Chapter 14, Section 1 - Discovering Cells PDF

# Discovering Cells ## Reading Preview **Key Concepts** * What are cells? * How did the invention of the microscope contribute to knowledge about living things? * What is the cell theory? * How do microscopes produce magnified images? **Key Terms** * cell * microscope * cell theory **Target Re...

# Discovering Cells ## Reading Preview **Key Concepts** * What are cells? * How did the invention of the microscope contribute to knowledge about living things? * What is the cell theory? * How do microscopes produce magnified images? **Key Terms** * cell * microscope * cell theory **Target Reading Skill** Sequencing: A sequence is the order in which a series of events occurs. As you read, construct a flowchart showing how the work of Hooke, Leeuwenhoek, Schleiden, Schwann, and Virchow contributed to scientific understanding of cells. ## Lab Discover Activity ### Is Seeing Believing? 1. Cut a black-and-white photograph out of a page in a newspaper. With only your eyes, closely examine the photo. Record your observations. 2. Examine the same photo with a hand lens. Again, record your observations. 3. Place the photo on the stage of a microscope. Use the clips to hold the photo in place. Shine a light down on the photo. Focus the microscope on part of the photo. (See Appendix B for instructions on using the microscope). Record your observations. ### Think It Over Observing: What did you see in the photo with the hand lens that you could not see with only your eyes? What additional details could you see with the microscope? A forest is filled with an amazing variety of living things. Some are easy to see, but you have to look closely to find others. If you look carefully at the floor of a forest, you can often find spots of bright color. A beautiful pink coral fungus grows beneath tall trees. Beside the pink fungus, a tiny red newt perches on a fallen leaf. What do you think a fungus, a tree, and a red newt have in common? They are all living things, or organisms, and, like all organisms, they are made of cells. ## An Overview of Cells You are made of cells. Cells are the basic units of structure and function in living things. This means that cells form the parts of an organism and carry out all of an organism's processes, or functions. ### Cells and Structure When you describe the structure of an object, you describe what it is made of and how its parts are put together. The structures of many buildings, for example, are determined by the way in which bricks, steel beams, and other materials are arranged. The structures of living things are determined by the amazing variety of ways in which cells are put together. A tall tree, for example, consists of cells arranged to form a high trunk and leafy branches. A red newt's cells form a body with a head and four legs. ### Cells and Function An organism's functions are the processes that enable it to stay alive and reproduce. Some functions in organisms include obtaining oxygen, getting rid of wastes, obtaining food, and growing. Cells are involved in all these functions. For example, cells in your digestive system absorb food. The food provides your body with energy and materials needed for growth. ### Many and Small Figure 2 shows human skin cells. One square centimeter of your skin's surface contains more than 100,000 cells. But no matter how closely you look with your eyes alone, you won't be able to see individual skin cells. That is because, like most cells, those of your skin are very small. Until the late 1600s, no one knew cells existed because there was no way to see them. **Reading Checkpoint**: What are some functions that cells perform in living things? ## First Observations of Cells Around 1590, the invention of the microscope enabled people to look at very small objects. The invention of the microscope made it possible for people to discover and learn about cells. A microscope is an instrument that makes small objects look larger. Some microscopes do this by using lenses to focus light. The lenses used in light microscopes are similar to the clear, curved pieces of glass or plastic used in eyeglasses. A simple microscope contains only one lens. A light microscope that has more than one lens is called a compound microscope. ## Robert Hooke ## Development of the Cell Theory Leeuwenhoek's exciting discoveries caught the attention of other researchers. Like Hooke, Leeuwenhoek, and all good scientists these other researchers were curious about the world around them including things they couldn't normally see. Many other people began to use microscopes to discover what secrets they could learn about cells. ### Schleiden, Schwann, and Virchow Three German scientists made especially important contributions to people's knowledge about cells. These scientists were Matthias Schleiden (SHLY dun), Theodor Schwann, and Rudolf Virchow (FUR koh). In 1838, Schleiden concluded that all plants are made of cells. He based this conclusion on his own research and on the research of others before him. The next year, Theodor Schwann concluded that all animals are also made up of cells. Thus, stated Schwann, all living things are made up of cells. Schleiden and