Page 1 of 8 KEY CONCEPT Cells capture and release energy. BEFORE, you learned NOW, you will learn • The cell is the basic unit • Why cells need energy of all living things • How energy is captured • Plant cells and animal and stored cells have similarities • How plants and animals and differences get energy • Plants and animals need energy and materials leaf cells VOCABULARY TH INK ABOUT chemical energy p. 47 What do these cells glucose p. 47 photosynthesis p. 48 have in common? chlorophyll p. 48 Both muscle cells and plant cells need cellular respiration p. 50 energy to live. Your muscle cells need fermentation p. 52 energy to help you move and perform other functions. Even though plant cells don’t move in the same way that muscles move, they still need energy. How do human muscle cells and plant cells get energy? muscle cells All cells need energy. OUTLINE To stay alive, cells need a constant supply of energy. Animal cells get Remember to include this energy from food, while plant cells get energy from sunlight. All heading in your outline of this section. cells use chemical energy.Chemical energy is the energy stored in the bonds between atoms of every molecule. To stay alive, cells must I. Main idea A. Supporting idea be able to release the chemical energy in the bonds. 1. Detail 2. Detail A major energy source for most cells is stored in a sugar molecule B. Su pp ortin g idea called glucose. When you need energy, cells release chemical energy from glucose. You need food energy to run, walk, and even during sleep. Your cells use energy from food to carry out all of their activities. Think about muscle cells. When you run, muscle cells release chemical energy from glucose to move your legs. The more you run, the more glucose your muscle cells need. You eat food to restore the glucose supply in muscles. But how do plant cells get more glucose? Plants transform the energy in sunlight into the chemical energy in glucose. Chapter 2: How Cells Function 47 AE Page 2 of 8 Some cells capture light energy. The source of energy for all organisms ultimately comes from the Sun. Plants change the energy in sunlight into a form of energy their cells can use—the chemical energy in glucose. All animals benefit from the ability of plants to convert sunlight to food energy. Animals either eat plants, or they eat other animals that have eaten plants. Photosynthesis (FOH -toh-SIHN-thih-sihs) is the process that plant cells use to change the energy from sunlight into chemical energy. Photosynthesis takes place in plant cells that have chloroplasts. Chloroplasts contain chlorophyll (KLAWR-uh-fhil), a light-absorbing pigment, or colored substance, that traps the energy in sunlight. The process of photosynthesis involves a series of chemical steps, or reactions. The illustration on the next page shows an overview of how photosynthesis changes starting materials into new products. reading tip 1 The starting materials of photosynthesis are carbon dioxide and As you read each numbered water. The plant takes in carbon dioxide from the air and water item here, find the number from the soil. on the diagram on page 49. 2 The process takes place when carbon dioxide and water enter the plant’s chloroplasts. Chlorophyll captures energy from sunlight, which is used to change carbon dioxide and water into new prod- ucts. 3 The products of photosynthesis are glucose and oxygen. The plant releases most of the oxygen to the air as a waste product and keeps the glucose for its energy needs. Check Your Reading Summarize photosynthesis. Remember that a summary includes only the most important information. Plants do not immediately use all of the glucose they make. Starch Some of the glucose molecules are linked together to build large carbohydrates called starch. Plants can store starch and later break it back down into glucose or other sugars Chloroplast when they need energy. Sugars and starches supply food for The starch in this plant cell stores energy. animals that eat plants. AE 48 Unit: Cells and Heredity Page 3 of 8 Photosynthesis Chloroplast Leaf cell (magnified 2200 ϫ) 1 The starting materials 2 The process Inside the 3 The products Glucose Carbon dioxide from the air chloroplasts, chlorophyll supplies energy and is a and water from the soil enter captures energy from source of materials for the the chloroplasts. sunlight. This energy is used plant; most oxygen is to change starting materials released into the air. into new products. Ca rbon d io xid O ) e (6 (C 6H12 6 CO Glucose 2) Light Energy (6H 2O) Water Oxyg en (6O 2 ) Chloroplast Carbon Water Light Glucose Oxygen ؉ dioxide ؉ ؉ 6H 2O Energy C H O 6O 6CO 2 6 12 6 2 What part of the diagram shows starting materials being changed? Chapter 2: How Cells Function 49 AE Page 4 of 8 All cells release energy. All cells