Chapter 4
An Introduction to Cells and
Procaryotic Cell
Structure and Function
All living things (single and multicellular) are
made of cells that share some common
characteristics:
◦ basic shape – spherical, cubical, cylindrical
◦ internal content – cytoplasm, surrounded by a
membrane
◦ DNA chromosome(s), ribosomes, metabolic
capabilities
Two basic cell types: eucaryotic and procaryotic
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Eucaryotic cells: animals, plants, fungi, and
protists
◦ contain double-membrane bound nucleus with DNA
chromosomes
◦ contain membrane-bound organelles that
compartmentalize the cytoplasm and perform specific
functions
Procaryotic cells: bacteria and archaea
◦ no nucleus or other membrane-bound organelles
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Growth and development
Reproduction and heredity – genome
composed of DNA packed in chromosomes;
produce offspring sexually or asexually
Metabolism – chemical and physical life
processes
Movement and/or irritability – respond to
internal/external stimuli; self-propulsion of
many organisms
Cell support, protection, and storage
mechanisms – cell walls, vacuoles, granules
and inclusions
Transport of nutrients and waste
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Appendages
◦ two major groups of appendages:
 Motility – flagella and axial filaments (periplasmic
flagella)
 Attachment or channels – fimbriae and pili
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Glycocalyx – surface coating
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3 parts:
◦ filament – long, thin, helical structure composed of
protein flagellin
◦ hook- curved sheath
◦ basal body – stack of rings firmly anchored in cell
wall
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Number and arrangement of flagella varies:
◦ monotrichous, lophotrichous, amphitrichous,
peritrichous
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Functions in motility of cell through
environment
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1.
2.
3.
4.
Monotrichous – single flagellum at one end
Lophotrichous – small bunches arising from
one end of cell
Amphitrichous – flagella at both ends of cell
Peritrichous – flagella dispersed over
surface of cell; slowest
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Guide bacteria in a direction in response to
external stimulus:
chemical stimuli – chemotaxis; positive and negative
light stimuli – phototaxis
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Fine, proteinaceous, hairlike bristles from
the cell surface
Function in adhesion to other cells and
surfaces
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Rigid tubular structure made of pilin protein
Found only in Gram negative cells
Function to join bacterial cells for partial DNA
transfer called conjugation
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Coating of molecules external to the
cell wall, made of sugars and/or
proteins
Two types:
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1. slime layer - loosely organized and attached
2. capsule - highly organized, tightly attached
Functions:
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protect cells from dehydration and nutrient
loss
inhibit killing by white blood cells by
phagocytosis contributing to pathogenicity
attachment - formation of biofilms
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External covering outside the cytoplasm
Composed of two basic layers:
◦ cell wall and cell membrane
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Maintains cell integrity
Two generally different groups of bacteria
demonstrated by Gram stain:
◦ Gram-positive bacteria: thick cell wall composed
primarily of peptidoglycan and cell membrane
◦ Gram-negative bacteria: outer cell membrane, thin
peptidoglycan layer, and cell membrane
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Insert figure 4.12
Comparative cell envelopes
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Determines cell shape, prevents lysis
(bursting) or collapsing due to changing
osmotic pressures
Peptidoglycan is primary component:
◦ unique macromolecule composed of a repeating
framework of long glycan chains cross-linked by
short peptide fragments
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Thick, homogeneous sheath of peptidoglycan
◦ 20-80 nm thick
◦ function in cell wall maintenance and enlargement
during cell division; move cations across the cell
envelope; stimulate a specific immune response
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Composed of an outer membrane and a thin
peptidoglycan layer
Outer membrane is similar to cell membrane
bilayer structure
◦ outermost layer contains lipopolysaccharides and
lipoproteins (LPS)
 endotoxin that may become toxic when released during
infections
 may function as receptors and blocking immune response
 contains porin proteins in upper layer – regulate
molecules entering and leaving cell
◦ Bottom layer composed of phospholipids and
lipoproteins
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Single, thin sheet of peptidoglycan
Protective structure while providing some
flexibility and sensitivity to lysis
Periplasmic space surrounds peptidoglycan
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Differential stain that distinguishes cells with a
Gram-positive cell wall from those with a
Gram-negative cell wall
◦ Gram-positive - retain crystal violet and stain purple
◦ Gram-negative - lose crystal violet and stain red from
safranin counterstain
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Important basis of bacterial classification and
identification
Practical aid in diagnosing infection and guiding
drug treatment
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Some bacterial groups lack typical cell wall
structure i.e. Mycobacterium and Nocardia
◦ Gram-positive cell wall structure with lipid mycolic
acid (cord factor)
 pathogenicity and high degree of resistance to certain
chemicals and dyes
 basis for acid-fast stain used for diagnosis of
infections caused by these microorganisms
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Some have no cell wall i.e. Mycoplasma
◦ cell wall is stabilized by sterols
◦ pleomorphic
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Phospholipid bilayer with embedded proteins
Functions in:
◦ providing site for energy reactions, nutrient
processing, and synthesis
◦ transport into and out of the cell
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Cell cytoplasm:
◦ dense gelatinous solution of sugars, amino acids,
and salts
◦ 70-80% water
 serves as solvent for materials used in all cell functions
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Chromosome
◦ single, circular, double-stranded DNA molecule
that contains all the genetic information
required by a cell
◦ DNA is tightly coiled around a protein,
aggregated in a dense area called the nucleoid.
