Chapter 3: The Cell PDF

CHAPTER 3. The Cell  How are Prokaryotes Different from Eukaryotes? The way their DNA is packaged  No nucleus  Not wrapped around histones The makeup of their cell wall  Bacteria- peptidoglycan  Archae- tough and made of other chemicals, distinct to them Their internal structur...

CHAPTER 3. The Cell  How are Prokaryotes Different from Eukaryotes? The way their DNA is packaged  No nucleus  Not wrapped around histones The makeup of their cell wall  Bacteria- peptidoglycan  Archae- tough and made of other chemicals, distinct to them Their internal structures  No complex, membrane-bound organelles  Structures common to all bacterial cells  Cell membrane  Cytoplasm  Ribosomes  One (or a few) chromosomes  Structures found in most bacterial cells Cell wall Surface coating or glycocalyx  Structures found in some bacterial cells  Flagella  Pili  Fimbriae  Capsules  Slime layers  Inclusions  Actin cytoskeleton  Endospores 6 Bacterial Arrangements and Sizes  Bacterial cells are capable of carrying out all necessary life activities  Reproduction  Metabolism  Nutrient processing  Bacteria can also act as a group  Colonies  Biofilms  Arrangement, or Grouping  Cocci- greatest variety in arrangement  Single  Pairs (diplococcic)  Tetrads  Irregular clusters (staphylococci and micrococci)  Chains (streptococci)  Cubical packet (sarcina)  Bacilli- less varied  Single  Pairs (diplobacilli)  Chain (streptobacilli)  Row of cells oriented side by side (palisades)  Spirilla  Occasionally found in short chains Figure 3.13 Figure 3.14  GLYCOCALYX 3-11 3-11  External Structures  Appendages: Cell extensions  Common but not present on all species  Can provide motility (flagella and axial filaments)  Can be used for attachment and mating (pili and fimbriae)  Flagella  Three parts: Filament, hook (sheath), and basal body  Vary in both number and arrangement › Polar arrangement: flagella attached at one or both ends of the cell  Monotrichous- single flagellum  Lophotrichous- small bunches or tufts of flagella emerging from the same site  Peritrichous- dispersed randomly over the structure of the cell FLAGELLA http://www.professorcrista.com/ cellular-structure-animations.html Arrangements of Flagella Monotrichous Lophotrichous 3-16 3-16  Arrangements of Flagella Amphitrichous Peritrichous 3-17 3-17  Flagellar Function  Chemotaxis- positive and negative  Phototaxis  Move by runs and tumbles Figure 3.33  Bacteria achieve directional movement by changing the rotation of their flagella. In a cell with peritrichous flagella, the flagella bundle when they rotate in a counterclockwise direction, resulting in a run. However, when the flagella rotate in a clockwise direction, the flagella are no longer bundled, resulting in tumbles. Figure 4.5 Periplasmic Flagella  Axial filament:  Two or more long coiled threads found in spirochetes  Internal flagellum enclosed between the cell wall and cell membrane  Impart a twisting or flexing motion to the cell Periplasmic Flagella Figure 3.29 (a)Capsules are a type of glycocalyx composed of an organized layer of polysaccharides. (b)A capsule stain of Pseudomonas aeruginosa, a bacterial pathogen capable of causing many different types of infections in humans. (credit b: modification of work by American Society for Microbiology) Figure 3.30  Bacteria may produce two different types of protein appendages that aid in surface attachment. Fimbriae typically are more numerous and shorter, whereas pili (shown here) are longer and less numerous per cell. (credit: modification of work by American Society for Microbiology)  Pili  Elongate, rigid tubular structures Made of the protein pilin  Longer than fimbriae but shorter than flagella  Bacteria typically only have one or two per cell  Join two bacterial cells and mediate the transfer of DNA from one cell to another (conjugation) Also known as