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Microbiology, 4/e Prescott, Harley, Klein | ||||||
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Microbiology, 4/e Prescott, Harley, Klein | ||||||
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3 Procaryotic Cell Structure and Function CHAPTER OVERVIEW This chapter provides a description of the procaryotic cell, beginning with the general features of size, shape, and arrangement. Then the general features of biological membranes and the specific features of procaryotic membranes are given. Important internal structures of procaryotes, such as the cytoplasmic matrix, the ribosomes, the inclusion bodies, and the nucleoid are described, in addition to structures external to the cell, such as the cell wall, capsule, pili, and flagella. The differences between the cell walls of gram-positive organisms and gram-negative organisms are discussed and the mechanism of this differential staining reaction is explained. The chapter concludes with a discussion of bacterial chemotaxis and bacterial endospores. CHAPTER OBJECTIVES After reading this chapter you should be able to:
! describe the various sizes, shapes, and cellular arrangements exhibited by bacteria
! describe the bacterial plasma membrane and the limited internal membrane structures found in procaryotes
! describe the appearance, composition, and function of the various internal structures found in procaryotic organisms (such as inclusion bodies, ribosomes, and the nucleoid)
! define the composition of gram-positive and gram-negative cell walls and explain how these differences contribute to the differential reaction to the Gram-staining procedure
! describe external structures such as capsules, fimbriae and flagella
! diagram and describe the various arrangements of bacterial flagella
! describe how bacteria use their locomotive ability to swim toward chemical attractants and away from chemical repellents
! describe the production of the bacterial endospore and how it enables spore-forming bacteria to survive harsh environmental conditions and renew growth when the environment becomes conducive to growth
CHAPTER OUTLINE
I. An Overview of Procaryotic Cell Structure
A. Size, shape, and arrangement
1. Procaryotes come in a variety of shapes including spheres (cocci), rods (bacilli), ovals (coccobacilli), curved rods (vibrios), rigid helices (spirilla), and flexible helices (spirochetes)
2. During the reproductive process, some cells remain attached to each other to form chains, clusters, square planar configurations (tetrads), or cubic configurations (sarcinae)
3. A few bacteria are flat and some lack a single, characteristic form and are called pleiomorphic
B. Procaryotic cells vary in size although they are generally smaller than most eucaryotic cells; recently, however, a large procaryote, Epulopiscium fisheloni was discovered that grows as large as 600Tm H 80Tm, a littler smaller than a printed hyphen
C. Procaryotic cells contain a variety of internal structures
D. Not all structures are found in every genus, but procaryotes are consistent in their fundamental structure and most important components
E. Procaryotic cell organizationCprocaryotes are morphologically distinct from eucaryotic cells and have fewer internal structures.
II. Procaryotic Cell Membranes
A. The plasma membrane
1. The plasma membrane consists of a phospholipid bilayer with hydrophilic surfaces (interact with water) and a hydrophobic interior (insoluble in water); such asymmetric molecules are said to be amphipathic
2. Archaeobacterial membranes have a monolayer instead of a bilayer structure
3. Proteins are associated with the membrane and may be either peripheral (loosely associated and easily removed) or integral (embedded within the membrane and not easily removed)
4. The membrane is highly organized, asymmetric, flexible, and dynamic
5. The plasma membrane serves several functions for the cell
a. It retains the cytoplasm and separates the cell from its environment
b. It serves as a selectively permeable barrier, allowing some molecules to pass into or out of the cell while preventing passage of other molecules
