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Prokaryotes
- 1. Prokaryotes
- 2. Learning Objectives Explain how prokaryotes are classified. Describe how prokaryotes vary in structure and function. Explain the role of prokaryotes in the living world.
- 3. Classifying Prokaryotes • Unicellular • No nucleus • DNA in cytoplasm • Bacteria or Archaea
- 4. Bacteria Bacteria include a wide range of organisms; many phyla are needed to classify this group.
- 5. Archaea vs. Bacteria Important differences: • Cell walls • Cell membranes • DNA sequences • Harsh environments
- 6. Prokaryotes: Size, Shape, and Movement Prokaryotes vary in size, shape, and movement. Bacilli Cocci Spirilla • None • Propelled by flagella • Glide on secretions
- 7. Prokaryotes: Nutrition and Metabolism
- 8. Prokaryotes: Nutrition and Metabolism
- 9. Prokaryotes: Reproduction and Change Prokaryotes reproduce using binary fission. conjugation endospore
- 10. Decomposers By assisting in breaking down dead organisms, prokaryotes supply raw materials in the environment. Bacteria of the genus Rhizobium Bacteria of the genus Rhizobium Cyanobacteria of the genus Anabaena Cyanobacteria of the genus Anabaena Bacteria called actinomycetes Bacteria called actinomycetes
- 11. Producers Food chains everywhere are dependent upon prokaryotes as producers of food and biomass.
- 12. Nitrogen Fixers Nitrogen-fixing bacteria and archaea provide 90% of atmospheric nitrogen.
- 13. Prokaryotes Overview Classification: •Bacteria •Archaea Classification: •Bacteria •Archaea • Diverse • Size • Shape • Movement • Metabolism • Diverse • Size • Shape • Movement • Metabolism Ecologically importantEcologically important
Notes de l'éditeur
- Read the lesson title aloud to students.
- Click to show each of the learning objectives. Ask: What are the three domains into which all living things are classified? Answer: Eukaryo, Bacteria, and Archaea Explain that prokaryotes make up the domains Bacteria and Archaea. All other living things are classified into the third domain, Eukarya. Tell students that at the end of the presentation, they should be able to explain how prokaryotes are classified and their role in the living world. They should also be able to describe how prokaryotes vary in structure and function.
- Explain to students that microscopic life covers nearly every square centimeter of Earth. The smallest and most abundant of these microorganisms are prokaryotes—unicellular organisms that lack a nucleus. Click to reveal this information. Tell students: Prokaryotes have DNA like all other cells, but their DNA is not found in a membrane-bound nuclear envelope as it is in eukaryotes. Prokaryote DNA is located in the cytoplasm. Click to reveal this information. Explain to students that, for many years, most prokaryotes were simply called “bacteria.” Tell students: We now know that the classification of prokaryotes is more complex. Ask: How are prokaryotes classified? Answer: Prokaryotes are classified as Bacteria or Archaea—two of the three domains of life. Click to reveal the answer. Explain that these groups are as different from each other as both are from eukaryotes. The domain Bacteria corresponds to the kingdom Eubacteria. The domain Archaea corresponds to the kingdom Archaebacteria.
- Tell students: Bacteria is the larger of the two domains of prokaryotes. Bacteria include a wide range of organisms with lifestyles so different that biologists do not agree about exactly how many phyla are needed to classify this group. They live almost anywhere: in fresh water, in salt water, on land, and on and within the bodies of humans and other eukaryotes. Explain to students that the illustration is of Escherichia coli, a typical bacterium that lives in human intestines. E. coli has the basic structure typical of most prokaryotes. Tell students: Bacteria are usually surrounded by a cell wall that protects the cell from injury and determines its shape. The cell walls of bacteria contain peptidoglycan—a polymer of sugars and amino acids that surrounds the cell membrane. Click to highlight the cell wall label. Explain that some bacteria, such as E. coli, have a second membrane outside the peptidoglycan wall that makes the cell especially resistant to damage. Click to highlight the outer membrane label. Tell students: Some prokaryotes have flagella that they use for movement, or pili, which in E. coli serve mainly to anchor the bacterium to a surface or to other bacteria. Click to highlight the flagellum and pili labels.
