The term metabolism refers to all of the chemical reactions by which complex molecules taken into an organism are broken down to produce energy and by which energy is used to build up complex molecules. All metabolic reactions fall into one of two general categories: catabolic and anabolic reactions, or the processes of breaking down and building up, respectively. The best example of metabolism from daily life occurs in the process of taking in and digesting nutrients, but sometimes these processes become altered, either through a person's choice or through outside factors, and metabolic disorders follow. Such disorders range from anorexia and bulimia to obesity. These are all examples of an unhealthy, unnatural alteration to the ordinary course of metabolism; on the other hand, hibernation allows animals to slow down their metabolic rates dramatically as a means of conserving energy during times when food is scarce.
HOW IT WORKS
The Body's Furnace
The term metabolism, strangely enough, is related closely to devil, with which it shares the Greek root ballein, meaning "to throw." By adding dia ("through" or "across"), one arrives at devil and many related words, such as diabolical ; on the other hand, the replacement of that prefix with meta ("after" or "beyond") yields the word metabolism. The connection between the two words has been obscured over time, but it might be helpful to picture metabolism in terms of an image that goes with that of a devil: a furnace.
Metabolism is indeed like a furnace, in that it burns energy, and that is the aspect most commonly associated with this concept. But metabolism also involves a function that a furnace does not: building new material. All metabolic reactions can be divided into either catabolic or anabolic reactions. Catabolism is the process by which large molecules are broken down into smaller ones with the release of energy, whereas anabolism is the process by which energy is used to build up complex molecules needed by the body to maintain itself and develop new tissue.
One way to understand the metabolic process is to follow the path of a typical nutrient as it passes through the body. The digestive process is discussed in Digestion, while nutrients are examined in Nutrients and Nutrition as well as in Proteins, Amino Acids, Enzymes, Carbohydrates, and Vitamins. Here we touch on the process only in general terms, as it relates to metabolism.
The term digestion is not defined in the essay on that subject, because it is an everyday word whose meaning is widely known. For the present purposes, however, it is important to identify it as the process of breaking down food into simpler chemical compounds as a means of making nutrients absorbable by the body. This is a catabolic process, because the molecules of which foods are made are much too large to pass through the lining of the digestive system and directly into the bloodstream. Thanks to the digestive process, smaller molecules are formed and enter the bloodstream, from whence they are carried to individual cells throughout a person's body.
The smaller molecules into which nutrients are broken down make up the metabolic pool, which consists of simpler substances. The metabolic pool includes simple sugars, made by the breakdown of complex carbohydrates; glycerol and fatty acids, which come from the conversion of lipids, or fats; and amino acids, formed by the breakdown of proteins. Substances in the metabolic pool provide material from which new tissue is constructed—an anabolic process.
The chemical breakdown of substances in the cells is a complex and wondrous process. For instance, a cell converts a sugar molecule into carbon dioxide and water over the course of about two dozen separate chemical reactions. This is what cell biologists call a metabolic pathway: an orderly sequence of reactions, with particular enzymes (a type of protein that speeds up chemical reactions) acting at each step along the way. In this instance, each chemical reaction makes a relatively modest change in the sugar molecule—for example, the removal of a single oxygen atom or a single hydrogen atom—and each is accompanied by the release of energy, a result of the breaking of chemical bonds between atoms.
Cells capture and store the energy released in catabolic reactions through the use of chemical compounds known as energy carriers. The most significant example of an energy carrier is adenosine triphosphate, or ATP, which is formed when a simpler compound, adenosine diphosphate (ADP), combines with a phosphate group. (A phosphate is a chemical compound that contains oxygen bonded to phosphorus, and the term group in chemistry refers to a combination of atoms from two or more elements that tend to bond with other elements or compounds in certain characteristic ways.)
