Download Functional Anatomy of Prokaryotic and Eukaryotic Cells

Biol 3400
Tortora et al – Chap 4
Functional Anatomy of Prokaryotic and Eukaryotic Cells
I.
Introduction
Prokaryotic and eukaryotic cells are similar in a number of ways including
• Chemically similar – contain macromolecules: Nucleic acids, proteins, lipids and polysaccharides’
• Similar metabolic reactions to metabolize food, synthesis proteins and nucleic acids and store energy
• Contain a membrane, cytoplasm, DNA and ribosomes
Prokaryotic and eukaryotic cells differ in a number of ways (Table 4.2) including
Prokaryotic cells
• DNA is usually in the form of a single circular dsDNA chromosome and not enclosed in a
membrane
• DNA is not associated with histones; other proteins are associated with DNA
• They lack membrane bound organelles
• Usually divide by binary fission
Eukaryotic cells
• DNA is usually in the form of multiple linear dsDNA chromosomes found in a membrane
bound nucleus
• The DNA is consistently associated with chromosomal proteins called histones and with
nonhistone proteins
• They may have a number of membrane bound organelles, including endoplasmic reticulum,
Golgi complex, lysosomes, vacuoles, mitochondria and chloroplasts
• Cell division usually involves mitosis
Prokaryotic cells: Archaea (member of Archaeal domain = archaeon) and Bacteria (member of Bacterial
domain = bacterium)
Prokaryotic cell structures include the following (Fig. 4.6; Note: Underlined structures are found in all
prokaryotic cells):
• Plasma membrane
• Cytoplasm
• Nucleoid region
• Ribosomes
• Cell wall (and periplasmic space)
• Flagellum
• Pili and Fimbriae
• Inclusions
• Gas vacuole
• Capsule and slime layers
• Endospores
1
Biol 3400
Tortora et al – Chap 4
Eukaryotic cells: Protozoa, Fungi and Algae
Eukaryotic cell structures include the following (Fig. 4.22; Note: not all cells possess all of these
structures at all times):
⇒ Plasma membrane
⇒ Cell wall
⇒ True nucleus - membrane bound nucleus
⇒ Ribosomes
⇒ Membrane bound organelles
⇒ chloroplast
⇒ mitochondrion
⇒ endoplasmic reticulum
⇒ vacuoles
⇒ golgi apparatus
⇒ lysosomes
⇒ peroxisomes
⇒ Cytoskeleton
⇒ Flagellum/Cilium
II.
Cell Morphology
A. Prokaryotic Cells
Two most common cell shapes
• coccus (pl. cocci) e.g.,
• bacillus (pl. bacilli) e.g.,
Other shapes include:
• spirillum (pl. spirilli) e.g.,
• Mycelial – e.g., Actinomycetes
• Stalked – e.g., Caulobacter, Hypomicrobium
• Plates
• Star shape
Some prokaryotes are variable in shape and lack a single characteristic form = pleomorphic
e.g.,
B. Eukaryotic Cells
• Highly variable: cell shapes vary in shape from sphere and cylinders to very irregular nerve cells.
2
Biol 3400
Tortora et al – Chap 4
C. Cell Size
• Cells come in a variety of sizes & shapes
• Size limit is set by the logistics required to carry out metabolism
•
•
The smallest cells are nanobacteria (diameter of 0.05 to 0.2 µm).
Mycoplasmas = 0.2 µm in diameter
What factors constrain the lower size limit of cells?
• Most bacterial cells are 10 times larger than mycoplasmas (1 to 10 µm in diameter)
• Eukaryotic cells are typically 10 times larger than bacteria (10 - 100 µm in diameter).
What factors constrain the upper size limit of cells?
Generally prokaryotic cells are smaller than eukaryotic cells. But there are exceptions.
e.g.,
Epulopiscium fishelsoni
Thiomargarita namibiensis
size up to 80 x 600 µm
size up to 750 µm in diameter
Nanochlorum eukaryotum
1 to 2 µm in diameter
Generally cells are microscopic. But there are exceptions
Loligo - Atlantic squid has neurons with axon diameters as large as 1.0 mm.