must have energy to function. Glucose and other sugars are cell food—they are the power source for cell activities in almost all living things. When glucose is stored as glycogen or taken in as starch, it must be broken down into individual molecules before cells are able to use it. Chemical energy is stored in the bonds of sugars. When the bonds of a sugar molecule are broken, a burst of energy is released that the cell can use. Cells can release energy in two basic processes: cellular respiration and fermentation. Cellular respiration requires oxygen, but fermenta- tion does not. In addition, cellular respiration releases much more usable energy than does fermentation. Check Your Reading What is released when a sugar molecule is broken down? Cellular Respiration In cellular respiration, cells use oxygen to release energy stored in sugars such as glucose. In fact, most of the energy used by the cells in your body is provided by cellular respiration. Just as photosynthesis occurs in organelles called chloroplasts, cellular respiration takes place in organelles called mitochondria. Remember that mitochondria are in both plant cells and animal cells, so both kinds of cells release energy through cellular respiration. Like photosynthesis, cellular respiration is a process that changes starting materials into new products. 1 The starting materials of cellular respiration are sugars—such as glucose—and oxygen. reading tip 2 The process begins when glucose in the cytoplasm is broken down Reread step 2 to make into smaller molecules. This releases a small amount of energy. sure you understand what These molecules then move into the mitochondria. At the same happens to oxygen and glucose. time, oxygen enters the cell and travels into the mitochondria. As the smaller molecules are broken down even further, hydrogen is released along with a large amount of energy. The hydrogen combines with oxygen to make water. 3 The products are energy, carbon dioxide, and water. Some of the energy produced during cellular respiration is trans- ferred to other molecules, which then carry the energy where it is needed for the activities of the cell. The rest of the energy is released as heat. Carbon dioxide formed during cellular respiration is released by the cell. Check Your Reading What are the three products of cellular respiration? AE 50 Unit: Cells and Heredity Page 5 of 8 Cellular Respiration Mitochondrion Leaf cell (magnified 2200 ϫ) Animal cell (magnified 2400 ϫ) 1 The starting materials 2 The process Inside the 3 The products Energy Glucose and oxygen enter mitochondria more chemi- is released, and water the cell. Glucose is split into cal bonds are broken in the and carbon dioxide are smaller molecules. smaller molecules. Oxygen produced. is needed for this process. Oxygen (6O Energy 2 ) ) Glucose (6CO 2 (C H S 6 1206) MALLER Carbon dioxide MOLECULES W ater (6H 2O) Energy Mitochondrion Glucose Oxygen Water Carbon ؉ Chemical ؉ ؉ dioxide C6H12 O6 6O 2 6H 2O Energy 6CO 2 Where in the process is energy released? Chapter 2: How Cells Function 51 AE Page 6 of 8 Photosynthesis and Respiration Cycle Light Photosynthesis Energy chloroplast glucose water oxygen carbon dioxide Cellular Chemical Respiration mitochondrion Energy You may find it interesting to compare cellular respiration with photosynthesis. The diagram above highlights the cycle that occurs between photosynthesis and cellular respiration. Notice that the start- ing materials of one process are also the products of the other process. This cycle does not necessarily occur in the same cell, or even in the same organism. Fermentation VOCABULARY Fermentation is the process by which cells release energy without Add a word triangle for oxygen. Recall that in cellular respiration the cell first breaks glucose fermentation to your notebook. Your triangle into smaller molecules. This releases a small amount of energy. could include a sketch of Without oxygen, cellular respiration cannot continue. Instead of a loaf of bread. entering the mitochondria, these smaller molecules stay in the cyto- plasm, where fermentation occurs. There are two main types of fermentation: alcoholic fermentation and lactic acid fermentation. Both types of fermentation start with the small molecules made from sugars at the beginning of cellular respira- tion. In the absence of oxygen, different reactions occur that produce either alcohol and carbon dioxide or lactic acid. In both cases, a small amount of energy is released. Check Your Reading Use a Venn diagram to compare and contrast fermentation and cellular respiration. AE 52 Unit: Cells and Heredity Page 7 of 8 The production of many foods that people eat every day involve either alcoholic fermentation or lactic acid fermentation. Three important foods are bread, yogurt, and cheese.