Prokaryotes: The First Single-Celled Creatures Chapter 16

Origin of Life The earth’s environment when life originated 2.5 billion years ago was very different than it is today. There was little or no oxygen in the atmosphere. The atmosphere was full of hydrogen-rich gases, such as SH2, NH3, and CH4. High energy electrons would have been freely created by energy from the sun (UV radiation) or electrical energy in lightning.

Origin of Life Stanley Miller and Harold Urey recreated an oxygen-free atmosphere similar to the early earth in the laboratory. When subjected to levels of lightning and UV radiation, many organic building blocks formed spontaneously. The researchers concluded that life may have evolved in a “primordial soup” of biological molecules.

Origin of Life The bubble model of Louis Lerman suggests an answer to the problems of the primordial soup model. Bubbles produced by volcanic eruptions under the sea provide various gases, and the gases could react in bubbles. The bubble surface would protect the gases from breakdown by UV radiation. At the surface of the ocean, the bubbles could pop, and the molecules could briefly be subject to energy sources. More complex molecules could have formed and then fell back into the sea to again be enclosed in bubbles and re-enter the cycle.

A chemical process involving bubbles may have preceded the origin of life Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 3 3 4 4 When the bubbles persisted long enough to rise to the surface, they popped, releasing their contents to the air. Bombarded by the sun's ultraviolet radiation, lightning, and other energy sources, the simple organic molecules released from the bubbles reacted to form more complex organic molecules. 2 2 The gases, concentrated inside the bubbles, reacted to produce simple organic molecules. 5 5 The more complex organic molecules fell back into the sea in raindrops. There, they could again be enclosed in bubbles and begin the process again. 1 1 Volcanoes erupted under the sea, releasing gases enclosed in bubbles.

How Cells Arose RNA was probably the first macromolecule to form. When combined with high energy phosphate groups, RNA nucleotides form polynucleotide chains. When folded up, these RNA macromolecules could have been capable of catalyzing the formation of the first proteins.

Origin of Life The first cells probably formed spontaneously as aggregates of macromolecules. For example, shaking together oil and water produces tiny bubbles called microspheres. Similar microspheres might have represented the first step in the evolution of cellular organization. Those microspheres better able to incorporate molecules and energy would have tended to persist longer than others.

How Cells Arose Because RNA molecules can behave as enzymes, catalyzing there own assembling, perhaps they could have served as early hereditary molecules as well.

A clock of biological time Invasion of land by animals Humans appear Invasion of land by plants Oldest multicellular organisms 4 BILLION YEARS Oldest known rocks Midnight 1 BILLION YEARS Afternoon Morning Oldest definite fossils of eukaryotes 3 BILLION YEARS Noon 2 BILLION YEARS Oldest definite fossils of prokaryotes Appearance of oxygen in atmosphere

The Simplest Organisms Prokaryotes have been plentiful on earth for over 2.5 billion years. Prokaryotes today are the simplest and most abundant form of life on earth.

The Simplest Organisms Prokaryotes occupy an important place in the web of life on earth. They play a key role in cycling minerals within the earth’s ecosystems. Photosynthetic bacteria were largely responsible for introducing oxygen into the earth’s atmosphere. Bacteria are responsible for some of the most deadly animal and plant diseases.

The Simplest Organisms Prokaryotes are small and simply organized. They are single-celled and lack a nucleus. Their single circle of DNA is not confined by a nuclear membrane. Both bacteria and archaea are prokaryotes.

Prokaryotes come in many shapes Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Rod-shaped Spherical Spirally-coiled E.coli Streptococcus Spirillum Flagellated Filamentous Stalked Helicobacter pylori Cyanobacteria Gliding bacteria (E. coli): © Getty RF; (Streptococcus): © S. Lowry/University Ulster/Getty Images; (Spirillum): © John D. Cunningham/Visuals Unlimited; (Helicobacter): © CAMR/A. Barry Dowsett/Photo Researchers, Inc.; (Cyanobacteria): © Dr. James Richardson/Visuals Unlimited, Inc.; (Gliding): © Phototake, Inc.

