Back to Biology 1402 pageProkaryotes and the
Origins of Metabolic Diversity
Chapter 27
Prokaryotic Cells
No nucleus
No extensive endomembrane system
Relatively few cell organelles
Single circular chromosome (DNA) located in "nucleoid" region (not membrane bounded) - DNA also in plasmids
Reproduction by binary fission
Importance of Prokaryotes
In terms of numbers, most numerous organisms on earth - outnumber all eukaryotes combined
Occur in virtually all environments in which life can exist - many in extreme environments
Many well-known pathogens - others perform activities essential to life (e.g., decomposition of organic matter into inorganic elements) - activities of prokaryotes essential for eukaryotes to exist
Structure & Function
Most unicellular - some colonial - a few exhibit organization with division of labor
Variety of shapes - cocci, bacilli, helical
Virtually all have cell wall containing peptidoglycans - major distinction from plants (cellulose in cell wall)
Protective layers and adhesion - capsules & pili
Motility - flagella and other means - may exhibit taxis (directed movement - toward or away from stimulus)
Lineages of Prokaryotes
(1) Domain Bacteria - formerly Eubacteria
Proteobacteria - nitrogen-fixing bacteria (Rhizobium) and pathogens (Salmonella)
Gram-positive bacteria - production of antibiotics (Streptomyces) and important pathogens (Clostridium)
Cyanobacteria - formerly blue-green algae (changed earth’s atmosphere)
Spirochetes - important pathogens (Treponema)
Chlamydias - pathogens (Chlamydia)
Lineages of Prokaryotes
(2) Domain Archaea - formerly Archaebacteria
Methanogens - unique form of energy metabolism - H2S used to reduce CO2 to methane - anaerobes
Extreme halophiles - thrive in extremely salty environments (salt lakes) - many require salinity 10x saltier than sea water
Extreme thermophiles - thrive in hot environments - many have optimal temperature ranges of 60-80oC - some live near sea vents at temperatures above boiling (105oC)
Cell - Genomic Organization
No membrane bound nucleus
No endomembrane system - compartments infolded areas of plasma membrane
Chromosome - single DNA molecule - circular - located in nucleoid region of cell
Plasmids - small rings of DNA - replicate independently of chromosome - not essential - confer resistance to anitibiotics
Reproduction in Prokaryotes
Reproduce exclusively by binary fission - cell divides and each daughter receives one copy of parental chromosome
No meiosis, mitosis or sex
Sources of Genetic Variation
in Prokaryotes
Mutation - change in DNA - is major source of genetic variation in prokaryotes
Three major mechanisms of genetic recombination:
transformation - genes taken up from environment
conjugation - transfer of DNA between 2 cells that are temporarily joined
transduction - genes transferred by viruses
Nutrition
Refers to how and where organism obtains two resources for synthesizing organic compounds:
Energy
sunlight - phototrophs
chemicals -
chemotrophsSource of Carbon
carbon dioxide - autotrophs
organic nutrient(s) - heterotrophs
Modes of Nutrition
Photoautotrophs- obtain energy from sunlight; carbon from CO2 - photosynthetic prokaryotes
Chemoautotrophs - energy from inorganic compounds (e.g., H2S); carbon from CO2 - unique to certain prokaryotes
Photoheterotrophs - energy from sunlight; carbon from organic compound(s) - unique to prokaryotes
Chemoheterotrophs - energy and carbon from organic compounds - occurs widely in prokaryotes and other groups (e.g., fungi, animals)
Role of Oxygen in Metabolism
Obligate aerobes - require oxygen for cellular respiration - cannot exist without oxygen
Facultative anaerobes - will use oxygen if present, but utilize fermentation in anaerobic environment
Obligate anaerobes - poisoned by oxygen - some utilize fermentation - others utilize anaerobic respiration (molecule other than oxygen acts as oxidizing agent)
Symbiosis
Ecological relationships between organisms of different species that live in direct contact - organisms referred to as symbionts - the larger is usually referred to as the host
Mutualism - both symbionts benefit
Commensalism - one benefits; other unaffected
Parasitism - parasite benefits; host adversely affected
Nitrogen Metabolism
Essential component of proteins & nucleic acids
Prokaryotes able to metabolize most nitrogenous compounds - most other organisms unable to do so
Nitrogen fixation - convert atmospheric nitrogen to ammonia - extremely important to life - this is only biological mechanism that makes atmospheric nitrogen available to most organisms - occurs only among certain prokaryotes
Survival Mechanisms
Under harsh conditions, cell duplicates chromosome - one copy encapsulated within durable wall - endospore - able to survive harsh conditions for long periods of time - resumes normal development under hospitable conditions
To minimize effects of competition, many produce substances that inhibit growth of competitors - antibiotics - useful to combat pathogenic bacteria
Prokaryotes and Disease
Pathogens - cause disease(s) - involves a minority of prokaryotes
Cause disease by two means:
exotoxins - proteins that cause disease with or without the pathogen being present - Chlostridium sp. which causes botulinism
endotoxins - components of outer membrane of certain gram-negative bacteria toxic - e.g., Salmonella sp. which causes typhoid fever
Koch’s Postulates
Protocol for establishing relationship between a suspected pathogen and specific disease
Four criteria must be met:
organism must be isolated from diseased host
organism grown in pure culture
when organism introduced into healthy host, must produce disease symptoms
organism must be recovered from diseased host
Prokaryotes in Research
Much of our present knowledge regarding metabolism and molecular biology has come from studies of prokaryotes (e.g., Escherichia coli)
Much research on use of prokaryotes to decompose effluent, pesticides and other petroleum distillates
Manufacture of antibiotics
Molecular biology - basic research on processes - has also revolutionized phylogenetic systematics