Bacteria and Viruses. What are bacteria?

Transcription

1 Bacteria and Viruses CHAPTER 13 LESSON 1 What are bacteria? What do you think? Read the two statements below and decide whether you agree or disagree with them. Place an A in the Before column if you agree with the statement or a D if you disagree. After you ve read this lesson, reread the statements to see if you have changed your mind. Before Statement After 1. A bacterium does not have a nucleus. Key Concept What are bacteria? 2. Bacteria cannot move. Characteristics of Bacteria You are surrounded by billions of tiny organisms too small to be seen. They even live inside your body. These organisms are called bacteria. Bacteria (singular, bacterium) are microscopic prokaryotes. As you learned, a prokaryote is a unicellular organism that has no nucleus or other membrane-bound organelles. Bacteria live almost everywhere. Their habitats include the air, glaciers, the ocean floor, and soil. Bacteria also live in or on almost every organism, both living and dead. Bacteria live both inside you and on your skin. In fact, your body has more bacterial cells than human cells! The bacteria in your body outnumber human cells by 10 to 1. Bacteria have many of the same traits as archaea (ar KEE uh; singular, archaean). Bacteria and archaea are prokaryotes. Both lack membrane-bound organelles. Archaea can live where most living things cannot survive, such as in very warm areas or areas with little oxygen. Both bacteria and archaea are important to life on Earth. Identify the Main Idea As you read, highlight the main idea of each paragraph. Use a different color to highlight a detail or example that might help you understand that main idea. Key Concept Check 1. Define What are bacteria? Reading Essentials Bacteria and Viruses 219

2 Make a folded book. Use it to organize your notes on the characteristics of bacteria. Characteristics of Bacteria Structure of Bacteria Look at the typical bacterium shown below. The bacterium is made up of cytoplasm and. These parts are surrounded by a cell membrane and a cell wall. The cytoplasm also contains ribosomes. Most bacteria have that is one coiled, circular chromosome. Many bacteria also have one or more plasmids. Plasmids are small circular pieces of that are separate from other. Visual Check 2. Identify Circle the names of the two structures that surround the bacterium s and cytoplasm. Ribosome Cytoplasm Cilia Cell wall Capsule Cell membrane Reading Check 3. State the three basic shapes of bacteria. Flagellum Some bacteria have specialized structures. The bacterium that causes pneumonia (noo MOH nyuh) has a thick covering around its cell wall. This structure, called a capsule, helps the bacterium survive. The capsule keeps the bacterium from drying out. It also keeps things from getting into the bacterium. Many bacteria have capsules with pili (PI li). These hairlike structures help bacteria stick to surfaces. Size and Shapes of Bacteria Bacteria are much smaller than plant or animal cells. Bacteria are generally only 1 5 micrometers (μm) (1 m = 1 million μm) wide. An average eukaryotic cell is μm wide. Scientists estimate that as many as 100 bacteria could be lined up across the head of a pin. Bacteria have three basic shapes. Bacteria can be shaped like spheres, rods, or spirals. 220 Bacteria and Viruses Reading Essentials

3 Obtaining Food and Energy Bacteria live in many different places. Because their environments are different, bacteria get food in different ways. Some bacteria take in food and break it down to get energy. Many of these bacteria feed on dead organisms or organic waste. Other bacteria take in nutrients from living hosts. Bacteria that cause tooth decay, for example, live in the plaque on your teeth. These bacteria feed on sugars in the things you eat and drink. Some bacteria make their own food. They use light energy and make food, like most green plants do. These bacteria live where there is much light, such as on the surface of a lake. Other bacteria use energy from chemical reactions and make their food. These bacteria live where there is no sunlight, such as on the dark ocean floor. Most organisms, including humans, cannot survive without oxygen. However, not all bacteria need oxygen to survive. Bacteria that can live without oxygen are called anaerobic (a nuh ROH bihk) bacteria. Bacteria that need oxygen are called aerobic (er OH bihk) bacteria. Most bacteria in the environment are aerobic. 4. Apply Why does going to the dentist to have plaque removed from your teeth help prevent cavities? Key Concept Check 5. Describe How do bacteria obtain food? Movement Some bacteria can move around to find the resources they need to survive. These bacteria move using special structures. Many bacteria have long whiplike structures called flagella (fluh JEH luh; singular, flagellum). If you look again at the diagram of the bacterium, you can see a flagellum. Some bacteria twist or spiral as they move. Still others use their pili as hooks as they move. Some bacteria make threadlike structures that they use to push away from a surface. Reproduction You learned that organisms reproduce asexually or sexually. Bacteria reproduce asexually by a process called fission. Fission is cell division that forms two genetically identical cells. Fission can occur quickly. A cell might divide as often as every 20 minutes if conditions are right. Genetic Diversity Bacteria produced by fission are identical to the parent cell. However, genetic variation can be increased by a process called conjugation. Reading Check 6. Identify Name a structure that helps bacteria move. 7. Calculate How many bacteria would there be if 500 bacteria underwent fission every 20 minutes for one hour? Reading Essentials Bacteria and Viruses 221