Prokaryote Structure A cell wall surrounds the prokaryotic cell’s plasma membrane. The plasma membrane of bacteria is encased within a cell wall of peptidoglycan. In some bacteria, the peptidoglycan layer is thin and covered over by an outer membrane of lipopolysaccharide. Bacteria who have this layer are gram-negative. Bacteria who lack this layer are gram-positive.

Prokaryote Structure Outside the cell wall and membrane, many bacteria have a gelatinous layer called a capsule. Many kinds of bacteria have long, threadlike outgrowths, called flagella, that are used in swimming. Some bacteria also possess shorter outgrowths, called pili (singular, pilus) that help the cell to attach to surfaces or other cells.

Prokaryote Structure Under harsh conditions, some bacteria form thick-walled endospores around their DNA and some cytoplasm. These endospores are highly resistant to environmental stress and can be dormant for centuries before germinating a new active bacterium. For example, Clostridium botulinum can persist in cans and bottles that have not been heated to high enough temperature to kill the spores.

The Simplest Organisms Prokaryotes reproduce by binary fission. The cell increases in size and divides in two. Some bacteria can exchange genetic information by passing plasmids from one cell to another. This process is called conjugation. A conjugation bridge forms between a donor cell and a recipient cell.

Bacterial conjugation 1 Donor cell Recipient cell Conjugation bridge 2 3 Plasmid Bacterial chromosome 4 5

The Simplest Organisms In addition to conjugation, bacteria can also transfer or obtain genetic material by: Taking up DNA from the environment (transformation), or Infection by bacterial viruses.

Comparing Prokaryotes to Eukaryotes Internal compartmentalization. Unlike eukaryotic cells, prokaryotic cells contain no internal compartments, no internal membrane system, and no cell nucleus. Cell size. Most prokaryotic cells are only about 1 micrometer in diameter, whereas most eukaryotic cells are well over 10 times that size Unicellularity. All prokaryotes are fundamentally single celled. Even though some may adhere together in a matrix or form fi laments, their cytoplasms are not directly interconnected, and their activities are not integrated and coordinated, as is the case in multicellular eukaryotes. Chromosomes. Prokaryotes do not possess chromosomes in which proteins are complexed with the DNA, as eukaryotes do. Instead, their DNA exists as a single circle in the cytoplasm. Cell division. Cell division in prokaryotes takes place by binary fission (see chapter 8). The cells simply pinch in two. In eukaryotes, microtubules pull chromosomes to opposite poles during the cell division process, called mitosis. Flagella. Prokaryotic flagella are simple, composed of a single fiber of protein that is spun like a propeller. Eukaryotic flagella are more complex structures, with a 9 + 2 arrangement of microtubules, that whip back and forth rather than rotate. Metabolic diversity. Prokaryotes possess many metabolic abilities that eukaryotes do not: Prokaryotes perform several different kinds of anaerobic and aerobic photosynthesis; prokaryotes can obtain their energy from oxidizing inorganic compounds (so-called chemoautotrophs); and prokaryotes can fix atmospheric nitrogen. Eukaryotic cell Prokaryotic cell Unicellular bacteria Eukaryotic chromosomes Prokaryotic chromosome Mitosis in eukaryotes Binary fission in prokaryotes Simple bacterial flagellum Chemoautotrophs cell Example Feature TABLE 16.1 PROKARYOTES COMPARED TO EUKARYOTES Comparing Prokaryotes to Eukaryotes Unlike eukaryotes, prokaryotes: Lack internal compartmentalization. Are very small and are unicellular only. Have a single circle of DNA. Have simple cell division and simple flagella. Have diverse metabolism.

Comparing Prokaryotes to Eukaryotes Prokaryotes are far more metabolically diverse than eukaryotes. Prokaryotes have evolved many more ways than eukaryotes to acquire the carbon atoms and energy necessary for growth and reproduction. Many are autotrophs, organisms that obtain their carbon from inorganic CO2. Others are heterotrophs, organisms that obtain at least some of their carbon from organic molecules.

Comparing Prokaryotes to Eukaryotes Photoautotrophs use the energy of sunlight to build organic molecules from CO2. Cyanobacteria use chlorophyll a as a pigment. Other bacteria use bacteriochlorophyll.

