Key Terms
archaea spirochete
bacteria binary fission
coccus endospore
bacillus cyanobacteria
There are more prokaryotes living in a handful of fertile soil than the total number of people who have ever lived. In fact, prokaryotic organisms outnumber all eukaryotic organisms combined. Prokaryotes inhabit some of the most extreme environments imaginable. But they also inhabit "normal"
environments in huge numbers. Though individual prokaryotes are small organisms, their combined impact on Earth and life is enormous.
Diversity of Prokaryotic Life
Some prokaryotes are well-known because they cause diseases in humans–a fact first recognized only in the 1860s. For example, the disease tuberculosis (TB) is caused by a bacterium called Mycobacterium tuberculosis. This tiny microbe causes more deaths worldwide than any other infection. Other bacteria cause strep throat (Streptococcus pyogenes), diphtheria (Corynebacterium diphtheriae), many sexually transmitted diseases, and certain kinds of food poisoning. These diseases may give the impression that all microorganisms are harmful. Far more common, however, are prokaryotes that are either not harmful or are actually helpful to humans and other organisms. For example, the bacterium Escherichia coli (E. coli) that lives in your intestines releases certain vitamins that are important to your health. Other bacteria in your mouth prevent harmful fungi from growing there. On a global scale, prokaryotes cycle vital chemicals between organic matter and the soil and atmosphere. For example, some species of soil bacteria convert nitrogen gas (N2) from the atmosphere to nitrogen-containing compounds that plants can absorb from the soil. Plants use these nitrogen-containing compounds to build proteins. Without prokaryotes, larger organisms on this planet could not survive.
As you read in Chapter 15, many biologists now classify organisms into three domains–Archaea, Bacteria, and Eukarya. The third domain, Eukarya, consists of all the unicellular and multicellular organisms made of eukaryotic cells, including protists, fungi, plants, and animals. The domains Archaea and Bacteria both consist of prokaryotes. Recall that prokaryotes lack membrane-bound nuclei like the nuclei found in eukaryotes. Prokaryotes instead have their DNA concentrated in nucleoid regions. However, archaea and bacteria differ in many other characteristics as you will soon read.
Park or deep-sea vents where super-heated water exceeds 100°C. Halophiles (salt lovers) thrive in such environments as Utah's Great Salt Lake, or in seawater evaporating ponds. Still other species of archaea live in oxygen-free environments such as the mud at the bottom of lakes and swamps where they produce bubbles of "swamp gas" (methane). Some archaea also live in less extreme environments, such as cool seawater.
Biologists hypothesize that archaea and bacteria diverged very early in their evolution from ancient prokaryotic ancestors. Genetic analysis of organisms from all three domains indicates that archaea may be at least as closely related to eukaryotes as they are to bacteria.
Bacteria Prokaryotic organisms classified as bacteria differ from archaea in several features of cell structure and chemical makeup (Figure 16-8). Scientists place the two groups of prokaryotes in separate domains partly because of key differences in the information contained in their nucleic acids (DNA and RNA). There are also differences in RNA polymerases, the enzymes that catalyze the synthesis of RNA. Bacteria polymerases are relatively small and simple, while archaea polymerases are complex and similar to those of eukaryotes. Introns, the noncoding portions of genes, are absent in bacteria but are present in some genes of archaea. Certain antibiotics that kill bacteria have no effect on archaea. And finally, bacterial cell walls contain a polymer called peptidoglycan, which consists of sugars and short polypeptides and is not found in archaea or eukaryotes.
Figure 16-8
Bacteria have a basic cell structure that includes a cell wall, plasma membrane, ribosomes, DNA that is not enclosed in a membrane, pili, and flagella for movement. Structure and Function of Bacteria
Cell Shape Bacteria come in three basic shapes: spherical, rod-shaped, and spiral-shaped. Spherical bacteria, such as the bacteria that cause pneumonia, are called cocci (singular, coccus), from the Greek word for "berries." Some species form clusters of spherical cells, while others, such as Streptococcus, form chains. Rod-shaped bacteria are called bacilli (singular, bacillus). The E. coli bacteria in your intestine are bacilli. A third group of bacteria is curved or spiral-shaped. The largest spiral-shaped bacteria are called spirochetes. The bacterium that causes syphilis and another that causes Lyme disease are spirochetes.
Cell Wall Structure Nearly all bacteria have a cell wall outside their plasma membrane. As in plants, the wall maintains cell shape and protects the cell. But the cell walls of bacteria differ greatly from the cell walls of plants, fungi, and protists.
Bacteria have one of two types of cell walls. One type is composed mostly of peptidoglycan, while the other has less peptidoglycan and an additional outer membrane. The two types can be distinguished by a testing method called Gram staining. In this test, technicians place bacteria on a glass slide. Next they wash a violet-colored dye over the slide, which stains the bacteria. The violet dye is then rinsed off and the slide is washed a second time with pink dye. Then a microscope is used to observe the color of the bacteria. If the bacteria appear purple, they are Gram-positive (Figure 16-10, left). This is because the extra-thick cell wall of Gram-positive bacteria retains the violet dye. In contrast, Gram-negative bacteria do not retain the violet dye, but take on the pink dye. As a result, they appear pink (Figure 16-10, right). How is this classification useful? Some antibiotics only work against Gram-positive bacteria. Doctors use Gram staining to identify bacteria in order to prescribe the correct antibiotics.
