Prokaryotes are unicellular organisms that lack membrane-bound structures, the most noteworthy of which is the nucleus. Prokaryotic cells tend to be small, simple cells, measuring around 0.
While prokaryotic cells do not have membrane-bound structures, they do have distinct cellular regions. In prokaryotic cells, DNA bundles together in a region called the nucleoid. Here is a breakdown of what you might find in a prokaryotic bacterial cell.
Bacteria and archaea are the two types of prokaryotes. No, prokaryotes do not have mitochondria. Mitochondria are only found in eukaryotic cells. This is also true of other membrane-bound structures like the nucleus and the Golgi apparatus more on these later. One theory for eukaryotic evolution hypothesizes that mitochondria were first prokaryotic cells that lived inside other cells. Over time, evolution led to these separate organisms functioning as a single organism in the form of a eukaryote.
Eukaryotes are organisms whose cells have a nucleus and other organelles enclosed by a plasma membrane. Organelles are internal structures responsible for a variety of functions, such as energy production and protein synthesis. 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. Up to 90 percent of the cell wall in gram-positive bacteria is composed of peptidoglycan, with most of the rest 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.
Gram-positive and gram-negative bacteria : 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, proteins in this cell membrane, 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. Prokaryotes reproduce asexually by binary fission; they can also exchange genetic material by transformation, transduction, and conjugation.
Reproduction in prokaryotes is asexual and usually takes place by binary fission. The DNA of a prokaryote exists as as a single, circular chromosome. Prokaryotes do not undergo mitosis; rather 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. Modes of prokaryote reproduction : Besides binary fission, there are three other mechanisms by which prokaryotes can exchange DNA. In a transformation, the cell takes up prokaryotic DNA directly from the environment. In b transduction, a bacteriophage injects DNA into the cell that contains a small fragment of DNA from a different prokaryote.
In c conjugation, DNA is transferred from one cell to another via a mating bridge that connects the two cells after the pilus draws the two bacteria close enough to form the bridge. In transformation, the prokaryote takes in DNA found in its environment that is shed by other prokaryotes.
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, sometimes also move short pieces of chromosomal DNA from one bacterium to another.
Transduction results in a recombinant organism. Most but not all prokaryotic cells have a cell wall , but the makeup of this cell wall varies. All cells prokaryotic and eukaryotic have a plasma membrane also called cytoplasmic membrane or cell membrane that exhibits selective permeability, allowing some molecules to enter or leave the cell while restricting the passage of others.
The structure of the plasma membrane is often described in terms of the fluid mosaic model , which refers to the ability of membrane components to move fluidly within the plane of the membrane, as well as the mosaic-like composition of the components, which include a diverse array of lipid and protein components Figure 8.
The plasma membrane structure of most bacterial and eukaryotic cell types is a bilayer composed mainly of phospholipids formed with ester linkages and proteins. These phospholipids and proteins have the ability to move laterally within the plane of the membranes as well as between the two phospholipid layers.
Figure 8. The bacterial plasma membrane is a phospholipid bilayer with a variety of embedded proteins that perform various functions for the cell. Note the presence of glycoproteins and glycolipids, whose carbohydrate components extend out from the surface of the cell. The abundance and arrangement of these proteins and lipids can vary greatly between species. Archaeal membranes are fundamentally different from bacterial and eukaryotic membranes in a few significant ways. First, archaeal membrane phospholipids are formed with ether linkages, in contrast to the ester linkages found in bacterial or eukaryotic cell membranes.
Second, archaeal phospholipids have branched chains, whereas those of bacterial and eukaryotic cells are straight chained. Finally, although some archaeal membranes can be formed of bilayers like those found in bacteria and eukaryotes, other archaeal plasma membranes are lipid monolayers. Membrane proteins and phospholipids may have carbohydrates sugars associated with them and are called glycoproteins or glycolipids, respectively.
These glycoprotein and glycolipid complexes extend out from the surface of the cell, allowing the cell to interact with the external environment Figure 8. Glycoproteins and glycolipids in the plasma membrane can vary considerably in chemical composition among archaea, bacteria, and eukaryotes, allowing scientists to use them to characterize unique species.
Plasma membranes from different cells types also contain unique phospholipids, which contain fatty acids. As described in Using Biochemistry to Identify Microorganisms , phospholipid-derived fatty acid analysis PLFA profiles can be used to identify unique types of cells based on differences in fatty acids.