ADP will combine with a phosphate group only if energy is added to it. In cells, that energy comes from the catabolism of compounds in the metabolic pool, including sugars, glycerol (related to fats), and fatty acids. The ATP molecule formed in this manner has taken up the energy previously stored in the sugar molecule, and thereafter, whenever a cell needs energy for some process, it can obtain it from an ATP molecule. The reverse of this process also takes place inside cells. That is, energy from an ATP molecule can be used to put simpler molecules together to make more complex molecules. For example, suppose that a cell needs to repair a rupture in its cell membrane. To do so, it will need to produce new protein molecules, which are made from hundreds or thousands of amino-acid molecules. These molecules can be obtained from the metabolic pool.
The reactions by which a compound is metabolized differ for various nutrients. Also, energy carriers other than ATP may play a part. For example, the compound known as nicotinamide adenine dinucleotide phosphate (NADPH) also has a role in the catabolism and anabolism of various substances. The general outline described here, however, applies to all metabolic reactions.
Catabolism and Anabolism
Energy released from organic nutrients (those containing carbon and hydrogen) during catabolism is stored within ATP, in the form of the high-energy chemical bonds between the second and third molecules of phosphate. The cell uses ATP for synthesizing cell components from simple precursors, for the mechanical work of contraction and motion, and for transport of substances across its membrane. ATP's energy is released when this bond is broken, turning ATP into ADP. The cell uses the energy derived from catabolism to fuel anabolic reactions that synthesize cell components. Although anabolism and catabolism occur simultaneously in the cell, their rates are controlled independently. Cells separate these pathways because catabolism is a "downhill" process, or one in which energy is released, while anabolism is an "uphill" process requiring the input of energy.
Catabolism and anabolism share an important common sequence of reactions known collectively as the citric acid cycle, the tricarboxylic acid cycle, or the Krebs cycle. Named after the German-born British biochemist Sir Hans Adolf Krebs (1900-1981), the Krebs cycle is a series of chemical reactions in which tissues use carbohydrates, fats, and proteins to produce energy; it is part of a larger series of enzymatic reactions known as oxidative phosphorylation. In the latter reaction, glucose is broken down to release energy, which is stored in the form of ATP—a catabolic sequence. At the same time, other molecules produced by the Krebs cycle are used as precursor molecules for reactions that build proteins, fats, and carbohydrates—an anabolic sequence. (A precursor is a substance, cellular component, or cell from which another substance, cellular component, or cell—different in kind from the precursor—is formed.)
Introduction to Lipids
As noted earlier, many practical aspects of metabolism are discussed elsewhere, particularly in the essays Digestion and Nutrients and Nutrition. Also, two types of chemical compound, proteins and carbohydrates, are so important to a variety of metabolic processes that they are examined in detail within entries of their own. In the present context, let us focus on the third major kind of nutrient, lipids or fats.
Lipids are soluble in nonpolar solvents, which is the reason why a gravy stain or other grease stain is difficult to remove from clothing without a powerful detergent or spot remover. Water molecules are polar, because the opposing electric charges tend to occupy opposite sides or ends of the molecule. In a molecule of oil, whether derived from petroleum or from animal or vegetable fat, electric charges are very small, and are distributed evenly throughout the molecule.
Whereas water molecules tend to bond relatively well, like a bunch of bar magnets attaching to one another at their opposing poles, oil and fat molecules tend not to bond. (The "bond" referred to here is the fairly weak one between molecules. Much stronger is the chemical bond within molecules—a bond that, when broken, brings about a release of energy, as noted earlier.) Their functions are as varied as their structures, but because they are all fat-soluble, lipids share in the ability to approach and even to enter cells. The latter have membranes that, while highly complex in structure, can be identified in simple terms as containing lipids or lipoproteins (lipids attached to proteins). The behavior of lipids and lipid-like molecules, therefore, becomes very important in understanding how a substance may or may not enter a cell. Such a substance may be toxic, as in the case of some pesticides, but if they are lipid-like, they are able to penetrate the cell's membrane. (See Food Webs for more about the biomagnification of DDT.)
