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Learning Objectives Connection for AP® Courses The Prokaryotic Cell Reproduction References

Learning Objectives

In this section, you will explore the following questions:

What are similarities in the structures of the prokaryotes, Archaea and Bacteria?
What are examples of structural differences between Archaea and Bacteria?

Connection for AP® Courses

Connection for AP^® Courses

Domains Archaea and Bacteria contain single-celled organisms lacking a nucleus and other membrane-bound organelles. The two groups have substantial biochemical and structural differences. Most have a cell wall external to the plasma cell membrane, the composition of which can vary among groups, and many have additional structures such as flagella and pili. Prokaryotes also have ribosomes, where protein synthesis occurs. For the purpose of AP^®, you do not have to memorize the various groups of bacteria. You should, however, be able to distinguish between prokaryotes and eukaryotes and know the domains.

Information presented and the examples highlighted in the section support concepts outlined in Big Idea 2 and Big Idea 3 of the AP^® Biology Curriculum Framework. The AP^® Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP^® Biology course, an inquiry-based laboratory experience, instructional activities, and AP^® exam questions. A Learning Objective merges required content with one or more of the seven Science Practices.

Big Idea 2	Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis.
Enduring Understanding 2.B	Growth, reproduction and dynamic homeostasis require that cell create and maintain internal environments that are different form their external environment.
Essential Knowledge	2.B.3 Archaea and Bacteria generally lack internal membranes and organelles.
Science Practice	1.4 The student can use representations and models to analyze situations or solve problems qualitatively and quantitatively.
Learning Objective	2.14 The student is able to use representations and models to describe differences in prokaryotic and eukaryotic cells.
Big Idea 3	Living systems store, retrieve, transmit, and respond to information essential to life processes.
Enduring Understanding 3.C	The processing of genetic information is imperfect and is a source of genetic variation.
Essential Knowledge	3.C.2 Prokaryotes contain circular chromosomes and plasmid DNA.
Science Practice	6.2 The student can construct explanations of phenomena based on evidence produced through scientific practices.
Learning Objective	3.27 The student is able to compare and contrast processes by which genetic variation is produced and maintained in organisms from multiple domains.
Essential Knowledge	3.C.2 Prokaryotes contain circular chromosomes and plasmid DNA.
Science Practice	7.2 The student can connect concepts in and across domain(s) to generalize or extrapolate in and/or across enduring understandings and/or big ideas.
Learning Objective	3.28 The student is able to construct an explanation of the multiple processes that increase variation within a population.

The Science Practices Assessment Ancillary contains additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards:

[APLO 2.5]
[APLO 2.13]
[APLO 2.14]
[APLO 4.9]

There are many differences between prokaryotic and eukaryotic cells. 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 jelly-like substance inside the cell; nucleic acids, the genetic material 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).

Part a: The micrograph shows ball-shaped cocci about 0.9 microns long. Part b: The micrograph shows hotdog-shaped bacilli about 2 microns long. Part c: The micrograph shows corkscrew-shaped spirilli that are quite long and 2 microns in diameter.

Figure 22.9 Prokaryotes fall into three basic categories based on their shape, visualized here using scanning electron microscopy: (a) cocci, or spherical—a pair is shown—(b) bacilli, or rod-shaped; and (c) spirilli, or spiral-shaped. (credit a: modification of work by Janice Haney Carr, Dr. Richard Facklam, CDC; credit c: modification of work by Dr. David Cox; scale-bar data from Matt Russell)

The Prokaryotic Cell

Recall that prokaryotes (Figure 22.10) 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.

Figure 22.10 The features of a typical prokaryotic cell are shown.

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).

The trunk of the phylogenetic tree is a universal ancestor. The tree forms two branches. One branch leads to the domain bacteria, which includes the phyla proteobacteria, chlamydias, spirochetes, cyanobacteria, and Gram-positive bacteria. The other branch branches again, into the eukarya and archaea domains. Domain archaea includes the phyla euryarchaeotes, crenarchaeotes, nanoarchaeotes, and korarchaeotea.

Figure 22.11 Bacteria and Archaea are both prokaryotes but differ enough to be placed in separate domains. An ancestor of modern Archaea is believed to have given rise to Eukarya, the third domain of life. Archaeal and bacterial phyla are shown; the evolutionary relationship between these phyla is still open to debate.

The composition of the cell wall differs significantly between the domains Bacteria and Archaea. 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 it is responsible for the organism’s shape. Some bacteria have an outer capsule outside the cell wall. Other structures are present in some prokaryotic species, but not in others (Table 22.2). 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 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. Plasmids, which consist of extra-chromosomal DNA, are also present in many species of bacteria and archaea.

Characteristics of phyla of Bacteria are described in Figure 22.12 and Figure 22.13; Archaea are described in Figure 22.14.

Characteristics of the five phyla of bacteria are described. The first phylum described is proteobacteria, which includes five classes, alpha, beta, gamma, delta and epsilon. Most species of Alpha Proteobacteria are photoautotrophic but some are symbionts of plants and animals, and others are pathogens. Eukaryotic mitochondria are thought be derived from bacteria in this group. Representative species include Rhizobium, a nitrogen-fixing endosymbiont associated with the roots of legumes, and Rickettsia, ob

Figure 22.12 Phylum Proteobacteria is one of up to 52 bacteria phyla. Proteobacteria is further subdivided into five classes, Alpha through Epsilon. (credit Rickettsia rickettsia: modification of work by CDC; credit Spirillum minus: modification of work by Wolframm Adlassnig; credit Vibrio cholera: modification of work by Janice Haney Carr, CDC; credit Desulfovibrio vulgaris: modification of work by Graham Bradley; credit Campylobacter: modification of work by De Wood, Pooley, USDA, ARS, EMU; scale-bar data from Matt Russell)

A table describing four different types of bacteria: Chlamydia, Spirochaetae, Cyanobacteria, and Gram-positive. The table is made up of three columns to describe the bacteria: phylum, representative organisms, and representative micrograph.

