Chapter 3. Cell Structure and Function

Transcription

1 Chapter 3 Cell Structure and Function

2 How do you define life? Growth Reproduction Response to stimulus Metabolism

3 Prokaryotic and Eukaryotic Cells: An Overview Cells Prokaryotes Eukaryotes Figure 3.1

4 Prokaryotes Composed of bacteria and archaea Lack nucleus Lack various internal structures bound with phospholipid membranes Are typically 1.0 µm in diameter or smaller Have a simple structure

5 Eukaryotes Have nucleus Have internal membrane-bound organelles Are larger: µm in diameter Have more complex structure Composed of algae, protozoa, fungi, animals, and plants Will be covered in detail in Chapter 12

6 Approximate size of various types of cells. Figure 3.4

7 Structure plan of a prokaryote External Cell envelope Internal Glycocalyx Capsule Slime layer Appendages Flagella Fimbriae Pili Cell wall Cell membrane Cytoplasm Ribosomes Inclusions Nucleoid Cytoskeleton Endospore

8 Cross section of a typical prokaryotic cell Inclusions Ribosome Cytoplasm Nucleoid Glycocalyx Flagellum Cell wall Cytoplasmic membrane Figure 3.2

9 Model of a bacterial cell Cytoplasm = consists of a gel-like network Cell membrane = encloses the cytoplasm Cell wall = covers the cell membrane Nucleoid = area of the cytoplasm that contains the chromosome in the form of looped coils Flagellum = external helical filament whose rotary motor propels the cell

10 External structures Appendages Motility Flagella Axial filaments (periplasmic flagella) Attachment Fimbriae Channels Pili Surface coating- Glycocalyx

11 Glycocalyx Gelatinous, sticky substance surrounding the outside of the cell Composed of polysaccharides or polypeptides Bacillus anthracis = polypeptide capsule Streptococcus pneumoniae = polysaccharide capsule Functions Protect cells from dehydration and nutrient loss Inhibit killing by white blood cells by phagocytosis, contributing to pathogenicity Attachment - formation of biofilms

12 Glycocalyx Slime layer Loosely attached to cell surface Water soluble Sticky layer allows prokaryotes to attach to surfaces (capsule) Capsule Firmly attached to cell surface Composed of organized repeating units of organic chemicals May prevent bacteria from being recognized by host immune system (slime layer) Figure 3.5

13 Flagella Are responsible for movement (motility) Are not present on all bacteria Composed of protein flagellin Three parts make up a flagellum: filament, hook, and basal body Basal body anchors the filament and hook to cell wall Rotation propels bacterium through environment Bacteria move in response to stimuli (taxis): Runs or tumbles

14 Flagella: Structure Embed movie? PLAY Flagella: Structure

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16 Flagella: Movement Guide bacteria in a direction in response to external stimulus: Chemical stimuli chemotaxis; positive and negative Light stimuli phototaxis ; positive Signal sets flagella into motion clockwise or counterclockwise: Counterclockwise results in smooth linear direction run Clockwise tumbles Embed movie?

17 Flagellar arrangements Monotrichous single flagellum at one end Vibrio cholerae Amphitrichous flagella at both ends of cell Aquaspirillum sp. Lophotrichous small bunches emerging from the same site Peritrichous flagella dispersed all over surface of cell Proteus vulgaris Spirillum serpens

18 Periplasmic flagella (Axial filament) Internal flagella, enclosed in the space between the outer sheath and the cell wall peptidoglycan Produce cellular motility by contracting and imparting twisting or flexing motion Examples: Spirochetes like Borrelia burgdorferi Axial filament Endoflagella rotate Axial filament rotates around cell Outer membrane Cytoplasmic membrane Figure 3.8 Spirochete corkscrews and moves forward Axial filament

19 Fimbriae Sticky, bristle like projections Flagellum Fimbria Used by bacteria to adhere to one another and to substances in environment Shorter than flagella Serve an important function in biofilms Figure 3.10

20 Pili (Pilus) Rigid tubular structure made of pilin protein Pilus Mostly found in Gram-negative bacteria Longer than fimbriae but shorter than flagella Function to join bacterial cells for partial DNA transfer called conjugation Pili involved in conjugation are called sex pilus or F-pilus Figure 3.11

