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 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 Number and arrangement of flagella varies:

◦ monotrichous, lophotrichous, amphitrichous, peritrichous

Functions in motility of cell through environment

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1. Monotrichous – single flagellum at one end

2. Lophotrichous – small bunches arising from one end of cell

3. Amphitrichous – flagella at both ends of cell

4. 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:1. slime layer - loosely organized and attached2. capsule - highly organized, tightly attached

Functions:◦ 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 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 4.12Comparative cell envelopes

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

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

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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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--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:

singles diplococci – in pairs tetrads – groups of four irregular clusters chains cubical packets

◦ bacilli: chains palisades

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1. Microscopic morphology2. Macroscopic morphology – colony

appearance3. Physiological / biochemical

characteristics4. Chemical analysis5. Serological analysis6. Genetic and molecular analysis

• 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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