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Cell Structure.docx

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Department
Biology
Course
BIO3124
Professor
Angela Yeung
Semester
Fall

Description
Cell Structure & Function Page 79 in book *Review of morphology and arrangements The significance of smallness - Size range for prokaryotes: 0.2 µm to >700 µm in diameter o Most rod-shaped bacteria are between 0.5 & 4.0 µm wide and <15 µm long - Examples of very large prokaryotes: o Epulopiscium fishelsoni (Figure 3.2a)  Unusual form of cell division  Multiple offspring are formed and are then released from the Epulopiscium “mother cell.” A mother cell of Epulopiscium contains several thousand genome copies (p 80) o Thiomargarita namibiensis (Figure 3.2b)  can be 750 µm in diameter  chemolithotroph - Size range for eukaryotic cells: 10 to >200 µm in diameter - Surface-to-Volume Ratios, Growth Rates, and Evolution – Advantages to being small: o Small cells have more surface area relative to cell volume than large cells (i.e., higher S/V  Surface-to-volume ratio (S/V) inversely proportional to size of cell)  support greater nutrient exchange per unit cell volume  tend to grow faster than larger cells o Greater evolutionary possibilities  Cell division can occur more rapidly  Increase the chances of advantageous mutations - Lower Limits of Cell Size o Cellular organisms <0.15 µm in diameter are unlikely o Open oceans tend to contain small cells (0.2–0.4 µm in diameter) o Mycoplasma pneumoniae  0.2µm – relies on host factors to acquire nutrients  no cell wall Cytoplasmic membrane structure Same as eukaryotic membrane Different in bacteria/Must Learn In bacteria and archaea… - 6-8nm thick - Vital barrier that separates cytoplasm from environment (prevents osmotic lysis) - Highly selective permeable barrier; enables concentration of specific metabolites and excretion of waste products o Proteins allow substances to cross membrane - Maintain concentration and electrical gradient - Energy storage - Harvest light energy in photosynthetic bacteria - General structure is phospholipid bilayer o Contain both hydrophobic and hydrophilic components o Fatty acids point inward to form hydrophobic environment; hydrophilic portions remain exposed to external environment or the cytoplasm o Fluidity is increased by unsaturated fatty acids (there are double bonds) o Rigidity is increased by saturated fatty acids o Fatty acid + glycerol linked by ester linkages in bacteria (OC=O) - Can exist in many different chemical forms as a result of variation in the groups attached to the glycerol backbone (like we learned in BCH2733) - Sta2+lized b2+hydrogen bonds and hydrophobic interactions - Mg and Ca help stabilize membrane by forming ionic bonds with negative charges on the phospholipids o Somewhat fluid - Hopanoids strengthen and stabilize the membrane of bacteria (similar to sterols in eukaryotes, which are rigid, planar lipids -- cholesterol) o Eukaryotes contain 5-25% sterols o Mycoplasma (smallest bacteria – no cell wall) membranes contain sterols Embedded proteins o Outer surface of cytoplasmic membrane can interact with a variety of proteins that bind substrates or process large molecules for transport o Inner surface of cytoplasmic membrane interacts with proteins involved in energy-yielding reactions and other important cellular functions o Integral membrane proteins: Firmly embedded in membrane by hydrophobic amino acids in the proteins; Peptide chain can span membrane multiple times (G protein) o Peripheral membrane proteins: One portion anchored in the membrane; Firmly associated with membrane (ex. Lipoproteins) o Arranged in clusters  Allow molecules to move across membrane  Interactions between groups of proteins  Protein interactions in energy metabolism Archaeal cytoplasmic membrane – page 83 - Can exist as lipid monolayers, bilayers, or mixture - Isoprene side chains instead of fatty acid o Isoprenoid (hydrophobic) side chain is ether-linked to sn-glycerol-1-phosphate  Very compact and stable – capable of resisting high temperatures Major lipids of archaea: 1. Glycerol diether (C20) 2. Diglycerol tetraether (C40) or Cardacheols - Found in Thermoplasma - Forms Monolayer instead of bilayer 3. Crenarchaeols - Rings within hydrocarbon chains - four cyclopentyl rings and one cyclohexyl ring - o 4. Glycolipids - Carbohydrate bonded to glycerol - Predominant in methanogens and extreme halophiles Cytoplasmic Membrane Functions 1. Permeability Barrier - Small hydrophobic molecules may pass through by diffusion - Polar and charged molecules must be transported 2. Protein Anchor - Provide site for proteins involved in transport, bioenergetics and chemotaxis 3. Energy Conservation - Provides site for the generation and use of the proton motive force  The membrane has an energetically charged form in which proton (H ) are separated from hydroxyl ions (OH ) across its surface. This charge separation is a form of [potential] energy, analogous to the potential energy present in a charged battery. This energy source, called the proton motive force, is responsible for driving many energy-requiring functions in the cell, including some forms of transport, motility, and biosynthesis of ATP. 1 – Permeability