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22.2A: Basic Structures of Prokaryotic Cells - Biology


Prokaryotes, found in both Domain Archaea and Bacteria, are unicellular organisms that lack membrane-bound organelles and a defined nucleus.

Learning Objectives

  • Describe the basic structure of a typical prokaryote

Key Points

  • Prokaryotic cells lack a defined nucleus, but have a region in the cell, termed the nucleoid, in which a single chromosomal, circular, double-stranded DNA molecule is located.
  • Archaeal membranes have replaced the fatty acids of bacterial membranes with isoprene; some archaeal membranes are monolayer rather than bilayer.
  • Prokaryotes can be further classified based on the composition of the cell wall in terms of the amount of peptidoglycan present.
  • Gram-positive organisms typically lack the outer membrane found in gram-negative organisms and contain a large amount of peptidoglycan in the cell wall, roughly 90%.
  • Gram-negative bacteria have a relatively thin cell wall composed of a few layers of peptidoglycan.
  • Gram-negative bacteria have a relatively thin cell wall composed of a few layers of peptidoglycan.

Key Terms

  • nucleoid: the irregularly-shaped region within a prokaryote cell where the genetic material is localized
  • plasmid: a circle of double-stranded DNA that is separate from the chromosomes, which is found in bacteria and protozoa
  • osmotic pressure: the hydrostatic pressure exerted by a solution across a semipermeable membrane from a pure solvent

The Prokaryotic Cell

Prokaryotes are unicellular organisms that lack organelles or other internal membrane-bound structures. Therefore, they do not have a nucleus, but, instead, generally have a single chromosome: a piece of circular, double-stranded DNA located in an area of the cell called the nucleoid. Most prokaryotes have a cell wall outside the plasma membrane.

The composition of the cell wall differs significantly between the domains Bacteria and Archaea, the two domains of life into which prokaryotes are divided. The composition of their cell walls also differs from the eukaryotic cell walls found in plants (cellulose) or fungi and insects (chitin). The cell wall functions as a protective layer and is responsible for the organism’s shape. Some bacteria have a capsule outside the cell wall. Other structures are present in some prokaryotic species, but not in others. For example, the capsule found in some species enables the organism to attach to surfaces, protects it from dehydration and attack by phagocytic cells, and increases its resistance to our immune responses. Some species also have flagella used for locomotion and pili used for attachment to surfaces. Plasmids, which consist of extra-chromosomal DNA, are also present in many species of bacteria and archaea.

The Plasma Membrane

The 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, preventing them from diffusing into the extracellular environment, while other molecules may move through the membrane. 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.

The Cell Wall

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 walls varies between archaea and bacteria. It 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 100 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 (1853–1938). 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.


22.2 Structure of Prokaryotes: Bacteria and Archaea

By the end of this section, you will be able to do the following:

  • Describe the basic structure of a typical prokaryote
  • Describe important differences in structure between Archaea and Bacteria

There are many differences between prokaryotic and eukaryotic cells. The name "prokaryote" suggests that prokaryotes are defined by exclusion—they are not eukaryotes, or organisms whose cells contain a nucleus and other internal membrane-bound organelles. However, all cells have four common structures: the plasma membrane, which functions as a barrier for the cell and separates the cell from its environment the cytoplasm, a complex solution of organic molecules and salts inside the cell a double-stranded DNA genome, the informational archive of the cell and ribosomes, where protein synthesis takes place. Prokaryotes come in various shapes, but many fall into three categories: cocci (spherical), bacilli (rod-shaped), and spirilli (spiral-shaped) (Figure 22.9).

The Prokaryotic Cell

Recall that prokaryotes are unicellular organisms that lack membrane-bound organelles or other internal membrane-bound structures (Figure 22.10). Their chromosome—usually single—consists of a piece of circular, double-stranded DNA located in an area of the cell called the nucleoid . Most prokaryotes have a cell wall outside the plasma membrane. The cell wall functions as a protective layer, and it is responsible for the organism’s shape. Some bacterial species have a capsule outside the cell wall. The capsule enables the organism to attach to surfaces, protects it from dehydration and attack by phagocytic cells, and makes pathogens more resistant to our immune responses. Some species also have flagella (singular, flagellum) used for locomotion, and pili (singular, pilus) used for attachment to surfaces including the surfaces of other cells. Plasmids, which consist of extra-chromosomal DNA, are also present in many species of bacteria and archaea.

