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Prokaryotic Cells a Prokaryote Is a Simple, Unicellular Organism That

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Prokaryotic Cells a Prokaryote Is a Simple, Unicellular Organism That Prokaryotic cells A prokaryote is a simple, unicellular organism that lacks an organized nucleus or other membrane-bound organelle. CELL SHAPES Procaryotes come in a variety of shapes including spheres (cocci), rods (bacilli), ovals (coccobacilli), curved rods (vibrios), rigid helices (spirilla), and flexible helices (spirochetes) Some lack a single, characteristic form and are called pleiomorphic cocci (s., coccus) – spheres diplococci (s., diplococcus) – pairs streptococci – chains staphylococci – grape-like clusters tetrads – 4 cocci in a square sarcinae – cubic configuration of 8 cocci CELL ARRANGEMENT. During the reproductive process, some cells remain attached to each other to form chains, clusters, square planar configurations (tetrads) 1 SIZE OF BACTERIAL CELLS Procaryotic cells vary in size although they are generally smaller than most eucaryotic cells (smaller than 0.2µ in diameter to those more than 50 µm in diameter). A few very large prokaryotes, such as surgeonfish symbiont Epulopiscium fishelsoni (80 µm in diameter and can be more than 0.6 mm in length). The bacterium Escherichia coli, are about 1x3 µm and are typical for the vast majority of prokaryotes. 2 Reasons most prokaryotes are very small 1.Nutrients and waste products pass more readily into and out of a small cell than a large cell, thus accelerating cellular metabolism and growth. This is because relative to cell volume, small cells contain more surface area than do large cells. The surface to-volume (S/V) ratio . A cell with a smaller radius (r) value therefore has a higher S/V ratio than a cell with a larger r value – . 2.Because growth rate depends to some extent on the rate of nutrient exchange, the higher S/V of small cells typically supports more rapid growth than for larger cells. 3.Small cells will typically develop larger populations than will large cells . 4.The small size of prokaryotes may ultimately explain why these organisms tend to adapt rapidly to changing environmental conditions and easily exploit new habitats (conditions that allow beneficial mutations to be immediately expressed 3 A.THE PLASMA MEMBRANE 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 4 Archaeobacterial membranes have a monolayer instead of a bilayer structure The lipids of Archaea have ETHER linkages between glycerol and their hydrophobic side chains. 5 MEMBRANE PROTEINS 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) The membrane is highly organized, asymmetric, flexible, and dynamic Chemical Composition of Membranes The general structure of biological membranes is a phospholipid bilayer. Phospholipids contain both hydrophobic (fatty acid) and hydrophilic (glycerol-phosphate) components and can exist in many different chemical form as a result of variation in the groups attached to the glycerol backbone. In a phospholipid, the fatty acids point inward toward each other to form a hydrophobic environment, while the hydrophilic portions remain exposed to the aqueous external environment. 6 Thin sections of the cytoplasmic membrane can be seen with the electron microscope. The cytoplasmic membrane appears as two light- colored lines separated by a darker area. This unit membrane, as it is called (because each phospholipid leaf forms half of the “unit”), consists of a phospholipid bilayer with proteins embedded in it. The major proteins of the cytoplasmic membrane typically have very hydrophobic external surfaces in the regions of the protein that span the membrane, and hydrophilic surfaces that make contact with the environment and the cytoplasm. The overall structure of the cytoplasmic membrane is stabilized by hydrogen bonds and hydrophobic interactions. In addition, cations such as Mg2+ and Ca2+ help stabilize the membrane by combining ionically with negative charges of the phospholipids. 7 THE PLASMA MEMBRANE SERVES SEVERAL FUNCTIONS FOR THE CELL a. Forms a boundary between cytoplasm and cell external, 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 e. The genetic material is bound to the cell membrane during cell division which appears to have some role in the redistribution of this material between the two new cells B. INTERNAL MEMBRANE SYSTEMS 8 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 C. THE CYTOPLASMIC MATRIX The cytoplasmic matrix is the substance between the membrane and the nucleoid It is featureless in electron micrographs but is often packed with ribosomes and inclusion bodies Despite the homogenous appearance, the matrix is highly organized with respect to protein location Cytoskeletal elements 9 Cell division Cell shape Cell shape INCLUSION BODIES Inclusion bodies are granules of organic or inorganic material that are stored by the cell for future use. ORGANIC: o Glycogen o Poly-β-hydroxybutyrate(PHB) o Cyanophycin proteins (N storage) o Carboxysomes (reserve Rubisco, site of CO2 fixation) Some are not bounded by a membrane Others are enclosed by a single-layered membrane INORGANIC: o Polyphosphate granules (=Volutin) (=metachromic) o Magnetosome: Contain iron in the form of magnetite FUNCTION OF INCLUSION BODIES ??????? Gas vacuoles Aggregates of gas vesicles with a membrane made of protein 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 RIBOSOMES Ribosomes are complex structures consisting of protein and RNA Sites for the synthesis of cellular proteins Procaryotic ribosomes are similar in structure to, but smaller and less complex than, eucaryotic ribosomes 70S (sveldberg unit): 50S+30S 11 Nucleoid The chromosome (CMS) • Closed circular, double-stranded DNA molecule • Looped and coiled extensively • DNA+RNA+proteins • Nucleoid proteins probably aid in folding Nucleoid proteins differ from histones The Plasmids • usually small, closed circular DNA molecules • exist and replicate independently of chromosome • not required for growth and reproduction • may carry genes that confer selective advantage (e.g., drug resistance) • Virulence and Metabolic What are EPISOMES????? The loss of plasmid is called ……………….. The cell envelope The cell wall in addition to cell membrane (together making Periplasm Gap between plasma membrane and cell wall (gram-positive bacteria), or between plasma membrane and outer membrane (gram-negative bacteria). Periplasmic enzymes: • Found in periplasm of gram-negative bacteria • Function: – nutrient acquisition – electron transport – peptidoglycan synthesis – modification of toxic compounds 11 Exoenzyme • secreted by gram-positive bacteria • perform many of the same functions that periplasmic enzymes do for gram-negative bacteria CELL WALL • Bacteria are divided into two major groups based on the response to Gram-stain procedure. – gram-positive bacteria stain purple – gram-negative bacteria stain pink • staining reaction due to cell wall structure Cell Wall Components Major component (~50%) is peptidoglycan • Important component of both gram-positive and gram-negative bacteria • Glycan chain cross-linked with peptides • Glycan: Two alternating sugars form backbone (sugar derivatives) – N-acetylglucosamine (composed of ?????) – N-acetylmuramic acid (composed of ?????) 12 13 Some amino acids are not observed in proteins 14 Importance of D-form of A.A. ????? Peptidoglycan cross-links Gram Positive Cross links between the peptides= Peptide interbridge (in Gram positive walls only) • Composed primarily of peptidoglycan • May also contain large amounts of teichoic acids • Some gram-positive bacteria have layer of proteins on surface of peptidoglycan • Polymers of Glycerol or Ribitol joined by phosphate groups 15 GRAM NEGATIVE • Consist of a thin layer of peptidoglycan surrounded by an outer membrane • Outer membrane composed of lipids, lipoproteins, and lipopolysaccharide (LPS)(lipids+carbohydrates) • No teichoic acids • more permeable than plasma membrane due to presence of porin proteins and transporter proteins • porin proteins form channels through which small molecules (600-700 daltons) can pass • LPS Consist of three parts: lipid A core polysaccharide O side chain (O antigen) 16 Functions of LPS • protection from host defenses
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