21 Bacterial Cell Walls Provide structure and shape and protect cell from osmotic forces Assist some cells in attaching to other cells or in resisting antimicrobial drugs Can target cell wall of bacteria with antibiotics Composed of peptidoglycan Scientists describe two basic types of bacterial cell walls Gram-positive and Gram-negative

22 Bacterial shapes, arrangements, and sizes Vary 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

23 Bacterial arrangements Arrangement of cells is dependent on pattern of division and how cells remain attached after division: Cocci: Singles in pairs (diplococci) groups of four (tetrads) Irregular clusters (staphylococci) Chains (streptococci) Cubical packets- groups of 8 (sarcina) Bacilli: Diplobacilli Chains Palisades

24 Peptidoglycan (Murein) Peptidoglycan is unique to bacteria Thus, the enzymes responsible for its biosynthesis make excellent targets for antibiotics Penicillin inhibits the transpeptidase that cross-links the peptides Vancomycin prevents cross-bridge formation by binding to the terminal D-Ala-D-Ala dipeptide Unfortunately, the widespread use of such antibiotics selects for evolution of resistant strains

25 Possible structure of peptidoglycan Sugar backbone Tetrapeptide (amino acid) crossbridge Connecting chain of amino acids Figure 3.13

26 Gram-Positive Bacterial Cell Walls Relatively thick layer of peptidoglycan Contain unique polyalcohols called teichoic acids and lipoteichoic acids Up to 60% mycolic acid in acid-fast bacteria helps cells survive desiccation Peptidoglycan layer (cell wall) Cytoplasmic membrane Gram-positive cell wall Lipoteichoic acid Teichoic acid Integral protein Figure 3.14a

27 Gram-Negative Bacterial Cell Walls Have only a thin layer of peptidoglycan Bilayer membrane outside the peptidoglycan contains phospholipids, proteins, and lipopolysaccharide (LPS) Lipid A portion of LPS can cause fever, vasodilation, inflammation, shock, and blood clotting Contain porin proteins in upper layer regulate molecules entering and leaving cell Porin Gram-negative cell wall Outer membrane of cell wall Peptidoglycan layer of cell wall Cytoplasmic membrane Porin (sectioned) Periplasmic space Lipopolysaccharide (LPS) layer, containing lipid A Integral proteins Phospholipid layers

28 Structures of Gram-positive and Gram-negative bacterial cell walls

29 Nontypical 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, e.g. Mycoplasma sp. Cell wall is stabilized by sterols Pleomorphic

30 Mycoplasmas Lack cell wall Unusual forms of medically significant bacteria Cell membrane has sterols- prevents lysis of cells Pleomorphic Mycoplasma pneumoniae: primary atypical pneumonia Colony shows fried egg morphology G.A. Meloni et al. J. Gen Microbiol. 1980

31 Bacterial Cytoplasmic Membranes Structure Referred to as phospholipid bilayer Composed of lipids and associated proteins Integral proteins Peripheral proteins Fluid mosaic model describes current understanding of membrane structure

32 The structure of a prokaryotic cytoplasmic membrane: a phospholipid bilayer Head, which contains phosphate (hydrophilic) Tail (hydrophobic) Phospholipid Integral proteins Cytoplasm Phospholipid bilayer Integral protein Integral protein Peripheral protein Figure 3.15

33 Bacterial Cytoplasmic Membranes Function Energy storage Harvest light energy in photosynthetic bacteria Selectively permeable Naturally impermeable to most substances Proteins allow substances to cross membrane Maintain concentration and electrical gradient

34 Transport across cell membrane (Passive processes)

35 Figure 3.19 Effects of isotonic, hypertonic, and hypotonic solutions on cells. Cells without a wall (e.g., mycoplasmas, animal cells) H 2 O H 2 O H 2 O Cell wall Cell wall Cells with a wall (e.g., plants, fungal and bacterial cells) H 2 O H 2 O H 2 O Cell membrane Isotonic solution Cell membrane Hypertonic solution Hypotonic solution