Recall that prokaryotes are divided into two different domains, Bacteria and Archaea, which together with Eukarya, comprise the three domains of life (Figure 22.11).

Characteristics of bacterial phyla are described in Figure 22.12 and Figure 22.13. Major bacterial phyla include the Proteobacteria, the Chlamydias, the Spirochaetes, the photosynthetic Cyanobacteria, and the Gram-positive bacteria. The Proteobacteria are in turn subdivided into several classes, from the Alpha- to the Epsilon proteobacteria. Eukaryotic mitochondria are thought to be the descendants of alphaproteobacteria, while eukaryotic chloroplasts are derived from cyanobacteria. Archaeal phyla are described in Figure 22.14.

The Plasma Membrane of Prokaryotes

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 22.15).

The Cell Wall of Prokaryotes

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 100 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 (1853–1938). 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 22.16). 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.

Visual Connection

Which of the following statements is true?

  1. Gram-positive bacteria have a single cell wall anchored to the cell membrane by lipoteichoic acid.
  2. Porins allow entry of substances into both Gram-positive and Gram-negative bacteria.
  3. The cell wall of Gram-negative bacteria is thick, and the cell wall of Gram-positive bacteria is thin.
  4. Gram-negative bacteria have a cell wall made of peptidoglycan, whereas Gram-positive bacteria have a cell wall made of lipoteichoic acid.

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 22.2. Note that features related to DNA replication, transcription and translation in Archaea are similar to those seen in eukaryotes.

Structural Characteristic Bacteria Archaea
Cell type Prokaryotic Prokaryotic
Cell morphology Variable Variable
Cell wall Contains peptidoglycan Does not contain peptidoglycan
Cell membrane type Lipid bilayer Lipid bilayer or lipid monolayer
Plasma membrane lipids Fatty acids-glycerol ester Phytanyl-glycerol ethers
Chromosome Typically circular Typically circular
Replication origins Single Multiple
RNA polymerase Single Multiple
Initiator tRNA Formyl-methionine Methionine
Streptomycin inhibition Sensitive Resistant
Calvin cycle Yes No

Reproduction

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 22.17.

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. 2 The model they derived from their data indicates that three important groups of bacteria—Actinobacteria, Deinococcus, and Cyanobacteria (collectively called Terrabacteria by the authors)—were the first to colonize land. 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 2.5 and 3.2 billion years ago, whereas the Archaea diverged earlier: between 3.1 and 4.1 billion years ago. Eukarya later diverged from the archaean line. The work further suggests that stromatolites that formed prior to the advent of cyanobacteria (about 2.6 billion years ago) photosynthesized in an anoxic environment and that because of the modifications of the Terrabacteria for land (resistance to drying and the possession of compounds that protect the organism from excess light), photosynthesis using oxygen may be closely linked to adaptations to survive on land.


Biology 171


Close your eyes and picture a brick wall. What is the wall’s basic building block? It is a single brick. Like a brick wall, cells are the building blocks that make up your body.

Your body has many kinds of cells, each specialized for a specific purpose. Just as we use a variety of materials to build a home, the human body is constructed from many cell types. For example, epithelial cells protect the body’s surface and cover the organs and body cavities within. Bone cells help to support and protect the body. Immune system cells fight invading bacteria. Additionally, blood and blood cells carry nutrients and oxygen throughout the body while removing carbon dioxide. Each of these cell types plays a vital role during the body’s growth, development, and day-to-day maintenance. In spite of their enormous variety, however, cells from all organisms—even ones as diverse as bacteria, onion, and human—share certain fundamental characteristics.

Learning Objectives

By the end of this section, you will be able to do the following:

  • Name examples of prokaryotic and eukaryotic organisms
  • Compare and contrast prokaryotic and eukaryotic cells
  • Describe the relative sizes of different cells
  • Explain why cells must be small

Cells fall into one of two broad categories: prokaryotic and eukaryotic. We classify only the predominantly single-celled organisms Bacteria and Archaea as prokaryotes (pro- = “before” -kary- = “nucleus”). Animal cells, plants, fungi, and protists are all eukaryotes (eu- = “true”).

