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Transcript
The bacterial cell wall! Major functions: Protection from osmotic changes Determination and maintenance of cell shape Bacterial Cell Walls are made of • PEPTIDOGLYCAN • Peptidoglycan has a polysaccharide portion and a peptide portion. • The polysaccharide is a complex polymer consisting of alternating monosaccharides called N-acetylglucosamine (NAG) and N-acetyl muramic acid (NAM). • Short polypeptide chains cross-link, or join together, many polysaccharide chains; the polypeptides are always attached to the NAM monosaccharide. • Also attached to the NAM is a tetrapeptide side chain, which is involved in the binding of the peptides to hold together the whole structure. Peptidoglycan Structure The gram-positive cell wall • Consists of many layers of peptidoglycan surrounding the plasma membrane • Also contains techoic acids and lipotechoic acids (probably function in joining layers of peptidoglycan and anchoring wall to plasma membrane) The gram-negative cell wall • MORE COMPLEX!! • Consists of only a single (or very few layers) of peptidoglycan surrounding the plasma membrane, and • ANOTHER MEMBRANE (The outer membrane)! The outer membrane • The outer membrane is a phospholipid bilayer like the plasma membrane, but also contains – Lipoproteins which anchor it to the cell wall and plasma membrane – Lipopolysaccharides (which contain Lipid A, an endotoxin, and O polysaccharide, an antigen) – Porin proteins, which allow transport of small hydrophilic molecules across the outer membrane Outer membrane structures and functions • Protect cell by excluding large toxic molecules (like penicillin, to which they are relatively resistant) • Creates a periplasmic space (between the two membranes) in which there are many enzymes and transport proteins better control of what enters and exits the cell • No techoic acids!!!!! Gram-negative cell walls • Because they are thin, gram negative cell walls are more susceptible to mechanical breakage than grampositive ones. • Because of the outer membrane, gramnegative cell walls are more successful in avoiding poor osmotic conditions, especially when an antibiotic like penicillin is present. Cell walls can be damaged by… • Lysozyme, an enzyme which breaks the glycosidic bonds between NAG and NAM. • Penicillin, an antibiotic that interferes with cell wall synthesis by inhibiting peptide bridge formation. • Gram-negative cells are much less susceptible to both of these than gram-positive cells. • If cell wall is weakened or destroyed, cell lysis may occur if the cell is in a hypotonic environment. Structures internal to the cell wall The plasma membrane The chromosome (nucleoid region) Ribosomes Inclusions Endospores (sometimes) The plasma membrane • Consists of a phospholipid bilayer associated with various proteins (the fluidmosaic model) Major functions • Delineate cell boundary, and hold in the contents • To control transportation of substances into and out of the cell (they are selectively permeable). • There are four ways for things to get across a plasma membrane (next lecture) The Chromosome or "nucleoid” • Bacteria always possess one circular piece of DNA, their chromosome. • While they have no membrane bounded nucleus, the chromosome is attached to the plasma membrane and somewhat localized; this region is sometimes referred to as the "nucleoid” region. Ribosomes • Ribosomes exist in all bacterial cells - they are responsible for one and only one vital job: MAKING PROTEINS!!! (Remember ribosomes have no membrane) • They consist of two subunits made of RNA and Proteins – Many antibiotics kill bacteria by interfering with ribosome function – A given cell may contain thousands of ribosomes Endospores • Only certain gram-positive cells can form endospores, dormant structures that can survive essentially forever and are very difficult to destroy. (Bacillus, Clostridium) • Endospores form when the environment is unfriendly, in other words, when the cell might otherwise die. Usually this is due to a lack of an adequate carbon or nitrogen source. Endospores cont'd • Environmental signals trigger the beginning of sporulation (spore formation). • A double plasma membrane surrounds the chromosome, forming the core, and a layer of peptidoglycan forms between the two membranes. In addition, a salt called calcium dipicolinate is laid down. This has been shown to be important in various aspects of endospore hardiness. • A very thick and impenetrable proteinaceous spore coat forms. • So the spore forms INTERNAL to the plasma membrane, and is then released to the environment, where it will wait for conditions to improve. Endospore Formation Endospores are amazing The presence of water and nutrients reverses the dormancy, and the cell begins to grow and function again. 25-million year old endospores trapped in amber have germinated when placed in nutrient media!!!!! It is almost impossible to "kill" an endospore, so the bacteria that form them can be particularly dangerous pathogens, for example, Bacillus anthracis, which causes anthrax, and Clostridium botulinum, which causes botulism.