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Cell Structure and Taxonomy Hani Masaadeh, MD, Ph.D Structure and Taxonomy Compound light microscope role. Electron Microscope role. Cell. (The fundamental unit of any living organism). Metabolism. DNA. Species. Procaryotes and Eucaryotes. Viruses, viroides (Kuru disease) and virions (Creutzfeldt–Jakob disease). Selective toxicity. Cytology. Types of Living Cells All living cells can be classified into two groups: Prokaryotes and Eukaryotes. Prokaryote comes from the Greek words for prenucleus. Eukaryote comes from the Greek words for true nucleus. Plants and animals are entirely composed of eukaryotic cells. In microbial world, bacteria and archaea are prokaryotes. Other cellular microbes – fungi (yeasts and molds), protozoa, and algae (Sea weeds) are eukaryotes. Viruses are noncellular elements. Eucaryotes Cell structure Cell membrane. Both eucaryotic and procaryotic cells possess a cell membrane. Cell membranes have selective permeability, allowing only certain substances to pass through them. proteins and phospholipids. True nucleus. Motile eucaryotic cells possess either cilia or flagella. Cell Structure Cytoplasm. Endoplasmic reticulum. Rough. Smooth. Ribosomes. Golgi apparatus. Lysosomes and Peroxisomes. Mitochondria. Plastids. Cytoskeleton. Cell wall. Flagella and Cilia. Nucleus Command center. Nucleoplasm. Chromosome. Nuclear membrane. Genes. RNA. Genotype. Nucleolus. The Prokaryotic Cell: Size, Shape 1 µm wide and 2 to 3 µm long. Basic shapes: 1. Spherical coccus 2. rod-shaped bacillus 3. Spiral : have one or more twists, called vibrios when they look like curved rods, and spirilla when they look like a corckscew, spirochetes if helical and flexible. Figures 4.1a, 4.2a, 4.2d, 4.4b, 4.4c The Prokaryotic Cell: Size, Shape Unusual shapes Star-shaped (rectangular) Stella Square Haloarcula Most bacteria are monomorphic (maintain a single shape) A few are pleomorphic (have many shapes, not just one) Figure 4.5 The Prokaryotic Cell: Arrangements Cocci and bacilli Pairs: Diplococci, diplobacilli Chains: Streptococci, streptobacilli Tetrads: division in 2 planes and remain in groups of four. Sarcinae: division in 3 planes and remain attached in cubelike groups of eight. Clusters: division in multiple planes forming clusters called Staphylococci Figures 4.1a, 4.1d, 4.2c Plasma Membrane Selective permeability allows passage of some molecules. Permeability depends on: Size: large molecules (Proteins) can not pass while small molecules (H20, O2, C02, simple sugars) pass easily. Solubility: Lipid soluble molecules (02, CO2, nonpolar organic molecules) pass easily. Presence of transporter proteins. Enzymes for ATP production Photosynthetic pigments on foldings called chromatophores or thylakoids Damage to the membrane by alcohols, quaternary ammonium (detergents), and polymyxin antibiotics causes leakage of cell contents Cytoplasm: Composition and Functions Cytoplasm is the substance of the cell inside the plasma membrane. 80% water and contains proteins (enzymes), carbohydrates, lipids, and inorganic ions. It is thick, aqueous, semitransparent, and elastic. Major structures: Nucleoid, ribosomes, inclusion bodies, protein filaments Nucleoid, the nuclear area, contains a single long, continuous, circular thread of double stranded DNA called the bacteria chromosome which carries the genetic information rquired for the cell’s sturctures and functions. Moreover, bacteria often contain an extrachromosomal, small circular, double stranded DNA molecules called Plasmids. Plasmids usually contain 5 to 100 genes responsible for antibiotic resistance, production of toxins, and synthesis of enzymes. Cytoplasm: Ribosomes Ribosomes The sites of protein synthesis. Composed of two subunits, each consists of protein and ribosomal RNA. Prokaryotic ribosomes differ from eukaryotic ribosomes in number