The prokaryotic plasma membrane is a thin lipid bilayer 6 to 8 nanometers that completely surrounds the cell and separates the inside from the outside. Its selectively permeable nature keeps ions, proteins, and other molecules within the cell and prevents them from diffusing into the extracellular environment, while other molecules may move through the membrane.
Recall that the general structure of a cell membrane is a phospholipid bilayer composed of two layers of lipid molecules. In archaeal cell membranes, isoprene phytanyl chains linked to glycerol replace the fatty acids linked to glycerol in bacterial membranes. Some archaeal membranes are lipid monolayers instead of bilayers Figure 2. Figure 2. Archaeal phospholipids differ from those found in Bacteria and Eukarya in two ways.
First, they have branched phytanyl sidechains instead of linear ones. Second, an ether bond instead of an ester bond connects the lipid to the glycerol. The cytoplasm of prokaryotic cells has a high concentration of dissolved solutes.
Therefore, the osmotic pressure within the cell is relatively high. The cell wall is a protective layer that surrounds some cells and gives them shape and rigidity. It is located outside the cell membrane and prevents osmotic lysis bursting due to increasing volume. The chemical composition of the cell wall varies between Archaea and Bacteria, and also varies between bacterial species. Bacterial cell walls contain peptidoglycan , composed of polysaccharide chains that are cross-linked by unusual peptides containing both L- and D-amino acids including D-glutamic acid and D-alanine.
Proteins normally have only L-amino acids; as a consequence, many of our antibiotics work by mimicking D-amino acids and therefore have specific effects on bacterial cell-wall development. There are more than different forms of peptidoglycan. S-layer surface layer proteins are also present on the outside of cell walls of both Archaea and Bacteria. Bacteria are divided into two major groups: Gram positive and Gram negative , based on their reaction to Gram staining.
Note that all Gram-positive bacteria belong to one phylum; bacteria in the other phyla Proteobacteria, Chlamydias, Spirochetes, Cyanobacteria, and others are Gram-negative. The Gram staining method is named after its inventor, Danish scientist Hans Christian Gram — The different bacterial responses to the staining procedure are ultimately due to cell wall structure.
Gram-positive organisms typically lack the outer membrane found in Gram-negative organisms Figure 3. Up to 90 percent of the cell-wall in Gram-positive bacteria is composed of peptidoglycan, and most of the rest is composed of acidic substances called teichoic acids. Teichoic acids may be covalently linked to lipids in the plasma membrane to form lipoteichoic acids. Lipoteichoic acids anchor the cell wall to the cell membrane. Gram-negative bacteria have a relatively thin cell wall composed of a few layers of peptidoglycan only 10 percent of the total cell wall , surrounded by an outer envelope containing lipopolysaccharides LPS and lipoproteins.
This outer envelope is sometimes referred to as a second lipid bilayer. The chemistry of this outer envelope is very different, however, from that of the typical lipid bilayer that forms plasma membranes.
Bacteria are divided into two major groups: Gram positive and Gram negative. Both groups have a cell wall composed of peptidoglycan: in Gram-positive bacteria, the wall is thick, whereas in Gram-negative bacteria, the wall is thin. In Gram-negative bacteria, the cell wall is surrounded by an outer membrane that contains lipopolysaccharides and lipoproteins. Porins are proteins in this cell membrane that allow substances to pass through the outer membrane of Gram-negative bacteria. In Gram-positive bacteria, lipoteichoic acid anchors the cell wall to the cell membrane.
Figure 3. Archaean cell walls do not have peptidoglycan. There are four different types of archaean cell walls. One type is composed of pseudopeptidoglycan , which is similar to peptidoglycan in morphology but contains different sugars in the polysaccharide chain. The other three types of cell walls are composed of polysaccharides, glycoproteins, or pure protein. Other differences between Bacteria and Archaea are seen in Table 4. Note that features related to DNA replication, transcription and translation in Archaea are similar to those seen in eukaryotes.