conjugation pili or sex pili  Found on gram-negative bacteria  Pilus Versus Fimbriae 3-26 3-26  Fimbriae  Small, bristlelike fibers  Most contain protein  Tend to stick to each other and to surfaces Figure 4.7  The Glycocalyx  Develops as a coating of repeating polysaccharide units, protein, or both  Protects the cell  Sometimes helps the cell adhere to the environment  Differ among bacteria in thickness, organization, and chemical composition  Slime layer- a loose shield that protects some bacteria from loss of water and nutrients  Capsule- when the glycocalyx is bound more tightly to the cell and is denser and thicker  Polysaccharides and Proteins Proteins Figure 4.9  Functions of the Glycocalyx  Formed by many pathogenic bacteria- protect the bacteria against phagocytes  Important in formation of biofilms  Slime Layer  Loosely attached to cell surface  Water soluble  Protects cells from drying out  Sticky layer that allows prokaryotes to attach to surfaces 3-32 3-32  Bacteria on Human Tooth : Slime layer helps in the attachment 3-33 3-33  Capsule  Composed of organized repeating units of organic chemicals  Firmly attached to cell surface  Protects cells from drying out  May prevent bacteria from being recognized and destroyed by host 3-34 3-34  Cell wall  Functions of Cell Wall: › Prevents cell from bursting when osmotic pressure inside is stronger than outside the cell › Maintains shape and size. › Anchors flagella › Site of action of some antibiotic and antibodies › Differentiates major types of bacteria › Enhances virulence in some bacterial species. 3-35 3-35  Differences in Cell Envelope Structure  The differences between gram- positive and gram-negative bacteria lie in the cell envelope  Gram-positive › Two layers › Cell wall and cytoplasmic membrane  Gram-negative › Three layers › Outer membrane, cell wall, and cytoplasmic membrane  Structure of the Cell Wall, cont. Gram-positive cell wall  A thick (20 to 80 nm), homogeneous sheath of peptidoglycan  Contains tightly bound acidic polysaccharides Gram-Negative Cell Wall  Single, thin (1 to 3 nm) sheet of peptidoglycan  Periplasmic space surrounds the peptidoglycan  Cell wall  Cell wall composed of peptidoglycan  Peptidoglycan is an alternating series of two simple sugar subunits :  N- acetylmuramic acid (NAM)  N-acetylglucosamine (NAG)  Many sheets of peptidoglycan can be present, depending on the organism.  Glycan unit  Contains peptidoglycan  Gram-positive 3-39 3-39 Figure 4.14  Gram-Positive Cell Walls  Relatively thick layer of peptidoglycan  Contains unique polysaccharides called teichoic acids  Some covalently linked to lipids, forming lipoteichoic acids that anchor peptidoglycan to cell membrane  Retains crystal violet dye in Gram staining procedure; appear purple  Acid-fast bacteria contain up to 60% mycolic acid: helps cells survive desiccation 3-41 3-41 GRAM POSITIVE CELL WALL: TEICHOIC ACID AND LIPOTEICHOIC ACIDS Teichoic acid: -Negative charge due to phosphate group -Checks movements of ions -Prevents Cell lysis Lipoteichoic acid -Plays an important part in our body’ s defense system. Phagocytic WBC can bind to lipoteichoic acid and recognize 3-42 3-42 ANTIBACTERIAL COMPOUNDS THAT TARGET PEPTIDOGLYCAN: PENICILLIN and CEPHALOSPORINS Binds to the enzymes involved with peptidoglycan synthesis and prevents cross linkages of adjacent glycan units. LYSOZYME Breaks the bonds of NAM and NAG thus destroying the structural integrity of the cell wall 3-43 3-43  Gram-Negative Cell Walls Have only a thin layer of peptidoglycan Have a bilayer membrane composed of phospholipids, channel proteins (porins) and lipopolysaccharide (LPS) May be impediment to the treatment of disease Following Gram staining procedure, cells appear pink 3-44 3-44  LPS  Union of lipid with sugar  Also known as endotoxin  Lipid portion known as lipid A  Released from dead cells when cell