Figure 22.13 Chlamydia, Spirochetes, Cyanobacteria, and Gram-positive bacteria are described in this table. Note that bacterial shape is not phylum-dependent; bacteria within a phylum may be cocci, rod-shaped, or spiral. (credit Chlamydia trachomatis: modification of work by Dr. Lance Liotta Laboratory, NCI; credit Treponema pallidum: modification of work by Dr. David Cox, CDC; credit Phormidium: modification of work by USGS; credit Clostridium difficile: modification of work by Lois S. Wiggs, CDC; scale-bar data from Matt Russell)

Characteristics of the four phyla of archaea are described. Euryarchaeotes includes methanogens, which produce methane as a metabolic waste product, and halobacteria, which live in an extreme saline environment. Methanogens cause flatulence in humans and other animals. Halobacteria can grow in large blooms that appear reddish, due to the presence of bacterirhodopsin in the membrane. Bacteriorhodopsin is related to the retinal pigment rhodopsin. Micrograph shows rod-shaped Halobacterium. Members of the ubi

Figure 22.14 Archaea are separated into four phyla: the Korarchaeota, Euryarchaeota, Crenarchaeota, and Nanoarchaeota. (credit Halobacterium: modification of work by NASA; credit Nanoarchaeotum equitans: modification of work by Karl O. Stetter; credit korarchaeota: modification of work by Office of Science of the U.S. Dept. of Energy; scale-bar data from Matt Russell)

The Plasma Membrane

The plasma membrane is a thin lipid bilayer (six to eight 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 22.14).

This illustration compares phospholipids from Bacteria and Eukarya to those from Archaea. In Bacteria and Eukarya, fatty acids are attached to glycerol by an ester linkage, while in Archaea, isoprene chains are linked to glycerol by an ether linkage. In the ester linkage, the first carbon in the fatty acid chain has an oxygen double-bonded to it, whereas in the ether linkage, it does not. In Archaea, the isoprene chains have methyl groups branching off from them, whereas such branches are absent in Bacter

Figure 22.15 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 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, 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 22.15). 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

The left illustration shows the cell wall of Gram-positive bacteria. The cell wall is a thick layer of peptidoglycan that exists outside the plasma membrane. A long, thin molecule called lipoteichoic acid anchors the cell wall to the cell membrane. The right illustration shows Gram-negative bacteria. In Gram-negative bacteria, a thin peptidoglycan cell wall is sandwiched between an outer and an inner plasma membrane. The space between the two membranes is called the periplasmic space. Lipoproteins anchor

Figure 22.16 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. (credit: modification of work by Franciscosp2/Wikimedia Commons)

Which of the following statements is true?

Gram-positive bacteria have a cell wall anchored to the cell membrane by lipoteichoic acid.
Porins allow entry of substances into both Gram-positive and Gram-negative bacteria.
The cell wall of Gram-negative bacteria is thick, and the cell wall of Gram-positive bacteria is thin.
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.

Structural 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	Phytanyl groups

Table 22.2

Reproduction

Reproduction in prokaryotes is asexual and usually takes place by binary fission. Recall that the DNA of a prokaryote exists 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.

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. Archaea are not affected by bacteriophages but instead have their own viruses that 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. The DNA transferred can be in the form of a plasmid or as a hybrid, 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.

Illustration A shows a small, circular piece of DNA being absorbed by a cell. Illustration C shows a bacteriophage injecting DNA into a prokaryotic cell. The DNA then gets incorporated in the genome. Illustration C shows two bacteria connected by a pilus. A small loop of DNA is transferred from one cell to another via the pilus.

Figure 22.17 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. The DNA may remain separate as plasmid DNA or be incorporated into the host genome. 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 sex pilus draws the two bacteria close enough to form the bridge.

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. The model they derived from their data indicates that three important groups of bacteria—Actinobacteria, Deinococcus, and Cyanobacteria, which the authors call Terrabacteria—were the first to colonize land. Recall that Deinococcus is a genus of prokaryote—a bacterium—that is highly resistant to ionizing radiation. Cyanobacteria are photosynthesizers, while Actinobacteria are a group of very common bacteria that include species important in decomposition of organic wastes.

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 archaea diverged earlier: between 3.1 and 4.1 billion years ago. Eukarya later diverged off 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.

Science Practice Connection for AP® Courses

Think About It

What features and metabolic processes do all cells, both prokaryotes and eukaryotes, have in common? How do prokaryotes and eukaryotes differ?

References

Battistuzzi, F. U., Feijao, A., & Hedges, S. B. (2004, Nov. 9). A genomic timescale of prokaryote evolution: Insights into the origin of methanogenesis, phototrophy, and the colonization of land. BioMed Central: Evolutionary Biology 4, 44. doi: 10.1186/1471-2148-4-44.

22.2 Structure of Prokaryotes

22.2 Structure of Prokaryotes

Learning Objectives