NATIONAL SCIENCE EDUCATION STANDARDS

UNIFYING CONCEPTS AND PROCESSES

I, V

CONTENT

B.3, C.1.a, C.1.b, C.5.a, C.5.c, C.5.d

INQUIRY

A.1.c, A.2.a, A.2.d

Cellular Respiration: An Overview

Key Questions Where do organisms get

energy?

What is cellular respiration?

What is the relationship between photosynthesis and cellular respiration?

Vocabularycalorie • cellular respiration •aerobic • anaerobic

Taking NotesPreview Visuals Before you read, study Figure 9–2 on page 252. Make a list of ques-tions that you have about the dia-gram. As you read, write down the answers to the questions.

BUILD VocabularyPREFIXES The prefi x macro- means “large” or “elongated.” Macromolecules are made up of many smaller molecular subunits. Carbohydrates, proteins, and lipids are important macromolecules found in living things.

Chemical Energy and Food Where do organisms get energy?

Food provides living things with the chemical building blocks they need to grow and reproduce. Recall that some organisms, such as plants, are autotrophs, meaning that they make their own food through photo-synthesis. Other organisms are heterotrophs, meaning that they rely on other organisms for food. For all organisms, food molecules contain chemical energy that is released when their chemical bonds are broken.

Organisms get the energy they need from food. How much energy is actually present in food? Quite a lot, although

it varies with the type of food. Energy stored in food is expressed in units of calories. A caloriecalorie is the amount of energy needed to raise the temperature of 1 gram of water 1 degree Celsius. The Calorie (capital C) that is used on food labels is a kilocalorie, or 1000 calories. Cells can use all sorts of molecules for food, including fats, proteins, and carbo-hydrates. The energy stored in each of these macromolecules varies because their chemical structures, and therefore their energy-storing bonds, differ. For example, 1 gram of the sugar glucose releases 3811 calories of heat energy when it is burned. By contrast, 1 gram of the tri-glyceride fats found in beef releases 8893 calories of heat energy when its bonds are broken. In general, carbohydrates and proteins contain approximately 4000 calories (4 Calories) of energy per gram, while fats contain approximately 9000 calories (9 Calories) per gram.

Cells, of course, don’t simply burn food and release energy as heat. Instead, they break down food molecules gradually, capturing a little bit of chemical energy at key steps. This enables cells to use the energy stored in the chemical bonds of foods like glucose to produce com-pounds such as ATP that directly power the activities of the cell.

250 Lesson 9.1 • Lesson Overview • Lesson Notes

THINK ABOUT IT When you are hungry, how do you feel? If you are like most people, you might feel sluggish, a little dizzy, and—above all—weak. Weakness is a feeling triggered by a lack of energy. You feel weak when you are hungry because food serves as a source of energy. Weakness is your body’s way of telling you that your energy supplies are low. But how does food get converted into a usable form of energy? Car engines have to burn gasoline in order to release its energy. Do our bodies burn food the way a car burns gasoline, or is there something more to it?

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Getting Started

Objectives9.1.1 Explain where organisms get the energy they need for life processes.

9.1.2 Define cellular respiration.

9.1.3 Compare photosynthesis and cellular respiration.

Student ResourcesStudy Workbooks A and B, 9.1 Worksheets

Spanish Study Workbook, 9.1 Worksheets

Lab Manual B, 9.1 Data Analysis Worksheet

Lesson Overview • Lesson Notes • Activity: Art in Motion • Assessment: Self-Test, Lesson Assessment

For corresponding lesson in the Foundation Edition, see pages 212–215.

Activate Prior Knowledge Write the term cellular respiration on the board. Then, make a T-Chart below it. Label one column Facts and the other Questions. Have each student come up to the board and write either a fact they know or a question they have about cellular respira-tion. Discuss the T-Chart as a class. Answer any ques-tions that students will need to know before reading the lesson.

Study Wkbks A/B, Appendix S30, T-Chart. Transparencies, GO15.

Teach for Understanding

ENDURING UNDERSTANDING A cell is the basic unit of life; the processes that occur at the cellular level provide the energy and basic structure organisms need to survive.

GUIDING QUESTION Why do most organisms undergo the process of cellular respiration?

EVIDENCE OF UNDERSTANDING At the end of the lesson, have students complete this assessment to show they understand the importance of cellular respiration for life on Earth. Have students make a case for cellular respiration. Ask them to write a short persuasive argument that explains why cellular respiration is an important process on an individual organism level as well as on a global scale.

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Cellular Respiration and Fermentation 251

calorie

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Overview of Cellular Respiration What is cellular respiration?

