hlw_ch26

CHAPTER 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.
Bacteria are small and lack membrane-bound organelles.
The bacterial genome is circular. Some bacteria also carry smaller circles of DNA called plasmids.
Diffusion limits size in bacterial cells.
Bacteria can obtain DNA by horizontal gene transfer from organisms that may be distantly related.
Like Bacteria, Archaea lack a nucleus, but form a second prokaryotic domain distinct from Bacteria.
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.

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⁺.
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.
Many bacteria and archaeons obtain energy from fermentation, which involves the partial oxidation of organic molecules and the production of ATP by substrate-level phosphorylation.
Many photosynthetic bacteria are photoheterotrophs, obtaining energy from sunlight and using preformed organic compounds as a source of carbon instead of CO₂.
Chemoautotrophy, in which chemical energy is used to convert CO₂ to organic molecules, is unique to Bacteria and Archaea.

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.
Bacteria and archaeons can to reduce nitrogen gas to ammonia in a process called nitrogen fixation.
The nitrogen cycle also involves oxidation and reduction reactions that are linked to the carbon cycle.

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26.4 THE EXTENT OF BACTERIAL DIVERSITY WAS RECOGNIZED ONLY 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.
Direct sequencing of ribosomal RNA genes from organisms in soil and seawater samples revealed new groups of bacteria.
Proteobacteria are the most diverse group of bacteria and are involved in many of the biogeochemical cycles that are linked to the carbon cycle.
Gram-positive bacteria include important disease-causing strains as well as species that are principal sources of antibiotics.
Photosynthetic bacteria are not limited to a single branch of the bacterial tree.

26.5 THE DIVERSITY OF ARCHAEA HAS ONLY RECENTLY BEEN RECOGNIZED.

Archaeons tend to thrive where energy available for growth is limited.
Archaea are divided into three major groups, the Crenarchaeota, Thaumarchaeota, and Euryarchaeota.
Archaeons at the base of the Crenarchaeota and Euryarchaeota are hyperthermophiles, meaning that they grow at high temperatures.
A number of archaeons grow in highly acidic waters, such as those associated with acid mine drainage.
Some archaeons generate methane, and others tolerate high-salt conditions.
Thaumarchaeotes may be the most abundant cells in the oceans.

26.6 THE EARLIEST FORMS OF LIFE ON EARTH WERE BACTERIA AND ARCHAEA.

Evidence for the early history of life on Earth comes from microfossils, fossilized structures called stromatolites, and the isotopic composition of rocks and organic matter.
Fossils indicate that life on Earth originated more than 3.5 billion years ago.
The early atmosphere and ocean contained little or no free oxygen.
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.
Prokaryotic metabolisms were not only essential in the early history of the Earth, but are also vital today, as many forms of life depend on biogeochemical cycles and metabolisms unique to Bacteria and Archaea.
Most animals, including humans, live in intimate association with bacteria, which in turn affect our health.

Self-Assessment Question 1

Name and describe the three domains of life.

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Model 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 for many 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.

Self-Assessment Question 2

Describe shared and contrasting features of bacterial and archaeal cells.

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Model Answer:

Prokaryotic cells (both bacteria and archaeons) do not contain a nucleus or other membrane-bound organelles; do not 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; are 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 to bacteria. Furthermore, many of the antibiotics that target protein synthesis in bacteria are ineffective against archaeons, suggesting fundamental differences in translation as well.

Self-Assessment Question 3

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

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Model 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.

Self-Assessment Question 4

Describe how surface area and volume change with size.

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Model 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.

Self-Assessment Question 5

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

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Model Answer:

Anoxygenic photosynthetic processes use 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 when alternative electron acceptors such as nitrogen sulfur, manganese, iron, or arsenic are used instead of O₂.

Self-Assessment Question 6

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

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Model 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.

Self-Assessment Question 7

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

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Model 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.

Self-Assessment Question 8

Name and describe three major groups of Bacteria.

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Model Answer:

Proteobacteria are the most diverse of the bacterial groups and contain many organisms that populate 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 undergo oxygenic photosynthesis.

Self-Assessment Question 9

Name and describe three major groups of Archaea.

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Model Answer:

The 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 survive in colder, oxygen-poor environments, like the deep ocean.

Self-Assessment Question 10

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

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Model Answer:

The Earth is about 4.5 billion years old, and life is thought to have first originated around 3.5 billion years ago.