c. It is the location of a variety of crucial metabolic processes including respiration, photosynthesis, lipid synthesis, and cell wall synthesis
d. It may contain special receptor molecules that enable bacterial detection of and response to chemicals in the surroundings
B. Internal membrane systems
1. Mesosomes are structures formed by invaginations of the plasma membrane that may play a role in cell wall formation during division, in chromosome replication and distribution, and in secretory processes; however, mesosomes may be artifacts generated during chemical fixation for electron microscopy
2. Photosynthetic bacteria may have complex infoldings of the plasma membrane that increase the surface area available for photosynthesis
3. Bacteria with high respiratory activity may also have extensive infoldings that provide a large surface area for greater metabolic activity
4. These internal membranes may be aggregates of spherical vesicles, flattened vesicles, or tubular membranes
III. The Cytoplasmic Matrix
A. The cytoplasmic matrix is the substance between the membrane and the nucleoid
B. It is featureless in electron micrographs but is often packed with ribosomes and inclusion bodies
C. Despite the homogenous appearance, the matrix is highly organized with respect to protein location
D. Inclusion Bodies
1. Inclusion bodies are granules of organic or inorganic material that are stockpiled by the cell for future use
a. Some are not bounded by a membrane
b. Others are enclosed by a single-layered membrane
2. Gas vacuoles are a type of inclusion body found in cyanobacteria and some other aquatic forms; they provide buoyancy for these organisms and keep them at or near the surface of their aqueous habitat
E. Ribosomes
1. Ribosomes are complex structures consisting of protein and RNA
2. They are responsible for the synthesis of cellular proteins
3. Procaryotic ribosomes are similar in structure to, but smaller and less complex than, eucaryotic ribosomes
F. Molecular Chaperones
1. Helper proteins that aid the folding of nascent polypeptides during protein synthesis
2. Many molecular chaperones are heat-shock proteins that increase in concentration after cells are subjected to environmental stress; they promote proper folding of new proteins that are replacing heat-damaged existing proteins
3. Molecular chaperones also function to keep secretory proteins in an export-competent state until they are translocated across the plasma membrane
VI. The NucleoidCan irregularly shaped region in which the single circular chromosome of the procaryote will be found; in most procaryotes it is not bounded by a membrane, but is sometimes found to be associated with the plasma membrane or with mesosomes; two genera of planctomycete bacteria have been shown to have membrane-bounded DNA-containing regions
A. The bacterial chromosome is an efficiently packed, closed circular DNA molecule that is looped and coiled extensively
B. In actively growing bacteria, the nucleoid has projections that extend into the cytoplasmic matrix; these projections probably contain DNA being actively transcribed
C. Plasmids are small, closed circular DNA molecules that can exist and replicate independently of the bacterial chromosome; they are not required for bacterial growth and reproduction, but they may carry genes that give the bacterium a selective advantage (e.g., drug resistance, enhanced metabolic activities, etc.)
VII. The Procaryotic Cell WallCa rigid structure that results in the characteristic shapes of the various procaryotes and protects them from osmotic lysis
A. Peptidoglycan (murein) is a polysaccharide polymer found in procaryotic cell walls that consists of polysaccharide chains cross-linked by peptide bridges
B. Gram-positive cell walls consist of a thick layer of peptidoglycan and large amounts of teichoic acids
C. Gram-negative cell walls are more complex; they consist of a thin layer of peptidoglycan surrounded by an outer membrane composed of lipids, lipoproteins, and a large molecule known as lipopolysaccharide (LPS). There are no teichoic acids in gram-negative cell walls.