wall disintegrates  May trigger fever, vasodilation, inflammation, shock, and blood clotting  Can be released when antimicrobial drugs kill bacteria 3-45 3-45  Periplasmic Space  Between outer membrane and cell membrane  Contains peptidoglycan and periplasm  Contains water, nutrients, and substances secreted by the cell, such as digestive enzymes and proteins involved in transport 3-46 3-46  The Gram-Negative Outer Membrane  Similar to the cell membrane, except it contains specialized polysaccharides and proteins  Uppermost layer- contains lipopolysaccharide  Innermost layer- phospholipid layer anchored by lipoproteins to the peptidoglycan layer below  Outer membrane serves as a partial chemical sieve ◦ Only relatively small molecules can penetrate ◦ Access provided by special membrane channels formed by porin proteins Practical Considerations of Differences in Cell Envelope Structure  Outer membrane- an extra barrier in gram-negative bacteria  Makes them impervious to some antimicrobial chemicals  Generally more difficult to inhibit or kill than gram- positive bacteria  Cell envelope can interact with human tissues and cause disease 3-49 3-49  Nontypical Cell Walls  Some aren’t characterized as either gram- positive or gram-negative  Some don’t have a cell wall at all  For example, Mycobacterium and Nocardia- unique types of lipids  Archae- unusual and chemically distinct cell walls: Pseudomurein (NAG and N acetyltalosoamineuronic acid)  Mycoplasmas- lack cell wall entirely  Some cells lack a cell wall – ex. Mycoplasma 3-52 3-52 Mycoplasmas and Other Cell- Wall-Deficient Bacteria  Mycoplasma cell membrane is stabilized by sterols and is resistant to lysis  Very small bacteria (0.1 to 0.5 µm)  Range in shape from filamentous to coccus  Not obligate parasites  Can be grown on artificial media  Found in many habitats  Important medical species: Mycoplasma pneumonia  Archael Cell Walls  Do not have peptidoglycan  Cell walls contain variety of specialized polysaccharides and proteins  Gram-positive archaea stain purple  Gram-negative archaea stain pink 3-54 3-54 CELL WALL COMPOSITION OF: - Plants and Algae : CELLULOSE - Fungi : CHITIN - Yeast : GLUCAN AND MANNAN - Protozoa : PELLICLE - Animals : GLYCOCALYX (GLYCOPROTEIN) - Bacteria : PEPTIDOGLYCAN - Archaea : NO PEPTIDOGLYCAN 3-55 3-55  Cell Membrane Structure  Also known as the cytoplasmic membrane  Very thin (5-10 nm)  Contain primarily phospholipids and proteins  The exceptions:  Mycoplasma (sterols called hopanoids) and archaea. Sterols are present in eukaryots.  Functions  Provides a site for functions such as energy reactions, nutrient processing, and synthesis  Regulates transport (selectively permeable membrane)  Secretion Bacterial plasma membrane structure 3-58 3-58  Effects of Solutions on Figure 3.18 Organisms 3-59 3-59  Bacterial Internal Structure  Contents of the Cell Cytoplasm  Gelatinous solution  Site for many biochemical and synthetic activities  70%-80% water  Also contains larger, discrete cell masses (chromatin body, ribosomes, granules, and actin strands)  Bacterial Chromosome  Single circular strand of DNA  Aggregated in a dense area of the cell- the nucleoid Figure 4.17  Plasmids  Nonessential pieces of DNA  Double-stranded circles of DNA  Often confer protective traits such as drug resistance or the production of toxins and enzymes  Ribosomes  Made of RNA and protein  Special type of RNA- ribosomal RNA (rRNA)  Characterized by S units- the prokaryotic ribosome is 70S Figure 4.18  Inclusions  Inclusions- also known as inclusion bodies  Some bacteria lay down nutrients in these inclusions during periods of nutrient abundance  Serve as a storehouse when nutrients become depleted  Some enclose condensed, energy-rich organic substances  Some aquatic bacterial inclusions include gas vesicles to provide buoyancy and flotation  Granules  A type of inclusion