Questions for Discussion6

STUDENT LEARNING OBJECTIVES

At the completion of this chapter, you should be able to do the following:

1.Define and outline the differences between nutrition and metabolism.

2.Define these terms: assimilation, catabolism, anabolism.

3.Outline the process of carbohydrate metabolism.

4.Discuss the roles of glycolysis, the citric acid cycle, and the electron transport chain in the production of cellular energy.

5.List the hormones involved in the control of glucose metabolism.

6.Outline the role of lipids, their transport, and their metabolism.

7.Outline the role of proteins and their metabolism.

8.Discuss the difference between vitamins and minerals and their roles in metabolism.

9.Discuss the factors that control and influence metabolic rate.

LANGUAGE OF SCIENCE AND MEDICINE

Before reading the chapter, say each of these terms out loud. This will help you avoid stumbling over them as you read.

amino acid (ah-MEE-no ASS-id)

[amino NH2, acid sour]

anabolism (ah-NAB-oh-liz-em)

[anabol- build up, -ism action]

antioxidant (an-tee-OK-seh-dent)

[anti- against, -oxi- sharp (oxygen), -ant agent]

appetite center

assimilation (ah-sim-ih-LAY-shun)

[assimila- make alike, -tion process]

ATP synthase (SIN-thays)

[ATP adenosine triphosphate, syn- together, -ase enzyme]

basal metabolic rate (BMR) (BAY-sal met-ah-BAHL-ik)

[bas- basis, -al relating to, metabol- change, -ic relating to]

calcitriol (kal-SIT-ree-ol)

[calci- lime (calcium), -tri- three, -ol alcohol (after 1,25-D3 or 1,25-dihydroxycholecalciferol)]

catabolism (kah-TAB-oh-liz-em)

[catabol- break down, -ism action]

cellulose (SELL-yoo-lohs)

[cell- storeroom (cell), -ul- small, -ose carbohydrate]

chylomicron (kye-loh-MYE-kron)

[chylo- juice (chyle), -micro- small, -on particle]

citric acid cycle (SIT-rik ASS-id SYE-kul)

[citr- citron tree, -ic relating to, acidus sour, kyklos circle]

coenzyme (koh-EN-zyme)

[co- together, -en- in, -zyme ferment]

deamination (dee-am-ih-NAY-shun)

[de- undo, -amin- ammonia compound, -ation process]

electron transport system (eh-LEK-tron TRANZ-port SIS-tem)

[electr- electric, -on unit, trans- across, -port carry, system organized whole]

essential fatty acid

[acid sour]

free fatty acid (FFA)

[acid sour]

glucose phosphorylation (GLOO-kohs fos-for-ih-LAY-shun)

[gluco- sweet, -ose carbohydrate (sugar), phos- light, -phor- carry, -yl- chemical, -ation process]

glycolysis (glye-KOHL-ih-sis)

[glyco- sweet (glucose), -o- combining form, -lysis loosening]

hyperglycemia (hye-per-gly-SEE-mee-ah)

[hyper- above, -glyc- sweet (glucose), -emia blood condition]

hypoglycemia (hye-poh-gly-SEE-mee-ah)

[hypo- below, -glyc- sweet (glucose), -emia blood condition]

lipid (LIP-id)

[lip- fat, -id form]

lipogenesis (lip-oh-JEN-eh-sis)

[lipo- fat, -gen- produce, -esis process]

lipoprotein (lip-oh-PROH-teen)

[lipo- fat, prote- primary, -in substance]

macronutrient (MAK-roh-NOO-tree-ent)

[macro- large, -nutri- nourish, -ent agent]

metabolic rate (met-ah-BOL-ik)

[metabol- change, -ic relating to]

metabolism (meh-TAB-oh-liz-em)

[metabol- change, -ism process]

micronutrient (MYE-kroh-NOO-tree-ent)

[micro- small, -nutri- nourish, -ent agent]

mineral

[mineral- mine]

nutrition (noo-TRIH-shun)

[nutri- nourish, -tion process]

oxidative phosphorylation (ahk-sih-DAY-tiv fos-for-ih-LAY-shun)

[oxi- sharp (oxygen), -id- chemical (-ide), -at- action of (-ate), -ive relating to, phos- light, -phor- carry, -yl- chemical, -ation process]

phosphorylation (fos-for-ih-LAY-shun)

[phos- light, -phor- carry, -yl- chemical, -ation process]

satiety center (sah-TYE-eh-tee SEN-ter)

[sati- enough, -ety state]

saturated

(SATCH-yoo-ray-ted)

total metabolic rate (met-ah-BOL-ik)

[metabol- change, -ic relating to]

triglyceride (try-GLISS-er-yde)

[tri- three, -glycer- sweet, -ide chemical]

unsaturated

(un-SATCH-yoo-ray-ted)

vitamin (VYE-tah-min)

[vita- life, -amin- ammonia compound]

WALTER heard that pizza was a complete meal because it contained all the major macronutrients. “Maybe I could invent a pizza diet and make millions,” he thought. He went over the macronutrients in his head, “carbohydrates, lipids… hmm, and then there are macronutrients such as sodium, carbon, potassium …”

A friend pointed out that some of his assumptions concerning nutrients were incorrect. Do you know which ones should not be part of Walter's list?

Now that you have read this chapter, see if you can answer these questions about the new “pizza diet” Walter wanted to invent.

1. Which item doesn't belong in Walter's list?

a. Protein

b. Lipid

c. Vitamin C

d. Carbohydrate

2. Which of the minerals Walter listed is NOT considered a macronutrient?

a. Sodium

b. Carbon

c. Potassium

d. All are considered macronutrients

3. The olive oil on pizza crust is mostly triglyceride lipids containing monounsaturated fatty acids. What is the first step in catabolizing the triglycerides in the olive oil?

a. Conversion to glycerol and three fatty acids

b. Glycolysis

c. Lipogenesis

d. Deamination

To answer these questions, you may have to refer to the glossary or index, other chapters in this textbook, A&P Connect, Mechanisms of Disease, and other resources.

Chapter 21 explained the processes of getting nutrients into our internal environment. This chapter takes the story further by discussing how our body manages the nutrients after they are absorbed—that is, how they are stored and how they are used by the cells of our bodies.

OVERVIEW OF NUTRITION AND METABOLISM

Nutrition refers to the foods that we eat and the types of nutrients they contain. As you undoubtedly know, healthful nutrition requires a balance of different nutrients in appropriate amounts. In contrast, malnutrition is a deficiency or imbalance in the consumption of food, vitamins, and minerals. As a matter of convenient communication, many nutrition experts divide the essential (required) nutrients into two major categories:

1.Macronutrients—usually include those nutrients that we need in large amounts, such as carbohydrates, fats, and proteins. Water and minerals that we need in large quantities to remain in good health are often included among the macronutrients. For example, sodium, chloride, potassium, calcium, magnesium, and phosphorus are sometimes considered to be macronutrients.

2.Micronutrients—usually include nutrients that we need in very small amounts, such as vitamins and some minerals. Minerals in this group include iron, iodine, zinc, manganese, cobalt, and a few others. Mineral micronutrients are also called trace elements.