D. Archeaobacteria cell walls lack peptidoglycan and are composed of proteins, glycoproteins, or polysaccharides
E. The periplasmic space (or periplasm) is the gap between the plasma membrane and the cell wall (gram-positive organisms) or between the plasma membrane and the outer membrane (gram-negative organisms)
1. Periplasmic enzymes are found in the periplasm of gram-negative bacteria; they generally participate in nutrient acquisition
2. Exoenzymes are secreted by gram-positive bacteria and perform many of the same functions that periplasmic enzymes do for gram-negative bacteria
3. Denitrifying and chemolithoautotrophic bacteria often have electron transport proteins in periplasm
4. Enzymes involved in peptidoglycan synthesis and in the modification of toxic compounds may also be found in the periplasm
F. Adhesion sites are sites of direct contact or possibly true membrane fusions between the plasma membrane and the outer membrane of gram-negative bacteria; it has been proposed that substances can move into the cell through these sites rather than travel through the periplasm
G. The outer membrane is more permeable than the plasma membrane because of porin proteins that form channels through which small molecules (600-700 daltons) can pass
H. The mechanism of Gram staining involves constricting the thick peptidoglycan layer of gram-positive cells, thereby preventing the loss of the crystal violet stain during the brief decolorization step; the thinner, less cross-linked peptidoglycan layer of gram-negative organisms cannot retain the stain as well, and these bacteria are thus more readily decolorized when treated with alcohol
I. The cell wall and osmotic protection: The cell wall prevents swelling and lysis of bacteria in hypotonic solutions. However, in hypertonic habitats, the plasma membrane shrinks away from the cell wall in a process known as plasmolysis
VIII. Components External to the Cell Wall
A. Capsules and slime layers are layers of polysaccharides lying outside the cell wall; they protect the bacteria from phagocytosis, viral infection, pH fluctuations, osmotic stress, hydrolytic enzymes, or the predacious bacterium Bdellovibrio
1. Capsules are well organized
2. Slime layers are diffuse and unorganized
B. A glycocalyx is a network of polysaccharides extending from the surface of bacteria and other cells
C. S layers are regularly structured layers of protein or glycoprotein
D. S layers are common among the archeaobacteria where it may be the only structure outside the plasma membrane
E. Pili and fimbriae are short, thin, hairlike appendages that mediate bacterial attachment to surfaces (fimbriae) or to other bacteria during sexual mating (pili)
F. Flagella and motility
1. Flagella are threadlike locomotor appendages extending outward from the plasma membrane and cell wall
2. Flagella may be arranged in various patterns:
a. MonotrichousCa single flagellum
b. AmphitrichousCa single flagellum at each pole
c. LophotrichousCa cluster (tuft) of flagella at one or both ends
d. PeritrichousCa relatively even distribution of flagella over the entire surface of the bacterium
3. Flagellar ultrastructure: The flagellum consists of a hollow filament composed of a single protein known as flagellin. The hook is a short curved segment that links the filament to the basal body, a series of rings that drives flagellar rotation.
4. Flagellar synthesis involves many genes for the hook and basal body, as well as the gene for flagellin. New molecules of flagellin are transported through the hollow filament so that the growth of the flagellum is from the tip, not from the base.
5. The mechanism of flagellar movement appears to be rotation; the hook and helical structure of the flagellum causes the flagellum to act as a propeller, thus driving the bacterium through its watery environment
a. Counterclockwise rotation causes forward motion (called a run)
b. Clockwise rotation disrupts forward motion (resulting in a tumble)
6. Axial filaments cause flexing and spinning movements that allow spirochetes to move
H. Gliding motility is a mechanism used by some procaryotes by which they coast along solid surfaces; no visible structure is associated with this form of motility
IX. ChemotaxisCdirected movement of bacteria either towards a chemical attractant or away from a chemical repellent
A. The concentrations of these materials is detected by chemoreceptors in the surfaces of the bacteria
B. Directional travel toward a chemoattractant is caused by lowering the frequency of tumbles (twiddles), thereby lengthening the runs when traveling up the gradient, but allowing tumbling to occur at normal frequency when traveling down the gradient
C. Directional travel away from a chemorepellent involves similar but opposite responses
D. The mechanism of control of tumbles and runs is complex with several protein intermediates; nevertheless, it is fast with responses occurring in as little as 200 meters/second
E. Responses are triggered by methylation and/or phosphorylation of target proteins called methyl-accepting chemotaxis proteins (MCPs) to cycle them between active and inactive forms
X. The Bacterial EndosporeCa special, resistant, dormant structure formed by some bacteria, which enables them to resist harsh environmental conditions
A. Spore formation (sporulation) normally commences when growth ceases because of lack of nutrients, and is a complex, multistage process
B. Transformation of dormant spores into active vegetative cells is also a complex, multistage process that includes activation (preparation) of the spore, germination (breaking of the spores dormant state), and outgrowth (emergence of the new vegetative cell)