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Biology notes Created by
Waqar Ahmed Mirani
Chapter 06
Prokaryotes
( Taxonomy of Prokaryotes )
In recent past all living organisms are grouped into
five kingdoms and prokaryotes were placed into the Kingdom Monera but now
the status of classification has been changed in late 90’s . The pioneer work
had been conducted by Carl Woese (1990) and his colleagues proposed domain
system of classtification baswed upon subcellular structure . He described that
life has evolved onm earth along three lineages called domain which include
Bacteria , Archaebacterial and Eubacteria . This domain is widely accepted
throughout the world .
In view of recent classification two out of three domains’ Eubacteria and
Archaebacteria are prokaryotes . The Domain Eubacterial are comprises all
bacterial organisms and recntly given status as kingdom bacterial , the
domain Archae comprises the rest of prokaryotes , and the domain Eukaryea
includes all eukaryotes , including organism in the kingdoms Protista , Fungi ,
Plantae and Animalae .
( Comparison of domain system Classification )
Characteristics Archae Bacteria Eukarya
Cell Membrane Ether – linked lipids , Ether – linked Ether linked
pseudopeptidoglycan peptidoglycan lipids
Gene Structure Circular Circular Multiple , linear
chromosomes , chromosomes , chromosomes ,
similar translation unique similar
and transcription to translation and transcription and
Eukarya . Transcription . translation .
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Internal cell No membrane bound No membrane Membrane bound
structure organelles . bound organelles organelles
Metabolism Various Various including Photosynthesis
methanogenesis photosynthesis , and cellular
unique to archae aerobic and respiration .
anaerobic
respiration , and
autotrophy .
Reproduction Asexual Asexual Sexual and
reproduction reproduction asexual
reproduction .
(Bacterial Habitat)
Bacteria can be found in almost all habitat on earth .
They are present in soil , water , plants , animals , radioactive waste , deep in
the earth’s crust , arctic ice and glaciers and in hot springs . The range
extending upto 30 miles up in the atmosphere and 10,000 meters deep in water
. A gram of soil typically contains about 40 million bacterial cells . The earth is
estimated to hold at least 5 nonillion bacteia , and much of the earth’s biomass
is thought to be made up of basteria .
( Structure of Bacteria )
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Bacteria are prokaryotic unicllular organisms . Structurally
bacterial cell has three distinct regions first appendages that include Flagella
and Pilli which helps bacterial cell in locomotion and floating respectively ,
Secondly cell coverings consisting of capsule that outermost covering in some
cells , cell wall and cell membrane and third is cytoplasmic region that
cobntain (DNA) , plasmid is extra molecule of (DNA) used in conjugation .
Bacteria also contain Ribosomes for protein synthesis , Mesosomes the
membranous invaginations which helps in DNA re[plication , cell division
replication and export of enzymes and various inclusions .
Generally bacterial cell have two protective coverings the outer cell
wall and inner cell membrane . Some bacteria have a third outermost
protective covering called glycocalyx . Glycocalyx may be condensed to form
capsule and slime which is soft and sticky according to surrounding
environment .
(Shapes and Size of bacteria)
Bacteria are found in different shapes and sizes . Bacteria
are classified into five groups according to their basic shapes .
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1)Cocci :- These are spherical and nobn – flagellated . Cocci exuists in
different arrangement like single cell , in chains as diplococci , in groups of
four called tetrads .
2)Bacilli :- These are rod shaped may be flagellated bacteria they exist in
single form , paired form as di[plobacillus .
3)Spirilla :- They are flagellated deeply curved from the middle of the body .
4)Vibrio :- They are slightly curved bacteria like comma shape with flahgella .
5)Spirochetes :- These are long , slender abd flexable cork screw like
bacteria .
(Endospore development in bacteria)
In certain bacteria like Bacillus , the cells tide iover
unfavourabl;e conditions by forming endospores . During this process , a
portion of the cytoplasm and a copyu of bacterial chromosome undergo
dehydration and get surrounded by a three – layered covering . The remaining
part of cytoplasm and cell wall degenerate . The resulting structure called
endospore can tolerate exterme environmental conditiobns and can remain
viable for several years .
( Mode of nutrition in bacteria )
Bacteria require carbon and energy to synthesize their
cell components by different processes . They can obtain energy and nutrients
in different ways like photosynthesis , chemosynthesis etc . They are classified
on the basis of modes of nutrition in two types i.e. autrotrophi bacteria and
Heterotrophic bacteria .