If oxygen is available, organisms can obtain energy from food by aprocess called cellular respiration. Cellular respiration is theprocess that releases energy from food in the presence of oxygen. Although cellular respiration involves dozens of separate reactions,an overall chemical summary of the process is remarkably simple:

In Symbols:6O2 + C6H12O6 6CO2 + 6H2O + Energy

In Words:Oxygen + Glucose Carbon dioxide + Water + Energy

As you can see, cellular respiration requires oxygen and a food molecule such as glucose, and it gives off carbon dioxide, water, and energy. Do not be misled, however, by the simplicity of this equation. If cellular respiration took place in just one step, all of the energy from glucose would be released at once, and most of it would be lost in the form of light and heat. Clearly, a living cell has to control that energy. It can’t simply start a fi re—the cell has to release the explosive chemical energy in food molecules a little bit at a time. The cell needs to fi nd a way to trap those little bits of energy by using them to make ATP.

In Your Notebook Do plants undergo cellular respiration? What organelle(s) do they have that helps you determine the answer?

Organisms get energy from the food they eat, but the energy con-tained in foods varies greatly. Most foods contain a combination of proteins, carbohydrates, and fats. One gram of protein or a carbo-hydrate such as glucose contains roughly 4 Calories. One gram of fat, however, contains about 9 Calories. The accompanying table shows the approximate composition of one serving of some common foods. 1. Interpret Data Per serving, which

of the foods included in the table has the most protein? Which has the most carbohydrates? Which has the most fat?

You Are What You Eat

2. Calculate Approximately how many more Calories are there in 2 slices of bacon than there are in 3 slices of roasted turkey? Why is there a difference?

3. Calculate Walking at a moderate pace consumesaround 300 Calories per hour. At that rate, how manyminutes would you have to walk to burn the Calories in one chocolate bar? (Hint: Start by calculating the number of Calories consumed per minute by walking.)

FIGURE 9 –1 A Controlled Release Cellular respiration involves a series of controlled reactions that slowly release the energy stored in food. If the energy were to be released too suddenly, most of it would be lost in the forms of light and heat—just as it is when a marshmallow catches fi re.

Food

0

5

3

12

8

2

11

Protein (g) Carbohydrate (g)

Apple, 1 medium

Bacon, 2 slices

Chocolate, 1 bar

Eggs, 2 whole

2% milk, 1 cup

Potato chips, 15 chips

Skinless roasted turkey, 3 slices

22

0

23

0

12

14

3

Fat (g)

0

6

13

9

5

10

1

Composition of Some Common Foods

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Teach

Lead a DiscussionMake sure students understand the overall chemical summary for cellular respiration. Reinforce that the bolded reactions shown are simplifi cations, or sum-mations, of many sub-reactions. Have students verify that the reaction shown is balanced by counting the molecules of each element on the right and left sides of the reaction.

DIFFERENTIATED INSTRUCTION

L1 Struggling Students Write the word form of the cellular respiration summary on the board. Then, read it aloud, pointing to each word and reaction symbol as you read. For example, you might say, “oxygen and glucose are converted into carbon diox-ide and water and energy.” As you say oxygen, point to the word; as you say and, point to the plus sign. Then, write the symbol form of the summary below it. Draw lines from each chemical formula to its cor-responding name in the word form of the summary.

ELL Focus on ELL: Build Background

BEGINNING AND INTERMEDIATE SPEAKERS Distribute copies of a BKWL Chart to your students. Then, show students a short animation, video, or several drawings of the process of cellular respiration. Have them take notes on the visuals in the build background column. Write the words obtain, release, extract, respiration, and energy on the board. Have them copy the words and define each in the build background column. Then, have students fill out the K and W columns of the chart. As students read the lesson, have them fill in the L column.

Study Wkbks A/B, Appendix S27, BKWL Chart. Transparencies, GO12.

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AnswersIN YOUR NOTEBOOK Yes; they contain mitochondria.

PURPOSE Students will examine and interpret data to find how the energy content in foods varies.

PLANNING Have a few of the foods listed in the table on hand, and display them for students before they do the activity. Ask students to predict which of the foods contain the most Calories, and have them explain their reasoning.

ANSWERS

Eggs have the most protein; chocolate 1. has the most carbohydrates; choco-late has the most fat.

There are approximately 9 more 2. Calories in 2 slices of bacon than in 3 slices of roast turkey. The primary difference is that the bacon contains so much more fat than the turkey.

a little over 44 minutes3.