Components of Prokaryotic Cells

All cells share four common components: 1) a plasma membrane, an outer covering that separates the cell’s interior from its surrounding environment 2) cytoplasm, consisting of a jelly-like cytosol within the cell in which there are other cellular components 3) DNA, the cell’s genetic material and 4) ribosomes, which synthesize proteins. However, prokaryotes differ from eukaryotic cells in several ways.

A prokaryote is a simple, mostly single-celled (unicellular) organism that lacks a nucleus, or any other membrane-bound organelle. We will shortly come to see that this is significantly different in eukaryotes. Prokaryotic DNA is in the cell’s central part: the nucleoid ((Figure)).


Most prokaryotes have a peptidoglycan cell wall and many have a polysaccharide capsule ((Figure)). The cell wall acts as an extra layer of protection, helps the cell maintain its shape, and prevents dehydration. The capsule enables the cell to attach to surfaces in its environment. Some prokaryotes have flagella, pili, or fimbriae. Flagella are used for locomotion. Pili exchange genetic material during conjugation, the process by which one bacterium transfers genetic material to another through direct contact. Bacteria use fimbriae to attach to a host cell.

Microbiologist The most effective action anyone can take to prevent the spread of contagious illnesses is to wash his or her hands. Why? Because microbes (organisms so tiny that they can only be seen with microscopes) are ubiquitous. They live on doorknobs, money, your hands, and many other surfaces. If someone sneezes into his hand and touches a doorknob, and afterwards you touch that same doorknob, the microbes from the sneezer’s mucus are now on your hands. If you touch your hands to your mouth, nose, or eyes, those microbes can enter your body and could make you sick.

However, not all microbes (also called microorganisms) cause disease most are actually beneficial. You have microbes in your gut that make vitamin K. Other microorganisms are used to ferment beer and wine.

Microbiologists are scientists who study microbes. Microbiologists can pursue a number of careers. Not only do they work in the food industry, they are also employed in the veterinary and medical fields. They can work in the pharmaceutical sector, serving key roles in research and development by identifying new antibiotic sources that can treat bacterial infections.

Environmental microbiologists may look for new ways to use specially selected or genetically engineered microbes to remove pollutants from soil or groundwater, as well as hazardous elements from contaminated sites. We call using these microbes bioremediation technologies. Microbiologists can also work in the bioinformatics field, providing specialized knowledge and insight for designing, developing, and specificity of computer models of, for example, bacterial epidemics.

Cell Size

At 0.1 to 5.0 μm in diameter, prokaryotic cells are significantly smaller than eukaryotic cells, which have diameters ranging from 10 to 100 μm ((Figure)). The prokaryotes’ small size allows ions and organic molecules that enter them to quickly diffuse to other parts of the cell. Similarly, any wastes produced within a prokaryotic cell can quickly diffuse. This is not the case in eukaryotic cells, which have developed different structural adaptations to enhance intracellular transport.


Small size, in general, is necessary for all cells, whether prokaryotic or eukaryotic. Let’s examine why that is so. First, we’ll consider the area and volume of a typical cell. Not all cells are spherical in shape, but most tend to approximate a sphere. You may remember from your high school geometry course that the formula for the surface area of a sphere is 4πr 2 , while the formula for its volume is 4πr 3 /3. Thus, as the radius of a cell increases, its surface area increases as the square of its radius, but its volume increases as the cube of its radius (much more rapidly). Therefore, as a cell increases in size, its surface area-to-volume ratio decreases. This same principle would apply if the cell had a cube shape ((Figure)). If the cell grows too large, the plasma membrane will not have sufficient surface area to support the rate of diffusion required for the increased volume. In other words, as a cell grows, it becomes less efficient. One way to become more efficient is to divide. Another way is to develop organelles that perform specific tasks. These adaptations lead to developing more sophisticated cells, which we call eukaryotic cells.


Prokaryotic cells are much smaller than eukaryotic cells. What advantages might small cell size confer on a cell? What advantages might large cell size have?

Section Summary

Prokaryotes are single-celled organisms of the domains Bacteria and Archaea. All prokaryotes have plasma membranes, cytoplasm, ribosomes, and DNA that is not membrane-bound. Most have peptidoglycan cell walls and many have polysaccharide capsules. Prokaryotic cells range in diameter from 0.1 to 5.0 μm.

