Thurs, Jan 23, 2003 I. Archaeal cell structure. (Chap 2 pg. 450-453, Supplemental notes 3, 5) The Archaea are a diverse group of prokaryotic organisms that are very different from bacteria and from eucaryotes. Chap. 20 describes some of the major characteristics associated with archaea that distinguishes them from bacteria and eukaryotes. A. Morphology (Size, shape, and arrangment). Like the bacteria, members of the Archaea have diverse morphologies. 1. Size: Single cells range in diameter from 0.1 to 15 microns (100 to 15,000 nm). 2. Shape: cocci (spherical), rods, spiral, lobed, plate-shaped, pleomorphic (variable in shape and lack a single, characteristic form). 3. Arrangements: single cells, filaments, or aggregates B. Archaeal cell walls. 1. Differences between the cell walls of bacteria and archaea. a) Archaeal cell walls lack N-acetylmuramic acid (NAM) and D-amino acids, which are found in bacterial cell walls. b) Archaea are resistant to agents that cause bacteria to lyse, e.g. the enzyme lysozyme and antibiotics, such as pencillin. 2. Gram staining of Archaea: Archaea can stain either gram positive or gram negative depending on the thickness of the cell wall. 3. Cell walls of Gram-positive Archaea (Fig. 20.1, 20.2) a) Both Gram-positive bacteria and Gram-positive archaea have thick cell walls that retain the crystal violet dye used in Gram stain and thus stain purple. b) Unlike Gram-positive bacteria whose cell walls are made of peptidoglycan, Gram-positive archaea have a variety of polymers that make up their cell walls. (1) Pseudomurein (Fig 20.2). (a) The structure of pseudomurein is similar to that of bacterial peptidoglycan. Pseudomurein is a polymer made up of alternating sugar residues. One of the two sugars has an attached pentapeptide. Individual strands of pseudomurein are crosslinked to one another by covalent joining of the pentapeptides in adjacent strands. Some of the differences between pseudomurein and peptidoglycan. (b) Differences between pseudomurein and peptidoglycan.
(i) The sugars are different. The backbone consists of alternating residues of of NAG (N-acetylglucosamine) and NHAc (Nacetyltalosaminuronic acid). There is no N-acetylmuramic acid in pseudomurein. (ii) The chemical linkages between the sugars are different. In peptidoglycan, the beta-glycosidic bond is formed between the C1 of NAM and the C4 of NAG. In pseudomurein, the beta-glycosidic bond is formed between the C1 of NHAc and the C3 of NAG. (iii) The types of amino acids present in the cell wall peptides are different. Bacterial peptidoglycan includes D-amino acids. Pseudomurein contains only L-amino acids. (2) Other cell wall polymers found in Gram-positive Archaea. (a) A heteropolysaccharide that is similar to the chondroitin sulfate found in animal connective tissue. (Halococcus) (b) (b). A heteropolysaccharide containing glucose, glucoronic acid, glucosamine, and acetate. (Methanosarcina) 4. Cell walls of Gram-negative Archaea (Fig. 20.1) (1) Gram-negative Archaea don't have a polysaccharide cell wall or an outer membrane. Instead there is a thick layer of protein or glycoprotein outside the plasma membrane. This protein layer can be 20-40 nm thick. Sometimes there are two layers.
C. Plasma Membrane of Archaea 1. Archaeal cell membranes contain lipids that are different from those found in either bacterial or eukaryotic cells. a) Bacterial and eukaryotic membranes contain unbranched fatty acids attached to glycerol through ester bonds. In contrast archaeal membranes contain branched chain hydrocarbons (usually 20 carbons in length) that are attached to glycerol by an ether bond. (Fig. 20.3) In some cases two glycerol groups are joined together to form a long tetraether approx. 40 carbons in length. (Fig. 20.3) b) Archaeal membranes also contain other polar lipids: phospholipids, sulfolipids, and glycolipids and nonpolar lipids, such as squalene. 2. Similar to the plasma membranes of bacteria and eukaryotes, some archaeal cells have a plasma membrane that consists of a lipid bilayer of C20 diether lipids (Fig. 20.5a). However, the archaeal plasma membrane can also contain C40 diether lipids. Some thermophilic Archaea have plasma membranes that are lipid monolayers containing only C40 diethers. 3. Similar to bacteria and eukaryotes, the plasma membrane of Archaea also contains proteins. Both integral and peripheral membrane proteins are found. II. Structure of Eukaryotic Cells- (Chapter 4 Suppl. notes pg. 3,5, 9) A.. Examples of eukaryotic microbes (Figure 4.1) 1. Paramecium (a protozoa) 2. diatoms 3. Filamentous fungi 4. Mushrooms (fungi) B. Eukaryotic Plasma membrane 1. A lipid bilayer of unbranched fatty acids containing integral and peripheral membranes proteins. It's composition and structure is very similar to the plasma membrane of bacterial cells. One difference is that most eukaryotic plasma membranes contain sterols, such as cholesterol, which are thought to increase the mechanical strength of the membrane. C. Cell Wall 1. Some eukaryotic microbes lack an external cell wall. 2. A variety of different types of cell wall structures are found among the eukaryotic microorganisms that have cell walls. a) Algal cell walls contain polysaccharides such as cellulose and pectin. Their cell walls may also contain inorganic compounds such as silica or calcium carbonate.
