Presentation on theme: "Prokaryotes: The First Single-Celled Creatures"— Presentation transcript:
1Prokaryotes: The First Single-Celled Creatures Chapter 16
2Origin of LifeThe 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.
3Origin of LifeStanley 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.
4Origin of LifeThe 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.
6How 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.
7Origin of LifeThe 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.
8How Cells AroseBecause RNA molecules can behave as enzymes, catalyzing there own assembling, perhaps they could have served as early hereditary molecules as well.
9A clock of biological time Invasion of landby animalsHumans appearInvasion ofland by plantsOldestmulticellularorganisms4 BILLION YEARSOldest known rocksMidnight1 BILLIONYEARSAfternoonMorningOldestdefinitefossils ofeukaryotes3 BILLIONYEARSNoon2 BILLION YEARSOldest definite fossilsof prokaryotesAppearance ofoxygen in atmosphere
10The 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.
11The 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.
12The 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.
14Prokaryote StructureA 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.
15Prokaryote StructureOutside 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.
16Prokaryote StructureUnder 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.
17The 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.
19The Simplest Organisms In addition to conjugation, bacteria can also transfer or obtain genetic material by:Taking up DNA from the environment (transformation),orInfection by bacterial viruses.
20Comparing Prokaryotes to Eukaryotes Internal compartmentalization.Unlike eukaryotic cells, prokaryoticcells contain no internalcompartments, no internalmembrane system, and no cellnucleus.Cell size. Most prokaryotic cellsare only about 1 micrometer indiameter, whereas most eukaryoticcells are well over 10 times thatsizeUnicellularity. All prokaryotes arefundamentally single celled. Eventhough some may adhere togetherin a matrix or form fi laments,their cytoplasms are not directlyinterconnected, and their activitiesare not integrated and coordinated,as is the case in multicellulareukaryotes.Chromosomes. Prokaryotes donot possess chromosomes in whichproteins are complexed with theDNA, as eukaryotes do. Instead,their DNA exists as a single circle inthe cytoplasm.Cell division. Cell division inprokaryotes takes place by binaryfission (see chapter 8). The cellssimply pinch in two. In eukaryotes,microtubules pull chromosomesto opposite poles during the celldivision process, called mitosis.Flagella. Prokaryotic flagellaare simple, composed of a singlefiber of protein that is spun like apropeller. Eukaryotic flagella aremore complex structures, with a9 + 2 arrangement of microtubules,that whip back and forth ratherthan rotate.Metabolic diversity. Prokaryotespossess many metabolic abilitiesthat eukaryotes do not: Prokaryotesperform several different kindsof anaerobic and aerobicphotosynthesis; prokaryotes canobtain their energy from oxidizinginorganic compounds (so-calledchemoautotrophs); and prokaryotescan fix atmospheric nitrogen.Eukaryotic cellProkaryotic cellUnicellular bacteriaEukaryoticchromosomesProkaryoticchromosomeMitosis ineukaryotesBinary fissionin prokaryotesSimple bacterialflagellumChemoautotrophscellExampleFeatureTABLE 16.1PROKARYOTES COMPAREDTO EUKARYOTESComparing Prokaryotes to EukaryotesUnlike 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.
21Comparing 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.
22Comparing 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.
23Comparing 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.
24Comparing Prokaryotes to Eukaryotes Photoheterotrophs use sunlight for energy but obtain carbon from organic molecules produced by other organisms.Purple nonsulfur bacteria are an example.
25Comparing 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.
26Importance 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.
28Prokaryotic Lifestyles Many of the archaea that survive today are methanogens.They use H2 gas to reduce CO2 to CH4They 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.
30Kingdom 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.
31The 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.
32The 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)ProteinsRNA
33The structure of bacterial, plant, and animal viruses DNAHeadCapsid(proteins heath)NeckWhiskersTail fiberTail(a) BacteriophageBaseplate(b) Tobacco mosaic virus (TMV)ProteinsRNA(c) Human immunodeficiency virus (HIV)EnvelopeproteinCapsidEnzymeRNA
34How Bacteriophages Enter Prokaryotic Cells DNAHeadCapsid(proteins heath)NeckWhiskersTail fiberTail(a) BacteriophageBaseplateBacteriophages 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.
35A T4 bacteriophage T4 bacteriophage DNA Bacterial chromosome Bacterial cell(b)(a)
36How 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)EnvelopeproteinCapsidEnzymeRNA
37How 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.
38Disease VirusesEmerging 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.
39Disease VirusesThe 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.