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Cell Structure Page 1 of 11 Prokaryotic Cell Structure Prokaryotic cell metabolism is extremely diverse, but the underlying structure of all bacteria is much more uniform. Just like there is a lot of plant diversity, but at the cellular level plants are very similar. We will look at diversity at the morphological level, then look at bacterial structure in detail. Diversity slides In class questions 2: 1. In general, why is classification of bacteria based on morphology not very useful? Can you think of a better way to classify organisms? 2. Why classify organisms at all? If you discovered a new bacterium, growing in Diet Pepsi, why bother to classify it at all? What is gained by being able to classify it? Best to compare prokaryotic structure to eukaryotic structure: •Most obviously bacteria are much smaller Both prokaryotes and eukaryotes have: Cytoplasm Cell membrane DNA, RNA protein Prokarotes have NO: •Nucleus- DNA is in the cytoplasm packaged into the nucleoid •Internal membrane systems (ER, Golgi structures, vacuoles, etc) though there are exceptions •Obvious ctyoskeleton, though there are non-obvious cytoskeleton like structures •Mitochondria Unlike eukaryotes, prokaryotes have: •Unique structures that help them swim (flagella) and stick to substrate (pili) •Rigid cell walls different in structure from rigid plant cell walls (however some bacteria lack a rigid cell wall) •Some have an external membranes with complex associated molecules call lipopolysaccharides (Gram -) •Small ribosomes •Unique, very tough and hard to kill, spores that form inside the cells of some bacterial species (endospores) Cell Structure Page 2 of 11 Cytoplasm (work way from inside to outside) All prokaryotes have cytoplasm with -70S ribosomes -DNA not packaged into a membrane bound nucleus (chromosome is usually circular, but not always, it is sometimes linear. Called a nucleoid -Prokaryotes have coupled transcription and translation because transcription takes place in the cytoplasm, where ribosomes are. (Draw picture) Cell wall is outside the cell membrane and here Gram+ and Grambacteria differ (more later): The Nucleoid The region of the cytoplasm that contains the bacterial chromosome. E.G. The E. coli chromosome is 3.8 million basepairs and is 1,400 um long about 700x longer that the cell itself (2 um). It must be carefully packaged to get it into the cell, but also it needs to be replicated and genes need to be available for transcription when needed. Nucleoid picts The Ribosomes Prokaryotic ribosomes are smaller than eukaryotic ribosomes (70S vs. 80S, where S = Svedberg Unit which measures rates of sedimentation, bigger S=faster sedimentation=bigger protein generally) Prokaryotic 70S: 30S(16S rRNA+21 proteins) and 50S(23S rRNA+ 5S rRNA +34 proteins) Eukaryotic 80S: 40S(18S rRNA + 30 proteins) and 60S(28S rRNA+ 28S rRNA+50 proteins). XRay crystal pict Many important antibiotics target the prokaryotic ribosome and do not affect eukaryotic ribosomes: tetracyline and its derivatives (doxycycline) streptomycin Cell Structure Page 3 of 11 neomycin spectinomycin erythromycin chloramphenicol (all of these compounds are made by microbes to kill or inhibit competing bacteria Cell membranes Gram-, Gram+ and Archea all differ in chemistry of cell membranes and cell walls. We will look at these both, but membranes first. Membrane lipids have polar and hydrophobic regins. This allows them to orient to form the lipid bilayer that makes up most biological membranes. Bacterial and eukaryotic lipids have ester linkages lipid bilayer and H2O pict A phospolipid found in prok and euks: phosphotidlyethanolamine Fluid Mosaic Slide Archeal lipids have ether linkages(more stable in H2O than ester linkages) with branched hydrocarbons, eg phytanol: Bilayer schematic Archea in hot environments also make very long tetraether lipids, which is essentially a bilayer in molecule Tetra “Bilayer” schematic Cell Structure Page 4 of 11 Slide of lipids compared In class questions 3 1. It is thought that perhaps there was a self-maintaining organic chemistry of lipids, amino acids, etc. on early earth, and that some of the interacting chemicals were captured in lipid bubbles (vesicles). Such capture is considered a very important step in the evolution of life--why? In what important ways are enclosed “metabolisms (interacting organic chemicals) different that unenclosed metabolisms? Cell Structure Page 5 of 11 Cell wall structure and synthesis -Cell walls of both Gram+ and Gram- bacteria are composed of a protein/sugar compostie called peptidoglycan. -The peptide forms crosslinks that link together chains made of sugar. - The glycan (sugar) chains are composed of repeating units of N-acetylglucosamine (G) and N-acetylmuramic acid in long chains. peptide chain pict G-M with peptide -Look at crosslinking reaction--it’s very important because it gives bacterial cell walls their strengthf and is the site of action of many our most important antibiotics (penicillin, cephalexin, etc.) Crosslinking rxn Cell Structure Page 6 of 11 M-G depiction of crosslinked strands -Slides of peptidoglycan formation -Peptidoglycan with missing crosslinks becomes