Comparing Prokaryotes to Eukaryotes Chemoautotrophs obtain their energy by oxidizing inorganic substances. Different types of these prokaryotes use substances including ammonia, sulfur, hydrogen gas, and hydrogen sulfide.

Comparing Prokaryotes to Eukaryotes Photoheterotrophs use sunlight for energy but obtain carbon from organic molecules produced by other organisms. Purple nonsulfur bacteria are an example.

Comparing Prokaryotes to Eukaryotes Chemoheterotrophs are the most common prokaryote and obtain both carbon atoms and energy from organic molecules. These types of prokaryotes include decomposers and most pathogens.

Importance of Prokaryotes Prokaryotes affect our lives today in many important ways. Responsible for creating the properties of earth’s atmosphere and soil; play key ecological roles. Can be genetically modified to help with pest-control in crops and to produce therapeutic proteins. Oil-degrading bacteria are used to clean up spills. Cause major diseases in plants and animals. May be used in bioterrorism.

Using bacteria to clean up oil spills

Prokaryotic Lifestyles Many of the archaea that survive today are methanogens. They use H2 gas to reduce CO2 to CH4 They are strict anaerobes and are found in swamps and marshes where other microbes have consumed all of the oxygen. Other archaea are extremophiles, which live in unusually harsh environments. For example, thermoacidophiles favor hot, acidic springs.

Thermoacidophiles live in hot springs

Kingdom Bacteria Most prokaryotes are members of the Kingdom Bacteria. Cyanobacteria are among the most prominent of the photosynthetic bacteria. Other forms of bacteria are nonphotosynthetic. Most bacteria are unicellular but some form layers on the surface of a substrate. This layer of cells is called a biofilm.

The Structure of Viruses Viruses do not satisfy all of the criteria for being considered “alive” because they possess only a portion of the properties of living organisms. Viruses are literally segments of DNA (or sometimes RNA) wrapped in a protein coat. They cannot reproduce on their own.

The Structure of Viruses Viruses are extremely small, with most detectable only through the use of an electron microscope. Most viruses form a protein sheath, or capsid, around a nucleic acid core. Many viruses form a membranelike envelope around the capsid. (b) Tobacco mosaic virus (TMV) Proteins RNA

The structure of bacterial, plant, and animal viruses DNA Head Capsid (proteins heath) Neck Whiskers Tail fiber Tail (a) Bacteriophage Base plate (b) Tobacco mosaic virus (TMV) Proteins RNA (c) Human immunodeficiency virus (HIV) Envelope protein Capsid Enzyme RNA

How Bacteriophages Enter Prokaryotic Cells DNA Head Capsid (proteins heath) Neck Whiskers Tail fiber Tail (a) Bacteriophage Base plate Bacteriophages are viruses that infect bacteria. They are structurally and functionally diverse. When the virus kills the infected host in which it is replicating, this is called a lytic cycle. At other times the virus integrates their nucleic acid into the host genome but does not replicate. This is called the lysogenic cycle.

A T4 bacteriophage T4 bacteriophage DNA Bacterial chromosome Bacterial cell (b) (a)

How Animal Viruses Enter Cells Animal viruses typically enter their host cells by membrane fusion. A diverse array of viruses occur among animals. One example of an animal virus is human immunodeficiency virus (HIV). (c) Human immunodeficiency virus (HIV) Envelope protein Capsid Enzyme RNA

How Animal Viruses Enter Cells HIV infects only certain cells. Macrophages are attacked by HIV. The normal role of macrophages is to pick up cellular debris.

Disease Viruses Emerging viruses are viruses that originate in one organism and pass to another organism. For example, influenza is fundamentally a bird virus and smallpox is thought to have passed from cows to humans. Air travel and world trade in animals make emerging viruses a greater threat today than in the past.

Disease Viruses The influenza virus is one of the most lethal viruses in human history. New strains of influenza usually originate in the Far East, where influenza hosts are common. The most common hosts are ducks, chickens, and pigs. These hosts live in close proximity to each other and to humans.