As a cell increases in size, its surface area-to-volume ratio decreases. If the cell grows too large, the plasma membrane will not have sufficient surface area to support the rate of diffusion required for the increased volume.

Art Connections

(Figure) Prokaryotic cells are much smaller than eukaryotic cells. What advantages might small cell size confer on a cell? What advantages might large cell size have?

(Figure) Substances can diffuse more quickly through small cells. Small cells have no need for organelles and therefore do not need to expend energy getting substances across organelle membranes. Large cells have organelles that can separate cellular processes, enabling them to build molecules that are more complex.

Free Response

Antibiotics are medicines that are used to fight bacterial infections. These medicines kill prokaryotic cells without harming human cells. What part or parts of the bacterial cell do you think antibiotics target? Why?

The cell wall would be targeted by antibiotics as well as the bacteria’s ability to replicate. This would inhibit the bacteria’s ability to reproduce, and it would compromise its defense mechanisms.

Explain why not all microbes are harmful.

Some microbes are beneficial. For instance, E. coli bacteria populate the human gut and help break down fiber in the diet. Some foods such as yogurt are formed by bacteria.

Glossary


111 Structure of Prokaryotes: Bacteria and Archaea

By the end of this section, you will be able to do the following:

  • Describe the basic structure of a typical prokaryote
  • Describe important differences in structure between Archaea and Bacteria

There are many differences between prokaryotic and eukaryotic cells. The name “prokaryote” suggests that prokaryotes are defined by exclusion—they are not eukaryotes, or organisms whose cells contain a nucleus and other internal membrane-bound organelles. However, all cells have four common structures: the plasma membrane, which functions as a barrier for the cell and separates the cell from its environment the cytoplasm, a complex solution of organic molecules and salts inside the cell a double-stranded DNA genome, the informational archive of the cell and ribosomes, where protein synthesis takes place. Prokaryotes come in various shapes, but many fall into three categories: cocci (spherical), bacilli (rod-shaped), and spirilli (spiral-shaped) ((Figure)).


The Prokaryotic Cell

Recall that prokaryotes are unicellular organisms that lack membrane-bound organelles or other internal membrane-bound structures ((Figure)). Their chromosome—usually single—consists of a piece of circular, double-stranded DNA located in an area of the cell called the nucleoid . Most prokaryotes have a cell wall outside the plasma membrane. The cell wall functions as a protective layer, and it is responsible for the organism’s shape. Some bacterial species have a capsule outside the cell wall. The capsule enables the organism to attach to surfaces, protects it from dehydration and attack by phagocytic cells, and makes pathogens more resistant to our immune responses. Some species also have flagella (singular, flagellum) used for locomotion, and pili (singular, pilus) used for attachment to surfaces including the surfaces of other cells. Plasmids, which consist of extra-chromosomal DNA, are also present in many species of bacteria and archaea.


Recall that prokaryotes are divided into two different domains, Bacteria and Archaea, which together with Eukarya, comprise the three domains of life ((Figure)).


Characteristics of bacterial phyla are described in (Figure) and (Figure). Major bacterial phyla include the Proteobacteria, the Chlamydias, the Spirochaetes, the photosynthetic Cyanobacteria, and the Gram-positive bacteria. The Proteobacteria are in turn subdivided into several classes, from the Alpha- to the Epsilon proteobacteria. Eukaryotic mitochondria are thought to be the descendants of alphaproteobacteria, while eukaryotic chloroplasts are derived from cyanobacteria. Archaeal phyla are described in (Figure).




The Plasma Membrane of Prokaryotes

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)).


The Cell Wall of Prokaryotes

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 100 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 (1853–1938). 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?

  1. Gram-positive bacteria have a single cell wall anchored to the cell membrane by lipoteichoic acid.
  2. Porins allow entry of substances into both Gram-positive and Gram-negative bacteria.
  3. The cell wall of Gram-negative bacteria is thick, and the cell wall of Gram-positive bacteria is thin.
  4. Gram-negative bacteria have a cell wall made of peptidoglycan, whereas Gram-positive bacteria have a cell wall made of lipoteichoic acid.