Ostrich egg
III
Cell Components
A. Plasma membrane
•
•
•
•
Every cell is surrounded by a plasma membrane (also known as a cytoplasmic or cell membrane)
Encloses the cytoplasm
This is an important feature distinguishing archaea from bacteria and eukaryotes
Membranes are selectively permeable barriers - Why?
What is the function of the plasma membrane?
Fluid mosaic model (Fig 4.14)
• S.J. Singer and G. Nicolson (1972)
• Dynamic structure
• bifacial quality – sidedness
• 5 – 10 nm thick
3
Biol 3400
Tortora et al – Chap 4
1. Membrane components
i. Lipids
• backbone or basic fabric
• often as a lipid bilayer (amphipathic)
hydrophobic core
hydrophilic exterior surfaces
ii. Proteins
• integral (70 to 80 % of the membrane proteins)
• peripheral (20 to 30% of the membrane proteins)
Functions
o Transport – import and export
o Enzymes
o Receptors
o Intracellular junctions
o Cell to cell recognition
o Metabolic processes
iii. Carbohydrates
• Usually associated with the outer surface of the membrane
2. Differences between bacterial, eukaryotic and archaeal membranes
i. Bacterial membranes
• Like eukaryotes, most of the membrane lipids are phospholipids
• unlike eukaryotes bacteria lack sterols such cholesterol but contain sterol like compounds called
hopanoids. The role of hopanoids is likely similar to steroids – stabilized membranes.
• Some bacteria may have extensive in folding of the plasma membrane to increase membrane surface
area for the purpose of greater metabolic activity
Glycerol diesters (phospholipids)
o Glycerol bonded to phosphate (negatively charged) and two fatty acids
o Linkage between fatty acids and glycerol is an ester linkage
Ester linkage
O
||
R-O-C-(CH2)n CH3
4
Biol 3400
Tortora et al – Chap 4
•
Environmental conditions affect fatty acid composition of membranes
o Fatty acids are generally unbranched and 16 to 18 carbons long
e.g., palmitic acid CH3(CH2)14 COOH
stearic acid - 18 carbons
o Fatty acids may be saturated or unsaturated
•
Membrane composition also varies with species and these differences can be used to identify bacteria
(Fatty acid methyl ester analysis - FAME).
ii. Eukaryotic membranes
o generally the same structure as bacterial membranes including phospholipids but differs in the
major lipids: phospholipids, sphingolipids and cholesterol.
o Microdomains that differ in protein and lipid composition may be found – lipid rafts – span
membrane and appear to be involved in a variety of cellular processes (e.g., signal transduction
and cell movement)
Sterols
o compounds consisting of carbon skeleton of 4 interconnected rings
o targets for polyene antibiotics - damage cell membranes
o Mycoplasmas acquire sterols from eukaryotic cells
iii.
•
•
•
•
Archaeal membranes
Many are thought to be “Extremophiles”
Membranes are distinctive
Phospholipids are not the main structural components
Have branched chain hydrocarbons attached to glycerol by ether linkages
Glycerol diethers
Ether linkage
R-C-O-C-R
Bilayers
o glycerol bound to branched hydrocarbons (e.g., phytanyl) by ether linkage
o glycerol diethers
Monolayers
o diglycerol tetraether
o often found in extreme thermophiles
•
•
Phosphate, sulfur and sugar containing groups are attached to the third carbon of glycerol resulting in a
polar lipids that are the predominant lipids (70 – 93%) in the membrane
Nonpolar lipids (squalene derivatives) make up the rest of the membranes
5
Biol 3400
Tortora et al – Chap 4
3. The movement of materials across membranes
Review this material on your own – it is largely review from Biol 1010. You should be familiar with the
following
Passive Transport
• Simple diffusion
• Facilitated diffusion
o Transporter proteins
• Osmosis
o Aquaporin
o Osmotic pressure
Active Transport
o Group translocation
B. Cell walls
1. Bacterial cell walls
Functions
• Define cell shape
• Protection from osmotic shock (turgor pressure) and toxic substances
• Point of anchorage for flagella
• May contribute to pathogenicity
• Most bacteria have cell walls but there are exceptions – e.g., mycoplasmas
• Forms a strong, protective layer that is relatively porous, elastic and somewhat stretchable
Gram positive and Gram negative (Fig. 4.13)
Can be differentiated through the Gram stain - an important diagnostic tool.
