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Plasma membrane
The plasma membrane or bacterial cytoplasmic membrane is composed of
a phospholipid bilayer and thus has all of the general functions of a cell
membrane such as acting as a permeability barrier for most molecules and serving
as the location for the transport of molecules into the cell.
In addition to these functions, prokaryotic membranes also function in energy
conservation ,However, many microbes do contain structurally related compounds
called hopanoids which
likely
fulfill
the
same
function.
Unlike eukaryotes, bacteria can have a wide variety of fatty acids within their
membranes. Along with typical saturated and unsaturated fatty acids, The relative
proportions of these fatty acids can be modulated by the bacterium to maintain the
optimum fluidity of the membrane (e.g. following temperature change).
As a phospholipid bilayer, the lipid portion of the outer membrane is impermeable
to charged molecules. However, channels called porins are present in the outer
membrane that allow for passive transport of many ions, sugars and amino
acids across the outer membrane.
These molecules are therefore present in the periplasm, the region between the
cytoplasmic and outer membranes. The periplasm contains the peptidoglycan layer
and many proteins responsible for substrate binding or hydrolysis and reception of
extracellular signals.
The periplasm is thought to exist in a gel-like state rather than a liquid due to the
high concentration of proteins and peptidoglycan found within it. Because of its
location between the cytoplasmic and outer membranes, signals received and
substrates bound are available to be transported across the cytoplasmic
membrane using transport and signalling proteins imbedded there.
How do bacteria store genetic information?
Genetic information in bacteria is stored in the sequence of DNA in two forms,
that is bacterial chromosome and plasmid.
The following are the properties of a bacterial chromosome.
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Location: Within nucleoid region , not surrounded by nuclear envelope.
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Number: 1 chromosome each cell.
Size: E.coli 4640 kbp.
Component: Single, double stranded, circular DNA. Also contains RNA
and proteins that take part in DNA replication, transcription and regulation of
gene expression. DNA does not interact with protein histone.
Information: Contain genes essential for cellular functions.
In addition to chromosome, bacterial cells may also contain another genetic
element, plasmid. Features of plasmid are analysed below.
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Location: In cytosol of bacterial cells.
Number: From 1 to several.
Size: Much smaller than chromosomes.
Components: Single, double stranded, circular DNA.
Information: Contains drug resistant genes as well as heavy metal
resistant genes. Not essential for growth and metabolism of bacteria.
What helps bacteria synthesise protein ?
Protein synthesis is a very important process for both eukaryotes and
prokaryotes. In this process, nucleotide sequence in a segment of DNA is
translated into the specific sequence of amino acids in a protein. Translation
occurs at ribosomes; ribosomes consist of RNA and proteins. While
eukaryotic cells have 80S ribosomes, bacterial cells contain 70S ribosomes,
which have the folllowing components.
70S ribosome
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30S subunit: 21 proteins and 16S rRNA.
50S subunit: 34 proteins, a 23S rRNA and a 5S rRNA
Combination of 2 subunits to form functional ribosome requires
magnesium ions and chemical energy.
Activity of 70S ribosomes is blocked by antibiotics like erythromycin and
streptomycin.