There are many small differences in individual genetic makeup, as well as differences in individual lifestyles and environments, that influence how nutrients affect our bodies. Fortunately, we have some advice that we can rely on to help us make healthy choices. For example, the United States government makes use of an individually customized food pyramid as a general nutrition guide (Figure 22-1). The Canadian government uses a similar individualized food guide to advise eating a healthy, balanced diet.

Now let's see how nutrients and metabolism are related.

Metabolism refers to the complex interactions of chemical processes that make life possible. It is essentially how the body uses foods and their nutrients after they have been digested, absorbed, and transported to the cells of our bodies. Your body cells use nutrients from food in several ways: as fuel (energy), as material for growth and maintenance, and for regulation of body functions. Before they can be used in these different ways, nutrients have to be assimilated. Assimilation occurs when nutrient molecules enter cells and undergo many chemical changes.

Metabolism is a complex process made up of many other processes. Two of the major metabolic processes are termed catabolism and anabolism. Each of these processes, in turn, consists of a series of enzyme-driven chemical reactions known as metabolic pathways.

Catabolism breaks food molecules down into smaller molecular compounds and, in so doing, releases energy from them. Anabolism does the opposite. It builds nutrient molecules up into larger molecular compounds and, in so doing, uses energy. Thus catabolism is a decomposition process, whereas anabolism is a synthesis process. Both catabolism and anabolism take place inside cells. Both catabolic and anabolic processes go on continually and concurrently.

Catabolism releases energy in two forms: heat energy and chemical energy. The amount of heat generated is relatively large—so large, in fact, that it would effectively “cook” cells if it were released in one large burst! Fortunately, catabolism is regulated by enzymes so that heat is released in frequent small bursts. Most of this heat is used to maintain the

FIGURE 22-1 United States Food Guide Pyramid. Simple pyramid diagrams help educate the public on building a diet with a balance of foods from different categories illustrated in the diagram. This is an abbreviated version of the comprehensive food guide that can be found at www.mypyramid.gov. The full version includes recommended servings per day and other nutrition advice.

FIGURE 22-2 The role of ATP in metabolism. ATP temporarily stores energy in its last high-energy phosphate bond. When water is added and phosphate breaks free, energy is released to do cellular work. The ADP and phosphate groups that result can be resynthesized into ATP, capturing additional energy from nutrient catabolism. This cycle is called the ATP/ADP system.

homeostasis of body temperature. In contrast, chemical energy released by catabolism is more obviously useful. It cannot, however, be used directly for biological reactions. First, it must be transferred to the high-energy molecule adenosine triphosphate (ATP). ATP supplies energy directly to the energy-using reactions of all cells in all living cellular organisms.

Look now at Figure 22-2. The structural formula at the top of the diagram shows three phosphate groups attached to the rest of the ATP molecule, two of them by high-energy bonds. Adding water to ATP yields an inorganic phosphate group (Pi), adenosine diphosphate (ADP), and energy, which is used for anabolism and other cell work. The diagram also shows that Pi and ADP then use energy released by catabolism to recombine to form ATP.

Metabolism is not identical in all cells. More active cells have a higher metabolic rate than do less active cells. In addition, anabolism in different kinds of cells produces different compounds. In liver cells, for example, anabolism synthesizes various blood protein compounds. But in beta cells of the pancreas, anabolism produces insulin.

Metabolism is a broad and complex mix of biological chemistry. This chapter discusses only the essential concepts related to the many and varied metabolic pathways of the human body.

CARBOHYDRATES

Dietary Sources of Carbohydrates

Carbohydrates are found in most of the foods that we eat. Polysaccharides—such as starches in vegetables, grains, and other plant tissues—are broken down into simpler carbohydrates before they are absorbed.

Cellulose, a major component of most plant tissues, is an important exception. Because we do not make enzymes that chemically digest this complex carbohydrate, it passes through our digestive system without being broken down. Also called dietary fiber or “roughage,” cellulose and other indigestible polysaccharides mix with chyme and keep it thick enough to push easily through our digestive system. Most biologists now agree that a high-fiber diet has many health benefits.

Disaccharides such as those in refined sugar must also be chemically digested before they can be absorbed. Monosaccharides in fruits and some “diet foods” are already in an absorbable form, so they can move directly into the internal environment without initially being processed. As you will see, the monosaccharide glucose is the carbohydrate that is most useful to the typical human cell.

1. Name the two types of metabolism and distinguish between them.

2. Why must energy in nutrient molecules be transferred to ATP?

3. Why is cellulose indigestible?

A&P CONNECT

Nutritionists often talk about the “energy value” of food—that is, how much energy the body can get from that food. Do you know what it means when a label states that food energy in calories? Do you know the difference between a calorie and a Calorie? Or a calorie and a joule or kilojoule? Find answers to these questions, and also learn the energy values of major nutrients and the amount of energy expended by different physical activities, in Measuring Energy online at A&P Connect.

Carbohydrate Metabolism

The body metabolizes carbohydrates by both catabolic and anabolic processes. Most of our cells use carbohydrates—mainly glucose—as their first or preferred energy fuel. When the amount of glucose entering cells is inadequate for their energy needs, they may make more glucose by using a pathway that catabolizes fats or proteins.

As you read through the following sections outlining the basic process of carbohydrate metabolism, remember the ultimate result of catabolism: the transfer of energy from a nutrient molecule to ATP. It is the continued production of ATP, the energy currency of the cell, which makes nutrient catabolism so incredibly vital to all of life's processes.

Glucose Transport and Phosphorylation

Carbohydrate metabolism begins with the movement of glucose through cell membranes. In the interior of a cell, glucose reacts with ATP to form glucose 6-phosphate, which cannot move back across the cell membrane. This step, named glucose phosphorylation, prepares glucose for further metabolic reactions. Phosphorylation is the process of adding a phosphate group to a molecule. Depending on their energy needs of the moment, cells either catabolize (break apart) or anabolize (bind together) glucose 6-phosphate.

Glycolysis

Glycolysis is the first step in the process of carbohydrate catabolism. This pathway consists of a series of anaerobic chemical reactions that take place in the cytoplasm (see Figure 4-13, page 71). In the end, glycolysis breaks apart one glucose molecule (made of six carbon atoms) to form two pyruvic acid molecules, each of which has three carbon atoms (Figure 22-3). As you can see, a specific enzyme catalyzes each of these reactions. Glycolysis is an essential process because it produces a small amount of ATP (a net of two molecules for every sugar molecule) and also prepares glucose for the second step in catabolism, namely, the citric acid cycle. As we will see below, glucose itself cannot enter the cycle: It must first be converted to a compound called acetyl coenzyme A (acetyl CoA).

Citric Acid Cycle

Essentially, the citric acid cycle is a series of chemical reactions mediated by enzymes that converts the two acetyl molecules from each six-carbon glucose to four carbon dioxide and six water molecules (see Figure 4-14, page 72). The citric

FIGURE 22-3 Catabolism of glucose. Glycolysis splits one molecule of glucose (six carbon atoms) into two molecules of pyruvic acid (three carbon atoms each). The glycolytic pathway does not require oxygen, so it is termed anaerobic. The removal of a carbon dioxide molecule converts each pyruvic acid molecule into a two-carbon acetyl group that is “escorted” by coenzyme A (CoA) into the citric acid cycle, where it joins a four-carbon compound (oxaloacetic acid) to form a six-carbon compound (citric acid). Now, two more carbon dioxide molecules (one carbon atom each) are released from each citric acid molecule formed. The carbon and oxygen atoms in the original glucose molecule are thus released as waste products. However, the real metabolic prize is energy, which is released as the molecule is broken down. Because this part of the pathway requires oxygen, it is termed aerobic.

acid cycle occurs in the mitochondria (recall that glycolysis takes place only in the cytoplasm).