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 (Figure). Note that features related to DNA replication, transcription and translation in Archaea are similar to those seen in eukaryotes.

Differences and Similarities between Bacteria and Archaea
Structural Characteristic Bacteria Archaea
Cell type Prokaryotic Prokaryotic
Cell morphology Variable Variable
Cell wall Contains peptidoglycan Does not contain peptidoglycan
Cell membrane type Lipid bilayer Lipid bilayer or lipid monolayer
Plasma membrane lipids Fatty acids-glycerol ester Phytanyl-glycerol ethers
Chromosome Typically circular Typically circular
Replication origins Single Multiple
RNA polymerase Single Multiple
Initiator tRNA Formyl-methionine Methionine
Streptomycin inhibition Sensitive Resistant
Calvin cycle Yes No

Reproduction

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.


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. 1 The model they derived from their data indicates that three important groups of bacteria—Actinobacteria, Deinococcus, and Cyanobacteria (collectively called Terrabacteria by the authors)—were the first to colonize land. 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 2.5 and 3.2 billion years ago, whereas the Archaea diverged earlier: between 3.1 and 4.1 billion years ago. Eukarya later diverged from the archaean line. The work further suggests that stromatolites that formed prior to the advent of cyanobacteria (about 2.6 billion years ago) photosynthesized in an anoxic environment and that because of the modifications of the Terrabacteria for land (resistance to drying and the possession of compounds that protect the organism from excess light), photosynthesis using oxygen may be closely linked to adaptations to survive on land.

Section Summary

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. 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 peptidoglycan layer fortified with teichoic acids. Gram-negative organisms have a thin cell wall and an outer envelope containing lipopolysaccharides and lipoproteins.

Prokaryotes can transfer DNA from one cell to another by three mechanisms: transformation (uptake of environmental DNA), transduction (transfer of genomic DNA via viruses), and conjugation (transfer of DNA by direct cell contact).


Prokaryotic Cell

Diversity of Prokaryotic Cells


Prokarya ( pro – before, karya – nucleus ) is the superdomain containing Bacteria and Archaea. Prokaryotes do not possess nuclei, however chromosomal DNA is present in a nucleoid region of the cytoplasm. In addition to the genomic DNA, small mobile DNA elements called plasmids may exist in some bacteria that provide additional genetic diversity.

Bacteria have two common shapes: spherical ( cocci ) or rod-shaped ( bacili ). Other morphologies like filamentous and spiral ( spirochete ) exist, as well as more complicated shapes.

Cell Walls of Bacteria

Cell walls of bacteria exist immediately outside of the cell membrane and protect the cell from internal turgor pressure. These cell walls are composed of a carbohydrate called peptidoglycan . The cell walls of bacteria are relatively porous and not a permeability barrier. Because the cell wall provides rigidity and structure, compromising the integrity is detrimental to the cell. Penicillin is an antibiotic called a β -Lactam that interferes the cross-linking of the peptidoglycan and can effectively control bacterial populations.

Bacteria are generally separated into two categories based on their cell wall composition. The Gram stain is a methodology that detects the peptidoglycan by a dye called crystal violet. Gram-positive cells stain purple because of the thick layer of peptidoglycan at the surface of the cell. Gram negative cells stain pink with a counterstain since the crystal violet can not access the thin layer of peptidoglycan beneath an outer membrane of lipopolysaccharides. β -Lactams are less effective on Gram-negative cells due to these structural differences. Properties of the outer membrane in Gram-negative bacteria vary greatly between species and render antibodies less effective in some cases.


Prokaryotic Cells

Patrick has been teaching AP Biology for 14 years and is the winner of multiple teaching awards.

Prokaryotic cells are much simpler than eukaryotic cells. Prokaryotes don't have many membrane bound internal structures such as organelles or nuclei. Their genetic information floats freely within the cell in circular loops called plasmids. Cellular processes within prokaryotes are much simpler because of the lack of organelles.

Prokaryotic cells are one of the most basic kinds of cells that are out there in Biology. They are the cells of bacteria, so let's take a look at what makes up the key characteristics of a prokaryote.