• Gram negative
• Gram positive
• Gram variable
i. Peptidoglycan (murein)
Components
a. Polysaccharide
• β1,4 linkages between N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) monomers
(Fig 4.12).
• Note: N-acetylmuramic acid is found only in Bacteria
• Polymers 10 to 65 monomers long
• ...NAM – NAG - NAM - NAG - NAM - NAG...
6
Biol 3400
Tortora et al – Chap 4
b. Peptide chains
Tetrapeptide chains linked to M residues - composed of unusual amino acids (D- amino acids) – Why?
• L-alanine
• D-glutamic acid
• L-lysine (Gram positive) or diaminopimelic acid (DAP; Gram negative)
• D-alanine
There are either peptide interbridges (G-G-G-G-G: Gram positive) or direct peptide linkages (Gram
negative) between tetrapeptides → Results in a strong multilayer sheet or sacculus
Autolysins - enzymes used by the bacteria to recycle, reshape or restructure cell wall
Lysozyme
• Hydrolyes the β1,4 linkages between M and G monomers
Sources of lysozyme
• predators
• tears
• saliva
• chicken egg white
•
spheroplast - partial removal of cell wall
•
protoplast - complete removal of cell wall
Penicillin
• Prevents formation of peptides cross linkages between tetrapeptides
• Penicillin binds to transpeptidase
• Not effective against bacteria lacking cell walls such as mycoplasmas
ii. Gram positive cell wall
• Up to 20% of the organism
• peptidoglycan represents 50-90% of this structure
• relatively thick (ca. 40 nm – ranges from 20 to 80 nm)
• a thick peptidoglycan layer is more resistant to desiccation
• Polysaccharides such as teichoic acids (e.g., glycerophosphate or ribitol phosphate residues –
negatively charged) or teichuronic acids are bonded to the peptidoglycan or plasma membrane
lipids
• Putative function
- bind to cations and regulate movement into and out of the cell
- Prevent extensive cell wall lysis
• Responsible for cell wall’s antigenicity
e.g., Bacilli, Staphylococci, Streptococci
7
Biol 3400
Tortora et al – Chap 4
iii. Gram negative cell envelope
• more complex than Gram positive cell wall
Cell wall
• thin layer of peptidoglycan (ca. 2 to 7 nm)
• 1 - 10 % of the cell wall
• lipoproteins instead of teichoic acids
Outer membrane - lipid bilayer
• phospholipids
• proteins - e.g., porins
• lipoproteins – anchored to peptidoglycan
• The outer membrane may be linked to the plasma membrane in a number of places
• lipopolysaccharides (LPS)
• composed of i) lipid A, ii) the core polysaccharide and iii) the O polysaccharide side chain.
• main structural component of outer half of outer membrane believed to aid in creating a
permeability barrier as well as contributes to negative charge of the cell surface.
• Protects pathogenic bacteria from host defenses
• The LPS (Lipid A componenet in particular) is frequently toxic to animals and known as
endotoxin (i.e., because it is still attached to cell).
• O polysaccharide is an antigen that is used to distinguish species of bacteria
•
•
•
The outer membrane is more permeable than the plasma membrane to most small molecules due to
the presence of porin proteins
Less permeable than the plasma membrane to hydrophobic and amphipathic molecules - makes
cells less susceptible to certain antibiotics.
Keeps periplasmic enzymes from diffusing away.
Periplasmic space (30 - 70 nm wide)
• Gel-like in consistency due to abundance of proteins
Import region where number of chemical reactions occur:
• oxidation-reduction reactions
• osmotic regulation
• solute transport
• hydrolysis
• protein secretion
8
Biol 3400
Tortora et al – Chap 4
2. Archaeal cell wall
• No peptidoglycan, in particular lacking in N-acetylmuramic acid
• May be composed of
• Pseudopeptidoglycan – N-acetylglucosamine and N-acetyltalosaminuronic acid linked by β1,3
linkages
• Protein or glycoproteins - most common form of cell wall
• Polysaccharide
NOTE: Some Bacteria and Archaea do not have cell walls
e.g., mycoplasma
Thermoplasma
3. Eukaryotic cell wall
• Like Bacteria and Archaea, not all eukaryotes have cell walls
Algal cell wall
• Polysaccharides are the major components
e.g., cellulose
• May contain high concentrations of calcium or silicon - diatoms
Fungi
• Many contain chitin - polysaccharide consisting of N-acetylglucosamine monomers.
