CHAPTER 27 Bacteria and Archaea The organisms that make up the two prokaryotic domains Bacteria and Archaea were the first organisms to arise on earth about 3 5 billion years ago 1 Some general characteristics Unicellular but they do grow in colonies No membrane bound organelles Circular chromosomes Most prokaryotes range in size from 0 2 5 0 micrometers in diameter see fig 27 2 How big or small is a micrometer 1 1 000 000 of a meter But just how small is that The thickness of a dime is about 1 millimeter 1000 micrometers The two domains appear similar in many respects however they are different in some very basic ways One main distinction between the Bacteria and the Archaea is the type of a semi rigid permeable cell wall This bacterial cell wall is composed of peptidoglycan Peptidoglycan a type of polymer in bacterial cell walls consisting of modifited sugars cross linked by short polypeptides While only about 5000 prokaryotic species have been described there may be as many as 100 1000 times that number The cell wall of a bacterium gives characteristic shapes see fig 27 2 o Bacilli rod shaped prokarytoes o Cocci spherical prokaryotes o Spirilla range from comma like shapes to loose coils These are some shapes but there are many variations of these basic shapes The peptidoglycan cell wall of bacteria can be stained by a specific stain known as a Gram stain named for Hans Christian Gram a Danish physician The Domain Bacteria can be divided into two groups based on the ability to be stained with Gram stain Gram positive bacteria have the cell wall exposed to the environment Some bacteria are not stained by Gram stain Gram negative These bacteria still have a peptidoglycan cell wall but they also have About half the prokaryotic species are capable of motion using rotating an extra outer membrane flagella see fig 27 6 o flagella singular flagellum a long cellular appendage 2 specialized for locomotion o The anchor of the bacterial flagellum is an axle and wheel arrangement which allows the flagellum to rotate like a propeller see fig 27 6 Prokaryotes with flagella exhibit either very little movement or random movement when in a uniform environment However in a heterogeneous environment mobile bacteria demonstrate a behavior known as taxis o Taxis an oriented movement toward or away from a stimulus If the movement is toward the stimulus then it s called positive taxis Alternatively if movement is away from the stimulus then the taxis is negative taxis to chemicals stimulus of light move in response slime layers proteins The three main types of taxis seen in prokaryotes are Chemotaxis prokaryotes change their movement pattern in response Phototaxis occurs when a prokaryote moves towards or away from Magnetotaxis describes an ability to sense a magnetic field and All of these require that the organism have the ability to sense the stimulus The sensors are usually in the cell membrane Surrounding the cell walls of some prokaryotes are either capsules or Both these structures are composed of polysaccharides and or The capsule or the slime layer both function to attach the prokaryote to a surface and may also act as protective covering see fig 27 4 The major difference between the two Capsule in many prokaryotes a dense and well defined layer of polysaccharide or protein that surrounds the cell wall and is sticky protecting the cell and enabling it to adhere to substrates or other cells dense and well defined Slime layer protect against dehydration and some shield pathogenic prokaryotes from attacks by their hosts immune system less well organized Another way that prokaryotes can attach themselves to a surface e g a host cell or another prokaryotic cell is via fimbriae singular fimbria see fig 27 5 Fimbriae a short hair like appendage of a prokaryotic cell that helps it adhere to the substrate or to other cells o Fimbriae and pilli allow prokaryotes to stick to their substrate or other individuals in a colony o While prokaryotes do not have membrane bound organelles some do have specialized membranes that perform metabolic functions see fig 27 7 3 Genomic Organization region The prokaryotic chromosome is circular and located in the nucleoid There can also be plasmid smaller circular DNA rings which contain additional genes not always necessary for basic survival e g antibiotic resistance o Plasmid is small ring of DNA that carries accessory genes separate from those of a bacterial chromosome also found in some eukaryotes such as yeast Some bacteria especially the bacilli can form a protective endospore in response to harsh environmental conditions see fig 27 9 o Endospores are thick coated resistant structures produced within bacterial cells that have been exposed to harsh conditions o These structures are resistant to dehydration extreme heat or o The endospore will persist until conditions are favorable and then cold and most poisons develop into a bacterium Bacteria have evolved to live in almost every possible environmental condition found on Earth including some very extreme conditions While the range of conditions that the domains bacteria and archaea can in live in is large the conditions a single species requires may be very specific o In order for the prokaryotic domains to be able to occupy so many different ecological niches they have had to develop the ability to use many different energy sources o Some prokaryotes grow using autotrophic methods and derive their energy from inorganic chemicals chemosynthesis o Other autotrophic prokayotes like cyanobacteria obtain their energy from sunlight via photosynthesis see fig 27 14 There are also heterotrophic prokaryotes that must have an organic food source for their energy This may occur in the presence of oxygen aerobic or without oxygen anaerobic Prokaryote use asexual reproduction in the form of binary fission The entire bacterial chromosome is replicated and passed on to each daughter cell 4 Some prokaryotes are able to transfer genetic information between cells by conjugation See fig 27 12 Conjugation is the direct transfer of DNA between two cells that are temporarily joined Bacteria occupy many diverse habitats however some of the most extreme environments are the realms of the Archaea Some examples include Halophiles salt loving that inhabit the extremely salty Dead Sea and Thermophiles heat loving including thermoacidophiles which live Great Salt Lake in o acidic hot springs or in the hot 105 oC water surrounding o Deep
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