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
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.
- Provide students with multiple opportunities to summarize the similarities and differences between prokaryotic and eukaryotic cells and between cells in the three domains (Eukarya, Archaea, Bacteria). You may wish to ask students to sketch typical cells of each class or domain, create tables comparing and contrasting the cellular and genomic organization in each, or complete other short activities. When discussing similarities and differences, be sure to offer or ask for qualifying details where it makes sense to do so. (For example, cell walls are found in prokaryotes and some eukaryotes; the material of which they are made is quite different.)
- When reviewing prokaryotic reproduction, take time to connect new information to students’ previous knowledge. For example, remind students of the importance of genetic diversity as discussed in chapters on evolutionary theory. Emphasize that although new mutations are a major source of variation (as they learned in previous chapters), additional diversity arises in prokaryotic populations from genetic recombination. Stress that while eukaryotes carry out the sexual processes of meiosis and fertilization that combine DNA from two individuals, prokaryotes uses other processes (transformation, transduction, and conjugation) to bring together DNA from different individuals. You may wish to ask students to consider the advantages of several modes of genetic recombination for a population.
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 Practice Challenge Questions contain 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).
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.
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 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.
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 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).
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.
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.
|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|
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.
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 (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. Stromatolites are some of the oldest fossilized organisms on Earth at around 3.5 million years ago. There is evidence that these prokaryotes were also some of the first photosynthesizes on Earth. In fact, bacterial prokaryotes were likely responsible for the first accumulation of oxygen in our atmosphere through photosynthesis. The group Terrabacteria possessed many adaptations for living on land, such as resistance to drying. Some of these adaptations were also related to photosynthesis, such as compounds that protect cells from excess light. These early prokaryotic pathways related to photosynthesis were the foundation for photosynthesis in eukaryotic cells. This is evidenced by the similarity in structure and function between some photosynthetic prokaryotes and eukaryotic chloroplasts.
What features and metabolic processes do all cells, both prokaryotes and eukaryotes, have in common? How do prokaryotes and eukaryotes differ?
Students should identify similarities between prokaryotes and eukaryotes such as the use of DNA, ribosomes, and ATP. Prokaryotes and eukaryotes are different in that prokaryotes do not have membrane-bound organelles such as mitochondria and nuclei. Both questions above are applications of AP® Learning Objective 2.14 and Science Practice 1.4 because students are asked to describe differences and similarities in prokaryotic and eukaryotic cells in addition to differences between bacteria and archaea.
- 2Battistuzzi, FU, Feijao, A, and Hedges, SB. A genomic timescale of prokaryote evolution: Insights into the origin of methanogenesis, phototrophy, and the colonization of land. BioMed Central: Evolutionary Biology 4 (2004): 44, doi:10.1186/1471-2148-4-44.