• Composition is used in classification schemes
e.g., primitive Chytridiomycetes fungal cell walls lack chitin but contain cellulose
Protists
• Many protists have a pellicle for support – rigid layer of components beneath the plasma membrane.
• Pellicle is composed of protein
9
Biol 3400
Tortora et al – Chap 4
C.
Layers External to the Cell Wall
Prokaryotes
• Prokaryotes have a variety of layers outside the cell wall
Functions
• protection
⇒ ingestion
⇒ dehydration
⇒ loss of nutrients
• attachment
• pathogenicity
•
•
Form thick or thin, rigid or flexible layers depending upon composition
Variable composition – glycoproteins and polysaccharides
1. Capsule
• Rigid – tight matrix that excludes particles
• protein or polysaccharide
2. Slime layer
• loosely bound layer – easily deformed
3. Extracellular Polymeric Substance (EPS)
• glycocalyx that helps cells bind to surfaces and other cells – formation of biofilm
Glycocalyx – term that collectively refers to both capsule and slime layer
3. Surface layer (S-layer)
• nearly all bacteria and Archaea
• crystalline protein layer
• unknown function – may function as permeability or protective barrier
D.
Genetic Information
Deoxyribonucleic acid (DNA) - macromolecule consisting of nucleotide monomers
• Backbone = 5'...-Phosphate-Sugar-Phosphate-Sugar -Phosphate-Sugar-…3'
Nucleotide = nucleoside plus phosphate
Nucleoside = deoxyribose + nitogenous base
Purines – adenine & guanine
Pyrimidines - cytosine & thymine
10
Biol 3400
Tortora et al – Chap 4
Review structure of DNA – Chapter 2
1. Bacterial and Archaeal DNA
• These organisms do not have a nucleus – the bulk of the genetic material is found in the nucleoid
region
Chromosome
• single double stranded DNA molecule in the form of a covalently closed circular chromosome –
also known at the genophore
• Exceptions: Borrelia burgdorferi and some Streptomyces spp. have a linear chromosome;
Rhodobacter sphaeroides has two circular chromosomes
•
usually only one chromosome and it may be present as multiple copies in rapidly growing cells
•
DNA arranged into supercoiled domains that are stabilized with structural proteins
•
In some Archaea – DNA is extensively complexed with proteins that closely resemble histone
proteins of eukaryotic organisms
Plasmids
• autonomously replicating units that usually contain only a few genes (< 30; 1 – 5% of the size of the
chromosome). Most the bacterial and archaeal genomes sequenced contain plasmids
• Usually covalently closed circles (CCC) but may be linear
• May contain one to many plasmids
• Range in size from several kbp to Mbp
Examples of plasmid coded factors (Table 3.3)
1. R-plasmids - antibiotic resistance genes
2. Bacteriocins
3. Conjugal factors
4. Metabolic factors - substrate utilization or fixation
5. Virulence factors - toxin production
2.
•
•
•
•
Eukaryotic DNA
Most possess a nucleus that contains the bulk of the genetic material
Nucleus contains a number of linear chromosomes
Chromosomes composed of DNA complexed with protein → chromatin
DNA associated with Histones in structural subunits called nucleosomes
•
Chloroplasts and mitochondria also contain small circular genomes
11
Biol 3400
Tortora et al – Chap 4
E.
•
•
•
•
Ribosomes
Sites of protein synthesis
Approximately 20 - 25 nm in diameter
Large numbers may be present in cells (>10,000 in bacterial cells and more in eukaryotic cells)
Number varies depending on the level of protein synthesis going on in the cell.
