Objectives. Key Terms

Transcription

1 Objectives Distinguish between kinetic and potential energy. Explain what chemical energy is and how cells release it from food. Define calories and kilocalories as units of energy. Key Terms kinetic energy potential energy thermal energy chemical energy calorie If you've ever felt tired or groggy from being hungry, you probably know that you need food for energy. But how does your body use the energy stored in food? To understand this, you first need to know some basic facts about energy. Introduction to Energy Energy is the ability to perform work. In the physical science sense of the word, work is performed whenever an object is moved against an opposing force. For example, your leg muscles do work when you climb the steps to the top of a water slide your legs move your body against the opposing force of gravity. The two basic forms of energy are kinetic energy and potential energy. While you climb the stairs, you have kinetic energy, the energy of motion. Anything that is moving has kinetic energy. In fact, the word kinetic comes from a Greek word meaning "motion." Once you reach the top of the stairs and are standing still, you have low kinetic energy. Where has the energy gone? Although it is not possible to destroy or create energy, energy can be converted from one form to another. By climbing the stairs, your body converted kinetic energy to potential energy. Potential energy is energy that is stored due to an object's position or arrangement. As you climb higher against the force of gravity, your body gains potential energy due to its position its higher location. The potential energy is

2 converted back to kinetic energy as you move down the slide. What becomes of the energy once you have splashed into the pool and come to a stop? As you go down the slide, your body collides with air and water molecules, increasing their motion. And when you splash into the pool, the rest of your motion is transferred to the water. The air and water molecules transfer their motion in random directions as they collide again and again. This type of kinetic energy random molecular motion is called thermal energy. (Thermal energy that is transferred from a warmer object to a cooler one is referred to as heat.) Overall, your trip down the slide converted potential energy into the directed kinetic energy of your sliding body, and then into the random kinetic energy of molecular motion (thermal energy). You cannot retrieve this thermal energy and put it to work again. So, to climb the stairs again, you need a fresh supply of energy. That energy is provided by food. Chemical Energy How do the organic compounds in food provide energy for a climb up a water slide? Just like the molecules in gasoline and other fuels, these organic compounds have a form of potential energy called chemical energy. In the case of chemical energy, the potential to perform work is due to the arrangement of the atoms within the molecules. Put another way, chemical energy depends on the structure of molecules. Organic molecules such as the carbohydrates, fats, and proteins you learned about in Chapter 5 have structures that make them especially rich in chemical energy (Figure 7-5).

3 Figure 7-5 The stored chemical energy of foods such as peanuts can be released through cellular respiration. Figure 7-6 compares potential energy due to position and chemical structure (chemical energy). In the case of potential energy due to position, the potential energy is converted to kinetic energy in the forms of motion and thermal energy (which is released to the surroundings as heat). In the case of chemical energy, the rearrangement of atoms during chemical reactions releases the potential energy. This energy is then available for work such as contracting a muscle.

4 Figure 7-6 Potential energy can be converted to other types of energy. Putting Chemical Energy to Work The organic molecules in food are high in chemical energy, just as the organic molecules in gasoline are. Cells and automobile engines make chemical energy available for work through similar processes. In both cases, a complex molecule is broken into smaller molecules that have less chemical energy than the original substance (Figure 7-7).

5 Figure 7-7 Both engines and cells use oxygen to convert the potential energy in complex molecules to energy that can be used for work. An automobile engine is called an internal combustion (burning) engine. This type of engine mixes oxygen with gasoline in a very fast chemical reaction that results in the molecules of gasoline breaking down. The reaction releases thermal energy as heat, which is then used to power the car. The main waste products emitted from the automobile's exhaust pipe are carbon dioxide and water. Only about 25 percent of the energy from the gasoline is converted into the car's kinetic energy (motion). The rest is lost to the surroundings as heat, which explains why it gets very hot under the hood of a running car. Within your cells, organic molecules such as glucose also react with oxygen in the process of cellular respiration. And similar to an automobile engine, working cells produce carbon dioxide and water as their "exhaust." Fortunately, the "burning" is much slower in your cells than in an automobile engine. Your cells are also more efficient than automobile engines they convert about 40 percent of the energy from food into useful work. The other 60 percent of the energy is converted to thermal energy, which is lost from your body in the form of heat. The heat generated by cellular respiration is not completely wasted. Retaining some of this heat enables your body to maintain a constant temperature, even when the surrounding air is cold. When you are sitting still in class, you radiate about as much heat as a 100-watt lightbulb. You've probably experienced the discomfort of this heat while sitting in a closed room crowded with other "human lightbulbs." When you exercise,

6 your cells increase the rate of cellular respiration. This is why you feel warm after exercise such as running or inline skating. Excess heat is lost through sweating and other cooling methods, much as a car's radiator keeps the engine from overheating. Calories: Units of Energy You have probably heard the term calorie used to refer to food or exercise. A calorie is the amount of energy required to raise the temperature of 1 gram (g) of water by 1 degree Celsius ( C). However, a calorie is such a tiny unit of energy that it is not very practical for measuring the energy content of food. Instead, people usually express the energy in food in kilocalories. One kilocalorie (kcal) equals 1,000 calories. The "calories" shown on a food label are actually kilocalories. You can actually measure the energy content of a food such as a peanut in the laboratory. First you dry the peanut and burn it under an insulated container of water. Burning the peanut converts its stored chemical energy to thermal energy, releasing heat. By measuring the increase in water temperature and using the definition of a calorie, you can calculate the number of calories in the peanut. One peanut has about 5,000 calories, or 5 kcal. That is enough chemical energy to raise the temperature of 1 kilogram (1,000 g) of water by 5 C. Of course, your cells don't burn molecules from peanuts or other foods with a flame. Cells use enzymes to break down organic molecules through the more controlled process of cellular respiration. As a result, the released energy is easier to manage for work. As shown in Figure 7-8, just a handful of peanuts provides enough fuel to power an hour-long walk. Figure 7-8

7 Different daily activities require different amounts of energy. Organic molecules in food are the source of this energy. Concept Check Identify the types of energy you have at the top of a staircase and as you go down the stairs. 2. Explain how your body uses chemical energy during exercise. 3. If a food has 10 kcal of energy, how much could it increase the temperature of 100 g of water? Copyright 2006 by Pearson Education, Inc., publishing as Pearson Prentice Hall. All rights reserved.