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Lesson 1 What Are Cells? Fast Fact ---see picture Nature's Kaleidoscope The beautiful designs on this page may look like those in a kaleidoscope, but these designs occur in nature. This photograph of tiny, singlecelled organisms called diatoms was taken through a microscope. In the Investigate, you'll observe other cells through a microscope. Observing Cells Materials dropper paper towels red food coloring microscope microscope slide colored pencils slice of onion prepared slide of animal skin coverslip Procedure CAUTION: Food coloring stains. Avoid getting it on your clothing. 1. Use the dropper to place one drop of food coloring in the center of the slide. Break the onion slice, and pull off a piece of onion skin. Put the onion skin in the drop of food coloring. Gently lower the coverslip at an angle so that it spreads the food coloring. Use a paper towel to remove any excess food coloring. 2. Observe the onion skin cells under the microscope. Use the colored pencils to record your observations in a drawing. 3. Observe the prepared slide of the animal skin cells. Use colored pencils to record your observations in another drawing. Draw Conclusions 1. Compare the onion skin cells and the animal skin cells. 2. Inquiry Skill When scientists observe, they use their senses to learn about objects and events. In the center of most cells are structures that direct how the cells function. Look for these structures. Based on what you observe, how many directing structures do you think each cell has? Investigate Further In what ways do you think all plant cells are alike? Plan and conduct a simple experiment to test your ideas. 31 Reading in Science VOCABULARY cell p.32 microscopic p. 32 organism p.32 cell membrane p.34 nucleus p. 34 cytoplasm p. 34 protist p. 36 SCIENCE CONCEPTS how living things are made of cells why different cells have different jobs READING FOCUS SKILL MAIN IDEA AND DETAILS Once you understand cells, look for details about each cell type. Main Idea detail detail detail Cells You've probably seen cork stoppers in jars and cork bulletin boards. But did you know that cork comes from the bark of an oak tree? In 1665, Robert Hooke, an English scientist, observed a layer of cork through a microscope. He saw the same kind of structures you observed in the onion skin. Because the structures he saw looked like tiny rooms, Hooke named them cells. The microscope you used in the Investigate enabled you to observe and compare cells from an animal and a plant. A cell is the basic unit of structure and function in living things. Most cells are microscopic—they can be seen only with a microscope. Hooke's description of cells encouraged scientists to learn more about them. Using microscopes, they found that all cells share some characteristics and that different cells do different jobs. They also know that all organisms are made up of cells. An organism is any living thing that maintains vital life processes. ---see picture Robert Hooke's seventeenth-century microscope looks much different from a modern microscope. But both can show cells in a layer of cork. 32 You may see hundreds of different organisms each day. Each of these is made up of cells. Some simple organisms are just a single cell. But most plants and animals are made up of huge numbers of cells. Plants and animals have different types of cells, each with its own job. For example, your body has digestive cells, which help you break down food, and nerve cells, which help you sense and respond to your surroundings. Different types of cells work together to carry out functions that keep an organism alive. To carry out its own functions, each cell has structures called organelles, which help keep the cell alive. MAIN IDEA AND DETAILS How do cells keep organisms alive and healthy? Insta-Lab A Cell Model Carefully use a plastic knife to cut a peeled hard-boiled egg in half. Compare it to the animal skin cells you observed in the Investigate. Why is an egg a good model for animal cells? ---see pictures The single cell of this amoeba carries out all the functions the organism needs to stay alive. The outer cells of this plant's leaf help keep the plant from losing too much water. Inner cells of the leaf make food for the plant. Each plant cell also carries out life functions for itself. A salamander's skin cells don't look like a plant's outer cells, but—like a plant's outer cells—they help keep the organism from drying out. As in the plant, each cell carries out life functions for itself. 