Before it can enter the citric acid cycle, each pyruvic acid molecule combines with coenzyme A, thus forming acetyl CoA. Coenzyme A then detaches from acetyl CoA, leaving a two-carbon acetyl group, which enters the citric acid cycle by combining with oxaloacetic acid to form citric acid. This is what gives the citric acid cycle its name.

Each pyruvic acid molecule generates three CO2 molecules, some ATP, and many high-energy electrons while going through the citric acid cycle. Most of the energy leaving the citric acid cycle is temporarily “stored” in these high-energy electrons. The next section describes how these high-energy electrons are used to generate ATP.

Electron Transport System and Oxidative Phosphorylation

High-energy electrons removed during the citric acid cycle enter a chain of carrier molecules embedded in the inner membrane of mitochondria that is known as the electron transport system.

Figure 22-4 shows that high-energy electrons—along with their accompanying protons (H+)—are shuttled to the electron transport system during the citric acid cycle by carrier molecules called nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD). The electrons quickly move down the chain, from one membrane protein complex to the next, and eventually to their final acceptor, oxygen.

As the electrons are transported, some of their energy is used to pump their accompanying protons (H+) to the intramembrane space between the inner and outer membranes of the mitochondrion. This creates a concentration gradient of protons, and the intermembrane space thus becomes a reservoir of protons. Like water behind a dam, the reservoir of protons temporarily stores energy. In much the same way as water flows through a dam and turns wheels to generate energy, the inner membrane has “proton wheels”—in the form of ATP synthase. ATP synthase is an enzyme that uses the proton movement down the concentration gradient to bind together ADP and a phosphate group to generate ATP (Figure 22-5).

FIGURE 22-4 Electron transport system. This system of energy transfer takes place entirely within each mitochondrion.

FIGURE 22-5 Generation of ATP by ATP synthase. This simplified model of the proton “wheel” in the mitochondrial inner membrane shows how protons (H+) moving down their concentration gradient drive the rotation of a molecular machine. The energy of rotation then phosphorylates (adds phosphate to) ADP to become ATP.

At this time, the low-energy electrons (e−) and their protons (H+) join oxygen, forming water. This oxygen-requiring joining of a phosphate group to ADP to form ATP is called oxidative phosphorylation. As you can see, although oxygen is not needed until the very last step of aerobic respiration, its role is vital. Without oxygen to oxidize the hydrogen

FIGURE 22-6 Cell machinery for glucose catabolism. 1, Glycolysis occurs in the cytoplasm. 2, Citric acid cycle takes place mostly in the mitochondrial matrix. 3, Electron transport and oxidative phosphorylation occur on the inner membrane of mitochondria.

FIGURE 22-7 Energy extracted from glucose. Energy released from the breakdown of glucose is released mostly as heat, but some of it is transferred to a usable form—the high-energy bonds of ATP. In most human cells, one glucose molecule produces enough usable chemical energy to synthesize or “charge up” 36 ATP molecules. Some cells, such as heart and liver cells, shuttle electrons more efficiently and may be able to synthesize up to 38 ATP molecules. This represents an energy conversion efficiency of 38% to 44%, much better than the 20% to 25% typical of most machines.

into water, the energy generation pathway would stop. In effect, oxygen serves as an “electron dump,” ridding the body of spent electrons derived from the breakdown of glucose.

The breakdown of ATP molecules, of course, provides virtually all the energy that does cellular work. Therefore, oxidative phosphorylation is the crucial part of glucose catabolism (Figures 22-6). The energy extracted during the various steps of the breakdown of glucose is given for you in Figure 22-7.

FIGURE 22-8 Hormonal control of blood glucose level. Simplified view of some of the major glucose-regulating hormones. Insulin lowers the blood glucose level and is therefore hypoglycemic. Most hormones shown here raise the blood glucose level and are called hyperglycemic, or anti-insulin, hormones.

We can now summarize the long series of chemical reactions in glucose catabolism with one short equation:

C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + 36 (or 38) ATP + Heat

Control of Glucose Metabolism

Levels of sugar in the blood are under hormone control as shown in Figure 22-8, which shows that most hormones cause the glucose blood level to rise. These hormones are called hyperglycemic because they tend to promote a high blood glucose concentration. The one notable exception is insulin, which is hypoglycemic (tends to decrease the blood glucose level). See Box 22-1 for more discussion of blood glucose problems.

4. What is glycolysis? How much energy is transferred to ATP through this process?

5. What happens to a nutrient molecule as it proceeds through the citric acid cycle?

6. What is the purpose of the electron transport system?

7. What is the difference between hyperglycemic hormones and hypoglycemic hormones?

BOX 22-1 FYI

Abnormal Blood Glucose Concentration

The term hyperglycemia, which literally means “condition of too much sugar in the blood,” is used anytime the blood glucose concentration becomes higher than the normal set point level. Hyperglycemia is most often associated with untreated diabetes mellitus, but it can occur in newborns when too much intravenous glucose is given or in other similar situations. If untreated, the excess glucose leaves the blood in the kidney—literally “spilling over” into the urine. This increases the osmotic pressure of urine, drawing an abnormally high amount of water into the urine from the bloodstream. Thus hyperglycemia causes loss of glucose in the urine and its accompanying loss of water—potentially threatening the fluid balance of the body. Dehydration of this sort can ultimately lead to death.

In contrast, hypoglycemia occurs when the blood glucose concentration dips below the normal set point level. Hypoglycemia can occur in various conditions, including starvation, hypersecretion of insulin by the pancreatic islets, or injection of too much insulin. Symptoms of hypoglycemia include weakness, hunger, headache, blurry vision, anxiety, and personality changes—sometimes leading to coma and death if untreated.

LIPIDS

Dietary Sources of Lipids

Recall from Chapter 2 that lipids are a class of organic compounds that includes fats, oils, and related substances. The most common lipids in the diet are triglycerides, which are composed of a glycerol subunit attached to three fatty acids. Other important dietary lipids include phospholipids and cholesterol.

Dietary fats are often classified as either saturated or unsaturated. Saturated fats contain fatty acid chains in which there are no double bonds—that is, all available bonds of its hydrocarbon chain are filled (saturated) with hydrogen atoms (see Figure 2-16, p. 32). Saturated fats are usually solid at room temperature. Unsaturated fats contain fatty acid chains in which there are double bonds, meaning that not all sites for hydrogen are filled. Because the double bonds change the shape of unsaturated fats, the molecules usually do not “fit” together as well and so are usually liquid at room temperature.

Triglycerides are found in nearly every food that we eat. However, the amount of triglycerides in each type of food varies considerably, as does the proportion of saturated to unsaturated types. Phospholipids are also found in nearly all foods because they make up the cellular membranes of all living organisms. Cholesterol, however, is found only in foods of animal origin. Cholesterol concentration also varies. For example, it is particularly high in liver, shrimp, and the yolks of eggs.

