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