In addition to lipoproteins, there are glycolipids, or lipids attached to sugars, as well as lipids attached to alcohols and some to phosphoric acids. The attachment with other compounds greatly alters the behavior of a lipid, often making them bipolar—that is, one end of the molecule is water-soluble. This is important, because it allows lipids to move out of the intestines and into the bloodstream. In the digestive process, lipids are made water-soluble either by being broken down into smaller parts or through association with another substance. The breaking down usually is done via two different processes: hydrolysis, or chemical reaction with water, and saponification. The latter, a reaction in which certain kinds of organic compounds are hydrolyzed to produce an alcohol and a salt, is used in making soap.
Putting Lipids to Use
Derived from living systems of plants, animals, or humans, lipids are essential to good health, not only for humans but also for other animals and even plants. Seeds, for example, contain lipids for the storage of energy. Because fat is a poor conductor of heat, lipids also can function as effective insulators, and for this reason, people living in Arctic zones seek fatty foods such as blubber. Some lipids function as chemical messengers in the body, while others serve as storage areas for chemical energy. There is a good reason why babies are born with "baby fat" and why children entering puberty often tend to become chubby: in both cases, they are building up energy reserves for the great metabolic hurdles that lie ahead, and within a few years, they will have used up those excessive fat stores.
FATS AND OILS.
Fats and oils are both energy-rich compounds that are basic components of the normal diet. Both have essentially the same chemical structure—a mixture of fatty acids combined with glycerol—and are insoluble (do not dissolve) in water. While fats remain solid or at least semisolid at room temperature, however, most oils very quickly become liquid at increased temperatures. Animal fats and oils include butter, lard, tallow, and fish oil. Numerous other oils, such as cottonseed, peanut, and corn oils, are derived from plants.
Fats have two main functions: they provide some of the raw material for synthesizing (creating) and repairing tissues, and they serve as a concentrated source of fuel energy. Fats, in fact, provide humans with roughly twice as much energy, per unit weight, as carbohydrates and proteins. Fats are not only an important source of day-to-day energy, but they also can be stored indefinitely as adipose (fat) tissue in case of future need. Fats also help by transporting fat-soluble vitamins, such as A and D (see Vitamins), throughout the system. They cushion and form protective pads around delicate organs, such as the heart, liver and kidneys, and the layer of fat under the skin helps insulate the body against too much heat loss. They even add to the flavor of foods that might otherwise be inedible.
NOT ALL FAT IS CREATED EQUAL.
Although normal amounts of certain kinds of fat in the diet are essential to good health, unnecessarily high amounts (especially of unhealthy fats) can lead to various problems. Healthy fats include those from fatty fish, such as salmon, mackerel, or tuna, or from fat-containing vegetables, such as the avocado. In addition, many vegetable oils, particularly olive oil, can be beneficial.
Bad fats, on the other hand, are usually ones that have been tampered with through a process known as hydrogenation. This is a term describing any chemical reaction in which hydrogen atoms are added to fill in chemical bonds between carbon and other atoms, but in the case of fatty foods, hydrogenation involves the saturation of hydrocarbons, organic chemical compounds whose molecules are made up of nothing but carbon and hydrogen atoms. When they are treated with hydrogen gas, they become "saturated" with hydrogen atoms. Saturated fats, as they are called, are harder and more stable and stand up better to the heat of frying, which makes them more desirable for use in commercial products. For this reason, many foods contain hydrogenated vegetable oil; however, saturated fats have been linked to a rise in blood cholesterol levels—and to an increased risk of heart disease.
Cholesterol is a variety of lipid, and, like other lipids, some of it is essential—but only some and only of the right kind. Most cholesterol is transported through the blood in low-density lipoproteins, or LDLs, which have been nicknamed bad cholesterol. These lipoproteins are received by LDL receptors on the cell membranes, but if there are more LDLs than LDL receptors, the excess LDLs will be deposited in the arteries. Thus, LDLs are not really "bad" unless there are too many of them. On the other hand, "good" cholesterol (HDLs, or high-density lipoproteins) help protect against damage to the artery walls by carrying excess LDLs back to the liver.
HOW MUCH IS TOO MUCH?
