Chapter 1
The structure and function of
prokaryotes
EUKARYOTES
PROKARYOTES
BACTERIA
ARCHAEA
1.Differences between prokaryotes/eukar
2.Shape and arrangement
3.Procaryoic cell organization
Prokaryotes (Bacteria)
• Eubacter "True" bacteria
– human pathogens
– clinical or environmental
– one kingdom
• Archaea
– Environmental organisms
– second kingdom
Eukaryotes
• Other cell-based life e.g.
– plants
– animals
– fungi
Differences between prokaryotes/eukaryotes
1. The prokaryotic cell, in contrast to the eukaryotic cell, is not
compartmentalized. Nuclear membranes, mitochondria,
endoplasmic reticulum, Golgi body, phagosomes and
lysosomes are not present.
2. Prokaryotes generally possess only a single circular
chromosome. Since there is no nuclear membrane, the
chromosome is bound to a specific site on the cell membrane the mesosome.
3. Prokaryotic ribosomes are 70S (S stands for Svedberg unit, a
measure of size), whereas eukaryotic ribosomes are larger
(80S). Prokaryotic ribosomal subunits are 30S and 50S
(eukaryotic are larger). The 30S ribosome has 16S RNA, whilst
the 50S ribosome contains 23S and 5S RNA. Ribosomal RNA is
larger in eukaryotes (e.g. 18S versus 16S rRNA).
4. Bacterial membranes generally do not contain sterols (甾醇)(e.g.
cholesterol).
Prokaryotic Cell (versus
Eukaryotic Cell)
•
•
•
•
Not compartmentalized
Cell membranes lack sterols (e.g. cholesterol)
Single circular chromosome
Ribosomal are 70S with
– subunits 30S (16S rRNA)
– 50S (5S & 23S rRNA)
Bacteria versus
Archaebacteria
• Eubacteria
– peptidoglycan (murein)
– muramic acid
• Archaebacteria
– pseudomurein
– no muramic acid
Bacteria versus
Archaebacteria
• 16S rRNA
– sequence very different
An overiew of procaryotic cell structre
1.size:width×length
E.coli(average): 1.1-1.5m × 2.0-6.0  m
? The smallest: Nanobacteria: 0.05-0.2 m
? monsteous microbe: >10-50 m
2.Shape and arrangement
Coccus or Cocci
Rod
Spirillum or Spirilla
Spirochete
Budding and appendaged
Filamentous
Streptococcus
Bacillus gram
Spirillum
Individual
Diplococci
Sarcina
Long chain: one plane
Tetrads: two planes
Cubical packets of eight cells: three planes
Grapelike form: random planes
Staphylococcus
Length-to-width ratio
The shape of the rod’s end:
flat;
rounded;
cigar-shaped;
bifurcated;分叉
Arrangement:
Single: many
Pairs:
Chain:
Curved: Vibrios
Other shapes:
Spirilla: rigid
Spirochetes:flexible
Long multimucleate
filaments:
Actinomycetes
Procaryoic cell organization
Structure of the cytoplasmic membrane
Phospholipids bilayer+protein, 5-10nm thick
Prokaryotes
Membrane Stabilizers:
hydrogen bonds
hydrophobic interactions
Mg2+, Ca2+, hopanoids
Many bacterial membrane
Strengthening Agents:
Hopanoids
类何帕烷
Eukaryotes membrane stabilizer:
Sterol
(a) General structure of a sterol;
(b) The structure of cholesterol;
(c) The structure of the hopanoid
diploptene.
Functions of the Cytoplasmic Membrane
The cytoplasmic membrane is
selectively permeable and
contains specific carrier
proteins that participate in
the transport process
Selectively Permeability:
nonpolar and fat-soluble
such as fatty acids, alcohols
and benzene (permeable);
charged molecules such as
organic acids, amino acids
and inorganic salts
(impermeable, must be
specifically transported)
Carrier Proteins: or Membrane
Transport Proteins:
Uniporters, Symporters and
Antiporters
1. Permeability barrier prevents leakage and function
as gate way for transport of
nutrients into and out of the
cell.
