Chapter 26 Summary

Core Concepts Summary

26.1 The tree of life has three main branches, called domains: Eukarya, Bacteria, and Archaea.

Prokaryotic cells are cells that lack a nucleus; they include Bacteria and Archaea. page 530

Bacteria are small and lack membrane-bounded organelles. page 530

The bacterial genome is circular. Some bacteria also carry smaller circles of DNA called plasmids. page 530

Diffusion limits size in bacterial cells. page 530

Bacteria can obtain DNA by horizontal gene transfer from organisms that may be distantly related. page 532

Like Bacteria, Archaea lack a nucleus, but form a second prokaryotic domain distinct from Bacteria. page 534

Some archaeons are extremophiles, living in extreme environments characterized by low pH, high salt, or high temperatures, but others live in less extreme environments like the upper ocean or soil. page 534

26.2 Bacteria and Archaea are notable for their metabolic diversity.

Bacteria are capable of oxygenic photosynthesis, using water as a source of electrons and producing oxygen as a by-product, and of anoxygenic photosynthesis, using electron donors other than water, such as H2S, H2, and Fe2+. page 535

Some bacteria and archaeons are capable of anaerobic respiration, in which NO3–, SO42–, Mn4+, and Fe3+ serve as the electron acceptor instead of oxygen gas. page 535

Many bacteria and archaeons obtain energy from fermentation, which involves the partial oxidation of organic molecules and the production of ATP in limited quantities. page 536

Some photosynthetic bacteria are photoheterotrophs, obtaining energy from sunlight but using preformed organic compounds rather than CO2 as a source of carbon. page 537

Chemoautotrophy, in which chemical energy is used to convert CO2 to organic molecules, is unique to Bacteria and Archaea. page 537

26.3 In addition to their key roles in the carbon cycle, Bacteria and Archaea are critical to the biological cycling of sulfur and nitrogen.

Plants and algae can take up sulfur and incorporate it into proteins, but bacteria and archaeons dominate the sulfur cycle by means of oxidation and reduction reactions that are linked to the carbon cycle. page 538

Some bacteria and archaeons can reduce nitrogen gas to ammonia in a process called nitrogen fixation. page 539

The nitrogen cycle also involves oxidation and reduction reactions by Bacteria and Archaea that are linked to the carbon cycle. page 540

26.4 The extent of bacterial diversity was recognized when sequencing technologies could be applied to non-culturable bacteria.

Traditionally, bacterial groups were recognized by morphology, physiology, and the ability to take up specific stains in culture. page 540

Direct sequencing of ribosomal RNA genes from organisms in soil and seawater samples revealed new groups of bacteria. page 540

552

Proteobacteria are the most diverse group of bacteria and are involved in many of the biogeochemical processes that are linked to the carbon cycle. page 543

Gram-positive bacteria include important disease-causing strains as well as species that are principal sources of antibiotics. page 543

Photosynthetic bacteria are not limited to a single branch of the bacterial tree. page 544

26.5 The diversity of Archaea has only recently been recognized.

Archaeons tend to thrive where energy available for growth is limited. page 545

Archaea are commonly divided into three major groups, the Crenarchaeota, Thaumarchaeota, and Euryarchaeota. page 545

Archaeons at the base of the Crenarchaeota and Euryarchaeota are hyperthermophiles, meaning that they grow at high temperatures. page 545

A number of archaeons grow in highly acidic waters, such as those associated with acid mine drainage. page 546

Some archaeons (but no bacteria or eukaryotes) generate methane as a by-product of their energy metabolism, contributing in important ways to the carbon cycle. page 546

Haloarchaea are archaeons that can live only in extremely salty environments. page 546

Thaumarchaeotes are among the most abundant cells in the oceans. page 546

26.6 The earliest forms of life on Earth were Bacteria and Archaea

Evidence for the early history of life comes from microfossils, fossilized structures called stromatolites, and the isotopic composition of rocks and organic matter. page 548

Fossils indicate that life on Earth originated more than 3.5 billion years ago. page 548

The early atmosphere and ocean contained little or no free oxygen. page 548

Oxygen began to accumulate in the atmosphere and oceans about 2.4 billion years ago as a result of the success of cyanobacteria utilizing oxygenic photosynthesis. page 549

Prokaryotic metabolisms were not only essential in the early history of Earth, but are also vital today, as many forms of life depend on biogeochemical cycles and metabolisms unique to Bacteria and Archaea. page 549

Most animals, including humans, live in intimate association with bacteria, which in turn affect health. page 550

Self-Assessment

  1. Name and describe the three domains of life.

    Self-Assessment 1 Answer

    The three domains of life are Eukarya, Archaea, and Bacteria. Eukarya consists of organisms composed of eukaryotic cells—cells with a membrane-bound nucleus and organelles that form separate compartments used for distinct cell functions. Archaea and Bacteria both consist of single-celled, prokaryotic organisms. Prokaryotic cells have a simpler internal organization than eukaryotic cells, with no membrane surrounding the cell’s DNA and very little in the way of internal organization. While bacterial and archaeal cells are both prokaryotic and share some characteristics, many of their features are quite distinctive.