Bacteria and
Archaea
Eukarya
Ribosome
70 S*
80S
Large subunit
50 S
23S rRNA and 5S rRNA
Large number of proteins
e.g., 34 proteins in E. coli
60S
25 - 28S rRNA and 5.8S rRNA
Large number of proteins
Small subunit
30S
16S rRNA
Large number of proteins
e.g., 21 proteins in E. coli
40S
18S rRNA
Large number of proteins
* Svedberg units (S)
•
•
Eukaryotic organisms have 70S ribosomes in their mitochondria and chloroplasts
Implications in the endosymbiotic theory
Practical implications of ribosome structural differences
•
Antibiotic treatment
chloramphenicol
tetracycline
kanamycin
erythromycin
streptomycin
all bind 70S bacterial
ribosomes and disrupt protein
synthesis
diptheria toxin
anisomycin
bind 70S archaeal and 80S eukaryotic ribosomes and
disrupts protein synthesis
12
Biol 3400
Tortora et al – Chap 4
F.
Cytoskeleton
Prokaryotic Cells
• For many years it was thought that prokaryotic cells lacked cytoskeletal elements. Recently homologues
have been discovered for all three elements of the eukaryotic cytoskeleton
e.g.,
FtsZ – tubulin homologue; widely observed in Bacteria and Archaea
MreB – actin homologue; Many rod shaped cells
Crescentin – intermediate filament proteins; Caulobacter
Eukaryotic Cells
• Have a well developed three dimensional network of fibrous proteins (microtubules, microfilaments and
intermediate filaments
• Microtubules and microfilaments are very dynamic structures – can be quickly disassembled and
assembled elsewhere
Functions
• Support
• Maintenance of cell shape
• Cell movement - e.g., muscle cell contraction, amoeboid movement, cilia
• Cell division (mitosis, cytokinesis and meiosis)
• Cell wall deposition
• Provides spatial organization and movement of organelles and cytosolic enzymes
• Regulation of biochemical activities in the cell – mechanical signaling
Cytoskeleton components
a) Microtubules
• thickest cytoskeletal elements - hollow rods - 25 nm
• found in the cytoplasm of all eukaryotes
• composed to α and β tubulin dimers
• readily assembled and disassembled
• grow by addition of subunits to ends
• centrosome - microtubule organizing centre (MOC) important in cell division
• a pair of centioles (9 sets of microtubule triplets) often found in the centrosome region of animal
cells. Replicate during cell division. May function in cell division but not necessary as they are
generally not found in plant cells.
Functions
• compression resisting function
• cell motility (flagella and cilia)
• chromosome movement
• organelles movement
• cell shape
13
Biol 3400
Tortora et al – Chap 4
b) Microfilaments
• thinnest cytoskeletal element - 7 nm in diameter
• composed of protein called actin
• readily assembled and disassembled
Functions
• tension bearing function
• muscle contraction (myosin motors molecules-burn ATP)
• cytoplasmic streaming (myosin motors molecules-burn ATP)
• cell motility (pseudopodia)
• cell division
• cell shape – three dimensional network just inside the plasma membrane
c) Intermediate filaments
• 8 to 12 nm in diameter
• diverse class composed of different protein subunits including keratins
• more permanent structures not readily assembled and disassembled like microtubules and
microfilaments
Functions
• tension bearing function
• maintenance of cell and organelle shape e.g. nuclear lamina
• fixing positions of certain organelles
G.
Motility
Bacterial and Archaeal Movement
1. Flagellum (pl. flagella)
• one to many flagella
• up to 60 cell lengths/s
Atrichous – no flagella
Monotrichous - single polar flagellum
Amphitrichous - single flagellum at each pole
Lophotrichous - polar tuft of flagella
Peritrichous - multiple flagella disgributed over the entire cell
14
Biol 3400
Tortora et al – Chap 4
Parts of a prokaryotic flagellum (Fig 4.8)
i. filament - whiplike extension that rotates - helical in shape – 15 to 20 µm long
composed of flagellin – highly conserved in bacteria
ii.
curved hook - single protein - connects filament to motor
iii. Basal apparatus or motor composed of many proteins (~30)
central rod → a series of rings embedded in the cell wall, plasma membrane and outer
membrane (Gram negative cells). This structure is around 20 nm in diameter
Mot proteins drive the flagellar motor - energy comes from proton motive force (about 1000
H+/rotation)
Fli proteins act as a switch
> 40 genes (fla, fli, flg) required for synthesis and motility (structural, export of components
and timing of synthesis)
2. Gliding
• cells glide along a surface.