A certain amount of excess adipose tissue can be valuable during periods of illness, overactivity, or food shortages. Too much, however, can be unsightly and also can overwork the heart and put added stress on other parts of the body. High levels of certain circulating fats may lead to atherosclerosis, which is a thickening of the artery walls, and they have been linked to various illnesses, including cancer.
With fat, as with many things where the body is concerned, if a little is a good, this does not mean that a lot is better. In the past, nutritionists considered a diet that obtained 40% of its calories from fats a reasonable one; today, however, they recommend that no more than 30% of all calories (and preferably an even smaller percentage) come from fat. Agreement on this point, however, is far from universal. Some physicians and scientists maintain that dietary fat does not contribute as much to body fat as do carbohydrates. Carbohydrates are good for someone who needs a boost of energy that can be consumed easily by the body, such as an athlete going into competition. But for in active people—and this includes a large portion of Americans—carbohydrates simply are stored as fat.
Experts do not even agree on the answer to a question much simpler than "How much is too much fat in the diet?"—the question "How much is too much fat on the body?" Some doctors classify a person as obese whose weight is at least 20% more than the recommended weight for his or her height, but others say that standard height-and-weight charts are misleading. After all, muscle weighs more than fat, and it is conceivable that a very muscular athlete with very little body fat might qualify as "overweight" compared with the recommended weight for his or her height.
BODY FAT, THE SEXES, AND NATURE.
Because of the complexity of the issue, many experts contend that the proportion of fat to muscle, measured by the skinfold "pinch" test, is a better measure of obesity. (Being obese is not the same as being overweight: the muscular athlete described in the last paragraph is overweight but not obese, a term that implies an excess of body fat.) In healthy adults, fat typically should account for about 18-25% of the body weight in females and 15-20% in males.
The reason for the difference between men and women is that fat naturally accumulates in a woman's buttocks and thighs, because nature "assumes" that she will bear children, in which case such excess fat will be useful. This is why women over the age of about 25 often complain that when they and their husbands or boyfriends embark on a fitness program together, the men usually see results faster. The reason is that there is no genetic or evolutionary benefit to be gained from a man having fat around his waist, which is where men usually gain. If anything—since our genetic codes and makeup have changed little since prehistory—the well-being and propagation of the human species are best served by a lean, muscular male capable of killing animals to feed and protect his family. All of this means, of course, that men should not gloat if they see better results from a regular workout program; instead, they should just recognize that nature is at work in their wives' or girlfriends' bodies as in their own.
Enzymes, as we noted earlier, are critical participants in metabolic reactions. They are like relay runners in a race, in this case a race along the metabolic pathways whereby nutrients are turned into energy or new bodily material. Therefore, if an enzyme is missing or does not function as it should, it can create a serious metabolic disorder. An example is phenylketonuria (PKU), caused by the lack of an enzyme known as phenylalanine hydroxylase. This enzyme is responsible for converting the amino acid phenylalanine to a second amino acid, tyrosine; when this does not happen, phenylalanine builds up in the body. It is converted to a compound called phenylpyruvate, which impairs normal brain development, resulting in severe mental retardation.
Other examples of metabolic disorders include alkaptonuria, thalassemia, porphyria, Tay-Sachs disease, Hurler syndrome, Gaucher disease, galactosemia, Cushing syndrome, diabetes mellitus, hyperthyroidism, and hypothyroidism. Most of these conditions affect a small population; however, diabetes mellitus (discussed in Noninfectious Diseases) is one of the leading killers in America. At present, no cures for metabolic disorders exist. The best approach is to diagnose such conditions as early as possible and then arrange a person's diet to deal as effectively as possible with that disorder.
Eating disorders are a different matter, because they are psychological rather than physiological conditions. No one is sure what causes eating disorders, but researchers think that family dynamics, biochemical abnormalities, and modern American society's preoccupation with thinness all may contribute. Eating disorders are virtually unknown in parts of the world where food is scarce, but in wealthy lands, such as the United States, problems of overeating, self-induced starvation, or forced purging have gained considerable attention.