Schwann had made an important discovery about living things. However, they didn't explain where cells came from. Until their time, most people thought that living things could come from nonliving matter. In 1855, Virchow proposed that new cells are formed only from cells that already exit. "All cells come from cells," wrote Virchow. ### What the Cell Theory Says Schleiden, Schwann, Virchow, and others helped develop the cell theory. The cell theory is a widely accepted explanation of the relationship between cells and living things. The cell theory states the following: * All living things are composed of cells. * Cells are the basic units of structure and function in living things. * All cells are produced from other cells. The cell theory holds true for all living things, no matter how big or how small. Since cells are common to all living things, they can provide information about the functions that living things perform. Because all cells come from other cells, scientists can study cells to learn about growth and reproduction. **Reading Checkpoint**: What did Schleiden and Schwann conclude about cells? ## Light and Electron Microscopes The cell theory could not have been developed without microscopes. For a microscope to be useful, it must combine two important properties: * Magnification * Resolution Scientists today use two kinds of microscopes: * Light microscopes * Electron microscopes ### Magnification and Lenses The first property, magnification, is the ability to make things look larger than they are. The lenses in light microscopes magnify an object by bending the light that passes through them. If you examine a hand lens, such as the one in Figure 4, you will see that the lens is curved, not flat. The center of the lens is thicker than the edge. A lens with this curved shape is called a convex lens. The light passing through the sides of the lens bends inward. When this light hits the eye, the eye sees the object as larger than it really is. ### Compound Microscope Magnification Since a compound microscope uses more than one lens, it can magnify an object more than one lens by itself. Light passes through a specimen and then through two lenses, as shown in Figure 5. The first lens, near the specimen, magnifies the object. Then a second lens, near the eye, further magnifies the enlarged image. The total magnification of the microscope is equal to the magnifications of the two lenses multiplied together. For example, suppose the first lens makes an object look 10 times bigger than it actually is, and the second lens makes the object look 40 times bigger than it actually is. The total magnification of the microscope is 10 x 40, or 400. ### Resolution To create a useful image, a microscope must also help you see individual parts clearly. The ability to clearly distinguish the individual parts of an object is called resolution. Resolution is another term for the sharpness of an image. For example, a photograph in a newspaper is really made up of a collection of small dots. If you put the photo under a microscope, you can see the dots. You see the dots not only because they are magnified but also because the microscope improves resolution. Good resolution is needed when you study cells. ## Electron Microscopes The microscopes used by Hooke, Leeuwenhoek, and other early researchers were all light microscopes. Since the 1930s, scientists have developed different types of electron microscopes. Electron microscopes use a beam of electrons instead of light to produce a magnified image. Electrons are tiny particles that are smaller than atoms. Electron microscopes can obtain pictures of extremely small objects--much smaller than those that can be seen with light microscopes. The resolution of electron microscopes is much better than the resolution of light microscopes. **Reading Checkpoint**: What do electron microscopes use to produce magnified images? ## Assessment ### Target Reading Skill: Sequencing Review your flowchart and use it to answer Questions 2 and 3 below. ### Reviewing Key Concepts 1. a. Defining: Define structure and function. b. Explaining: Explain this statement: Cells are the basis units of structure and function in organisms. c. Applying Concepts: In what important function are the cells in your eyes involved? 2. a. Reviewing: What does a microscope enable people to do? b. Summarizing: Summarize Hooke's observations of cork under a microscope. c. Relating Cause and Effect: Why would Hooke's discovery have been impossible without a microscope? 3. a. Reviewing: What are the main ideas of the cell theory? b. Explaining: What did Virchow contribute to the cell theory? c. Applying Concepts: Use the ideas of Virchow to explain why plastic plants and stuffed animals are not alive. 4. a. Defining: What is magnification? b. Comparing and Contrasting: Contrast the way light microscopes and electron microscopes magnify objects. ### Writing in Science Writing an Award Speech: Suppose you are a member of a scientific society that is giving an award to one of the early cell scientists. Choose the scientist, and write a speech that you might give at the award ceremony. Your speech should describe the scientist's accomplishments.