b) Most fungi have rigid cell walls, which usually contain cellulose, chitin, or glucan. (1) Cellulose (2) Chitin is a tough, nitrogen-containing polysaccharide. (3) Glucans are polysaccharides of glucose joined by alpha(1-3) and alpha(1-6) linkages.
D. Functions of eukaryotic organelles 1. Organelles are intracellular structures that perform specific functions. Many, but not all, eukaryotic organelles are bounded by membranes. Common eukaryotic organells are listed in Table 4.1. You need to know function of each of these organelles and be able to recognize the organelles in a drawing or photomicrograph of a cell. E. Endosymbiont theory for the origin of eukaryotic cells. The endosymbiont theory was first proposed by Lynn Margulis. 1. Box 4.1. 2. Observations. a) The RNA polymerases of mitochondria and plastids resemble those of eubacteria more than they do those of eucaryotes. Bacterial and organelle polymerases are sensitive to the same inhibitors and insensitive to inhibitors of eucaryotic RNA polymerases. b) The ribosomes found in mitochondria and plasmids are more similar to bacterial ribosomes than to the cytoplasmic ribosomes of eukaryotic cells. Protein synthesis in mitochondria and plastids is sensitive to the same inhibitors that inhibit protein synthesis in eubacteria and insensitive to some that inhibit c) Phylogenetic analysis of small subunit rrna nucleotide sequences suggest that mitochondrial rdna shared a common ancestor with modern endosymbiotic bacteria (rickettsia, Agrobacterium, Rhizobium). d) Similarly, 16S rdna phylogenetic analysis suggests that most plastid rdna genes shared common ancestors with a cyanobacterium. e) Intrageneric comparisons of organelle genomes reveal some species with the same gene in both organelle and nuclear genomes. One or the other may be inactive. Occasionally an active gene may be in the organelle for one species and in the nucleus for another of the same family. 3. Interpretations of the data above. a) Plastids almost certainly evolved from cyanobacteria. b) Mitochondria almost certainly evolved from Rickesettia. c) The endosymbiont theory hypothesizes that mitochondria and plastids evoloved from procaryotes that established symbiotic relationships within other cells. The endosymbionts must originally have had a larger set of genes, sufficient for autonomous growth. During the course of evolution, many of those genes are thought to have been transferred to the nuclear genome or lost. 4. References.
a) http://opbs.okstate.edu/~melcher/mg/mgw1/mg1378.html#info Ulrich Melcher b) Gray, M.W., Burger, G., and Lang, B.F. Science (1999) Mitochondrial evolution. 283(5407):1476-1481. Abstract. The serial endosymbiosis theory is a favored model for explaining the origin of mitochondria, a defining event in the evolution of eukaryotic cells. As usually described, this theory posits that mitochondria are the direct descendants of a bacterial endosymbiont that became established at an early stage in a nucleus-containing (but amitochondriate) host cell. Gene sequence data strongly support a monophyletic origin of the mitochondrion from a eubacterial ancestor shared with a subgroup of the alpha-proteobacteria. However, recent studies of unicellular eukaryotes (protists), some of them little known, have provided insights that challenge the traditional serial endosymbiosis-based view of how the eukaryotic cell and its mitochondrion came to be. These data indicate that the mitochondrion arose in a common ancestor of all extant eukaryotes and raise the possibility that this organelle originated at essentially the same time as the nuclear component of the eukaryotic cell rather than in a separate, subsequent event. III. Comparison of Bacteria, Archaea, and Eukaryotes. Table 19.8 and SN pg. 5 & 9