Anorexia nervosa, bulimia, and obesity are the most well known types of eating disorder. The word anorexia comes from the Greek for "lack of appetite," but the problem for people with anorexia is not that they are not hungry. On the contrary, they are starving, but unlike poor people in the Third World, they are not starving as the result of a shortage of food but because they are denying themselves nutrition. They do this because they fear gaining weight, even when they are so severely underweight that they look like skeletons.
The name of a related condition, bulimia, literally means "hungry as an ox." People with this problem go on eating binges, often gorging on junk food. Then they force their bodies to get rid of the food, either by vomiting or by taking large amounts of laxatives. A third type of eating disorder, obesity, also is characterized by uncontrollable overeating, but in this case the person does not force the body to eject the food that has been consumed. That, at least, makes obesity more healthy than bulimia, but there is nothing healthy about accumulating vast amounts of body fat, as severely obese people do.
ANOREXIA AND BULIMIA.
Young people are more likely than older people to suffer anorexia or bulimia, conditions that typically become apparent at about the age of 20 years. Although both men and women can experience the problem, in fact, only about 5% of people with these eating disorders are male. And though anorexia and bulimia are closely related—particularly inasmuch as they are psychological in origin but can exact a heavy biological toll—there are several important differences.
People who have anorexia or bulemia often come from families with overprotective parents who have unrealistically high expectations of their children. Frequently, high expectations go hand in hand with a wealthy background, and certainly anorexia and bulimia are not conditions that typically affect the poor. Anorexia and bulimia often seem to develop after some stressful experience, such as moving to a new town, changing schools, or going through puberty. Low self-esteem, fear of losing control, and fear of growing up are common characteristics of people with these conditions. Their need for approval manifests in a quest to meet or exceed our culture's idealized concept of extreme thinness. This quest is a part of our popular culture, promoted by waiflike models whose sunken eyes stare out of fashion magazines.
Like anorexia, bulimia results in starvation, but there are behavioral, physical, and psychological differences between the two. Bulimia is both less and more dangerous: on the one hand, people who have it tend to be of normal weight or are overweight, and unlike those with anorexia, they are aware of the fact that they have a problem. On the other hand, because the effects of their behavior are not so readily apparent, it is easier for a person with bulimia to persist in the pattern of bingeing and purging for much longer.
Approximately one in five persons with bulimia has a problem with drug or alcohol use, and they pursue their binges in a way not unlike that of a guilty addict or alcoholic hiding the spent needles or empty bottles from family members. They may go from restaurant to restaurant to avoid being seen eating too much in any one place, or they may pretend to be shopping for a large dinner party when, in fact, they intend to eat all the food themselves. Because of the expense of consuming so much food, some resort to shoplifting.
During a binge, people suffering from bulimia favor high-carbohydrate foods, such as doughnuts, candy, ice cream, soft drinks, cookies, cereal, cake, popcorn, and bread, and they consume many times the number of calories they would normally consume in one day. No matter what their normal eating habits, they tend to eat quickly and messily during a binge, stuffing the food into their mouths and gulping it down, sometimes without even tasting it. Some say they get a feeling of euphoria during binges, similar to the "runner's high" that some people get from exercise. Then, when they have gorged themselves, they force the food back out, either by causing themselves to vomit or by taking large quantities of laxatives.
Regular self-induced vomiting can cause all sorts of physical problems, such as damage to the stomach and esophagus, chronic heartburn, burst blood vessels in the eyes, throat irritation, and erosion of tooth enamel from the acid in vomit. Excessive use of laxatives can induce muscle cramps, stomach pains, digestive problems, dehydration, and even poisoning, while bulimia, in general, brings about vitamin deficiencies and imbalances of critical body fluids, which, in turn, can lead to seizures and kidney failure.
The self-imposed starvation of people with anorexia likewise takes a heavy toll on the body. The skin becomes dry and flaky, muscles begin to waste away, bones stop growing and may become brittle, and the heart weakens. Seeking to protect itself in the absence of proper insulation from fat, the body sprouts downy hair on the face, back, and arms in response to lower body temperature. In women, menstruation stops, and permanent infertility may result. Muscle cramps, dizziness, fatigue, and even brain damage as well as kidney and heart failure are possible. An estimated 10% to 20% of people with anorexia die either as a direct result of starvation or by suicide.