Now their name tells you one of their most important things pro means before kary means nucleus, so this tells you that this is what cells that evolved before the nucleus came about look like, so they do not possess internal organelles inside their cells that have double membranes around them like the nucleus or the mitochondria or the chloroplast. Similarly unlike Eukaryotes they only have one chlorop- one chromosome in them and it's actually a circular molecule now it's called “naked” not because it's going around there flashing the other organelles. It is because it's because it does not have a lot of proteins wrapped around it to help organize it, since they only have a little bit of DNA compared to a Eukaryotes so there's no need for that kind of organization.

Lastly they have their own special kind of ribosomes that are different from the Eukaryotic star ribosomes that your cells have or plant cells or fungi, so let's take a look at the structure of a Prokaryotic or bacterial cell.

Now these labels here are little bit small for you to see but they give you the right basic idea. This is a kind of bacteria here and you could see here is the DNA all kind of floating around inside the cytoplasm of the cell. Now they will have a cell membrane but that is often wrapped in an external wall sometimes called a capsule or a cell wall. Now this can be made out of various many various kinds of materials whether it's bacteria glycen or sorry bacteria peptide or glycen or other kind of molecules. You'll see the ribosomes those are the small little things floating around and then they may have a few other small organelles like some small little vacuoles or vesicles but that's pretty much it, the one thing that is a little bit different on the outside is they have flagella much like our cells do but our cells do this kind of motion called undulation. Bacterial cells their fungi rotate like a bored motor to make them go and that's a Prokaryotic cell.


How do Prokaryotic Cells Divide?

Prokaryotic cells divide through the process of binary fission. Unlike mitosis, this process does not involve the condensation of DNA or the duplication of organelles. Prokaryotic cells have only a small amount of DNA, which is not stored in complex chromosomes. Further, there are no organelles so there is nothing to divide.

When a prokaryote grows to a large size, the process of binary fission takes place. This process duplicates the DNA, then separates each new strand of DNA into individual cells. This process is simpler than mitosis, and as such bacteria can reproduce much faster.


Section Summary

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. 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 peptidoglycan layer fortified with teichoic acids. Gram-negative organisms have a thin cell wall and an outer envelope containing lipopolysaccharides and lipoproteins.

Prokaryotes can transfer DNA from one cell to another by three mechanisms: transformation (uptake of environmental DNA), transduction (transfer of genomic DNA via viruses), and conjugation (transfer of DNA by direct cell contact).


Text Preview

Prokaryotic cells are microscopic in size. They&rsquore typically measured in micrometers.

One micrometer is equal to one millionth of a meter. Most prokaryotic cells are in the range of 1 to 10 micrometers in length or width. Tens of thousands of average-sized prokaryotes could fit in one layer on the period at the end of this sentence.

One of the most obvious features of a prokaryotic cell is its overall shape. Most prokaryotic cells fall into one of three categories: Ball-shaped cells are called cocci singular, coccus. Rod-shaped cells are called bacilli singular, bacillus. And cells with a twisting, helical shape are called spirilla, singular, spirillum. A few species have cell shapes that don&rsquot quite fit into these categories. Cell shape is a morphological trait that helps taxonomists identify a bacterial species.

The defining characteristic of prokaryotes is that they lack a membrane-enclosed nucleus. The single chromosome, usually circular, is tightly wound and compact. The region of the cytoplasm containing the chromosome is called the nucleoid. Other smaller pieces of DNA, called plasmids, may also be present in the cytoplasm. Each plasmid contains only a few genes.

Prokaryotes lack the membrane-enclosed organelles of eukaryotes. There are no mitochondria, no chloroplasts, no endoplasmic reticulum, no Golgi apparatus. Many of the functions of these organelles take place at the prokaryotic cell&rsquos plasma membrane. A few prokaryotic species do have infoldings of the plasma membrane, where certain biochemical reactions occur.

Prokaryotes have nothing corresponding to the eukaryotic cytoskeleton. However, almost all prokaryotes have additional layers around the plasma membrane that give their cells structural support and provide additional protection against unfavorable conditions in their environment.

Most prokaryotes have a distinctive cell wall surrounding the plasma membrane. The cell wall is somewhat stiff, and partially permeable. The cell walls of eubacteria are made of a polymer called peptidoglycan, which is composed of amino acids and sugars, and forms a meshlike network around the cell. Eubacteria can be divided into two categories using the Gram stain, a technique developed by the Danish physician Hans Christian Gram in the late nineteenth century. This staining procedure discriminates between cells based on properties of their cell walls.