Archaea, bacteria, and eukaryotes each have a unique PFLA profile. One of the most important functions of the plasma membrane is to control the transport of molecules into and out of the cell. Internal conditions must be maintained within a certain range despite any changes in the external environment. The transport of substances across the plasma membrane allows cells to do so.
Cells use various modes of transport across the plasma membrane. For example, molecules moving from a higher concentration to a lower concentration with the concentration gradient are transported by simple diffusion , also known as passive transport Figure 9. Figure 9. Simple diffusion down a concentration gradient directly across the phospholipid bilayer. Some small molecules, like carbon dioxide, may cross the membrane bilayer directly by simple diffusion. However, charged molecules, as well as large molecules, need the help of carriers or channels in the membrane.
These structures ferry molecules across the membrane, a process known as facilitated diffusion Figure Figure Facilitated diffusion down a concentration gradient through a membrane protein. Active transport occurs when cells move molecules across their membrane against concentration gradients Figure Active transport against a concentration gradient via a membrane pump that requires energy. Group translocation also transports substances into bacterial cells. In this case, as a molecule moves into a cell against its concentration gradient, it is chemically modified so that it does not require transport against an unfavorable concentration gradient.
A common example of this is the bacterial phosphotransferase system, a series of carriers that phosphorylates i. Since the phosphorylation of sugars is required during the early stages of sugar metabolism, the phosphotransferase system is considered to be an energy neutral system. Some prokaryotic cells, namely cyanobacteria and photosynthetic bacteria , have membrane structures that enable them to perform photosynthesis.
These structures consist of an infolding of the plasma membrane that encloses photosynthetic pigments such as green chlorophylls and bacteriochlorophylls. In cyanobacteria, these membrane structures are called thylakoids; in photosynthetic bacteria, they are called chromatophores, lamellae, or chlorosomes. The primary function of the cell wall is to protect the cell from harsh conditions in the outside environment.
When present, there are notable similarities and differences among the cell walls of archaea, bacteria, and eukaryotes. The major component of bacterial cell walls is called peptidoglycan or murein ; it is only found in bacteria.
Structurally, peptidoglycan resembles a layer of meshwork or fabric Figure The structure of the long chains has significant two-dimensional tensile strength due to the formation of peptide bridges that connect NAG and NAM within each peptidoglycan layer. In gram-negative bacteria, tetrapeptide chains extending from each NAM unit are directly cross-linked, whereas in gram-positive bacteria, these tetrapeptide chains are linked by pentaglycine cross-bridges. Peptidoglycan subunits are made inside of the bacterial cell and then exported and assembled in layers, giving the cell its shape.
This provides the cell wall with tensile strength in two dimensions. Since peptidoglycan is unique to bacteria, many antibiotic drugs are designed to interfere with peptidoglycan synthesis, weakening the cell wall and making bacterial cells more susceptible to the effects of osmotic pressure see Mechanisms of Antibacterial Drugs.
The Gram staining protocol see Staining Microscopic Specimens is used to differentiate two common types of cell wall structures Figure 13 [5]. Gram-positive cells have a cell wall consisting of many layers of peptidoglycan totaling 30— nm in thickness.
These peptidoglycan layers are commonly embedded with teichoic acids TAs , carbohydrate chains that extend through and beyond the peptidoglycan layer.
Bacteria contain two common cell wall structural types. Gram-positive cell walls are structurally simple, containing a thick layer of peptidoglycan with embedded teichoic acid external to the plasma membrane.
Gram-negative cell walls are structurally more complex, containing three layers: the inner membrane, a thin layer of peptidoglycan, and an outer membrane containing lipopolysaccharide. TA is thought to stabilize peptidoglycan by increasing its rigidity. TA also plays a role in the ability of pathogenic gram-positive bacteria such as Streptococcus to bind to certain proteins on the surface of host cells, enhancing their ability to cause infection.
In addition to peptidoglycan and TAs, bacteria of the family Mycobacteriaceae have an external layer of waxy mycolic acids in their cell wall; as described in Staining Microscopic Specimens , these bacteria are referred to as acid-fast, since acid-fast stains must be used to penetrate the mycolic acid layer for purposes of microscopy Figure Acid-fast cells are stained red by carbolfuschin.
The outer membrane of a gram-negative bacterial cell contains lipopolysaccharide LPS , a toxin composed of Lipid A embedded in the outer membrane, a core polysaccharide, and the O side chain. Gram-negative cells have a much thinner layer of peptidoglycan no more than about 4 nm thick [7] than gram-positive cells , and the overall structure of their cell envelope is more complex.