33 Plant and Animal Cells All cells have similar structures and organelles. Every cell is surrounded by a thin covering called the cell membrane. This structure protects the cell, holds its contents together, and controls what goes in and out of the cell. Most organelles are also surrounded by membranes. Each type of organelle has a specific function that helps the cell. Most cells have a nucleus. The nucleus directs all of a cell's activities, including reproduction. Inside the nucleus are chromosomes (KROH•m u h•so h mz), threadlike structures that contain information about the characteristics of the cell. When a cell reproduces, the nucleus divides and each new cell gets identical chromosomes. Between the cell membrane and the nucleus is a jellylike material called cytoplasm (syt•oh•plaz•uhm). Cytoplasm contains chemicals that help keep a cell healthy. Several kinds of organelles are suspended in the cytoplasm. Mitochondria are the "powerhouses" of a cell. They release energy from nutrients. Vacuoles store nutrients, water, or waste materials in plant cells. Smaller vesicles in animal cells have similar functions. Plant cells have structures not found in animal cells. A thick cell wall helps support a plant cell. The cell wall lies outside the cell membrane. In the cytoplasm of many plant cells are chloroplasts. Chloroplasts make food for plant cells. * MAIN IDEA AND DETAILS What purposes does the cell membrane serve? Science Up Close Comparing Plant and Animal Cells Plant and animal cells have certain structures in common. They have a nucleus, cell membrane, cytoplasm, and organelles. Study the differences between plant and animal cells. ---see diagram Plant cells have different sizes, shapes, and functions, but most have the same organelles. 34 ---see diagram Like plant cells, animal cells have different sizes, shapes, and functions. They also have most of the same organelles. How is an animal cell different from a plant cell? Cell Structures Organelle Nucleus Function Kind of Cell directs a cell's activities inside nucleus; contains information about cell holds a cell together and separates it from its surroundings plant and animal plant Cytoplasm supports, and protects a plantcell a jellylike substance containing chemicals that help the cell stay healthy Chioroplast makes food for the cell plant Vacuole stores food, water, or wastes plant Vesicle stores food, water, or wastes animal Mitochondria release energy from nutrients plant and animal Chromosome Cell membrane Cell wall plant and animal plant and animal plant and animal For more links and activities, go to www.hspscience.com 35 Pond water contains many single-celled organisms. Single-Celled Organisms What do you think of when you hear the word bacteria? Many people think of "germs." But not all bacteria are harmful. Many of these single-celled organisms are helpful. Some enrich the soil by breaking down dead plants and animals. Others help animals digest food. Still others help make food! Cheese and yogurt are foods that form when certain bacteria mix with milk. Bacterial cells are different from plant and animal cells. Like plant cells, bacterial cells are surrounded by cell walls. But bacteria do not have a nucleus or membranebound organelles. Instead, their chromosomes and other materials are in the cytoplasm. Another kind of single-celled organism makes up most of the group called protists. A protist (pRoHT•ist) is a simple organism, usually a single cell, with a nucleus and organelles. Some protists have cell walls and chloroplasts. These protists are plantlike. Other protists have no cell walls or chloroplasts and are animal-like. The diatoms shown at the beginning of this lesson are plantlike—they have chloroplasts and make their own food. They are part of a group of protists called algae (AL•jee). Algae live mostly in water and produce a lot of Earth's oxygen. They also produce a lot of food for ocean life. Animal-like protists are called protozoa. They get food by "eating" other small organisms, such as algae and bacteria. MAIN IDEA AND DETAILS How do bacterial cells differ from plant and animal cells? 36 Reading Review 1. MAIN IDEA AND DETAILS Draw and complete the graphic organizer. ---see chart 2. SUMMARIZE Use your graphic organizer to write a lesson summary. 3. DRAW CONCLUSIONS You're looking at a slide through a microscope, and you see cells with cell walls but without chloroplasts. What kind of cells are you probably looking at? How do you know? 4. VOCABULARY Use lesson vocabulary and other lesson terms to make a crossword puzzle. Test Prep 5. Critical Thinking Suppose you observe cells through a microscope. How might you infer whether the cells are animal cells? 6. What is the function of a cell's nucleus? A. to make food B. to direct the cell's activities C. to protect the cell D. to keep the cell healthy Writing Expository Writing Imagine a cell as being a city. Write a description of how each part of the cell helps carry out a task important for the maintenance of the city. Math Extend a Pattern Imagine that a single cell divides into two cells every 15 minutes. If each of these cells also divides in two, and so on, how long will it take for a single cell to produce 500 cells? Language Arts Word Parts The word cytoplasm contains the word parts cyto- and -plasm. Use a dictionary to find the meanings of the parts. How do those meanings relate to the meaning of the whole word cytoplasm? For more links and activities, go to www.hspscience.com 37