  2. Describe shared and contrasting features of bacterial and archaeal cells.

    Self-Assessment 2 Answer

    Prokaryotic cells (both bacteria and archaeons) do not contain a nucleus or other membrane-bound organelles; they rarely have introns in their genes; their DNA occurs in a circular form; and they are relatively small in size when compared to eukaryotic cells. Archaeal cells also possess some features that differ from those of bacteria: Archaea have different lipids present in their membrane; do not undergo photosynthesis using chlorophyll; contain strains capable of methanogenesis; and have histone proteins in their cells. Importantly, DNA transcription in archaeons uses a RNA polymerase and ribosomes that are more similar to those of eukaryotes than those of bacteria. Furthermore, many of the antibiotics that target protein synthesis in bacteria are ineffective against archaeons, suggesting fundamental differences in translation as well.

  3. Explain how prokaryotic cells obtain nutrients and how this process puts constraints on their size.

    Self-Assessment 3 Answer

    Prokaryotic cells obtain nutrients through diffusion—the random motion of molecules. Nutrients diffuse from the environment across the cell membrane, and need to be able to reach all areas of the cell. This requirement limits the size of cells that can obtain nutrients by diffusion.

  4. Describe how surface area and volume change with size.

    Self-Assessment 4 Answer

    The surface area of a spherical cell—the area available for taking up molecules from the environment—increases as the square of the radius. However, the cell’s volume—the amount of cytoplasm that is supported by diffusion—increases as the cube of the radius. Therefore, a small cell has more surface area in proportion to its volume, whereas a bigger cell has less surface area in proportion to its volume. As cell size increases, it becomes harder to supply the cell with the materials needed for growth using diffusion alone.

  5. Explain how photosynthesis can occur without the production of oxygen, and how respiration can occur without requiring oxygen.

    Self-Assessment 5 Answer

    Anoxygenic photosynthesis uses hydrogen sulfide, hydrogen gas, ferrous iron, or arsenite instead of H2O as the electron donor; thus, they do not release O2 as a by-product. Respiration can occur without requiring oxygen, using alternative electron acceptors such as nitrogen, sulfur, manganese, iron, or arsenic oxides.

  6. Describe the roles of bacteria and archaeons in the sulfur and nitrogen cycles.

    Self-Assessment 6 Answer

    In the sulfur cycle, bacteria and archaeons reduce sulfur in a process called anaerobic respiration, and oxidize sulfur through chemosynthetic and photosynthetic processes. In the nitrogen cycle, bacteria and archaeons fix nitrogen gas to ammonia and through the processes of nitrification, denitrification, and anammox turn ammonia back into nitrogen gas.

  7. Explain how horizontal gene transfer complicates our understanding of evolutionary relationships among bacteria and archaeons.

    Self-Assessment 7 Answer

    Horizontal gene transfer complicates evolutionary relationships among bacteria and archaeons because phylogenies may falsely group distantly related bacteria by grouping genes passed on by conjugation, transformation, or transduction.

  8. Name and describe three major groups of Bacteria.

    Self-Assessment 8 Answer

    Proteobacteria are the most diverse of the bacterial groups and include many organisms that participate in the expanded carbon and other biogeochemical cycles. Gram-positive bacteria include both pathogens and bacteria that produce antibiotics. Cyanobacteria are species of bacteria that can carry out oxygenic photosynthesis.

  9. Name and describe three major groups of Archaea.

    Self-Assessment 9 Answer

    Three major groups of Archaea are the Crenarchaeota, Euryarchaeota, and Thaumarchaeota. The first two groups include acid-loving and heat-loving organisms. Euryarchaeota also include methane-producing and salt-loving organisms. Thaumarchaeota are nitrogen-metabolizing microbes that thrive in colder, oxygen-poor environments, like the deep ocean.

  10. State the age of Earth and the time when life is thought to have first originated.

    Self-Assessment 10 Answer

    Earth is about 4.6 billion years old, and life originated no later than 3.5 billion years ago.