- Tell students: Under a microscope, archaea look very similar to bacteria. Both are equally small, lack nuclei, and have cell walls. Explain to students that there are important differences between bacteria and archaea. Click to reveal the cell walls bullet point. Tell students: The walls of archaea lack peptidoglycan. Click to reveal the cell membranes bullet point. Tell students: Their membranes contain different lipids. Click to reveal the DNA sequences bullet point. Explain that the DNA sequences of key archaea genes are more like those of eukaryotes than those of bacteria. Based on these and other observations, scientists have concluded that archaea and eukaryotes are related more closely to each other than to bacteria. Ask: How would a diagram of an archaean be similar to the diagram of the bacteria shown on the previous slide? Answer: Like the bacterium, the archaean would lack a nucleus. It would also have a cell wall. Ask: How would a diagram of an archaean be different from the diagram of the bacteria shown on the previous slide? Answer: The cell wall would be labeled differently, because archaea do not have peptidoglycan in their cell walls. Tell students: Archaea live in extremely harsh environments. Click to reveal this point. One group of archaea produce methane gas and live in environments with little or no oxygen, such as thick mud and the digestive tracts of animals. Other archaea live in extremely salty environments, such as Utah’s Great Salt Lake, or in hot springs where temperatures approach the boiling point of water.
- Distribute worksheets to students. Encourage them to create a cluster diagram to record their notes about the ways in which prokaryotes vary. Tell students: Prokaryotes range in size from 1 to 5 micrometers, making them much smaller than most eukaryotic cells. Explain that prokaryotes come in a variety of shapes, though there are three basic shapes. Rod-shaped prokaryotes are called bacilli. Click to reveal the photo of bacilli. Spherical prokaryotes are called cocci. Click to reveal the photo of cocci. Spiral and corkscrew-shaped prokaryotes are called spirilla. Click to reveal the photo of spirilla. Explain to students that you can also distinguish prokaryotes by whether they move and how they move. Click to show different ways prokaryotes move. Tell students: Some prokaryotes do not move at all. Others are propelled by flagella. Some glide slowly along a layer of slime-like material they secrete.
- Tell students: Prokaryotes obtain energy in diverse ways. Explain to students that prokaryotes need a supply of chemical energy, which they store in the form of fuel molecules such as sugars. Direct students to consider the table, which compares the ways prokaryotes obtain energy. Point out that some species are able to change their method of energy capture or release depending on the conditions of their environment. Ask: What is the term for a prokaryote that uses only light as its energy source? Answer: A photoautotroph uses only light as its energy source. Click to highlight the location of this information in the table.
- Tell students: Prokaryotes release energy in diverse ways. Explain to students that energy is released from prokaryote’s fuel molecules during cellular respiration, fermentation, or both. Direct students to consider the table, which compares the ways prokaryotes release energy. Ask: Which modes of metabolism release energy through fermentation? Answer: obligate anaerobe and facultative anaerobe Click to highlight the location of this answer in the table.
- Tell students: When a prokaryote has grown so that it has nearly doubled in size, it replicates its DNA and divides in half, producing two identical cells. Explain to students that because binary fission does not involve the exchange or recombination of genetic information, it is a form of asexual reproduction. Tell students: When conditions are favorable, prokaryotes can grow and divide at astonishing rates. Some divide as often as once every 20 minutes. Ask: Prokaryotes reproduce asexually, so how do their populations evolve? Answer: mutation and conjugation Tell students: Mutations are one of the main ways prokaryotes evolve. Mutations are inherited by daughter cells produced by binary fission. Explain that many prokaryotes exchange genetic information by conjugation. Click to reveal the image of conjugation. Ask a volunteer to label the image as mutation or conjugation. Click to reveal the correct answer. Point out that, during conjugation, a hollow bridge forms between two bacterial cells, and genetic material, usually in the form of a plasmid, moves from one cell to the other. Many plasmids carry genes that enable bacteria to survive in new environments or to resist antibiotics that might otherwise prove fatal. This transfer of genetic information increases genetic diversity in populations of prokaryotes. Ask: What is the result of the processes of mutation and conjugation? Answer: genetic changes in prokaryotes Ask: Are mutation and conjugation forms of reproduction? Why or why not? Answer: No, these processes are not forms of reproduction; they do not result in the formation of daughter cells. Click to reveal the image of cells with endospores. Explain to students that when growth conditions become unfavorable, many prokaryotic cells form an endospore—a thick internal wall that encloses the DNA and a portion of the cytoplasm. Ask a volunteer to point to an endospore in the image. Click to reveal an endospore and its label. Tell students: Endospores can remain dormant for months or even years. The ability to form endospores makes it possible for some prokaryotes to survive in very harsh conditions.