Barrier - Prevents leakage o Gateway for transport of nutrients into and wastes out of the cell - Water can freely pass the membrane in both directions o Can be aided by aquaporins o E. coli AqpZ imports or exports water depending on osmotic conditions Passive process of movement Transport proteins: carrier-mediated transporters (active transport) - Accumulate solutes against the concentration gradient - All transport systems require energy in some form, either from the proton motive force, or ATP, or some other energy-rich organic compound. - Important characteristic properties (p. 86): 1. Saturation effect - Important to concentrate nutrient - „If the concentration of substrate is high enough to saturate the transporter, which can occur at even the very low substrate concentrations found in nature, the rate of uptake becomes maximal and the addition of more substrate does not increase the rate‟ 2. High specificity - Can be for a single substrate or closely related class of molecules 3. Highly regulated biosynthesis - Specific complement of transporters present in the cytoplasmic membrane of a cell at any one time is a function of both the resources available and their concentrations - Important because a particular nutrient may need to be transported by one type of transporter when the nutrient is present at high concentration and by a different, higher-affinity transporter, when present at low concentration. Three transport events are possible (direction of transport): 1. Uniporters transport in one direction across the membrane 2. Symporters function as co-transporters 3. Antiporters transport a molecule across the membrane while simultaneously transporting another molecule in the opposite direction Three transport systems exist: 1) Simple transport - Consists only of a membrane-spanning transport protein 2) Group translocation - Consists of a series of proteins in the transport event 3) ABC transporter (ATP binding cassette) - Consists of three components: a substrate-binding protein, a membrane- integrated transporter (transmembrane protein), and an ATP-hydrolyzing protein A – Simple transport Example: Lac Permease of E. Coli (p.87)  The bacterium Escherichia coli metabolizes the disaccharide sugar lactose.  Lactose is transported into cells of E. coli by the activity of a simple transporter, lac permease, a type of symporter.  Activity of lac permease is energy-driven  As each lactose molecule is transported into the cell, the energy in the proton motive force is diminished by the co-transport of protons into the cytoplasm (membrane reenergized through energy-yielding reactions)  Lac (galactoside) permease is one of three proteins required to metabolize lactose in E. coli and that the synthesis of these proteins is highly regulated by the cell by the lac operon  Net result of lac permease: energy-driven accumulation of lactose in the cytoplasm, against the concentration gradient. B – Group Translocation - Substance transported is chemically modified during its uptake across the membrane Example: Phosphotransferase system in E. Coli - Transports the sugars glucose, mannose, and fructose in E. coli, which are modified by phosphorylation during transport - Family of proteins that work in concert o Five proteins are necessary to transport any given sugar - Before the sugar is transported, the proteins in the phosphotransferase system are themselves alternately phosphorylated and dephosphorylated in a cascading fashion until the actual transporter, Enzyme II phc,phorylates the sugar during the transport event:  Energy for the phosphotransferase system comes from the energy-rich compound phosphoenolpyruvate (PE-P), an intermediate of glycolysis  HPr, Enzyme I, and Enzyme IIa are all cytoplasmic proteins; Enzyme IIb lies on the inner surface of the membrane; Enzyme IIc is cn integral membrane protein  HPr and Enzyme I are nonspecific components of the phosphotransferase system and participate in the uptake of several different sugars. Several different versions of Enzyme IIc exist, one for each different sugar transported C – ABC Binding Systems (ATP-Binding Cassette) - >200 different systems identified in prokaryotes - High substrate affinity  can bind their substrate(s) even when they are at extremely low concentration - often involved in uptake of:  organic compounds (e.g., sugars, amino acids)  inorganic nutrients (e.g., sulfate, phosphate)  trace metals - Once its substrate is bound, the periplasmic binding protein interacts with its respective membrane transporter to transport the substrate into the cell driven by ATP hydrolysis - ABC Transport Systems are comprised of: a. Periplasmic Binding proteins: - Gram-negative bacteria contain a region called the periplasm that lies between the cytoplasmic membrane and a second membrane layer called the outer membrane, which part of is the gram-negative cell wall - In Gram positive bacteria, substrate binding proteins are anchored to the external surface of the cytoplasmic membrane (they lack a periplasm) b. Membrane Transporters c. ATP-hydrolyzing proteins Example: Heme uptake in Haemophilus influenzae *to listen in podcast Tranport of large molecules Protein Exportation - Proteins are exported via translocases  Responsible for exporting