To save people with anorexia, force-feeding may be necessary. Some 70% of anorexia patients who are treated for about six months return to normal body weight, but about 15-20% can be expected to relapse. Bulimia is not as likely as anorexia to reach life-threatening stages, so hospitalization typically is not necessary. Treatment generally calls for psychotherapy and sometimes the administration of antidepressant drugs. Unlike people with anorexia, those with bulimia usually admit they have a problem and want help overcoming it.
Unlike anorexia or bulimia, obesity is more of a problem among people from lower-income backgrounds. This probably relates to a lack of education concerning nutrition, combined with the fact that healthier food is more expensive; by contrast, unhealthy items, such as white sugar, corn meal, and fatty cuts of pork and other meats can fill or overfill a person's stomach inexpensively. In addition, though men and women both tend to gain weight as they age, women are almost twice as likely as men to be obese.
Some cases of obesity relate to metabolic problems, while others stem from compulsive eating, which is psychologically motivated. Some studies suggest that obese people are much more likely than others to eat in response to stress, loneliness, or depression. And just as emotional pain can lead to obesity, obesity can lead to psychological scars. From childhood on, obese people are taunted and shunned, and throughout life they may face discrimination in school and on the job.
Physically, obesity is a killer, especially for those who are morbidly obese—that is, people whose obesity endangers their health. Obesity is a risk factor for diabetes, high blood pressure, arteriosclerosis, angina pectoralis (chest pains due to inadequate blood flow to the heart), varicose veins, cirrhosis of the liver, and kidney disease. Obese people are about 1.5 times more likely to have heart attacks than are other people, and the overall death rate among people ages 20-64 is 50% higher for the obese than for people of ordinary weight.
Having looked at several unnatural ways in which people alter their metabolisms, let us close with an example of a very natural way that animals sometimes temporarily change theirs. This is hibernation, a state of inactivity in which an animal's heart rate, body temperature, and breathing rate are decreased as a way to conserve energy through the cold months of winter. A similar state, known as estivation, is adopted by some desert animals during the dry months of summer.
Hibernation is a technique that animals have developed, as a result of natural selection over the generations (see Evolution), to adapt to harsh environmental conditions. When food is scarce, a nonhibernating animal would be like a business operating at a loss—that is, using more energy maintaining its body temperature and searching for food than it would receive from consuming the food. Hibernating animals use 70-100 times less energy than when they are active, allowing them to survive until food is once again plentiful.
CONTRAST WITH SLEEP.
Many animals sleep more often when food is scarce, but only a few truly hibernate. Bears, which many people think of as the classic hibernating animal, are actually just deep sleepers. By contrast, true hibernation occurs only in small mammals, such as bats and woodchucks and a few birds, among them nighthawks. Some insects also practice a form of hibernation. Hibernation differs from sleep, in that a hibernating animal shows a drastic reduction in metabolism and then awakes relatively slowly, whereas a sleeping animal decreases its metabolism only slightly and can wake up almost instantly if disturbed. Also, hibernating animals do not show periods of rapid eye movement (REM), the stage of sleep associated with dreaming in humans.
THE PROCESS OF HIBERNATION.
Animals prepare for hibernation in the fall by storing food; usually this storage is internal, in the form of fat reserves. A woodchuck in early summer may have only about 5% body fat, but as fall approaches, changes in the animal's brain chemistry cause it to feel hungry and to eat constantly. As a result, the woodchuck's body fat increases to about 15% of its total weight. In other animals, such as the dormouse, fat may constitute as much as 50% of the animal's weight by the time hibernation begins. A short period of fasting follows the feeding frenzy, to ensure that the digestive tract is emptied completely before hibernation begins.
Going into hibernation is a gradual process. Over a period of days, an animal's heart rate and breathing rate drop slowly, eventually reaching rates of just a few beats or breaths per minute. Their body temperatures also drop from levels of about 100°F (38°C) to about 60°F (15°C). The lowered body temperature makes fewer demands on metabolism and food stores. Electric activity in the brain ceases almost completely during hibernation, although some areas—those that respond to external stimuli, such as light, temperature, and noise—remain active. Thus, the hibernating animal can be aroused under extreme conditions.