• Mechanism is unknown but there are several models
a) excreted slime adheres to surface pulls cells along - cyanobacteria
b) movement of surface proteins - Flavobacterium
3. Gas Vesicles
• confer buoyancy on cells and allow to move up and down in a water column
• composed of two types of protein (97% of the gas vacuole is composed of GvpA (β-sheets). The
remainder is made of GvpC (α-helix) that acts like a cross linker between the GvpA molecules
4. Behavioural Responses to Stimuli
• In a heterogenous environment prokaryotes are capable of movement (taxis; pl. taxes) towards or
away from stimuli (light, heat, chemicals and electricity)
Chemotaxis
• movement towards or away from a chemical stimulus (chemoattractant or chemorepellent).
• Respond to temporal gradient
• Respond to very low levels of some materials – 10-8 M for some sugars
Types of Taxes
phototaxis - light stimulus
scotophobotaxis - entering darkness has a negative effect on a cell
geotaxis - gravitational stimuli
magnetotaxis - magnetic stimuli
15
Biol 3400
Tortora et al – Chap 4
aerotaxis – oxygen stimulus
osmotaxis – high osmotic strength stimulus
Bacteria lack spatial sensing capabilities- they are too small to sense a gradient along the cell length.
Consequently they respond in a temporal fashion.
1.
Periodically sample environment e.g., chemoreceptors – sensory proteins
2.
Process information through a signal transduction pathway
3.
Control of direction of flagella rotation
Movement is through a series of
a) alternating runs (straight line movement- counterclockwise rotation of flagella) and tumbles (reversal of
direction of flagella rotation)
b) The direction of the next run is random; however, if there is a gradient of a chemical attractant or
repellent present, the random movements become biased
• If the organisms sense an increase in attractant concentration then this results in longer runs and
tumbles are less frequent. This is also the case if the cell senses a decrease in repellant
concentration
Eukaryote Movement
• Cilia and flagella are the most prominent structures for movement
Cilia (sing. - cilium) and Flagella (singl. - flagellum) are composed of microtubules
•
•
•
•
•
•
Cilia and flagella are 0.25 µm in diameter
A plasma membrane encases a core of microtubules arranged in the 9 + 2 pattern (Fig 4.23): and
outer ring of 9 evenly spaced doublets around 2 two single microtubules
radial spokes and cross-linking proteins connect the microtubules;
dynein arms (named after the protein composing these structures) connect microtubules
anchored to cell by a basal body - identical in structure to centrioles
chemical energy required for movement
Comparison of Flagella and Cilia
Length
Cilia
2 - 20 µm
Flagella
•
10 - 200 µm
Number
large number
(100-1000’s)
Movement
back and forth - rowing
1 to several
undulating or snakelike
(sperm cell)
The eukaryotic flagellum is much different in structure than prokaryotic flagella (Another
difference between Prokaryotes and Eukaryotes!)
16
Biol 3400
Tortora et al – Chap 4
H.
Attachment Structures of Prokaryotes
Glycocalyx – discussed already
Pili and/or Fimbriae
• hair like proteinaceous projections used as attachment structures
fimbriae
• Slender tubes (3 – 10 nm in diameter) composed of helically arranged protein subunits
• function in attachment and twitching movement
• more numerous than pili
pili
• generally longer and thicker than fimbriae but fewer in number
• attachment between mating bacterial cells
• consist of phosphate-carbohydrate-protein (single type of pilin)
• F pilus - important structure involved in bacterial mating (conjugation) which is carried exclusively
by the donor cell
I.