proteins through and inserting into prokaryotic membranes (Ex. exoenzymes such as amylase and cellulase are secreted to cleave starch or cellulose) a. Sec translocase system (sec = secretory)  Exports proteins & inserts integral membrane proteins  Requires a signal sequence  Type III secretion system (toxins; ex. how bacteria infect humans) - Common in pathogenic bacteria - Secreted protein translocated directly into host Prokaryotic Cell Walls Bacterial Cell Walls - Provide structure and shape and protect cell from osmotic forces o Give bacterial cells characteristic shape - Assist in attaching to other cells or in resisting antimicrobial drugs o Antibiotics target cell wall of bacteria (lysozymes too) - Composed of peptidoglycan o Scientists describe two basic types of bacterial cell walls:  Gram-positive  much thicker and consists primarily of a single type of molecule  Gram-negative  cell wall, or cell envelope as it is sometimes called, is chemically complex and consists of at least two layers (thin) Gram staining: Listen to podcast for more details Peptidoglycan - Forms rigid layer which gives strength to cell wall o In gram –ve, outer membrane is present outside this layer - Polymer of 2 repeating sugar derivatives o N-acetylglucosamine & N-acetylmuramic acid o Linked by Beta-1,4 glycosidic bond o sensitive to lysozyme - Peptide linkage of muramic acid residues confer added strength o Peptides: L-alanine, D-glutamic acid, D-alanine o L-lysine in Gram +ve o diaminopimelic acid (DAP) in Gram –ve These constituents are connected to form a repeating structure, the glycan tetrapeptide - The polysaccharide chain layers are connected through cross-links of amino acids o The glycosidic bonds connecting the sugars in the glycan strands are covalent bonds, but these provide rigidity to the structure in only one direction (horizontally) o Only after cross-linking is peptidoglycan strong in both the X and Y directions (shown here as „peptide bonds‟) - In Gram -ve bacteria, peptidoglycan cross-linkage occurs by peptide bond formation from the amino group of DAP of top glycan chain to the carboxyl group of the D- alanine on the bottom glycan chain - Gram +ve cross-link can occur through a short interbridge of amino acids o Staphylococcus aureus, the interbridge peptide is composed of five glycine residues, a common interbridge amino acid o No DAP molecule but they have an interbridge instead Overall structure of peptidoglycan - Blue lines = amino acid interbridges or peptide bonds (depending whether + or -) - Lysozymes are enzymes that can cleave the β-1,4-glycosidic bonds between N- acetylglucosamine and N-acetylmuramic acid in peptidoglycan o weakening the wall; water can then enter the cell and cause lysis. - Penicillin is an antibiotic that destroys peptidoglycan in a different way than lysosyme o It prevents its biosynthesis, leading to osmotic lysis eventually Biosynthesis of peptidoglycan (page 153) - Synthesis of new peptidoglycan during growth requires the controlled cutting of preexisting peptidoglycan by autolysins along with the simultaneous insertion of peptidoglycan precursors. A lipid carrier molecule called bactoprenol plays a major role in this process. 1) Bactoprenol bonds to a N-acetylglucosamine/N- acetylmuramic acid/pentapeptide (extra D- alanine) peptidoglycan precursor; renders them sufficiently hydrophobic to pass through the membrane interior 2) Once inside the periplasm, bactoprenol interacts with enzymes called transglycosylases that insert cell wall precursors into the growing point of the cell wall and catalyze glycosidic bond formation 3) The final step in cell wall synthesis is transpeptidation. Transpeptidation forms the peptide cross-links between muramic acid residues in adjacent glycan chains i. In Gram negative bacteria (ex. E. coli), crosslinks form between DAP and D-alanine on adjacent peptides ii. Initially, there are 2 D-alanines in precursor, but one is cleaved off during transpeptidation: Diversity of peptidoglycan - Found only in Bacteria o Certain bacteria lack cell walls  Mycoplasma (pathogenic)  sterols (hopanoids) in their membrane confer strength  rely on host cell to live (feed off of it)  Thermoplasma (Archaea – lack pseudomurein/S-layer not peptidoglycan) - >100 different peptidoglycans o Peptide composition can vary o Peptide cross-links and interbridge - Alternating NAG and NAM remain invariant Gram positive cell wall (page 90) - Multiple layers of peptidoglycan (90%) o Laid down in „cables‟ about 50 nm wide, with each cable consisting of several cross-linked glycan strands (stronger) - L-lysine crosslink via peptide bridge to D-alanine - Contains teichoic acid o Acidic polysaccharides composed of repeating glycerol phosphate or ribitol phosphate units o Embedded cell wall, cytoplasmic membrane and capsular polymers o Teichoic acids also function to bind Ca2+ and Mg2+ for eventual transport into the cell o Function: these polysaccharides can be recognized by receptors on a host cell that it is trying to infect (a method for gram +ve to bind to host cell) - Lipoteichoic acids o Associated with lipids of cytoplasmic membrane (they go all the way down to the lipid bilayer from the ce
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