Introduction to Cells and Procaryotic Cell Structure

1. Chapter 4 An Introduction to Cells and Procaryotic 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: eucaryotic and procaryotic

3. Characteristics of Cells Eucaryotic cells: animals, plants, fungi, and protists • contain double-membrane bound nucleus with DNA chromosomes • contain membrane-bound organelles that compartmentalize the cytoplasmand perform specific functions Procaryotic 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/external stimuli; self-propulsion of many organisms • Cell support, protection, and storage mechanisms – cell walls, vacuoles, granules and inclusions • Transport of nutrients and waste

7. External Structures • Appendages • 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: • 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

9. 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

11. Flagellar Function Guide bacteria in a direction in response to external stimulus: chemical stimuli – chemotaxis; positive and negative light stimuli – phototaxis

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

13. 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

14. 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

17. 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 cell membrane, thin peptidoglycan layer, and cell membrane

18. Insert figure 4.12 Comparative cell envelopes

19. Structure of Cell Walls • Determines cell shape, prevents lysis (bursting) or collapsing due to changing osmotic pressures • Peptidoglycanis primary component: • unique macromolecule composed of a repeating framework of long glycan chains cross-linked by short peptide fragments

20. Gram-positive Cell Wall • 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

22. 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 lipopolysaccharidesand 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

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

25. 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 safranincounterstain • Important basis of bacterial classification and identification • Practical aid in diagnosing infection and guiding drug treatment

27. Atypical Cell Walls • Some bacterial groups lack typical cell wall structure i.e. Mycobacterium and Nocardia • Gram-positive cell wall structure with lipid mycolicacid (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

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

30. 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

31. 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.

32. 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

33. Bacterial Internal Structures • 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

34. 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)

36. 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

38. Endospores • 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

39. 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

41. 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: • chains • palisades

43. Classification Systems in the Procaryotae • 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

44. Bacterial Taxonomy Based on Bergey’s Manual • 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

45. 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

46. 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

47. 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)

48. Procaryotes 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

49. Procaryotes 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

50. 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