Figure 16-10
Gram-positive bacteria (left) have cell walls made mostly of peptidoglycan and retain the violet dye. Gram-negative bacteria (right) have an outer membrane in their cell walls and lose the violet dye but retain the pink dye.
Most motile bacteria have a bacterial flagellum (plural, flagella), which is different in structure from the flagella of eukaryotic cells. The filament of the bacterial flagellum is anchored in the plasma membrane and cell wall. Flagella may be scattered over the entire cell surface or concentrated at one or both ends of the cell. In addition, many bacteria possess structures called pili that are shorter and thinner than flagella (see Figure 16-8). Pili help bacteria stick to each other and to surfaces such as rocks in flowing streams or the lining of animal intestines.
Flagella are not the only mechanism of prokaryotic motility. Some bacteria form chains of cells that secrete slimy threads that anchor to surfaces. The bacteria glide along by extending their slime threads as they go.
Reproduction
Most prokaryotes can reproduce at a phenomenal rate under the right conditions. Prokaryotic cells copy their DNA almost continuously and divide repeatedly. With each division, called binary fission, the DNA copies move to opposite ends of the cell as the cell splits in the middle (Figure 16-11). Binary fission is much simpler than the process of mitosis that occurs in eukaryotes. This is another important difference between prokaryotes and eukaryotes.
Figure 16-11
Binary fission enables prokaryotes to reproduce very quickly in the right conditions.
Still, you can understand why certain bacteria can make you sick so soon after just a few cells infect you– or why food can spoil so rapidly. Refrigeration delays spoiling because low temperatures reduce the rate of reproduction in most microorganisms.
Binary fission produces a colony of cells that are clones–they are genetically identical. Chapter 13 discussed how this property of bacteria makes them especially useful in genetic engineering. But you may also recall that occasional errors called mutations occur in the DNA copying process. Most mutations are harmful, but occasionally a mutation aids survival in a particular environment. For example, as you read in Chapter 14, a mutation for resistance to a particular antibiotic favors the reproduction of the mutant cells over the non-resistant ones when the antibiotic is present. Soon, most of the bacteria in the population carry the mutated gene.
Figure 16-12
Bacteria are capable of receiving or exchanging genetic material. The result of these processes is greater variety within a population of bacteria.
Endospore Formation Some bacteria can survive extended periods of very harsh conditions by forming specialized "resting" cells, or endospores, within themselves. One example is Bacillus anthracis, the bacterium that causes the disease anthrax in cattle, sheep, and humans. The original cell copies its chromosome, and one copy becomes surrounded by a thick protective coat. The outer cell disintegrates, leaving the highly resistant endospore. Some endospores can survive lack of water and nutrients, heat, cold, and most poisons for many years. When the environment becomes more favorable, endospores can absorb water and grow again.
Modes of Nutrition
The phrase "mode of nutrition" describes how organisms obtain energy and carbon atoms. Some organisms obtain energy by photosynthesis (identified by the prefix photo-). Others obtain energy from chemical sources (chemo-). Autotrophs obtain carbon atoms from carbon dioxide. Heterotrophs obtain carbon from existing organic molecules (such as those in food).
compounds from carbon dioxide. Plants and many prokaryotes are photoautotrophs. Chemoautotrophs use carbon dioxide as a carbon source, but they extract energy from inorganic substances such as hydrogen sulfide or ammonia. All chemoautotrophs are prokaryotes. Photoheterotrophs use light energy to make ATP but obtain their carbon in organic form. This mode of nutrition is only found in certain prokaryotes. Chemoheterotrophs consume organic molecules for both energy and carbon. This nutritional mode exists in many prokaryotes and protists, as well as in all fungi and animals.
Figure 16-14
This table summarizes the four nutritional lifestyles of organisms.
Cyanobacteria and the "Oxygen Revolution"
Fossil evidence indicates that the photoautotrophic mode of nutrition is very ancient. As you read in Concept 16.1, scientists have inferred the early existence of photoautotrophs from the structure of prokaryote fossils in stromatolites. One photoautotrophic group of bacteria, called cyanobacteria, generates oxygen as a waste product of their photosynthesis.
Earth's early atmosphere was anaerobic, meaning it had very little or no free oxygen (O2). However, as cyanobacteria evolved, they began generating oxygen as a byproduct of photosynthesis. The oxygen would have bubbled to the surface of lakes and oceans and entered the atmosphere.
such as deep in the mud. Their descendants, called anaerobic organisms, survive in such oxygen-free environments today. Other organisms adapted and began using oxygen in extracting energy from food, the key process of cellular respiration. These are called aerobic organisms. The atmospheric oxygen required for cellular respiration is recycled by photosynthesis in cyanobacteria, algae, and plants. Today many prokaryotes and nearly all eukaryotes are aerobic. The "oxygen revolution" was a major episode in the history of life.
Concept Check 16.2
1. In what major ways are archaea different from bacteria? 2. What are the three major cell shapes of bacteria?
3. Describe three ways bacteria can recombine their genetic material.
4. Name the four major modes of prokaryotic nutrition. For each, identify the energy source and carbon source.