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Prokaryotic Cells Bacteria.

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Presentation on theme: "Prokaryotic Cells Bacteria."— Presentation transcript:

1 Prokaryotic Cells Bacteria

2 The Prokaryotic Cell Members of the prokaryotic world make up a vast heterogeneous group of very small single-celled organisms. Include bacteria and archae, although the majority are bacteria The thousands species of bacteria are differentiated by many factors such as: Morphology (shape), chemical composition (often detected by staining reactions), nutritional requirements, biochemical activities, and sources of energy (sunlight or chemicals) These differences can only be seen with the use of a microscope

3 Size, Shape, and Arrangement
Bacteria come in many sizes, and several shapes. Most range from 0.2 to 2.0 µm in diameter and 2-8 µm in length Basic shapes include: Cocci- round shaped Bacillus- rod shaped Spirillum- spiral shaped, helical shape like corkscrew, have rigid bodies, and use flagella to move Vibrio- curved Spirochetes- helical and flexible, move by axial filaments found within flexible external sheath Square Star

4 Cell Cluster Formation
Streptobacillus Bacteria are also classified according to cell cluster formation: Cluster formation Description Diplococci Two cocci cells paired Staphylococci Number of cells clustered together (grape-like) Streptococci & Streptobacillus Number of cells arranged in a chain Staphylobacilli do not exist because bacilli only divide along their long axis which means they will never form clusters of rods.

5 Learning Check: What Shape are these bacteria?
1. 2. 3. bacillus spirillum coccus 5. 6. spirochete vibrio bacillus

6 Shape and Arrangement The shape of bacteria is determined by heredity.
Most bacteria are monomorphic: maintain a single shape Environmental factors can alter that shape Some bacteria like Rhizobium and Corynebacterium are pleomorphic: can have many shapes, not just one. Source for figure:

7 Basic Components of Bacteria
Cell wall-this gives the cell its shape and retains the constituents Cell membrane-used for filtering in food constituents and discharging waste products Nucleoid-where the genetic material of the cell is stored; Cytoplasm- a semiliquid proteinaceous substance which contains starch, fat and enzymes. *Cell wall- Most bacteria have a cell wall but there are some that do not like Mycoplasma species

8 Structures External to the Cell Wall
Possible structures external to the prokaryotic cell wall are: Glycolax Flagella Axial filaments Fimbrae and Pili In certain species, capsules are important in contributing to bacterial virulence (the degree to which a pathogen causes disease). Capsules often protect pathogenic bacteria from phagocytosis by the cells of the host. For example Streptococus pneumoniae which causes pneumonia only when the cells are protected by a polysaccharide capsule

9 What is the Glycocalyx? Means sugar coat & is the general term used for substances that surround cells Bacterial glycocalyx is a viscous (sticky), gelatinous polymer that is external to the cell wall Composed of a polysaccharide, polypeptide, or both Two types: Capsule: substance is organized and is firmly attached to the cell wall Slime layer: substance is unorganized and only loosely attached to the cell wall glycocalyx Fig. 1 Polysaccharide: a carbohydrate (e.g., starch, cellulose, or glycogen) whose molecules consist of a number of sugar molecules bonded together. Polypeptide: a linear organic polymer consisting of a large number of amino-acid residues bonded together in a chain, forming part of (or the whole of) a protein molecule. Source for Fig. 1:

10 (a) Micrograph of Streptococcus pneumoniae, the common cause of pneumonia, showing a prominent capsule. (b) Bacteroides, a common fecal bacterium, has a slime layer surrounding the cell


12 Learning Check What advantage does a glycocalyx provide a cell? (think about its composition)

13 Glycocalyx Very important component of biofilms
Biofilms are densely packed communities of microbial cells that grow on living or inert surfaces A glycocalyx that helps cells in a biofilm attach to their target environment and to each other is called an extracellular polymeric substance (EPS) EPS protects the cells within it Facilitates communication among them and enables the cells to survive by attaching to various surfaces in their natural environment Fig. 2 Through attachment, bacteria can grow on diverse surfaces such as rocks in fast-moving streams, plant roots, human teeth. Streptococcus mutans, an important cause of dental cavities, attaches itself to the surface of teeth by a glycocalyx. Vibrio cholerae, the cause of cholera produces a glycocalyx that helps it attach to the cells of the small intestine. Source for Fig. 2

14 Flagella Some prokaryotes have flagella which are long filamentous appendages that propel bacteria Peritrichous: flagella distributed over the entire cell Polar: at one or both poles or ends of the cell Monotrichous: A single flagellum at one pole Lopothrichous: a tuft of flagella coming from one pole Amphitrichous: flagella at both poles of the cell Bacteria that lack flagella re referred to as atrichous (without projections) Motility enables a bacterium to move toward a favorable environment or away from an adverse on. This is called taxis. Motile bacteria contain receptors in various locations which pick up chemical stimuli, such as oxygen, ribose.

15 Flagellar Movement The flow of hydrogen ions (H ) or of sodium ions (Na ) through the cytoplasmic membrane near the basal body powers the rotation, propelling the bacterium through the environment at about 60 cell lengths per second—equivalent to a car traveling at 670 miles per hour Figure 3.9 Motion of a peritrichous bacterium. In peritrichous bacteria, runs occur when all of the flagella rotate counterclockwise and become bundled. Tumbles occur when the flagella rotate clockwise, become unbundled, and the cell spins randomly. In positive chemotaxis (shown), runs last longer than tumbles, resulting in motion toward the chemical attractant.

16 Axial Filaments Spirochetes have unique structure and motility
Move my means of axial filaments Bundles of fibrils that arise at the ends of the cell beneath an outer sheath and spiral around the cell The rotation of the filaments produces a movement of the outer sheath that propels the spirochetes in a spiral motion

17 Fimbriae and Pili Many gram-negative bacteria contain hair-like appendages that are shorter, straighter, and thinner than flagella are used for attachment and transfer of DNA rather than for motility Fimbrae can occur at the poles of the bacterial cell or entire surface of cell Have a tendency to adhere to each other and to surfaces Pili are usually longer than fimbrae but shorter than flagella and are found as one or two per cell Involved in motility and DNA transfer

18 Conjugation Pili

19 Learning Check Several Escherichia coli cells are connected by
conjugation pili. How are pili different from bacterial flagella? Bacterial flagella are flexible structures that rotate to propel the cell; pili are hollow tubes that mediate transfer of DNA from one cell to another

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