•
•
•
Inclusion bodies of Prokaryotes
Storage of energy or structural compounds
Often form in response to excess or imbalance in nutrients that are available
Usually bounded by a thin non-unit membrane
examples
• phosphate – polyphosphate – metachromacit granules
• sulfur
• glycogen or starch
• poly-β-hydroxyalkanoates (e.g., poly-β-hydroxybutyric acid - PHB) are common carbon reserves
produced by Bacteria and Archaea
e.g.,
B. thuringienisis
YPD mediium – very rich medium – PHB
NA medium – spore formation
J.
Spores
Functions
• reproduction
• dispersal - fungi and some actinomycetes produces spores that are readily dispersed
• survival under adverse conditions
17
Biol 3400
Tortora et al – Chap 4
1.
•
•
•
•
•
•
•
Bacterial endospore
Differentiated cells
Formed within the cell – refractile body
Dormant structures that are highly resistant to heat, desiccation, UV irradiation, chemical
disinfectant – Not a reproductive structure
Found commonly in soil
Contains little water
Complex multilayer spore coat contains peptidoglycan and calcium dipicolinate (required for heat
reistance) in its core
Endospore producing bacteria - at least 16 genera, including
⇒ Bacillus
⇒ Clostridium
⇒ Sporosarcina
⇒ Oscillospira
⇒ Desulfotomaculum
i) Endospore Structure
Exosporium
Spore coat
Cortex
Core wall
Core
Exosporium
• thin delicate proteinaceous covering
Spore coat
• several protein layers
• Impermeable and responsible for spores resistance to chemicals
Cortex
• up to half the spore volume
• peptidoglycan (less cross linked than vegetative cell)
Core or spore wall
c) usual cell wall
18
Biol 3400
Tortora et al – Chap 4
Spore core
• plasma membrane, cytoplasm, nucleoid
• energy supply → phosphoglycerate
• contains Ca2+ complexed with dipicolinic acid
• 10 – 30 % water content of vegetative cells – water moves freely in and out of endospores
• heat resistance of endospore is determined by state and amount of water in core
• pH is one unit lower than vegetative cells
• contains small acid-soluble spore proteins (SASP) which bind DNA and protect it from damage due
to UV, heat and desiccation. May also serve as a carbon and energy source during germination
• Ca2+ dipicolinic acid complexes and SASP form a gel in cytoplasm – gel like material excludes
water from the endospore protoplasm
• Low metabolic activity, no macromolecule synthesis and few or no mRNA molecules
• The core also contains DNA repair enzymes that repair the the DNA once the spore germinates
ii Sporulation (sporogenesis)
• a very complex inducible process and is controlled by > 200 sporulation specific genes.
• triggered by environmental conditions which are unfavourable for vegetative growth. In Bacillus
species → C, N or P limitation.
• a multistage process (seven stages have been described for Bacilli) that can last up to 8 hours or
more. Vegetative cell or mother cell becomes compartmentalized forming a spore compartment
through invagination of the plasma membrane (sporangium).
• Endospores dormant for many years
If conditions become favourable for vegetative growth
then the endospore can rapidly convert back to a vegetative cell.
Three stages
i) Activation
• prepares endospore for germination
• reversible process
• treatments like heating conditions endospore to germinate when placed in a nutrient solution
ii) Germination
• breaking of an endospore’s dormant state, germinants - such as glucose and certain amino acids may
induce gemination
• characterized by
• loss of refractility
• loss of resistance to heat and chemicals
• loss of Ca dipicolinate and cortex components, and degradation of SASPs
• and increased ability to be stained by dyes
iii) Outgrowth
• develops into a vegetative cell
• uptake of water → swelling
• macromolecule synthesis
• vegetative growth until encounters environmental triggers again
19
Biol 3400
Tortora et al – Chap 4
K.
Additional Features of Eukaryotic Cells
Review cell structures and concepts including
• Endomembrane system and all of its components
• Mitochondria
• Chloroplast
• Endosymbiotic theory – eukaryotic cells arose from larger cells engulfing smaller cells
Mitochondria and chloroplasts
• contain DNA – closed covalently circular form
• contain their own ribosomes – 70 S
• protein synthesis in these organelles is affected by antibiotics such as streptomycin that also kill
or inhibit bacteria
• rRNA sequences of mtDNA and cpDNA are more closely related to bacteria rRNA
20