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Plasmids
small circular, double-stranded DNA
free or integrated into the chromosome
duplicated and passed on to offspring
not essential to bacterial growth and metabolism
may encode antibiotic resistance, tolerance to toxic
metals, enzymes and toxins
◦ used in genetic engineering- readily manipulated
and transferred from cell to cell
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◦
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Ribosomes
◦ made of 60% ribosomal RNA and 40% protein
◦ consist of two subunits: large and small
◦ procaryotic differ from eucaryotic ribosomes in
size and number of proteins
◦ site of protein synthesis
◦ present in all cells
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Inclusions and granules
◦ intracellular storage bodies
◦ vary in size, number and content
◦ Bacterial cell can use them when environmental
sources are depleted.
◦ examples: glycogen, poly-b-hydroxybutyrate, gas
vesicles for floating, sulfur and phosphate granules
(metachromatic granules)
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Endospores
◦ inert, resting, cells produced by some G+ genera:
Clostridium, Bacillus and Sporosarcina
 have a 2-phase life cycle:
 vegetative cell – metabolically active and growing
 endospore – when exposed to adverse environmental conditions;
capable of high resistance and very long-term survival
◦ sporulation -formation of endospores
 hardiest of all life forms
 withstands extremes in heat, drying, freezing, radiation and
chemicals
 not a means of reproduction
◦ germination- return to vegetative growth
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Resistance linked to high levels of
calcium and dipicolinic acid
Dehydrated, metabolically inactive
thick coat
Longevity verges on immortality 25,250 million years.
Resistant to ordinary cleaning methods
and boiling
Pressurized steam at 120oC for 20-30
minutes will destroy
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Variety in shape, size, and arrangement
but typically described by one of three
basic shapes:
◦ coccus - spherical
◦ bacillus – rod
 coccobacillus – very short and plump
 vibrio – gently curved
◦ spirillum - helical, comma, twisted rod,
 spirochete – spring-like
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Arrangement of cells is dependent on pattern of
division and how cells remain attached after
division:
◦ cocci:
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singles
diplococci – in pairs
tetrads – groups of four
irregular clusters
chains
cubical packets
◦ bacilli:
 chains
 palisades
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1.
2.
3.
4.
5.
6.
Microscopic morphology
Macroscopic morphology – colony
appearance
Physiological / biochemical
characteristics
Chemical analysis
Serological analysis
Genetic and molecular analysis
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•
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G + C base composition
DNA analysis using genetic probes
Nucleic acid sequencing and rRNA analysis
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Bergey’s Manual of Determinative Bacteriology –
five volume resource covering all known
procaryotes
◦ classification based on genetic information –
phylogenetic
◦ two domains: Archaea and Bacteria
◦ five major subgroups with 25 different phyla
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Domain Archaea – primitive, adapted to
extreme habitats and modes of nutrition
Domain Bacteria -
◦ Phylum Proteobacteria – Gram-negative cell
walls
◦ Phylum Firmicutes – mainly Gram-positive with
low G + C content
◦ Phylum Actinobacteria – Gram-positive with high
G + C content
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Uses phenotypic qualities in identification
◦ restricted to bacterial disease agents
◦ divides based on cell wall structure, shape,
arrangement, and physiological traits
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Species –a collection of bacterial cells which
share an overall similar pattern of traits in
contrast to other bacteria whose pattern differs
significantly
Strain or variety – a culture derived from a
single parent that differs in structure or
metabolism from other cultures of that species
(biovars, morphovars)
Type – a subspecies that can show differences
in antigenic makeup (serotype or serovar),
susceptibility to bacterial viruses (phage type)
and in pathogenicity (pathotype)
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Free-living nonpathogenic bacteria
Photosynthetic bacteria - use
photosynthesis, can synthesize required
nutrients from inorganic compounds
◦ Cyanobacteria (blue-green algae)
 Gram-negative cell walls
 extensive thylakoids with photosynthetic chlorophyll
pigments and gas inclusions
◦ Green and purple sulfur bacteria
 contain photosynthetic pigment bacteriochlorophyll
 do not give off oxygen as a product of photosynthesis
◦ Gliding, fruiting bacteria
 Gram-negative
 Glide over moist surfaces
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Unusual forms of medically significant obligate
intracellular parasites
◦ Rickettsias
 Very tiny, Gram-negative bacteria
 Most are pathogens that alternate between mammals
and fleas, lice or ticks.
 Obligate intracellular pathogens
 Cannot survive or multiply outside of a host cell
 Cannot carry out metabolism on their own
 Rickettsia rickettisii – Rocky Mountain spotted fever
 Rickettsia prowazekii – epidemic typhus
 Coxiella burnetti – Q fever
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◦ Chlamydias
Tiny
Obligate intracellular parasites
Not transmitted by arthropods
Chlamydia trachomatis – severe eye infection and
one of the most common sexually transmitted
diseases
 Chlamydia psittaci – ornithosis, parrot fever
 Chlamydia pneumoniae – lung infections
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Constitute third Domain Archaea
Seem more closely related to Domain Eukarya
than to bacteria
Live in the most extreme habitats in nature,
extremophiles
Adapted to heat, salt, acid pH, pressure and
atmosphere
Includes: methane producers,
hyperthermophiles, extreme halophiles, and
sulfur reducers
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