Transport of Lipids

Lipids are transported in blood as chylomicrons, lipoproteins, and free fatty acids. Chylomicrons are small fat droplets found in blood soon after fat absorption takes place. Fatty acids and monoglyceride products of fat digestion combine during absorption to again form fats (triglycerides, or triacylglycerols). These triglycerides plus small amounts of cholesterol and phospholipids compose the chylomicrons.

Lipoproteins are produced mainly in the liver and, as their name suggests, consist of lipids and protein. Blood contains three types of lipoproteins: very-low-density lipoproteins, low-density lipoproteins, and high-density lipoproteins. Usually, they are designated by their abbreviations: VLDL, LDL, and HDL, respectively. Diets high in saturated fats and cholesterol tend to produce an increase in blood LDL concentration, which in turn is associated with a high incidence of coronary artery disease (CAD) and atherosclerosis (Figure 22-9 and Box 22-2). A high blood HDL concentration, in contrast, is associated with a low incidence of heart disease. You can remember this by thinking of the LDLs as the “lethal lipoproteins” and the HDLs as the “healthy lipoproteins.” Considerable evidence indicates that exercise

FIGURE 22-9 Cholesterol and heart disease. The graph shows a relationship between the total serum (blood plasma) cholesterol level and coronary artery disease (CAD).

tends to elevate HDL concentration and reduce the likelihood of coronary heart disease.

Fatty acids, on entering the blood from adipose tissue or other cells, combine with albumin to form free fatty acids (FFAs). Fatty acids are transported from cells of one tissue to those of another in the form of free fatty acids.

Lipid Metabolism

Lipid Catabolism

Lipid catabolism, like carbohydrate catabolism, consists of several processes. Each of these processes, in turn, consists of a series of chemical reactions. Triglycerides are first hydrolyzed to yield fatty acids and glycerol. Glycerol is then converted to glyceraldehyde 3-phosphate, which may be converted to glucose or it may enter the glycolysis pathway directly (see Figure 4-13, p. 71). Fatty acids are broken down into two-carbon pieces—the familiar acetyl CoA. These molecules are then catabolized via the citric acid cycle. The final process of lipid catabolism therefore consists of the same reactions as does carbohydrate catabolism. Catabolism of lipids, however, yields considerably more energy than does catabolism of carbohydrates. Whereas catabolism of 1 gram of carbohydrates yields only 4.1 kcal of heat, catabolism of 1 gram of fat yields 9 kcal.

Lipid Anabolism

Lipid anabolism, also called lipogenesis, consists of the synthesis of various types of lipids, notably triglycerides, cholesterol, phospholipids, and prostaglandins. Triglycerides and structural lipids (e.g., phospholipids that make up our cell membranes) are synthesized from fatty acids and glycerol or from excess glucose or amino acids. This is why it is possible to “get fat” from foods other than fat! Triglycerides are stored mainly in adipose tissue cells. These fat deposits constitute the body's largest reserve energy source. Enormous amounts of fat can be stored in our bodies. In contrast, only a few hundred grams of carbohydrates can be stored as liver and muscle glycogen.

BOX 22-2 Lipoproteins

As you've seen, high blood concentrations of low-density lipoproteins (LDLs) are associated with a high risk for atherosclerosis. Atherosclerosis is a form of “hardening of the arteries” that occurs when lipids accumulate in cells lining the blood vessels and promote the development of a plaque that eventually impedes blood flow and may trigger clot formation. Atherosclerosis may also weaken the wall of a blood vessel to the point that it ruptures. In any case, a person with atherosclerosis of the coronary arteries risks a heart attack when blood flow to cardiac muscle is impaired. If vessels in the brain are affected, there is risk of a cerebrovascular accident (CVA), or “stroke.”

According to a current model (see part A of figure), LDL delivers cholesterol to cells for use in synthesizing steroid hormones and stabilizing the plasma membrane. Most, if not all, cells have many LDL receptors embedded in the outer surface of their plasma membranes. These receptors attract cholesterol-bearing LDL. Once the LDL molecule binds to the receptor, specific mechanisms operate to release the cholesterol it carries into the cell. Excess cholesterol is stored in droplets near the center of the cell. It seems that, in some individuals at least, cells have so few LDL receptors that they accumulate too much cholesterol in the blood. Some mechanism in endothelial cells moves this excess LDL into the wall of blood vessels. This has been proposed as a cause for the lipid accumulation characteristic of atherosclerosis.

High blood concentrations of high-density lipoproteins (HDLs) have been associated with a low risk of developing atherosclerosis and its many possible complications. Although the exact details of how this works have yet to be worked out, scientists have made progress toward that end. According to one model (see part B of figure), HDL molecules are attracted to HDL receptors embedded in the plasma membranes. Once they bind to their receptors, the cell is stimulated to release some of its cholesterol from storage. The released cholesterol migrates to the plasma membrane, where it may attach to the HDL molecule and be whisked away to the liver for excretion in bile.

High blood LDL levels (more than 180 mg LDL per 100 ml of blood) signify that a large amount of cholesterol is being delivered to cells. High blood HDL levels (more than 60 mg HDL per 100 ml of blood) indicate that a large amount of cholesterol is being removed from cells and delivered to the liver for excretion from the body. Currently, researchers are using this information to develop treatments that may prevent—or even cure—atherosclerosis and the disorders it causes.

Role of blood lipoproteins. A, Simplified diagram of the role of low-density lipoprotein (LDL) in delivering cholesterol to cells. B, Proposed role of high-density lipoprotein (HDL) in removing cholesterol from cells.

Most fatty acids can be synthesized by the body. A certain number of the unsaturated fatty acids must be provided by the diet and are thus called essential fatty acids. Some of the essential fatty acids serve as a source within the body for synthesis of an important group of lipids called prostaglandins. These hormone-like compounds, first discovered in the 1930s from prostate fluids forming semen, have in recent years gained increasing recognition for their occurrence in various tissues, where they support a wide spectrum of biological activity. Certain essential fatty acids are also necessary for manufacturing the phospholipids in cell membranes (see Chapter 4) and the myelin in nerve tissue (see Chapter 11).

Control of Lipid Metabolism

Lipid metabolism is controlled mainly by the following hormones:

▪Insulin

▪Adrenocorticotropic hormone (ACTH)

▪Growth hormone

▪Glucocorticoids

As you may recall, these help regulate fat metabolism such that the rate of fat catabolism is inversely related to the rate of carbohydrate catabolism. If some condition such as diabetes mellitus causes carbohydrate catabolism to decrease below energy needs, increased secretion of growth hormone, ACTH, and glucocorticoids soon follows. These hormones, in turn, bring about an increase in fat catabolism. But, when carbohydrate catabolism equals energy needs, fats are not mobilized out of storage and catabolized. Instead, they are spared and stored in adipose tissue. So, it can be said that “Excessive carbohydrates have a ‘fat-storing’ effect.”