Bacteria and Archaea differ in the lipid composition of their cell membranes and the characteristics of the cell wall. In archaeal membranes, phytanyl units, rather than fatty acids, are linked to glycerol. Some archaeal membranes are lipid monolayers instead of bilayers. The cell wall is located outside the cell membrane and prevents osmotic lysis. The chemical composition of cell walls varies between species.
Bacterial cell walls contain peptidoglycan. Archaean cell walls do not have peptidoglycan, but they may have pseudopeptidoglycan, polysaccharides, glycoproteins, or protein-based cell walls. Bacteria can be divided into two major groups: Gram positive and Gram negative, based on the Gram stain reaction. Gram-positive organisms have a thick cell wall, together with teichoic acids.
Gram-negative organisms have a thin cell wall and an outer envelope containing lipopolysaccharides and lipoproteins. Improve this page Learn More. The Gram staining method is named after its inventor, Danish scientist Hans Christian Gram — The different bacterial responses to the staining procedure are ultimately due to cell wall structure.
Gram-positive organisms typically lack the outer membrane found in Gram-negative organisms Figure. Up to 90 percent of the cell-wall in Gram-positive bacteria is composed of peptidoglycan, and most of the rest is composed of acidic substances called teichoic acids.
Teichoic acids may be covalently linked to lipids in the plasma membrane to form lipoteichoic acids. Lipoteichoic acids anchor the cell wall to the cell membrane. Gram-negative bacteria have a relatively thin cell wall composed of a few layers of peptidoglycan only 10 percent of the total cell wall , surrounded by an outer envelope containing lipopolysaccharides LPS and lipoproteins. This outer envelope is sometimes referred to as a second lipid bilayer.
The chemistry of this outer envelope is very different, however, from that of the typical lipid bilayer that forms plasma membranes. Which of the following statements is true? There are four different types of archaean cell walls. One type is composed of pseudopeptidoglycan , which is similar to peptidoglycan in morphology but contains different sugars in the polysaccharide chain. The other three types of cell walls are composed of polysaccharides, glycoproteins, or pure protein.
Other differences between Bacteria and Archaea are seen in Figure. Note that features related to DNA replication, transcription and translation in Archaea are similar to those seen in eukaryotes. Reproduction in prokaryotes is asexual and usually takes place by binary fission. Recall that the DNA of a prokaryote is a single, circular chromosome.
Prokaryotes do not undergo mitosis; instead, the chromosome is replicated and the two resulting copies separate from one another, due to the growth of the cell. The prokaryote, now enlarged, is pinched inward at its equator and the two resulting cells, which are clones , separate. Binary fission does not provide an opportunity for genetic recombination or genetic diversity, but prokaryotes can share genes by three other mechanisms.
In transformation , the prokaryote takes in DNA shed by other prokaryotes into its environment. If a nonpathogenic bacterium takes up DNA for a toxin gene from a pathogen and incorporates the new DNA into its own chromosome, it too may become pathogenic. In transduction , bacteriophages, the viruses that infect bacteria, may move short pieces of chromosomal DNA from one bacterium to another. Transduction results in a recombinant organism.
Archaea also have viruses that may translocate genetic material from one individual to another. In conjugation , DNA is transferred from one prokaryote to another by means of a pilus , which brings the organisms into contact with one another, and provides a channel for transfer of DNA.
The DNA transferred can be in the form of a plasmid or as a composite molecule, containing both plasmid and chromosomal DNA. These three processes of DNA exchange are shown in Figure. Reproduction can be very rapid: a few minutes for some species. This short generation time coupled with mechanisms of genetic recombination and high rates of mutation result in the rapid evolution of prokaryotes, allowing them to respond to environmental changes such as the introduction of an antibiotic very quickly.
Evolution Connection The Evolution of Prokaryotes How do scientists answer questions about the evolution of prokaryotes? Unlike with animals, artifacts in the fossil record of prokaryotes offer very little information.
Fossils of ancient prokaryotes look like tiny bubbles in rock. Some scientists turn to genetics and to the principle of the molecular clock, which holds that the more recently two species have diverged, the more similar their genes and thus proteins will be.