2. Protein anchorsite of many proteins involved
in transport, bioenergetics,
and chemotaxis.
3. Energy conservation- site
of generation and use of the
proton motive force.
Mesosome:
invaginationa of the plasma membrane
in the shape of vesicles, tubules, or lamellae.
Functions:
1.cell wall formation during division.
2.play a role in chromosome replicaton
and distribution to daughter cells.
The cytoplasmic matrix: substance
lying between the plasma membrane and
the nucleoid.
Protoplast: the plasma membrane and
everything within.
Making Wall-less forms
• Result from action of:
–enzymes lytic for cell wall
–antibiotics inhibiting peptidoglycan
biosynthesis
• Wall-less bacteria that don’t replicate:
–spheroplasts (with outer membrane)?
–protoplasts (no outer membrane). ?
• Wall-less bacteria that replicate
–L forms
Naturally Wall-less Genus
Mycloplasma
INCLUSIONS
Within the cytoplasm of procaryotic (and eucaryotic)
cells are several kinds of reserve deposits, known as
inclusions. Some inclusions are common to a wide
variety of bacteria, whereas others are limited to a
small number of species and therefore serve as a
basis for identification. Among the more prominent
bacterial inclusions are the following:
Carbon storage polymers – PHB and glycogen
Phosphate polymers
Sulfur Granules
Gas Vacuoles
Polyhydroxybutyric acid
(PHB)
A Vibrio species
PHB is a lipidlike compound - one of the most common
inclusion bodies in prokaryotic organisms.
PHB is commonly found as a storage material and unique
to bacteria
Glycogen is a starchlike polymer of glucose subunits.
Glycogen granules are usually smaller than PHB granules.
Polyphosphate granule
in a bacterial cell
A Pseudomonas species
Many microorganisms accumulate granules of
polyphosphate, which are large reserves of
inorganic phosphates that can be used in the
synthesis of ATP
The sulfur globules
inside the cells of
purple sulfur bacterium
Chromatium buderi
Some bacteria, including many photosynthetic
bacteria, accumulate elemental sulfur granules
as a result of their metabolism.
Gas vacuoles (blue)
and storage granules (red)
in the cyanobacterium
Microcystis
The formation of gas vacuoles by aquatic bacteria
provides a mechanism for adjusting the buoyancy
of the cell.Many aquatic cyanobacteria use their
gas vacuoles to move up and down in the water
column.
Magnetosomes
Magnetic particles of Fe3O4 Isolated
from Aquaspirillum magnetotacticum
Ribosomes: protein synthesis
Procaryotic: 70S (50S 30S)
Eucaryotic: 80S ( 60S 40S)
What is S: Svedberg unit.
This is the unit of the sedimentation
coefficient, a measure of the sedimentation
velocity in a centrifuge; the faster a particle
travels when centrifuged, the greater its
Svedberg value or sedimentation coefficient.
The sedimentation coefficient is a function of a
particle’s molecular weigh, volume, and shape.
The Nucleoid
Usually procaryotics contain a single circle
of double-stranded DNA.
But some: a linear DNA chromosome
Recently:
Vibrio cholerae: >one chromosome.
Nucleoid: 60% DNA, 30% RNA, 10% protein
(by weight)
Exceptions:
1. Pirellula: a single membrane
2. Gemmata obscuriglobus: two membrane
Arrangement of DNA in Prokaryotes
DNA Supercoiling
To package the DNA
into the cell requires
that the DNA be
supercoiled.
There are over 50
supercoiled domains
in the E. coli
chromosome, they
are stabilized by
association with the
structural proteins.
• Plasmids: ds, circular or linear
DNA, exist and replicate
independently of the chromosome or
may be integrated with it, not
required for host growth and
reproduction.
• extrachromosomal DNA
• multiple copy number
• coding pathogenesis and antibiotic
resistance factors
• bacterial replication