Bacteria reach root cortex – Bacteria are released from infection thread
into several cells– Bacteria without cell walls are now called
bacteroids
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Symbiotic Relationships Between Prokaryotes and Plants
– Bacteroids become surrounded by special membrane called the peribacteroid membrane
– Chemicals secreted by Bacteria (or bacteroids) during formation of infection thread
• Induce cell division in root cortex and pericycle, forming nodule
• Induce synthesis of nodule proteins including leghemoglobin that buffers oxygen concentration in part of nodule where nitrogen is fixed
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Symbiotic Relationships Between Prokaryotes and Plants
• Other examples of symbiotic nitrogen fixing Bacteria– Frankia – lives within cells of root nodules of
alder trees and other plants– Anabaena – association with water fern,
Azolla– Nostoc – invades cavities in gametophytes of
hornworts and specialized cells of cycads
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Bacterial Parasites
• Parasitism– Symbiotic relationship in which one organism
benefits at the expense of the other• Plant pathogens divided into subgroups
called pathovars according to plants they infect
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Bacterial Parasites
• Pathovars of Pseudomonas syringae cause– Wildfire disease of tobacco– Blights of beans, peas, and soybeans
• Pathovars of Erwinia amylovora cause– Fire blight of apple and pear
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Bacterial Parasites
• Vectors for carrying Bacteria to uninfected plants include water, insects, humans, or other animals
• Bacteria enter plants through natural openings– Stomata– Lenticels– Hydathodes– Nectarthodes
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Bacterial Parasites
• Some Bacterial pathogens can overwinter in dead tissue– Return to infect new plant tissue during next
growing season• Plant defenses against infecting bacteria
– Hypersensitivity response• Produce antibiotic compounds
– Phytoalexins directly kill some pathogenic cells– Hydrogen peroxide may restrict spread of infection by
causing necroses of adjacent plant cells
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Viruses
• Subcellular parasites– Lack internal structures found in prokaryotic
and eukaryotic cells– Cannot reproduce on their own
• Invade host cells and use host’s metabolism to reproduce themselves
• Simple structure– Either DNA or RNA surrounded by protein
coat
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Viruses
• Shape varies• Well-studied plant virus – tobacco mosaic virus• Plant viruses
– Too large to pass through cell wall– In nature, almost always spread by insects that pick
up viral particles as they chew or suck on infected plants and then transmit them to uninfected plants
– Mites and fungi can also infect plants with viruses when they enter plant cells
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Viruses
• Viral infections usually do not kill plants• Infected plants usually stunted compared
to uninfected plants• Infection often causes changes in color or
shape of foliage