Conversely, species that diverged long ago will have more genes that are dissimilar. Scientists at the NASA Astrobiology Institute and at the European Molecular Biology Laboratory collaborated to analyze the molecular evolution of 32 specific proteins common to 72 species of prokaryotes. Actinobacteria are a group of very common Gram-positive bacteria that produce branched structures like fungal mycelia, and include species important in decomposition of organic wastes.
You will recall that Deinococcus is a genus of bacterium that is highly resistant to ionizing radiation. It has a thick peptidoglycan layer in addition to a second external membrane, so it has features of both Gram-positive and Gram-negative bacteria. Cyanobacteria are photosynthesizers, and were probably responsible for the production of oxygen on the ancient earth.
The timelines of divergence suggest that bacteria members of the domain Bacteria diverged from common ancestral species between 2. Eukarya later diverged from the archaean line. The work further suggests that stromatolites that formed prior to the advent of cyanobacteria about 2.
Prokaryotes domains Archaea and Bacteria are single-celled organisms that lack a nucleus. They have a single piece of circular DNA in the nucleoid area of the cell. Most prokaryotes have a cell wall that lies outside the boundary of the plasma membrane. Some prokaryotes may have additional structures such as a capsule, flagella, and pili.
Bacteria and Archaea differ in the lipid composition of their cell membranes and the characteristics of the cell wall. In archaeal membranes, phytanyl units, rather than fatty acids, are linked to glycerol. Some archaeal membranes are lipid monolayers instead of bilayers. The cell wall is located outside the cell membrane and prevents osmotic lysis. The chemical composition of cell walls varies between species.
Bacterial cell walls contain peptidoglycan. Archaean cell walls do not have peptidoglycan, but they may have pseudopeptidoglycan, polysaccharides, glycoproteins, or protein-based cell walls.
The Eukarya also spelled Eucarya possess the following characteristics:. The Eukarya are subdivided into the following four kingdoms:. It used to be thought that the changes that allow microorganisms to adapt to new environments or alter their virulence capabilities was a relatively slow process occurring within an organism primarily through mutations, chromosomal rearrangements, gene deletions and gene duplications. Those changes would then be passed on to that microbe's progeny and natural selection would occur.
This gene transfer from a parent organism to its offspring is called vertical gene transmission. It is now known that microbial genes are transferred not only vertically from a parent organism to its progeny, but also horizontally to relatives that are only distantly related, e.
This latter process is known as horizontal gene transfer. Through mechanisms such as transformation, transduction, and conjugation, genetic elements such as plasmids, transposons, integrons, and even chromosomal DNA can readily be spread from one microorganism to another.
Microbes are known to live in remarkably diverse environments, many of which are extremely harsh. This amazing and rapid adaptability is a result of their ability to quickly modify their repertoire of protein functions by modifying, gaining, or losing their genes. This gene expansion predominantly takes place by horizontal transfer. Learning Objectives Define phylogeny. Name the 3 Domains of the 3 Domain system of classification and recognize a description of each.
Name the four kingdoms of the Domain Eukarya and recognize a description of each. Define horizontal gene transfer. PLoS Biol 8 1 : e The Archaea archaebacteria The Archaea possess the following characteristics: Archaea are prokaryotic cells.
The cell walls of Archaea contain no peptidoglycan. Archaea are not sensitive to some antibiotics that affect the Bacteria , but are sensitive to some antibiotics that affect the Eukarya. The Bacteria and the Eukarya have membranes composed of unbranched fatty acid chains attached to glycerol by ester linkages. The Archaea have membranes composed of branched hydrocarbon chains attached to glycerol by ether linkages.
The Bacteria eubacteria Bacteria also known as eubacteria or "true bacteria" are prokaryotic cells that are common in human daily life, encounter many more times than the archaebacteria. The Bacteria possess the following characteristics: Bacteria are prokaryotic cells.
The cell walls of Bacteria , unlike the Archaea and the Eukarya, contain peptidoglycan. Bacteria are sensitive to traditional antibacterial antibiotics but are resistant to most antibiotics that affect Eukarya. The Eukarya eukaryotes The Eukarya also spelled Eucarya possess the following characteristics: Eukarya have eukaryotic cells.
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