body  Contain crystals of inorganic compounds  Are not enclosed by membranes  Example- sulfur granules of photosynthetic bacteria  Polyphosphate granules of Corynebacterium and Mycobacterium are called metachromatic granules because they stain a contrasting color in methylene blue  Magnetotactic bacteria contain granules with iron oxide- give magnetic properties to the cell Figure 3.19  Prokaryotic cells may have various types of inclusions. (a) A transmission electron micrograph of polyhydroxybutryrate lipid droplets. (b) A light micrograph of volutin granules. (c) A phase-contrast micrograph of sulfur granules. (d) A transmission electron micrograph of magnetosomes. (e) A transmission electron micrograph of gas vacuoles. (credit b, c, d: modification of work by American Society for Microbiology)  The Actin Cytoskeleton  Long polymers of actin  Arranged in helical ribbons around the cell just under the cell membrane  Contribute to cell shape  Bacterial Endospores: An Extremely Resistant Stage  Dormant bodies produced by Bacillus, Clostridium, and Sporosarcina Figure 4.21 (a)Sporulation begins following asymmetric cell division. The forespore becomes surrounded by a double layer of membrane, a cortex, and a protein spore coat, before being released as a mature endospore upon disintegration of the mother cell. (b)An electron micrograph of a Carboxydothermus hydrogenoformans endospore. (c)These Bacillus spp. cells are undergoing sporulation. The endospores have been visualized using Malachite Green spore stain. (credit b: modification of work by Jonathan Eisen)  Vegetative Cells Endospores Sensitive to extreme temperatures Resistant to extreme temperatures and radiation and radiation Gram-positive Do not absorb Gram stain, only special endospore stains (see Staining Microscopic Specimens) Normal water content and enzymatic Dehydrated; no metabolic activity activity Capable of active growth and Dormant; no growth or metabolic activity metabolism  Endospore-Forming Bacteria  These bacteria have a two-phase life cycle  Phase One- Vegetative cell  Metabolically active and growing  Can be induced by the environment to undergo spore formation (sporulation)  Phase Two: Endospore  Stimulus for sporulation- the depletion of nutrients  Vegetative cell undergoes a conversion to a sporangium  Sporangium transforms in to an endospore  Hardiest of all life forms  Withstand extremes in heat, drying, freezing, radiation, and chemicals  Heat resistance- high content of calcium and dipicolinic acid  Some viable endospores have been found that were more than 250 million years old  Germination › Breaking of dormancy › In the presence of water and a specific germination agent › Quite rapid (1 ½ hours) › The agent stimulates the formation of hydrolytic enzymes, digest the cortex and expose the core to water  Medical Significance › Several bacterial pathogens  Bacillus antracis  Clostridium tetani  Clostridium perfingens, Clostridium botulinum › Resist ordinary cleaning methods  Endospores Produced by Gram-positive Bacillus and Clostridium Each vegetative cell transforms into one endospore Each endospore germinates to form one vegetative cell Constitute a defensive strategy against hostile or unfavorable conditions 3-74 3-74 Endospores  Extremely resistant to drying, heat, radiation, and lethal chemicals  Stable resting stages  Can remain viable for tens to thousands of years  Serious concern to food processors, health care professionals, and governments 3-75 3-75 https://www.youtube.com/watch?v=JXV7qVkCnAM https://www.youtube.com/watch?v=RUV6mr7MVBI https://www.youtube.com/watch?v=sVkGmqx-b2c https://www.youtube.com/watch?v=q26YhisTg7U https://www.youtube.com/watch?v=lpI4WCM_9pM http://www.microbelibrary.org/images/kaiser/ pen.thumbnail.jpg http://highered.mheducation.com/sites/0072556781/ student_view0/chapter3/animation_quiz_1.html