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1)Autotrophic bacteria :- they synthesize their own food from simple
inorganic sources like CO2 . In this process , energy is obtained from either
sunlight or chemically by the oxidation of some inorganic substances like iron ,
sulphur etc . Autrotrophic bacteria are further divide in two types :
a)Photosynthetic Bacteria :- These bacteria have bacteriochlorophyll Or
Chlorobium as photosynthetic pigments . These pigments are present in
specific vesicles associated with bacterial membrane called
Chromatopphores . These bacteria use light energy as a source and hydrogen
sulphide or other H+ donor as reducing agent instead of water to make
carbohydrates , therefore doesn’t release oxygen but release sulphur as by
product .
b)Chemoautotrophic Bacteria :- They prepare their food by using inorganic
raw material in the absence of photosynthetic pigment . In this process ,
chemical energy is obtained from oxidation of certain inorganic substances
such as ammonia , nitrates , metallic and non—metallic substances . In this
reaction , chemical bonds are broken , to release energy , which is used to
drive the synthetic processes of cell .
2)Heterotrophic bacteria :- They don’t have photosynthetic pigments and
are not capable of synthesizing their own food from organic or inorganic
compounds . They are three types :
a)Saprotrophic bacteria :- They obtain food by decaying matter (humus)
and dead organisms . They secrete enzymes that break down complex organic
compounds into simpler products which are then absorbed as nutrients .
b)Parasitic Bacteria :- They live on and within the body of other living
organisms and gets nutrition from them . During their living they harm their
host and cause different diseases in both animals and plants hence they are
called pathopgenic bacteria .
c)Mutualistic bacteria :- They live in close association with other living
organism so that they both benefit from each other so neither of them is
harmed . Symbiotic bacteria fix free atmospheric nitrogen into nitrogenous
compounds which are utilized by the plants , and in return , the plant gives
nutrients and protection to bacteria .
(Growth in Bacteria)
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Growth of bacterial cultures is defined as an increase
in the number of bacteria in a population rather than in the size of indiidual
cells . The growth of a bacterial population occurs in geometrical or
exponential manner . Exponential describes a very rapid increase .
1)Lag Phase :- The word lag describes a kind of showness or delay .
Immediately after providing fresh nutritive medium , the bacterial population
initially remains unchanged . Although there is no apparent cell division
occuring but the cell growing in volume or mass .
2)Exponential (log) Phase :- The exponential phase of growth is a pattern
of balanced growth where in all the cells are dividing regularly and rapidly by
binary ission . The cells divide at a constant rate depending upon composition
of growth medium and the conditions of incubation . The growth of bacteria is
very high during this period .
3)Stationary Phase :- The log phase of bacterial growth is followed by
sttionary phase , in which the size of a population of bacteria remains
constant, increase in bacterial population has simply stopped . In this phase
rate of reproduction is equal to rate of death .
4)Death Phase:- In incubation continues after the population reaches
stationary phase , a death phase follows , in which the viabl cell population
declines . During this rate of death is higher than rate of reproduction . This
phase ends at complete exhaustion of nutrients .
(Reproduction in Bacteria)
Bacteria being prokaryotic organism reproduce by asexual
methods . The methods are : 1)Binary Fission 2)Conidia 3)Budding .
1)Binary Fission :-
It is the most common and fastest mode of asexual
reproduction in bacteria . In binary fission , single cell divides into two equal
cells . Before binary fission occurs , the cell must copy its genetic material
(DNA) . The double stranded DNA molecule i.e., incipient nuclei , are then
distributed into two poles of dividing cell without spindle formation , a
tranverse septum develops cytoplasm is cleaved in two regions of cell from the
middle separating the two daughter cells .
2)Conidia :-
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Like fungi conidia formation takes place in filamentous bacteria
like Streptomyces etc . Conidia appear as a small spores separated by the
formation of a transverse septum at the apex of the filament . The part of the
filament which bears conidia is called conidiophore . After maturity conidia
detached as a fragment from parent cell and germinates on suitable
substratum to gives rise to new bacteria .
3)Budding :-
Budding has ben observed in some members of the
Planctomycetes etc . In budding the bacterial cell develops small swelling at
one side which gradually increases in size . Simultanously the nucleic material
undergoes division , where one remains with the mother cell and other one
with some cytoplasm goes to the swelling . This outgrowth is the bud , which
gets separated from mother by partition wall .