4 Reading Check 8. Discuss How does conjugation give bacteria more genetic diversity? Conjugation During conjugation (kahn juh GAY shun), two bacteria of the same species attach to each other and combine their genetic material. As shown below, is transferred between the bacteria. This results in new combinations of genes and an increase in genetic diversity. Conjugation does not produce new organisms, so the process is not considered reproduction. Plasmid Conjugation tube Donor cell Recipient cell 1 The donor cell and recipient cell both have circular chromosomal. The donor cell also has as a plasmid. The donor cell forms a conjugation tube and connects to the recipient cell. Visual Check 9. Identify Circle the area in each of the first three steps where transfers between bacterial cells. 2 The conjugation tube connects both cells. The plasmid splits in two and one plasmid strand moves through the conjugation tube into the recipient cell. 3 4 The complimentary strands of the plasmids are completed in both bacteria. With the new plasmids complete, the bacteria separate from each other. The recipient cell now contains plasmid from the donor cell as well as its own chromosomal. 222 Bacteria and Viruses Reading Essentials

5 Endospore Formation Endospore Thick wall 1 Bacterial cells in favorable 2 As conditions become unfavorable, 3 The cell dissolves, leaving the conditions form without the cell forms an endospore endospore-protected to endospores. around some of its. survive in the harsh conditions. Endospores Sometimes environmental conditions are unfavorable for the survival of bacteria. Some of these conditions are extreme heat, cold, and drought. In these cases, some bacteria can form endospores. An endospore (EN duh spor) forms when a bacterium builds a thick internal wall around its chromosome and part of the cytoplasm. Endospores protect bacteria. A bacterium can stay dormant for months or even hundreds of years until conditions are better. Endospores enable bacteria to survive extreme conditions. The diagram at the top of the page shows how an endospore forms. Archaea Prokaryotes called archaea were once considered bacteria. Like a bacterium, an archaean has a cell wall and no nucleus or membrane-bound organelles. Its chromosome is also circular, like those in bacteria. However, archaea and bacteria differ in important ways. The ribosomes of archaea resemble the ribosomes of eukaryotes, rather than those of bacteria. Archaea also contain molecules in their plasma membranes that are not in any other organisms. Archaea often live in environments where other organisms cannot. Some archea live in hot springs, while others live in salty lakes. Some scientists refer to archaea as extremophiles (ik STREE muh filez), which is a word that means those that love extremes. Visual Check 10. Locate Color the in each of the three stages of endospore formation. Math Skills Each time bacteria undergo fission, the population doubles in size. To calculate how many bacteria there will be, use this formula: n = x 2 f Example: 100 bacteria undergo fission 3 times. f = 3, so 2 f is 2 multiplied by itself 3 times. (2 2 2 = 8) n = = 800 bacteria 11. Use a Formula How many bacteria would there be if a population of 10 bacteria u nderwent fission 5 times? Reading Essentials Bacteria and Viruses 223

6 Mini Glossary bacterium: a microscopic prokaryote (plural, bacteria) conjugation (kahn juh GAY shun): a process in which two bacteria of the same species attach to each other and combine their genetic material fission: cell division that forms two genetically identical cells flagellum (fluh JEH lum): a long whiplike structure that helps a bacterium move (plural, flagella) endospore (EN duh spor): what forms when a bacterium builds a thick internal wall around its chromosome and part of the cytoplasm 1. Review the terms and their definitions in the Mini Glossary. Write a sentence that explains what happens to a bacterium s cell during fission. 2. Fill in the diagram below to explain how bacteria can move. How Bacteria Move whipping a flagellum What do you think Reread the statements at the beginning of the lesson. Fill in the After column with an A if you agree with the statement or a D if you disagree. Did you change your mind? twisting or spiraling 3. Write a paragraph that explains what might happen to a bacterium when environmental conditions become too dry for survival. ConnectED Log on to ConnectED.mcgraw-hill.com and access your textbook to find this lesson s resources. END OF LESSON 224 Bacteria and Viruses Reading Essentials