of proteins and rRNA molecules. Smaller and less dense 70S ribosomes in prokaryotes and 80S ribosomes in eukaryotes. Several antibiotics work by inhibiting protein synthesis on prokaryotic ribosomes Figure 4.19 Cytoplasm: Inclusions (Reserve deposites) Metachromatic granules (volutin) Polysaccharide granules Lipid inclusions Sulfur granules Carboxysomes Phosphate reserves Energy reserves Energy reserves Energy reserves Ribulose 1,5-diphosphate carboxylase for CO2 fixation The Cell Wall A complex, semirigid structure responsible for the shape of the cell, surrounding the plasma membrane. CW prevents osmotic lysis of bacteria. CW maintain the shape of a bacterium and anchors the flagella. Clinically, CW increases the ability of some bacteria to cause disease and it is the site of action of some antibiotics. CW is used to differentiate major types of bacteria. Made of peptidoglycan (in bacteria) Figure 4.6a–b Gram-Positive Cell Walls Teichoic acids Lipoteichoic acid links to plasma membrane Wall teichoic acid links to peptidoglycan Negatively charged, may regulate movement of cations. Polysaccharides provide antigenic variation. Figure 4.13b CW of Gram +ve and Gram –ve bacteria Figure 4.13b–c Gram-Negative Outer Membrane Figure 4.13c Gram-Negative Outer Membrane Lipopolysaccharides (LPS), lipoproteins, phospholipids Forms the periplasm between the outer membrane and the plasma membrane. Protection from phagocytes and complement. Porins (proteins) form channels through membrane permit the passage of molecules (nutrients) O polysaccharide antigen, e.g., E. coli O157:H7 Lipid A is an endotoxin and is toxic in the host’s bloodstream or gastrointestinal tract. CW of Gram +ve and Gram –ve bacteria Gram +ve CW Thick peptidoglycan composed of many layers. Contains teichoic acids In acid-fast cells, contains mycolic acid Gram –ve CW Thin layer of peptidoglycan No teichoic acids Surrounded by outer membrane Peptidoglycan layer is connected to the outer membrane via lipoproteins. A periplasm (gel like fluid) is found between the outer membrane and plasma membrane. Atypical Cell Walls Mycoplasmas Lack cell walls Sterols in plasma membrane to protect them from lysis. Archaea May lack walls, or unusual walls composed of polysaccharides and proteins but not peptidoglycan. Archaea can not be Gram stained. External Structures: Glycocalyx Located outside cell wall Viscous (sticky), consists of gelatinous polysaccharide, polypeptide, or both. It is made inside the cell and secreted to the cell surface. If it is organized and firmly attached to the cell wall, it is called Capsule. Capsules increase bacterial virulence by preventing phagocytosis Extracellular polysaccharide (EPS) allows cell to attach to various surfaces Some bacteria use its capsule as a source of nutrition to produce energy. A glycocalyx can protect a cell against dehydration. Figure 4.6a–b External Structures: Flagella Located outside cell wall Long filamentous appendages that propel bacteria. It has 3 basic parts: 1. The filament: made of chains of flagellin 2. The hook: consisting of different protein 3. The basal body: Anchors the flagellum to the cell wall and plasma membrane. Flagellum moves the cell by rotating from the basal body. Figure 4.8a Axial Filaments and Fimbriae Endoflagella In spirochetes Anchored at one end of a cell Rotation causes cell to move Fimbriae allow attachment Pili are used to transfer DNA from one cell to another Cytoplasm: Endospores Resting cells formed when essential nutrients are depleted. Bacillus (anthrax and food poisoning), Clostridium (gangarene, tetanus, botulism, and food poisoning) Highly durable dehydrated cells with thick walls and additional layers, formed internal to the bacterial cell membrane. Resistant to desiccation, heat, chemicals, radiation. Sporulation: Endospore formation Germination: Return to vegetative state Figure 4.21b