8. In what forms are lipids transported to cells?

9. How can glycerol and fatty acids enter the citric acid cycle?

10. Which fatty acids cannot be made by the body?

11. Which hormones are involved in lipid metabolism?

PROTEINS

Sources of Proteins

Recall from Chapter 2 that proteins are very large molecules composed of chemical subunits called amino acids. Proteins are assembled from 20 different kinds of amino acids. If any one type of amino acid is deficient, vital proteins cannot be synthesized—a serious health threat. One way your body maintains a constant supply of amino acids is by synthesizing them from other compounds already present in the body. However, only about half of the required 20 types of amino acids can be made by the body. The remaining types of amino acids must be supplied in the diet. Nutritionists often refer to the amino acids that must be in the diet as essential amino acids. Table 22-1 lists amino acids according to whether they are considered essential in the diet or nonessential in the diet (synthesized by the body). Box 22-3 investigates the link between blood levels of amino acids and disease.

Proteins are obtained in the diet from various sources. Muscle meat and other animal tissues particularly high in proteins contain the essential amino acids. Food from a single plant or other nonanimal source does not usually contain an adequate amount of all the essential amino acids. Therefore, it is important to include meat (or other animal tissues), or a mixture of different vegetables that provide all the amino acids needed by the body, in the diet. Plant tissues that are particularly high in protein content include cereal grains, nuts, and legumes such as peas and beans.

TABLE 22-1 Amino Acids

ESSENTIAL

NONESSENTIAL

Histidine*

Isoleucine

Leucine

Lysine

Methionine

Phenylalanine

Threonine

Tryptophan

Valine

Alanine

Arginine

Asparagine

Aspartic acid

Cysteine

Glutamic acid

Glutamine

Glycine

Proline

Serine

Tyrosine†

* Essential in infants and, perhaps, adult males.

† Can be synthesized from phenylalanine; therefore, nonessential as long as phenylalanine is in the diet

Protein Metabolism

Protein Anabolism

Every cell synthesizes its own structural proteins and its own enzymes using ribosomes to read the DNA code and construct polypeptides. In addition, many cells, such as liver and glandular cells, synthesize special proteins for export. For example, liver cells manufacture the plasma proteins found in our blood. Any particular cell's genes, under the influence of signaling mechanisms, determine the specific proteins to be synthesized for that cell or for other body cells. Protein anabolism is truly “big business” in our body. For example, red blood cell replacement alone requires the production of millions of cells per second and by itself creates huge demands for protein anabolism.

BOX 22-3 Health Matters

Amino Acids and Disease

Recent research shows that the balance of amino acids circulating in the blood is associated with various diseases. High blood levels of homocysteine, one of several alternate forms of the amino acid cysteine (see Table 22-1), have been linked to heart disease, stroke, and dementias such as Alzheimer disease. Whether such abnormalities in homocysteine levels are the direct cause of these conditions is uncertain. Despite this uncertainty, many physicians recommend lowering abnormally high blood homocysteine levels to reduce the possible risk for these devastating conditions. Homocysteine can be reduced to normally low blood levels when there is adequate vitamin B6, B12, or B9 (folic acid) in the diet.

Protein Catabolism

The first step in protein catabolism takes place in liver cells. Called deamination, it consists of the splitting off of an amino (NH2) group from an amino acid molecule to form a molecule of ammonia and one of keto acid. Most of the ammonia is converted by liver cells to urea and later excreted in the urine. The keto acid may be oxidized via the citric acid cycle or may be converted to glucose or to fat. Both protein catabolism and anabolism go on continually. Only their rates differ from time to time. With a protein-deficient diet, for example, protein catabolism exceeds protein anabolism. Various hormones, as we shall see below, also influence the rates of protein catabolism and anabolism.

Control of Protein Metabolism

Protein metabolism, like that of carbohydrates and fats, is controlled largely by hormones rather than by the nervous system. Growth hormone and the male hormone testosterone both have a stimulating effect on protein synthesis, or anabolism. For this reason, they are referred to as anabolic hormones. The protein catabolic hormones of greatest consequence are glucocorticoids. They speed up the hydrolysis of cell proteins to amino acids, their entry into the blood, and their subsequent catabolism. ACTH functions indirectly as a protein catabolic hormone because of its stimulating effect on glucocorticoid secretion.

Thyroid hormone is necessary for and promotes protein anabolism and therefore growth when plenty of carbohydrates and fats are available for energy production. Under different conditions—for example, when the amount of thyroid hormone is excessive or when the energy foods are deficient—this hormone may then promote protein mobilization and catabolism.

12. What is meant by the term essential amino acid?

13. What happens when an amino acid is deaminated?

14. What is the purpose of the process of amino acid deamination?

15. How is protein metabolism controlled?

VITAMINS AND MINERALS

Vitamins

Vitamins are organic molecules needed in small quantities for normal metabolism throughout the body. Most vitamin molecules attach to enzymes or coenzymes and help them work properly. Coenzymes are organic, nonprotein catalysts that often act as “molecule carriers.” Many enzymes or coenzymes are not functional without the appropriate vitamins attached to them. This attachment gives coenzymes the proper functional shape. For example, coenzyme A (CoA)—an important carrier molecule associated with the citric acid cycle—has pantothenic acid (vitamin B5) as one of its major components.

Not all vitamins are involved directly with enzymes and coenzymes. Vitamins A, D, and E play a variety of different, but no less important, roles in the chemistry of the body. The form of vitamin A called retinal, for example, plays an important role in detecting light in sensory cells of the retina. Vitamin D can be converted to the hormone calcitriol, which plays a role in the regulation of calcium homeostasis in the body. One role of vitamin E (and vitamin C) is to serve as an antioxidant that prevents free radicals (highly reactive oxygen atoms) from damaging electron-dense molecules in the cell membranes and DNA molecules.

All but one vitamin, vitamin D, cannot be made by the body itself. Recent research suggests that vitamin D supplements may reduce risks for a range of diseases, including cancers of the breast, colon, ovaries, and prostrate. Bacteria living in the colon make two more: vitamin K and biotin. We must eat vitamins, or molecules we can convert into vitamins, in our food to get the rest. The body can store fat-soluble vitamins—A, D, E, and K—in the liver for later use. Because the body cannot store significant amounts of water-soluble vitamins such as B vitamins and vitamin C, they must be continually supplied in the diet. Table 22-2 lists some common vitamins, their sources and functions, and symptoms of deficiency.

Minerals

Minerals are at least as important as vitamins in our diet. Minerals are inorganic elements or salts that are found naturally in the earth. Like vitamins, mineral ions can attach to enzymes or other organic molecules and help them work. Of course, minerals such as sodium, chloride, and potassium are essential in relatively large amounts for maintaining the fluid/ion composition of the internal fluid environment.

Minerals such as sodium and calcium also function in nerve conduction and in the contraction of muscle fibers. Without these minerals, the brain, heart, and respiratory tract would cease to function. Iron is needed to manufacture hemoglobin in red blood cells, and iodine is needed to make thyroid hormones T3 and T4. Calcium, phosphorus, and magnesium are required to build the strong structural components of the skeleton. Information about some of the more important minerals is summarized for you in Table 22-3. Like vitamins, minerals are beneficial only when taken in the proper amounts.

Recommended adequate intakes (AIs) of minerals can change over the life span. For example, calcium intake should increase throughout childhood and remain high throughout adulthood. However, the actual intake of

FIGURE 22-10 Iron intake requirements. The chart compares male and female absorbable iron requirements over the life span.

calcium among females in the United States tends to fall short during adulthood—thereby increasing the risk for osteoporosis and other disorders.