- Ask: What type of organism is vital to recycling matter in an ecosystem? Answer: decomposer Tell students: Bacterial decomposers are also essential to industrial sewage treatment, helping to produce purified water and chemicals that can be used as fertilizers. Click to reveal the image and text for Rhizobium. Tell students: Bacteria of the genus Rhizobium often live symbiotically within nodules attached to roots of legumes such as clover, where they convert atmospheric nitrogen into a form that is useable by plants. Click to reveal the image and text for Anabaena. Tell students: Cyanobacteria of the genus Anabaena form filamentous chains in ponds and other aquatic environments, where they perform photosynthesis. Click to reveal image and text for actinomycetes. Tell students: Bacteria called actinomycetes are present in soil and in rotting plant material such as fallen logs, where they decompose complex organic molecules into simpler molecules. Point out that the growth of organisms such as bacteria can also disrupt the health of ecosystems. Share the example of when an oversupply of nutrients causes the explosive growth of algae and other producers in freshwater lakes. As these organisms die off, bacterial decomposers flourish. When this happens to excess, the bacteria can use up virtually all of the dissolved oxygen in the water, causing massive fish kills that further disrupt ecosystems.
- Ask: What type of organism is found at the base of every food web? Answer: producer Tell students: Photosynthetic prokaryotes are among the most important producers on the planet. Share with students the example of the tiny cyanobacterium Prochlorococcus, which alone may account for more than half of the primary production in the open ocean. Point out that food chains everywhere are dependent upon prokaryotes as producers of food and biomass.
- Tell students: While nitrogen gas makes up 80 percent of Earth’s atmosphere, only a few organisms—all of them prokaryotes—can convert nitrogen into useful forms. Explain to students that the process of nitrogen fixation converts nitrogen gas into ammonia (NH3). Ammonia can then be converted to nitrates that plants need, helping to maintain healthy ecosystems. Tell students: Nitrogen-fixing bacteria and archaea provide 90 percent of the nitrogen used by other organisms. Click to reveal this information. Tell students: The rest is provided by nitrogen-containing compounds from weathering rocks. Some even comes when lightning combines oxygen and nitrogen in the atmosphere. Help students understand the importance of the process of nitrogen fixation. Ask: How do humans and other animals get the nitrogen they need to make proteins? Answer: They eat plants or animals that have eaten plants. Plants get nitrogen from nitrogen-fixing bacteria. Tell students: Some plants have symbiotic relationships with nitrogen-fixing prokaryotes. Point out that the bacterium Rhizobium grows in nodules, or knobs, on the roots of legume plants such as clover and soybean. Ask a volunteer to point out the nodules in the image. Click to highlight some of the nodules. Tell students: The Rhizobium bacteria within these nodules convert nitrogen in the air into the nitrogen compounds essential for plant growth. In effect, these plants have fertilizer factories in their roots.
- Review with students the key points of this lesson. Tell students: Prokaryotes are classified as Bacteria or Archaea—two of the three domains of life. Click to reveal this information. Explain that prokaryotes vary in their size and shape, in the way they move, and in the way they obtain and release energy. Click to reveal this information and photo. Tell students: Prokaryotes are essential in maintaining every aspect of the ecological balance of the living world. In addition, some species have specific uses in human industry. Click to reveal this point and photo.
- Remind students that they should have used this worksheet to take notes on the structure and function of prokaryotes. Give students time to work individually or in small groups to review their notes and add to them as necessary. Point out to students that they can add circles to their cluster as needed. Worksheet Answers: Ways in which prokaryotes vary include size and shape, movement, method of obtaining energy, and method of releasing energy.