Periodically—perhaps every two weeks or so—the hibernating animal awakes and takes a few deep breaths to refresh its air supply. If the weather is particularly mild, some animals may venture from their lairs. An increase in heart rate signals that the time for arousal, or ending hibernation, is near. Blood vessels dilate, particularly around the heart, lungs, and brain, and this leads to an increased breathing rate. Eventually, the increase in circulation and metabolic activity spreads throughout the body, and the animal resumes a normal waking state.
WHERE TO LEARN MORE
Bouchard, Claude. Physical Activity and Obesity. Champaign, IL: Human Kinetics, 2000.
"KEGG Metabolic Pathways ." KEGG: Kyoto Encyclopedia of Genes and Genomes—GenomeNet, Bioinformatics Center, Institute for Chemical Research, Kyoto University (Web site). <http://www.genome.ad.jp/kegg/metabolism.html>.
Medline Plus: Food, Nutrition, and Metabolism Topics. Medline, National Library of Medicine, National Insti tutes of Health (Web site). <http://www.nlm.nih.gov/medlineplus/foodnutritionandmetabolism.html>.
Metabolic Pathways of Biochemistry. George Washington University (Web site). <http://www.gwu.edu/~mpb/>.
Metabolism (Web site). <http://www.ultranet.com/~jkimball/BiologyPages/M/Metabolism.html>.
Michal, Gerhard. Biochemical Pathways: An Atlas of Bio chemistry and Molecular Biology. New York: Wiley, 1999.
Pasternak, Charles A. The Molecules Within Us: Our Body in Health and Disease. New York: Plenum, 1998.
Pathophysiology of the Digestive System (Web site). <http://arbl.cvmbs.colostate.edu/hbooks/pathphys/digestion/>.
Spallholz, Julian E. Nutrition, Chemistry, and Biology. Englewood Cliffs, NJ: Prentice-Hall, 1989.
Wolinsky, Ira. Nutrition in Exercise and Sport. 3d ed. Boca Raton, FL: CRC Press, 1998.
Of or relating to animal fat.
Organic compounds made of carbon, hydrogen, oxygen, nitrogen, and (in some cases) sulfur bonded in characteristic formations. Strings of amino acids make up proteins.
The metabolic process by which energy is used to build up complex molecules that the body needs to maintain itself and develop new material.
The smallest particle of an element, consisting of protons, neutrons, and electrons. An atom can exist either alone or in combination with other atoms in a molecule.
Adenosine triphosphate, an energy carrier formed when a simpler compound, adenosine diphosphate (ADP), combines with a phosphate group.
The glucose in the blood.
Naturally occur ring compounds, consisting of carbon, hydrogen, and oxygen, whose primary function in the body is to supply energy. Included in the carbohydrate group are sugars, starches, cellulose, and various other substances. Most carbohydrates are produced by green plants in the process of undergoing photosynthesis.
The metabolic process by which large molecules are broken down into smaller ones with the release of energy. Compare with anabolism.
A substance in which atoms of more than one element are bond ed chemically to one another.
The process of breaking food down into simpler chemical compounds as a means of making the nutrients absorbable by the body or organism.
A protein material that speeds up chemical reactions in the bodies of plants and animals without itself taking part in, or being consumed by, these reactions.
A monosaccharide (sugar) that occurs widely in nature and which is the form in which animals usually receive carbohydrates. Also known as dextrose, grape sugar, and corn sugar. See also blood sugar.
Any organic chemical compound whose molecules are madeup of nothing but carbon and hydrogenatoms.
Fats and oils, which dissolve in oily or fatty substances but not in water-based liquids. In the body, lipids supply energy in slow-release doses, protect organs from shock and damage, and provide insulation for the body, for instance from toxins.
