Week 2 Bacteria Structure and Function PDF

Week 2 Lectures 3&4 Ch 4 Structure and function of Prokaryotic Cells Bacteria Objectives 1. Define The Size, Shape, and Arrangement of Bacter...

Week 2 Lectures 3&4 Ch 4 Structure and function of Prokaryotic Cells Bacteria Objectives 1. Define The Size, Shape, and Arrangement of Bacterial Cells 2. Describe The Bacterial Cell Wall 3-Structures External to the Cell Wall 4-Structures Internal to the Cell Wall 5-Explain the Movement of Materials across plasm Membranes 6-Define Endospores and free spores 7-Comparing Prokaryotic and Eukaryotic Cells  Structure and Function of Prokaryotic Cells Prokaryotic vs. Eukaryotic Cells 1. Prokaryotic and eukaryotic cells are similar in their chemical composition and chemical reactions. 2. Prokaryotic cells do not have a membrane-bound nucleus or other membrane-enclosed organelles. Eukaryotic cells do. 3. Peptidoglycan (complex polysaccharide) is found in prokaryotic cell walls but not in eukaryotic cell walls. 4. Most prokaryotic cells divide by binary fission. Eukaryotes divide by mitosis. 2 Comparison Between Prokaryotic and Eukaryotic Cells Characteristic Prokaryotes Eukaryotes Size of cell Typically 0.2-2.0 m m in diameter Typically 10-100 m m in diameter Nucleus No nuclear membrane or nucleoli (nucleoid) True nucleus, consisting of nuclear membrane & nucleoli Membrane-enclosed organelles Absent Present; examples include lysosomes, Golgi complex, endoplasmic reticulum, mitochondria & chloroplasts Flagella Consist of two protein building blocks Complex; consist of multiple microtubules Glycocalyx Present as a capsule or slime layer Present in some cells that lack a cell wall Cell wall Usually present; chemically complex (typical bacterial When present, chemically simple cell wall includes peptidoglycan) Plasma membrane No carbohydrates and generally lacks sterols Sterols and carbohydrates that serve as receptors present Cytoplasm No cytosketeton or cytoplasmic streaming Cytoskeleton; cytoplasmic streaming Ribosomes Smaller size (70S) Larger size (80S); smaller size (70S) in organelles Chromosome (DNA) Single circular chromosome; lacks histones Multiple linear chromosomes with histones arrangement Cell division Binary fission Mitosis Sexual reproduction No meiosis; transfer of DNA fragments only Involves meiosis (conjugation) 3  The Prokaryotic Bacterial Cell 1. Bacteria are unicellular, and most of them multiply by binary fission. 2. Bacterial species are differentiated by morphology, chemical composition, nutritional requirements, biochemical activities and source of energy. 4  Bacteria reproduce by binary fission Reproductive potential of E. coli Generation time 6  Size, Shape and Arrangement of Bacterial Cells 1. Most bacteria are from 0.20 to 2.0 µm in diameter and from 2 to 8 µm in length. 2. The three basic bacterial shapes are coccus (spheres), bacillus (rods), spiral (twisted), and Filamentous. 3. Pleomorphic bacteria can assume several shapes. The shape of a bacterium is determined by heredity. Genetically, most bacteria are monomorphic; that is, they maintain a single shape. However, a number of environmental conditions can alter that shape. If the shape is altered, identification becomes difficult. Moreover, some bacteria, such as Rhizobium (rī-ZŌ-bē-um) and Corynebacterium (kor’ī-nē-bak-TI-rē-um), are genetically pleomorphic, which means they can have many shapes, not just one. 7 shapes are coccus (spheres) with different arrangement bacillus (rods), with different arrangement 8 Filamentous Bacteria Streptomyces 9  I-Structures External to the Cell Wall 1-Glycocalyx 1. The glycocalyx (capsule, slime layer, or extracellular polysaccharide) is a gelatinous polysaccharide and/or polypeptide covering. 2. Capsules may protect pathogens from phagocytosis. 3. Capsules allow adherence to surfaces, prevent desiccation, and may provide nutrients. 2-Flagella 1. Flagella are relatively long filamentous appendages consisting of a filament, hook, and basal body. 2. Prokaryotic flagella rotate to push the cell. 3. Motile bacteria exhibit taxis; positive taxis is movement toward an attractant; negative taxis is movement away from a repellent. 4. Flagellar (H) protein functions as an antigen. Q-Do bacterial flagella push or pull a cell? Axial Filaments 1. Spiral cells that move by means of an axial filament (endoflagellum) are called spirochetes. 2. Axial filaments are similar to flagella, except that they wrap around the cell. 10 Bacterial Flagellum and Chemotaxis 11  Arrangement of Bacterial Flagella How Many Flagella Does a Bacterium Have? How Are They Arranged? There are basically four different types of flagellar arrangements: 1. A single flagellum can extend from one end of the cell - if so, the bacterium is said to be monotrichous. 2. A single flagellum (or multiple flagella; see below) can extend from both ends of the cell - amphitrichous. 3. Several flagella (tuft) can extend from one end or both ends of the cell - lophotrichous; or, 4. Multiple flagella may be randomly distributed over the entire bacterial cell - peritrichous. 12 3-Fimbriae and Pili 1. Fimbriae and pili are short, thin appendages on the surface of prokaryotic cells. 2. Fimbriae help cells adhere to surfaces. 3. Pili join cells for the transfer of DNA from one cell to another (conjugation via sex pili). Bacterial Pili Conjugation (Sex) Pilus 13  II-The Cell Wall Composition and Characteristics 1. The cell wall surrounds the plasma (cytoplasmic) membrane and protects the cell from changes in water pressure. 2. The bacterial cell wall consists of peptidoglycan, a polymer consisting of NAG and NAM and short chains of amino acids. 3. Penicillin interferes with peptidoglycan synthesis. 