In gram-negative cells , a gel-like matrix occupies the periplasmic space between the cell wall and the plasma membrane, and there is a second lipid bilayer called the outer membrane , which is external to the peptidoglycan layer Figure This outer membrane is attached to the peptidoglycan by murein lipoprotein. The outer leaflet of the outer membrane contains the molecule lipopolysaccharide LPS , which functions as an endotoxin in infections involving gram-negative bacteria, contributing to symptoms such as fever, hemorrhaging, and septic shock.
The composition of the O side chain varies between different species and strains of bacteria. Parts of the O side chain called antigens can be detected using serological or immunological tests to identify specific pathogenic strains like Escherichia coli OH7 , a deadly strain of bacteria that causes bloody diarrhea and kidney failure.
Archaeal cell wall structure differs from that of bacteria in several significant ways. First, archaeal cell walls do not contain peptidoglycan; instead, they contain a similar polymer called pseudopeptidoglycan pseudomurein in which NAM is replaced with a different subunit.
Other archaea may have a layer of glycoproteins or polysaccharides that serves as the cell wall instead of pseudopeptidoglycan. Last, as is the case with some bacterial species, there are a few archaea that appear to lack cell walls entirely. Although most prokaryotic cells have cell walls, some may have additional cell envelope structures exterior to the cell wall, such as glycocalyces and S-layers.
A glycocalyx is a sugar coat, of which there are two important types: capsules and slime layers. A capsule is an organized layer located outside of the cell wall and usually composed of polysaccharides or proteins Figure A slime layer is a less tightly organized layer that is only loosely attached to the cell wall and can be more easily washed off. Slime layers may be composed of polysaccharides, glycoproteins, or glycolipids. Glycocalyces allows cells to adhere to surfaces, aiding in the formation of biofilms colonies of microbes that form in layers on surfaces.
In nature, most microbes live in mixed communities within biofilms , partly because the biofilm affords them some level of protection. Biofilms generally hold water like a sponge, preventing desiccation. They also protect cells from predation and hinder the action of antibiotics and disinfectants.
All of these properties are advantageous to the microbes living in a biofilm, but they present challenges in a clinical setting, where the goal is often to eliminate microbes. As explained in Staining Microscopic Specimens , capsules are difficult to stain for microscopy; negative staining techniques are typically used. An S-layer is another type of cell envelope structure; it is composed of a mixture of structural proteins and glycoproteins.
In bacteria, S-layers are found outside the cell wall, but in some archaea, the S-layer serves as the cell wall. The exact function of S-layers is not entirely understood, and they are difficult to study; but available evidence suggests that they may play a variety of functions in different prokaryotic cells, such as helping the cell withstand osmotic pressure and, for certain pathogens, interacting with the host immune system.
After diagnosing Anika with pneumonia, the PA writes her a prescription for amoxicillin, a commonly-prescribed type of penicillin derivative. More than a week later, despite taking the full course as directed, Anika still feels weak and is not fully recovered, although she is still able to get through her daily activities. She returns to the health center for a follow-up visit. Many types of bacteria, fungi, and viruses can cause pneumonia.
Amoxicillin targets the peptidoglycan of bacterial cell walls. Another possibility is that the pathogen is a bacterium containing peptidoglycan but has developed resistance to amoxicillin.
Bacteria may produce two different types of protein appendages that aid in surface attachment. Fimbriae typically are more numerous and shorter, whereas pili shown here are longer and less numerous per cell. Many bacterial cells have protein appendages embedded within their cell envelopes that extend outward, allowing interaction with the environment.
These appendages can attach to other surfaces, transfer DNA, or provide movement. Filamentous appendages include fimbriae, pili, and flagella. Fimbriae and pili are structurally similar and, because differentiation between the two is problematic, these terms are often used interchangeably.
Fimbriae enable a cell to attach to surfaces and to other cells. For pathogenic bacteria, adherence to host cells is important for colonization, infectivity, and virulence. Adherence to surfaces is also important in biofilm formation. The term pili singular: pilus commonly refers to longer, less numerous protein appendages that aid in attachment to surfaces Figure A specific type of pilus, called the F pilus or sex pilus , is important in the transfer of DNA between bacterial cells, which occurs between members of the same generation when two cells physically transfer or exchange parts of their respective genomes see How Asexual Prokaryotes Achieve Genetic Diversity.
Before the structure and function of the various components of the bacterial cell envelope were well understood, scientists were already using cell envelope characteristics to classify bacteria. In doing so, Lancefield discovered that one group of S. She determined that various strains of Group A strep could be distinguished from each other based on variations in specific cell surface proteins that she named M proteins.
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