TABLE 22-2 Major Vitamins

VITAMIN

DIETARY SOURCE

FUNCTIONS

SYMPTOMS OF DEFICIENCY

Vitamin A

Green and yellow vegetables, dairy products, and liver

Maintains epithelial tissue and produces visual pigments

Night blindness and flaking skin

B-complex vitamins

B1 (thiamine)

Grains, meat, and legumes

Helps enzymes in the citric acid cycle

Nerve problems (beriberi), heart muscle weakness, and edema

B2 (riboflavin)

Green vegetables, organ meats, eggs, and dairy products

Aids enzymes in the citric acid cycle

Inflammation of skin and eyes

B3 (niacin)

Meat and grains

Helps enzymes in the citric acid cycle

Pellagra (scaly dermatitis and mental disturbances) and nervous disorders

B5 (pantothenic acid)

Organ meat, eggs, and liver

Aids enzymes that connect fat and carbohydrate metabolism

Loss of coordination (rare), decreased gut motility

B6 (pyridoxine)

Vegetables, meats, and grains

Helps enzymes that catabolize amino acids

Convulsions, irritability, and anemia

B9 (folic acid)

Vegetables

Aids enzymes in amino acid catabolism and blood production

Digestive disorders and anemia

B12 (cyanocobalamin)

Meat and dairy products

Involved in blood production and other processes

Pernicious anemia

Biotin (vitamin H)

Vegetables, meat, and eggs

Helps enzymes in amino acid catabolism and fat and glycogen synthesis

Mental and muscle problems (rare)

Vitamin C (ascorbic acid)

Fruits and green vegetables

Helps in manufacture of collagen fibers; antioxidant

Scurvy and degeneration of skin, bone, and blood vessels

Vitamin D (calciferol)

Dairy products and fish liver oil; also made in the body from cholesterol

Aids in calcium absorption

Rickets and skeletal deformity

Vitamin E (tocopherol)

Green vegetables and seeds

Protects cell membranes from being destroyed; antioxidant

Muscle and reproductive disorders (rare)

TABLE 22-3 Major Minerals

MINERAL

DIETARY SOURCE

FUNCTIONS

SYMPTOMS OF DEFICIENCY

Calcium (Ca)

Dairy products, legumes, and vegetables

Helps blood clotting, bone formation, and nerve and muscle function

Bone degeneration and nerve and muscle malfunction

Chlorine (Cl−)

Salty foods

Aids in stomach acid production and acid-base balance

Acid-base imbalance

Cobalt (Co)

Meat

Helps vitamin B12 in blood cell production

Pernicious anemia

Copper (Cu)

Seafood, organ meats, and legumes

Involved in extracting energy from the citric acid cycle and in blood production

Fatigue and anemia

Iodine (I)

Seafood and iodized salt

Required for thyroid hormone synthesis

Goiter (thyroid enlargement) and decrease in metabolic rate

Iron (Fe)

Meat, eggs, vegetables, and legumes

Involved in extracting energy from the citric acid cycle and in blood production

Fatigue and anemia

Magnesium (Mg)

Vegetables and grains

Helps many enzymes

Nerve disorders, blood vessel dilation, and heart rhythm problems

Manganese (Mn)

Vegetables, legumes, and grains

Helps many enzymes

Muscle and nerve disorders

Phosphorus (P)

Dairy products and meat

Aids in bone formation and is used to make ATP, DNA, RNA, and phospholipids

Bone degeneration and metabolic problems

Potassium (K)

Seafood, milk, fruit, and meats

Helps muscle and nerve function

Muscle weakness, heart problems, and nerve problems

Sodium (Na)

Salty foods

Aids in muscle and nerve function and fluid balance

Weakness and digestive upset

Zinc (Zn)

Many foods

Helps many enzymes

Inadequate growth

Figure 22-10 shows the requirement for iron over the life span for both men and women. Although both males and females require a large amount of iron during the spurt of growth in the teenage years, the iron requirement remains high only in women during the rest of adulthood. This difference is explained by the fact that adult women must continually replace the iron lost in the menstrual flow. Notice that female iron requirements drop to the level of males after menopause. Notice also that the iron requirement peaks during pregnancies—when fetal blood development requires large amounts of iron.

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16. What is a vitamin? How do they function in the body?

17. List two functions of minerals in the body.

18. Why should the intake of iron be more important for women during the childbearing years?

METABOLIC RATES

Metabolic rate refers to the amount of energy released in the body in a given time by catabolism. Metabolic rates are expressed in either of two ways: (1) in terms of the number of kilocalories of heat energy expended per hour or per day, or (2) as normal or as a definite percentage above or below normal.

Basal Metabolic Rate

The basal metabolic rate (BMR) is the body's rate of energy expenditure under “basal conditions,” namely, when the individual is tested under the following conditions:

▪Awake but resting, that is, lying down and, as far as possible, not moving a muscle.

▪In the postabsorptive state (12 to 18 hours after the last meal).

▪In a comfortably warm environment (the so-called thermoneutral zone, a temperature range at which metabolism is independent of ambient temperature).

Note that the BMR is not the minimum metabolic rate. It does not indicate the smallest amount of energy that must be expended to sustain life. It does, however, indicate the smallest amount of energy expenditure that can sustain life and also maintain the waking state and a normal body temperature in a comfortably warm environment.

Factors Influencing Basal Metabolic Rate

The BMR is not identical for all individuals because of the influence of various factors, some of which are described in the following paragraphs.

Most individuals have the same BMR per square meter of body surface, if other conditions are equal. However, because a larger individual has more square meters of surface area than does a smaller person, the BMR is greater than that of a smaller individual. Likewise, the higher the ratio of lean tissue to fat tissue in a person, the higher the BMR (Figure 22-11).

Other factors affecting BMR have to do with sex and age. Men oxidize their food approximately 5% to 7% faster than women do. This explains why their BMRs are about 5% to 7% higher for a given size and age. This gender difference in BMR probably results from the difference in the proportion

FIGURE 22-11 Body composition. Estimated values in healthy men and women.

of body fat, which is determined by sex hormones. In general, the younger the individual, the higher the BMR for a given size and sex.

Other factors affecting BMR are fever, drugs, and a person's physiological state. For example, fever increases the BMR. For every degree Celsius increase in body temperature, metabolism increases about 13%. A decrease in body temperature (hypothermia) has the opposite effect. In addition, certain drugs, such as caffeine, amphetamine, and levothyroxine, increase the BMR. Other factors, such as emotions, pregnancy, and lactation (milk production), also influence basal metabolism.

Total Metabolic Rate

Total metabolic rate is the amount of energy used or expended by the body in a given amount of time. It is often expressed in kilocalories per hour or per day. The main direct determinants of total metabolic rate are as follows:

Factor 1—the basal metabolic rate, which usually constitutes about 55% to 60% of the total metabolic rate.

Factor 2—the energy used to do skeletal muscle work.

Factor 3—the thermic effect of foods. The metabolic rate increases for several hours after a meal, apparently because of the energy needed for metabolizing foods.