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Stages of Cellular Respiration Cellular respiration captures the energy from food in three main stages—glycolysis, the Krebs cycle, and the electron transport chain. Although cells can use just about any food molecule for energy, we will concentrate on just one as an example—the simple sugar glucose. Glucose fi rst enters a chemical pathway known as glycolysis (gly kahl ih sis). Only a small amount of energy is captured to produce ATP during this stage. In fact, at the end of glycolysis, about 90 percent of the chemical energy that was available in glucose is still unused, locked in chemical bonds of a molecule called pyruvic (py roo vik) acid.

How does the cell extract the rest of that energy? First, pyruvic acid enters the second stage of cellular respiration, the Krebs cycle, where a little more energy is generated. The bulk of the energy, however, comes from the fi nal stage of cellular respiration, the elec-tron transport chain. This stage requires reactants from the other two stages of the process, as shown by dashed lines in Figure 9–2. How does the electron transport chain extract so much energy from these reactants? It uses one of the world’s most powerful electron acceptors—oxygen.

Oxygen and Energy Oxygen is required at the very end of the electron transport chain. Any time a cell’s demand for energy increases, its use of oxygen increases, too. As you know, the word respiration is often used as a synonym for breathing. This is why we have used the term cellular respiration to refer to energy-releasing pathways within the cell. The double meaning of respiration points out a crucial connection between cells and organisms: Most of the energy-releasing pathways within cells require oxygen, and that is the reason we need to breathe, to respire.

Pathways of cellular respiration that require oxygen are said to be aerobicaerobic (“in air”). The Krebs cycle and electron transport chain are both aerobic processes. Even though the Krebs cycle does not directly require oxygen, it is classifi ed as an aerobic process because it cannot run without the oxygen-requiring electron transport chain. Glycolysis, however, does not directly require oxygen, nor does it rely on an oxygen-requiring process to run. Glycolysis is therefore said to be anaerobicanaerobic (“without air”). Even though glycoly-sis is anaerobic, it is considered part of cellular respiration because its fi nal products are key reactants for the aerobic stages.

Recall that mitochondria are structures in the cell that convert chemical energy stored in food to usable energy for the cell.Glycolysis actually occurs in the cytoplasm of a cell, but the Krebs cycle and electron transport chain, which generate the majority of ATP during cellular respiration, take place inside the mitochon dria. If oxygen is not present, another anaerobic pathway, knownas fermentation, makes it possible for the cell to keep glycolysis run-ning, generating ATP to power cellular activity. You will learn more about fermentation later in this chapter.

In Your Notebook Make a fl owchart that shows the different steps of cellular respiration.

FIGURE 9 –2 The Stages of Cellular Respiration There are three stages to cellular respiration: glycolysis, the Krebs cycle, and the electron transport chain. Interpret Visuals Which stage(s) of cellular respiration occur in the mitochondrion?

Glucose Glycolysis

Krebs Cycle

Electron Transport

CO2

O2 H2O

Energy

Energy

Energy

If whales remain underwater for 45 minutes or more, do you think they rely primarily on aerobic or anaerobic pathways?

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Teach continued

Use VisualsUse Figure 9–2 to talk about the overall process of cellular respiration. Start by helping students make the connection between this visual and the chemical summary equations from the previous page. Point out where and how glucose and oxygen are used during the process and that water, carbon dioxide, and energy are released. Make sure students under-stand that cellular respiration can be divided into three basic stages. Tell them that they will learn more detailed information about each of these stages in later lessons.

DIFFERENTIATED INSTRUCTION

L1 Struggling Students Have students read the section, Stages of Cellular Respiration, in the text and look carefully at Figure 9–2. As a class, discuss the fi rst pictured stage, glycolysis. Then, have stu-dents write a one-sentence summary of the discus-sion. For example, students might write, “During glycolysis, glucose is broken down into pyruvic acid and energy is released.” Then, discuss and summa-rize each of the next two stages.

To help students answer the question, remind them that whales need to sur-face to breathe air. Make the connec-

tion between aerobic pathways and the whale’s breathing. Ask them to speculate on how long the breath might last and how the whale might obtain energy once that air runs out. Students can go online to Biology.com to gather their evidence.

Address MisconceptionsCellular Respiration v. Respiration Some of your stu-dents may have diffi culty distinguishing between the concepts of cellular respiration and respiration as breathing. Have students read the fi rst paragraph of the Oxygen and Energy section, research the connection between the two processes, and make a poster for the classroom wall that graphically shows the relationship.

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AnswersFIGURE 9–2 the Krebs cycle and the electron transport chain

IN YOUR NOTEBOOK Flowcharts should accurately show the connections between glycolysis, the Krebs cycle, and the electron transport chain.