To stain the bacteria, we treat a sample with the dye crystal violet, then with iodine.

Then we rinse the cells with alcohol. Finally, we use a pink counterstain to see the cells that didn&rsquot retain the violet dye. Eubacteria that retain the violet dye are called Gram-positive. They are characterized by cell walls with a thick layer of peptidoglycan. Eubacteria that do not retain the violet dye are called Gram-negative. These cells have a much thinner peptidoglycan wall, and a second, outer, lipid bilayer membrane!

Now see if you can match a description of cell wall type to the result of a Gram staining procedure. Drag the correct diagram of the outer layers of a eubacterial cell next to the image of the Gram-stained sample. Click Submit to check your answer. Click Jump Ahead to skip this step.

The violet cells are Gram-positive: they have a thick peptidoglycan wall but no outer membrane. The pink cells didn&rsquot retain the violet stain, so they&rsquore Gram-negative. They have a thin peptidoglycan wall and an outer membrane.

On top of cell walls and outer membranes, many prokaryotes secrete sticky substances to form a capsule around the cell. The capsule is an extra layer of protection, and it also helps prokaryotic cells stick to surfaces and to each other.

Now that we&rsquove seen the basic structure of the prokaryotic cell, let&rsquos explore some of the lifestyles and activities of prokaryotes.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education


Difference Between Prokaryotic and Eukaryotic Cells

Cell Characteristics Prokaryotic Cells Eukaryotic Cells
Featured in organisms Domain Bacteria and Archae plants, animals, fungi, algae, protozoans
Origin 3.5 billion years ago 1.5 billion years ago
Size of the cell 0.5 micrometer in diameter or may be less 5 micrometer in diameter or more
Cell number Single-celled Multicellular
Nucleus-brain of the cell Absent Present
Nuclear membrane or Nucleolus Absent Nucleus enclosed within the Nuclear membrane
The genetic material-Deoxyribonucleic acid(DNA) DNA is freely floating and circular, not bound by the nuclear membrane, popularly called Nucleoid DNA is linear and bound by nuclear membrane
Plasma Membrane or Cytoplasmic Membrane or Outer Membrane (phospholipid bilayer) Lacks sterols Consists of sterols and carbohydrates
Chromosomes Contain Histone-like proteins Contain Histone proteins
Reproduction By binary fission Mitotic and meioitic division seen
Zygote Is partially diploid (merozygotic) Is diploid
Cell wall Contains peptidoglycan Animal cells lack cell wall. Others have either cellulose or chitin
Capsules-slimy coverings Present in bacterial cells Absent
Ribosomes 70S type 80S type. Though mitochondria and plastids have 70S type
Mitochondria Absent Present
Chloroplast Absent Present in plants
Endoplasmic Reticulum Absent Present
Golgi apparatus Absent Present
Gas vacuoles May be present Present
Membrane vacuoles Absent Present
Centrioles Absent Present
Lysosymes Absent Present
Peroxisomes Absent Present
Mesosomes May be present Absent
Endocytosis and Exocytosis Absent Present
Locomotary organ Flagella present which are made up of less number of fibrils and do not show (9+2) microtubule arrangement Flagella and pili present which have complex mechanisms and show microtubule arrangement (9+2)
Energy yielding mechanisms (Electron transport chain) Occurs in cytoplasmic membrane Occurs in mitochondrial membrane by glycolysis cycle

As evident till now, all living beings are made up of cells and cell products, consisting of a number of cell organelles, which help in carrying out various life processes. A matter of debate between prokaryotic and eukaryotic cells is mitochondria – the energy-yielding organelle, which is supposed to be independent. They have their own genetic material – the DNA and hence, are capable of division. They have a symbiotic sort of relationship with the cell in which they reside. Thus, it might be obvious that they have originated from a single independent prokaryotic unit, since they are similar to them, in the sense that they have an unbound circular DNA. But the surprising thing is that the prokaryotic units lack mitochondria. Does this mean that it is a living organism residing inside another cell, or is it just an independent organ? If you are aware of all the scientific terms used, it will be easy to grasp.

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