4. Gram-positive cell walls consist of many layers of peptidoglycan; also contain teichoic acids. 5. Gram-negative bacteria have a lipoprotein-lipopolysaccharide-phospholipid outer membrane surrounding a thin peptidoglycan layer. 6. The outer membrane protects the cell from phagocytosis and from penicillin, lysozyme, and other chemicals. 7. Porins are proteins that permit small molecules to pass through the outer membrane; specific channel proteins allow other molecules to move through the outer membrane. 8. The lipopolysaccharide (LPS) component of the outer membrane consists of sugars that function as antigens and lipid A, as endotoxin. 14  Comparative Characteristics of Gram-Positive and Gram-Negative Bacteria Characteristic Gram-positive Gram-negative Gram reaction Retain crystal violet dye and stain dark violet Can be decolorized to accept counterstain or purple (safranin); stain red Peptidoglycan layer Thick (multilayered) Thin (single-layered) Teichoic acids Present in many Absent Periplasmic space Absent Present Outer membrane Absent Present Lipopolysaccharide (LPS) content Virtually none High Lipid and lipoprotein content Low (acid-fast bacteria have lipids linked to High (due to presence of outer membrane) peptidoglycan) Flagellar structure 2 rings in basal body 4 rings in basal body Toxins produced Primarily exotoxins Primarily endotoxins Resistance to physical disruption High Low Inhibition by basic dyes High Low Susceptibility to anionic High Low detergents Resistance to sodium azide High Low 16 Resistance to drying High Low Cell Walls and the Gram Stain Mechanism 1. Crystal violet-iodine complex combines with peptidoglycan. 2. Alcohol decolorizer removes the lipid outer membrane of Gram-negative bacteria and washes out the crystal violet. 3. Safranin counterstains the cell wall. Gram Stain of Staphylococcus epidermidis Gram Stain of Escherichia coli Note Gram-positive (purple) cocci in irregular clusters. Note Gram-negative (pink) bacilli. Atypical Cell Walls 1. Mycoplasma is a bacterial genus that naturally lacks cell walls. 2. Archaea have pseudomurein; they lack peptidoglycan. 17 3. L forms are mutant bacteria with defective cell walls.  Damage to the Cell Wall 1. In the presence of lysozyme, Gram-positive cell walls are destroyed, and the remaining cellular contents are referred to as a protoplast. 2. In the presence of lysozyme, Gram-negative cell walls are not completely destroyed, and the remaining cellular contents are referred to as a spheroplast. 3. Protoplasts and spheroplasts are subject to osmotic lysis. 4. Certain antibiotics (e.g.,penicillin) interfere with cell wall synthesis. Q-What are the major structural differences between Gram-positive and Gram negative cell walls? The Cell Wall and Glycocalyx 1. The cell walls of many algae and some fungi contain cellulose. 2. The main material of fungal cell walls is chitin. 3. Yeast cell walls consist of glucan and mannan. 4. Animal cells are surrounded by a glycocalyx, which strengthens the cell and provides a means of attachment to other cells. 18  III-Structures Internal to the Cell Wall 1-Plasma (Cytoplasmic or Cell) Membrane 1. The plasma membrane encloses the cytoplasm and is a phospholipid bilayer with peripheral and integral protein (fluid mosaic model). 2. The plasma membrane is selectively permeable. 3. Plasma membranes carry enzymes for metabolic reactions, such as nutrient breakdown, energy production, and photosynthesis. 4. Mesosomes, irregular infoldings of the plasma membrane, are artifacts. 5. Plasma membranes can be destroyed by alcohols and polymyxins. Diagram of a Cytoplasmic Membrane 19 Movement of Materials Across Membranes 1. Movement across the membrane may be by passive processes, in which materials move from areas of higher to lower concentration and no energy is expended by the cell. 2. In simple diffusion, molecules and ions move until equilibrium is reached. 3. In facilitated diffusion, substances are transported by transporter proteins across membranes from areas of high to low concentration. 4. Osmosis is the movement of water from areas of high to low concentration across a selectively permeable membrane until equilibrium is reached. 5. In active transport, materials move from areas of low to high concentration by transporter proteins, and the cell must expend energy. 6. In group translocation, energy is expended to modify chemicals and transport them across the membrane. 20 2-Cytoplasm 1. Cytoplasm is the fluid component inside the plasma membrane. 2. The cytoplasm is mostly water, with inorganic and organic molecules, DNA, ribosomes, and inclusions. 3-The Nuclear Area (Nucleoid) 1. The nuclear area contains the DNA of the bacterial chromosome. 2. Bacteria can also contain plasmids, which are extrachromosomal closed circular DNA. Ribosomes 1. The cytoplasm of a prokaryote contains numerous 70S ribosomes; ribosomes consist of rRNA and protein. 2. Protein synthesis occurs at ribosomes; it can be inhibited by certain antibiotics. 70S Ribosome During Translation 22 5-Inclusions 1. Inclusions are reserve deposits found in prokaryotic and eukaryotic cells. 2. Among the inclusions found in bacteria are metachromatic granules (inorganic phosphate), polysaccharide granules (usually glycogen or starch), lipid inclusions, sulfur granules, carboxysomes (ribulose 1,5-diphosphate carboxylase), magnetosomes (Fe3O4), and gas vacuoles. Endospores 1. Endospores are resting structures formed by some bacteria for survival during adverse environmental conditions. 2. The process of endospore formation is called sporulation; the return of an endospore to its vegetative state is called germination. 23