Check for Understanding

INDEX CARD SUMMARIES

Give students each an index card, and ask them to write one important idea about cellular respiration that they understand on the front of the card. Then, have them write something about cellular respiration that they don’t understand on the back of the card in the form of a question.

ADJUST INSTRUCTION

Read over students’ cards to get a sense of which concepts they understand and which they are struggling with. If a question will be answered by reading the rest of the chapter, use that card when the time comes to emphasize a concept. If the answer to a question is necessary to move forward, review the topic as a class to allow students to hear the concept discussed in different ways.

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aerobic

anaerobic

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Comparing Photosynthesisand Cellular Respiration

What is the relationship betweenphotosynthesis and cellular respiration?If nearly all organisms break down food by the process of cellular respiration, why doesn’t Earth run out of oxygen? Where does all of the carbon dioxide waste product go? How does the chemical energy stored in food get replaced? As it happens, cellular respiration is balanced by another process: photosynthesis. The energy in photosynthesis and cellular respiration fl ows in opposite directions. Look at Figure 9–3 and think of the chemical energy in carbohydrates as money in the Earth’s savings account. Photosynthesis is the process that “deposits” energy. Cellular respiration is the process that “withdraws” energy. As you might expect, the equations for photosynthesis and cellular respiration are the reverse of each other.

On a global level, photosynthesis and cellular respiration are also opposites. Photosynthesis removes carbon dioxide from the atmosphere, and cellular respiration puts it back. Photosynthesis releases oxygen into the atmosphere, and cellular respiration uses that oxygen to release energy from food. The release of energy by cellular respi-ration takes place in nearly all life: plants, animals, fungi, protists, and most bacteria. Energy capture by photosynthesis, however, occurs only in plants, algae, and some bacteria.

Review Key Concepts 1. a. Review Why do all organisms

need food? b. Relate Cause and Effect Why do

macromolecules differ in the amountof energy they contain?

2. a. Review Write the overall reaction for cellular respiration.

b. Apply Concepts How does the process of cellular respiration main-tain homeostasis at the cellular level?

3. a. Review In what ways are cellular respiration and photosynthesis considered opposite processes?

b. Use Analogies How is the chemical energy in glucose similar to money in a savings account?

4. The Greek word glukus means “sweet,” and the Latin word lysis refers to a process of loosening or decomposing. Based on this information, write a defi nition for the word glycolysis.

Light energy

ATP, Heat energyC6H12O6 + 6O2 6H2O + 6CO2

PHOTOSYNTHESIS

CELLULAR RESPIRATION

FIGURE 9 –3 Opposite Processes Photosynthesis and cellular respiration can be thought of as opposite processes. Compare and Contrast Exactly how is the equation for photosynthesis different from the equation for cellular respiration?

Cellular Respiration and Fermentation 253

Lesson 9.1 • Lesson Assessment • Self-Test • Art in Motion

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Cellular Respiration and Fermentation 253

To better understand the relationship between photosynthesis and cellular respira-tion, students can watch the animation, Art in Motion: Opposite Processes.

Assess and Remediate EVALUATE UNDERSTANDING

Have pairs of students use Figure 9–3 to help them summarize the content of the lesson. For example, ask them to explain why both the rabbit and the plant are producing ATP and heat. Listen to their discussions to help you evaluate their grasp of lesson concepts. Then, have them complete the 9.1 Assessment.

REMEDIATION SUGGESTION

LPR Less Profi cient Readers If students have trouble understanding Question 2b, suggest they use the Glossary at the back of this text to review the meaning of the term homeostasis.

Students can check their under-standing of lesson concepts with the Self-Test assessment. They can then take an online version of the Lesson Assessment.

AnswersFIGURE 9–3 The products of photosynthesis are the reactants of cellular respiration, and the reactants of cellular respiration are the products of photosynthesis.

1a. Food provides organisms with the energy they need to carry out life processes such as growth and reproduction.

1b. The amount of energy stored in macro-molecules varies because their chemical structures, and therefore the energy con-tained in their chemical bonds, differ.

2a. 6O2 + C6H12O6 → 6CO2 + 6H2O + Energy

2b. The process of cellular respiration pro-vides the energy a cell needs to carry out basic cell processes, which, in turn, control the cell’s internal conditions.

3a. Photosynthesis “deposits” energy, uses carbon dioxide and water, and pro-duces oxygen and glucose. By contrast, cellular respiration “withdraws” energy, uses oxygen and glucose, and produces carbon dioxide and water.

3b. Chemical energy is stored in the chemical bonds of glucose, just as money is stored in a savings account.

4. Sample answer: a process that involves the breaking down of a sweet substance

Assessment Answers

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