Energy Balance and Body Weight

Our bodies maintain a state of energy balance, in which the body's energy input equals its energy output. Energy input per day equals the total calories (kilocalories) in the food ingested per day. Energy output equals the total metabolic rate expressed in kilocalories. If calorie intake and energy output are not equal, changes in body weight may occur:

▪Body weight remains constant (except for possible variations in water content) when the total calories in the food ingested equal the total metabolic rate.

▪Body weight increases when energy input exceeds energy output.

▪Body weight decreases when energy input is less than energy output—when the total number of calories in the food eaten is less than the total metabolic rate.

Foods are stored primarily as glycogen and fats. Many cells (except for skeletal muscle) catabolize carbohydrates first, then fats. If there is no food intake, almost all of the glycogen is estimated to be used up in a matter of 1 or 2 days. Then, with no more carbohydrate to act as a fat sparer, fat is catabolized. The amount of fat available determines the length of time that an individual can catabolize fat as a reserve source of energy. Finally, with no more fat available, tissue proteins are catabolized. Because significant amounts of protein are not “stored” for use in catabolism, important structural and functional proteins are quickly depleted. For this reason, severe starvation will eventually lead to death.

MECHANISMS FOR REGULATING FOOD INTAKE

Mechanisms for regulating food intake are still not clearly established, though it is understood that the hypothalamus plays a major role in these mechanisms. A cluster of neurons in the lateral hypothalamus function as an appetite center—meaning that impulses from them bring about increased appetite.

It is likely that a group of neurons in the ventral medial nucleus of the hypothalamus functions as a satiety center—meaning that impulses from these neurons decrease appetite so that we feel satiated or, “full.”

The temperature of the blood circulating to the hypothalamus is important in regulating the action of these centers. Another factor is blood glucose concentration and the rate of glucose use.

The hypothalamus also produces several hormones and neurotransmitters that affect the feeding centers. Some appetite-altering hormones and neurotransmitters are produced in many other organs, such as the liver, adipose tissue, pancreas, GI tract, and vagal nerve. Of course, factors such as daily eating habits or patterns, emotional responses, the sensations of food, and many others must also be involved in regulating or affecting appetite.

19. Give one of the two ways in which metabolic rates can be expressed.

20. Name three of the factors that influence basal metabolic rate.

21. Distinguish between basal metabolic rate and total metabolic rate.

22. In which division of the brain would you find the control center for regulating food intake?

Cycle of LIFE

The importance of proper nutrition to an individual's well-being begins at the moment of conception and continues until death. In the womb, various nutrients must be obtained from the mother's blood in sufficient quantity to ensure normal growth and development.

One critical nutrient during fetal development, infancy, and childhood is protein. Sufficient proteins, containing all the essential amino acids, are required to permit normal development of the nervous system, muscle tissues, and other vital structures.

Another critical nutrient during the early years of life is the mineral calcium. Large quantities of calcium are needed by a growing body to maintain normal development of the skeleton and other tissues. In the womb, a steady supply of calcium in the mother's blood is maintained by increased levels of parathyroid hormone (PTH). PTH increases blood calcium levels by removing it from storage in the bones.

Unless a pregnant woman consumes enough calcium to replace this calcium lost from bones, she may suffer from the bone-softening effects of calcium deficiency. If proteins, calcium, or other necessary nutrients are in short supply anytime before the beginning of adulthood, the consequences may be permanent. For example, bone deformities resulting from a lack of calcium during childhood could become permanent if not corrected or compensated for before the skeleton ossifies completely.

In late adulthood, the number of food calories needed declines because the metabolic rate declines due to changes in the balance of some metabolic hormones. Even though the number of required food calories declines, the overall balance of nutrients consumed must be maintained to preserve proper metabolic function. Some nutrients, such as calcium, may be needed in greater quantity in older adults to compensate for age-related bone loss.

The BIG Picture

Every cell in the body must maintain the operation of its metabolic pathways to ensure its survival. Anabolic pathways are required to build the various structural and functional components of the cells. Catabolic pathways are required to convert energy to a usable form. Catabolic pathways are also needed to degrade large molecules into small subunits that can be used in anabolic pathways. These processes require the correct amounts of carbohydrates, fats, proteins, vitamins, and minerals in order to produce the structural and functional components necessary for cellular metabolism.

Various body systems operate to make sure that essential nutrients reach the cells as needed to maintain metabolism and homeostasis. For example, the nervous, skeletal, and muscular systems make it possible for us to take in complex foods from our external environment. The digestive system reduces these complex nutrients to simpler, more usable nutrients—then provides the mechanisms that allow their absorption into the internal environment. The circulatory and lymphatic systems transport absorbed nutrients to individual cells for immediate use or to the liver or other organs for temporary storage. The endocrine system regulates the balance between immediate use and storage. The respiratory and circulatory systems provide the oxygen needed for oxidative phosphorylation to generate ATP. These two systems also provide a mechanism for removing waste CO2 generated by the catabolism of nutrient molecules. Likewise, the urinary system provides a mechanism for removing waste urea generated by protein catabolism. Even the integumentary system becomes involved, by producing vitamin D in the presence of sunlight.

It should be easy for you to see now why metabolism is simply the sum total of all the biochemical processes required by a living organism!

MECHANISMS OF DISEASE

Nutritional and metabolic disorders are numerous and widespread in the United States. A number of inherited conditions are well known, but not common. Perhaps the most common disorder is obesity, followed by eating disorders such as anorexia nervosa and bulimia, which are well known because of media coverage. Beyond these conditions, however, there are serious nutritional disorders, including protein-calorie malnutrition and a number of vitamin deficiency disorders that were once common in this country and still can be seen in many developing countries around the world.

Find out more about these nutritional and metabolic disorders and diseases online at Mechanisms of Disease: Nutrition and Metabolism.

CHAPTER SUMMARY

To download an MP3 version of the chapter summary for use with your iPod or other portable media player, access the Audio Chapter Summaries online at http://evolve.elsevier.com.

Scan this summary after reading the chapter to help you reinforce the key concepts. Later, use the summary as a quick review before your class or before a test.

OVERVIEW OF NUTRITION AND METABOLISM

A. Nutrition refers to the foods that we eat and the types of nutrients they contain

1. Malnutrition—deficiency or imbalance in the consumption of food, vitamins, and minerals

B. Categories of nutrients

1. Macronutrients—nutrients that we need in large amounts

a. Carbohydrates, fats, and proteins

b. Water

c. Minerals

2. Micronutrients—nutrients that we need in very small amounts

a. Vitamins

b. Minerals (trace elements)

C. Metabolism—complex, interactive set of chemical processes that make life possible; the use the body makes of foods and their nutrients after they have been digested, absorbed, and transported to the cells of our bodies

1. Assimilation—occurs when nutrient molecules enter cells and undergo many chemical changes

2. Two major metabolic processes:

a. Catabolism—breaks food molecules down into smaller molecular compounds; releases energy from them (heat and chemical)

b. Anabolism—a synthesis process

c. Both catabolism and anabolism go on continually and concurrently and take place inside cells

3. ATP supplies energy directly to the energy-using reactions of all cells in all living cellular organisms (Figure 22-2)