36 Transport across cell membrane (Active processes) Extracellular fluid Uniport Cytoplasmic membrane ADP ATP Cytoplasm P Uniport ADP ATP Antiport Energy in the form of ATP is used for transport Carrier proteins involved in transport P Symport Coupled transport: uniport and symport

37 Group translocation across cell membrane (Active processes) Glucose Extracellular fluid PO 4 Cytoplasm Glucose 6-PO 4

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39 Cytosol Liquid portion of cytoplasm Cytoplasm of Bacteria Mostly water Contains cell's DNA in region called the nucleoid Ribosomes Cytoskeleton Organization and segregation of bacterial chromosomes Xindan Wang, Paula Montero Llopis & David Z. Rudner Nature Reviews Genetics 14, (March 2013) Inclusions May include reserve deposits of chemicals

40 Chromosome Single, circular, double-stranded DNA molecule that contains all the genetic information required by a cell Plasmids Free small circular, double-stranded DNA Not essential to bacterial growth and metabolism Used in genetic engineering - readily manipulated and transferred from cell to cell Nucleoid

41 Bacterial ribosome Ribosomes (70S) where S= Svedberg units of sedimentation Made of 60% ribosomal RNA and 40% protein Consist of two subunits: large (50S) and small (30S) Prokaryotic differ from eukaryotic ribosomes in size and number of proteins Site of protein synthesis Found in all cells

42 A portion of an E. coli chromosome being transcribed (left to right) and being simultaneously translated. The arrow points to the putative site where RNA polymerase is first bound to the DNA. The chains of dark bodies are polysomes, that is, several ribosomes on the same mrna molecule. Miller OL Jr, Hamkalo BA, Thomas CA Jr. Visualization of bacterial genes in action. Science Jul 24;169(3943):

43 Intracellular storage bodies poly β hydroxyl butyrate (PHB) granules Vary in size, number, and content Bacterial cell can use them when environmental sources are depleted Ralstonia eutropha Wahl et al. BMC Microbiology 2012, 12:262

44 Cytoskeleton Composed of three or four types of protein fibers Can play different roles in the cell Cell division Cell shape Segregate DNA molecules Move through the environment Figure 3.24

45 Endospores Unique structures produced by some bacteria Defensive strategy against unfavorable conditions Vegetative cells transform into endospores when multiple nutrients are limited. Dehydrated, metabolically inactive Longevity verges on immortality, 250 million years Resistant to extreme conditions such as heat, radiation, chemicals Resistant to ordinary cleaning methods and boiling Pressurized steam at 120 o C for minutes will destroy them

46 The formation of an endospore 1 DNA is replicated. Cell wall Vegetative cell Cytoplasmic membrane DNA 5 A cortex of peptidoglycan is deposited between the membranes; meanwhile, dipicolinic acid and calcium ions accumulate within the center of the endospore. Cortex 2 DNA aligns along the cell s long axis. 6 Spore coat forms around endospore. Spore coat 3 Cytoplasmic membrane invaginates to form forespore. Forespore 4 Cytoplasmic membrane grows and engulfs First membrane forespore within a second membrane. 8 Vegetative cell s DNA disintegrates. Second membrane 7 Endospore matures: completion of spore coat and increase in resistance to heat and chemicals by unknown process. Endospore is released from original cell. Outer spore coat Endospore Figure 3.23

47 External Structures of Archaea Glycocalyces Flagella Hamus Grappling hook Fimbriae and hami Prickles Many archaea have fimbriae Some make fimbria-like structures called hami Function to attach archaea to surfaces Figure 3.25

48 Archaeal Cell Walls and Cytoplasmic Membranes Most archaea have cell walls Do not have peptidoglycan Contain variety of specialized polysaccharides and proteins All archaea have cytoplasmic membranes Maintain electrical and chemical gradients Control import and export of substances from the cell Figure 3.26

49 Cytoplasm of Archaea Archaeal cytoplasm similar to bacterial cytoplasm 70S ribosomes Fibrous cytoskeleton Circular DNA Archaeal cytoplasm also differs from bacterial cytoplasm Different ribosomal proteins Different metabolic enzymes to make RNA Genetic code more similar to eukaryotes

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