weak and the cell eventually ruptures because the cell-wall can no longer contain the internal pressure of the cell. -Note: penicillin and other antibiotics that inhibit peptidoglycan formation are effective only against cells that GROWING and making cell wall--because they inhibit cosslink FORMATION (they can’t take apart crosslinks that are already formed). -Gram+ cells are most sensitve. Therefore, penicillin-type antibiotics are given for Staph and Strep infections. (Note: many strains of these are now resistant to most cell-wall disrupting antibiotics) Gram- cells are less sensitive because the large drugs cant’ easily pass through the outer membrane. Penicillin action slides Cell Structure Page 7 of 11 Outer Membrane Structure -Lies outside of the cell wall in Gram- bacteria (not found in Gram+ bacteria) -The inner leaflet is made of phospholipids and the outer leaflet is made of lipopolysaccharide (LPS- a very important molecule) -The membrane is held by Braun’s lipoprotein to the peptidoglycan cell wall. Outer membrane structure -The outer membrane is protective and keeps out large (often toxic) molecules such as antibiotics, bile salts etc). -The outer membrane contains porins which let in small molecules such as •sugars ( ) •amino acids ( ) E. coli porins let in molecules of 600 daltons MW, or less: OmpF OmpC OmpA -Also present are larger, gated channels that let in nutrients like vitamin B12 Slides of porins -In lab you will do antibiotic sensitivity testing and will see that Gram+ are generally more sensitive that Gram-. Cell Structure Page 8 of 11 LPS -Found in the outer leaflet of the OM -The base is composed of 2 N-acetylglucosamines hooked to 6 lipid molecules (called “lipid A’ -These are linked to a core polysaccharide. The lipd A + core is the same in all strains of a species. -Core is linked to a sugar chain called “O antigen”. This varies from strain to strain in a species (slide of LPS structure) •There are 167 different E. coli O-antigens •O-antigens named O55, O111, O127 are associated with infant diarrhea •E. coli O157 is the strain that often makes the news papers. It is associated with foods and particulary virulent (it produces Shiga toxin). Its natural habitat is the cow GI tract (guess how it gets into food) -Functions of LPS •To make the OM fairly impermeable •To make the cell surface less visible to host immune systems (important for pathogen and symbionts to colonize hosts) •Acts as an adhesion factor Related: Exopolysaccharides which are found loosely associate with the exterior of many (most?) bacteria (Slides) LPS drawing Cell Structure Page 9 of 11 Pili and Fimbriae -Hair-like protein extensions aht emminate from the cell surface -Used for attachment (Type I fimbriae). For example, the E. coli that commonly causes urinary tract infections has a type of Type I fimbriae that recognizes glycoproteins with D-mannose on urinary tract cells. -Type IV fimbriae are used for attachment twitching motility) in many organisms and motility (called Type IV motility Picts and movie Flagella -whip-like extension in the cell surface used obviously, but also can be used for attachment for movement Types of flagellar arrangements Structure and assembly slides. Assembly movie, motility movies most Cell Structure Page 10 of 11 Chemotaxis and directed movement Many bacteria such as E. coli swim smoothly when their flagella turn CCW and they tumble when the flagella rotate CW because the bundle of flagella fall fly apart into separate filaments(slide) -Bacteria are attracted to beneficial environments -Food, O2, temperature, pH light can act as attractants -They avoid detrimental environments -Toxins, non-optiman pH or temperature act as repellants This phenomenon is called “chemotaxis” and comes about by something called biased random movement: When moving toward attractants, or away from repellents bacteria swimm smoothly (flagella move in a CCW direction) When moving away from attractants, or toward repellents they tumble more frequently In class problem Given this drawing, apply the following rules: 1. When moving toward attractant, runs are 3x longer 2. when not moving toward attractant runs stay the same What happens in terms of how the bacterium moved with respect to the attractant? Cell Structure Page 11 of 11 Endospores -Small, very resistant, form made by a limited group of bacteria, mostly in the Firmicutes (Gram +). Best known are Bacillus sp Clostridium species Because they are so hard to destroy and sometimes cause serious illness (anthrax, tenanus, botulism) they are important and wellstudied. -Live a long time in natural environments: ~100,000 years (maybe as long as 2.5 million years!) -Resistant to heat can boil them and they survive. To kill they need dry heat or high temperatures and pressures together (autoclave, pressure cooker, etc) -UV resistant -Resistant to disinfectants such as alcohol, bleach, etc. thought they will eventually succumb. -Sporulation is triggered by a lack of nutrients. This makes sensethe cell is preparing to shut down for a long time and endure bad times. In a population that is undergoing sporulation, at most ~10% will form spores. -Germination occurs when conditions improve Slide