CARBOHYDRATES

A. Dietary sources of carbohydrates

1. Polysaccharides—starches found in vegetables, grains, and other plant tissues

2. Cellulose—major component of most plant tissues; passes through our digestive system without being broken down

3. Disaccharides—found in refined sugar; must be chemically digested before they can be absorbed

4. Monosaccharides—found in fruits; move directly into the internal environment without being processed; glucose

B. Carbohydrate metabolism—body metabolizes carbohydrates by both catabolic and anabolic processes

C. Glucose transport and phosphorylation—glucose reacts with ATP to form glucose 6-phosphate

1. Glucose phosphorylation—prepares glucose for further metabolic reactions

2. Phosphorylation—process of adding phosphate group to a molecule

D. Glycolysis—first step in the process of carbohydrate catabolism (Figure 22-3)

1. Occurs in cytoplasm

2. Produces small amount of ATP

3. Prepares glucose for the citric acid cycle

E. Citric acid cycle—series of chemical reactions mediated by enzymes; converts two acetyl molecules to four carbon dioxide and six water molecules

1. Occurs in the mitochondria

2. Most of the energy leaving the citric acid cycle is “stored” in high-energy electrons

F. Electron transport and oxidative phosphorylation

1. High-energy electrons (along with their protons) removed during the citric acid cycle enter a chain of carrier molecules (Figure 22-4)

2. As electrons are transported, some of their energy is used to pump their accompanying protons (H+) to the intramembrane space between the inner and outer membranes of the mitochondrion

3. Protons temporarily store energy; move down their concentration gradient across the inner membrane, driving ATP synthase

4. Low-energy electrons (e−) and their protons (H+) join oxygen, forming water; joining of a phosphate group to ADP to form ATP is called oxidative phosphorylation

G. Control of glucose metabolism—levels of sugar in the blood are under hormone control (Figure 22-8)

1. Hyperglycemic hormones—promote a high blood glucose concentration

2. Hypoglycemic hormones—decrease blood glucose level

LIPIDS

A. Dietary sources of lipids

1. Triglycerides—most common lipid; composed of a glycerol subunit attached to three fatty acids

2. Phospholipids—important lipids; found in nearly all foods

3. Cholesterol—an important lipid; found only in foods of animal origin

4. Dietary fats

a. Saturated—contain fatty acid chains in which there are no double bonds

b. Unsaturated—contain fatty acid chains in which there are double bonds

B. Transport of lipids—lipids are transported in blood as chylomicrons, lipoproteins, and free fatty acids

1. Chylomicrons—small fat droplets found in blood soon after fat absorption

2. Lipoproteins are produced by the liver and consist of lipids and proteins

3. Fatty acids are transported from cells of one tissue to those of another in the form of free fatty acids

C. Lipid metabolism

1. Lipid catabolism—consists of several processes

2. Lipid anabolism (lipogenesis)—consists of the synthesis of various types of lipids; triglycerides, cholesterol, phospholipids, and prostaglandins

D. Control of lipid metabolism—controlled mainly by the following hormones:

1. Insulin

2. ACTH

3. Growth hormone

4. Glucocorticoids

PROTEINS

A. Sources of proteins

1. Proteins are assembled from 20 different kinds of amino acids

2. Body synthesizes amino acids from other compounds in the body

3. Only about half of the required 20 types of amino acids can be made by the body (nonessential amino acids); rest must be supplied through diet (essential amino acids)

B. Protein metabolism

1. Protein anabolism—process by which proteins are synthesized by the ribosomes of the cells

2. Protein catabolism—deamination takes place in liver cells

a. Consists of the splitting off of an amino (NH2) group from an amino acid molecule to form a molecule of ammonia and one of keto acid (e.g., alpha-ketoglutaric acid)

3. Control of protein metabolism—protein metabolism is controlled largely by hormones

VITAMINS AND MINERALS

A. Vitamins—organic molecules needed in small quantities for normal metabolism throughout the body; attach to enzymes or coenzymes (Table 22-2)

1. Coenzymes—organic, nonprotein catalysts that often act as “molecule carriers”

2. The body does not make most of the necessary vitamins; they must be obtained through diet

B. Minerals—inorganic elements or salts that are found naturally in the earth; attach to enzymes or other organic molecules and help them work (Table 22-3)

1. Recommended adequate intakes (AIs) of minerals can change over the life span (Figure 22-10)

METABOLIC RATES

A. Metabolic rate—refers to the amount of energy released in the body in a given time by catabolism

1. Metabolic rates are expressed in either of two ways:

a. In terms of number of kilocalories of heat energy expended per hour or per day

b. As normal or as a definite percentage above or below normal

B. Basal metabolic rate (BMR)—the body's rate of energy expenditure under these “basal conditions”:

1. Awake but resting

2. In the postabsorptive state

3. In a comfortably warm environment

C. Factors influencing basal metabolic rate—BMR is not identical for all individuals because of the influence of various factors

1. Most people have the same BMR per square meter of body surface, but larger people have larger body surface, so BMR is greater

2. Gender differences based on proportions of body fat

3. Other factors are age, fever, drugs, physiological state

D. Total metabolic rate—the amount of energy used or expended by the body in a given amount of time

1. Factor 1—basal metabolic rate

2. Factor 2—energy used to do skeletal muscle work

3. Factor 3—thermic effect of foods

E. Energy balance and body weight—the body maintains a state of energy balance

1. Energy input equals its energy output

a. Body weight remains constant when the total calories in the food ingested equals the total metabolic rate

b. Body weight increases when energy input exceeds energy output

c. Body weight decreases when energy input is less than energy output

d. In starvation, the carbohydrates are used up first, then fats, then proteins

MECHANISMS FOR REGULATING FOOD INTAKE

A. Hypothalamus plays a major role in the mechanism of regulating food intake

1. Appetite center—cluster of neurons in the lateral hypothalamus function to bring about increased appetite

2. Satiety center—impulses from a group of neurons in the ventral medial nucleus of the hypothalamus decrease appetite; we feel satiated, or “full”

REVIEW QUESTIONS

Write out the answers to these questions after reading the chapter and reviewing the Chapter Summary. If you simply think through the answer without writing it down, you won't retain much of your new learning.

1. What is metabolism? Nutrition?

2. What two processes make up the process of metabolism?

3. Does the body digest dietary fiber? Why or why not?

4. Briefly describe glycolysis, the first process of carbohydrate catabolism.

5. Where does glycolysis occur?

6. How are dietary fats classified?

7. Explain how lipids are transported in blood.

8. List the hormones involved in the control of lipid metabolism.

9. What are the essential amino acids?

10. What does the term metabolic rate mean?

11. List the various factors that influence basal metabolic rate.

12. Describe various factors that influence the amount of food a person eats.

CRITICAL THINKING QUESTIONS

After finishing the Review Questions, write out the answers to these items to help you apply your new knowledge. Go back to sections of the chapter that relate to items that you find difficult.

1. How would you describe the mitochondria, and why do you think they are referred to as the “power plants” of the cells?

2. Can you identify and explain the processes and hormones involved in maintaining the homeostatic level of glucose in the blood?

3. State in your own words the process of lipid catabolism. How is it similar to the carbohydrate pathway?

4. Describe protein catabolism in your own words.

5. How would you compare and contrast the functions of proteins, carbohydrates, and fats?

6. What is the function of most vitamins in the body? What examples can you find that do not have this function? What functions do these vitamins have?

7. What is the difference between basal metabolic rate and total metabolic rate?