1. An Introduction to Cells and Prokaryotic Cell Structure and Function
Chapter 4
An Introduction to Cells and Prokaryotic Cell Structure and Function

2. Characteristics of Cells and Life
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: eukaryotic and prokaryotic

3. Characteristics of Cells
Eukaryotic 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
Prokaryotic cells: bacteria and archaea
no nucleus or other membrane-bound organelles

4. Characteristics of Life
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 or external stimuli; self-propulsion of many organisms
Cell support, protection, and storage mechanisms – cell walls, vacuoles, granules and inclusions
Capacity to transport nutrients and waste into and out of the cell

5. Prokaryotic cell

7. External Structures Appendages Glycocalyx – surface coating
two major groups of appendages:
motility – flagella and axial filaments (periplasmic flagella)
attachment or channels – fimbriae and pili
Glycocalyx – surface coating

8. Flagella 3 parts: Rotates 360o
filament – long, thin, helical structure composed of protein flagellin
hook- curved sheath
basal body – stack of rings firmly anchored in cell wall
Rotates 360o
Number and arrangement of flagella varies:
monotrichous, lophotrichous, amphitrichous, peritrichous
Functions in motility of cell through environment

10. Flagellar Arrangements
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

12. Flagellar Function
Guide bacteria in a direction in response to external stimulus:
chemical stimuli – chemotaxis; positive and negative
light stimuli – phototaxis
Signal sets flagella into rotary motion clockwise or counterclockwise:
counterclockwise – results in smooth linear direction – run
clockwise - tumbles

14. Chemotaxis in bacteria
Insert figure 4.5
Chemotaxis in bacteria

15. Axial Filaments
Periplasmic, internal flagella, enclosed between cell wall and cell membrane of spirochetes
Produce cellular motility by contracting and imparting twisting or flexing motion

17. Fimbriae Fine, proteinaceous, hairlike bristles from the cell surface
Function in adhesion to other cells and surfaces

18. Pili Rigid tubular structure made of pilin protein
Found only in Gram negative cells
Function to join bacterial cells for partial DNA transfer called conjugation

19. Glycocalyx
Coating of molecules external to the cell wall, made of sugars and/or proteins
Two types:
slime layer - loosely organized and attached
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

24. The Cell Envelope 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 membrane, thin peptidoglycan layer, and cell membrane

27. Structure of Cell Walls
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

29. Gram-positive Cell Wall
Thick, homogeneous sheath of peptidoglycan
20-80 nm thick
includes teichoic acid and lipoteichoic acid: function in cell wall maintenance and enlargement during cell division; move cations across the cell envelope; stimulate a specific immune response

30. Gram-negative Cell Wall
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

31. Gram-negative Cell Wall
Single, thin sheet of peptidoglycan
Protective structure while providing some flexibility and sensitivity to lysis
Periplasmic space surrounds peptidoglycan

33. The Gram Stain
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

35. Atypical Cell Walls
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

36. Cell Membrane Structure
Phospholipid bilayer with embedded proteins – fluid mosaic model
Functions in:
providing site for energy reactions, nutrient processing, and synthesis
transport into and out of the cell

39. Bacterial Internal Structures
Cell cytoplasm:
dense gelatinous solution of sugars, amino acids, and salts
70-80% water
serves as solvent for materials used in all cell functions

40. Bacterial Internal Structures
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

42. Bacterial Internal Structures
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

43. Bacterial Internal Structures
Ribosomes
made of 60% ribosomal RNA and 40% protein
consist of two subunits: large and small
Prokaryotic differ from Eukaryotic ribosomes in size and number of proteins
site of protein synthesis
present in all cells

45. Bacterial Internal Structures
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)

48. Bacterial Internal Structures
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

49. Endospores
Resistance linked to high levels of calcium and dipicolinic acid
Dehydrated, metabolically inactive
thick coat
Longevity verges on immortality – 25 to 250 million years
Resistant to ordinary cleaning methods and boiling
Pressurized steam at 120oC for minutes will destroy

52. Bacterial Shapes, Arrangements, and Sizes
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

56. Bacterial Shapes, Arrangements, and Sizes
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:
palisades

59. Classification Systems in the Prokaryotae
Microscopic morphology
Macroscopic morphology – colony appearance
Physiological / biochemical characteristics
Chemical analysis
Serological analysis
Genetic and molecular analysis
G + C base composition
DNA analysis using genetic probes
Nucleic acid sequencing and rRNA analysis

61. Bacterial Taxonomy Based on Bergey’s Manual
Bergey’s Manual of Determinative Bacteriology – five volume resource covering all known prokaryotes
classification based on genetic information – phylogenetic
two domains: Archaea and Bacteria
five major subgroups with 25 different phyla

62. Major Taxonomic Groups of Bacteria
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

64. Diagnostic Scheme for Medical Use
Uses phenotypic qualities in identification
restricted to bacterial disease agents
divides based on cell wall structure, shape, arrangement, and physiological traits

66. Species and Subspecies
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)

67. Prokaryotes with Unusual Characteristics
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

68. Prokaryotes with Unusual Characteristics
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

69. Unusual Forms of Medically Significant
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

70. Archaea: The Other Prokaryotes
Constitute third Domain Archaea
Seem more closely related to Domain Eukarya than to bacteria
Contain unique genetic sequences in their rRNA
Have unique membrane lipids and cell wall construction
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