An orderly sequence of reactions, with particular enzymes acting at each step along the way. Metabolic pathways may be either linear or circular, and sometimes they are linked, meaning that the product of one pathway becomes a reactant in another.
A group of relatively simple substances (e.g., amino acids) formed by the breakdown of relatively complex nutrients.
The chemical process by which nutrients are broken down and converted into energy or are used in the construction of new tissue or other material in the body. All metabolic reactions are either catabolic or anabolic.
A group of atoms, usually but not always representing more than one element, joined in a structure. Compounds typically are made up of molecules.
Materials essential to the survival of organisms. They include proteins, carbohydrates, lipids (fats), vitamins, and minerals.
The series of processes by which an organism takes in nutrients and makes use of them for its survival, growth, and development. The term nutrition also can refer to the study of nutrients, their consumption, and their use in the organism's body.
At one time chemists used the term organic only in reference to living things. Now the word is applied to compounds containing carbon and hydrogen.
A group (that is, a combination of atoms from two or more elements that tend to bond with other elements or compounds in certain characteristic ways) that includes a phosphate, or a chemical compound that contains oxygen bonded to phosphorus.
A substance or substances formed from the interaction of reactants in a chemical reaction.
Large molecules built from long chains of 50 or more amino acids. Proteins serve the functions of promoting normal growth, repairing damaged tissue, contributing to the body's immune system, and making enzymes.
A substance that interacts with another substance in a chemical reaction, resulting in the formation of a chemical or chemicals known as the product(s).
One of the three principal types of carbohydrate, along with starches and cellulose. Sugars can be defined as any of various water-soluble carbohydrates of varying sweetness. What we think of as "sugar" (i.e., table sugar) is actually sucrose; "blood sugar," on the other hand, is glucose.
A group of cells, along with the substances that join them, that forms part of the structural materials in plants oranimals.
"Metabolism." Science of Everyday Things. 2002. Encyclopedia.com. (June 28, 2016). http://www.encyclopedia.com/doc/1G2-3408600124.html
"Metabolism." Science of Everyday Things. 2002. Retrieved June 28, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1G2-3408600124.html
Metabolism refers to the physical and chemical processes that occur inside the cells of the body and that maintain life. Metabolism consists of anabolism (the constructive phase) and catabolism (the destructive phase, in which complex materials are broken down). The transformation of the macronutrients carbohydrates , fats, and proteins in food to energy , and other physiological processes are parts of the metabolic process. ATP (adinosene triphosphate) is the major form of energy used for cellular metabolism.
Carbohydrates made up of carbon, hydrogen, and oxygen atoms are classified as mono-, di-, and polysaccharides, depending on the number of sugar units they contain. The monosaccharides—glucose , galactose, and fructose—obtained from the digestion of food are transported from the intestinal mucosa via the portal vein to the liver. They may be utilized directly for energy by all tissues; temporarily stored as glycogen in the liver or in muscle; or converted to fat , amino acids , and other biological compounds.
Carbohydrate metabolism plays an important role in both types of diabetes mellitus. The entry of glucose into most tissues—including heart, muscle, and adipose tissue —is dependent upon the presence of the hormone insulin . Insulin controls the uptake and metabolism of glucose in these cells and plays a major role in regulating the blood glucose concentration. The reactions of carbohydrate metabolism cannot take place without the presence of the B vitamins , which function as coenzymes. Phosphorous, magnesium, iron , copper, manganese, zinc and chromium are also necessary as cofactors.
Carbohydrate metabolism begins with glycolysis , which releases energy from glucose or glycogen to form two molecules of pyruvate, which enter the Krebs cycle (or citric acid cycle), an oxygen-requiring process, through which they are completely oxidized. Before the Krebs cycle can begin, pyruvate loses a carbon dioxide group to form acetyl coenzyme A (acetyl-CoA). This reaction is irreversible and has important metabolic consequences. The conversion of pyruvate to acetyl-CoA requires the B vitamins.
The hydrogen in carbohydrate is carried to the electron transport chain, where the energy is conserved in ATP molecules. Metabolism of one molecule of glucose yields thirty-one molecules of ATP. The energy released from ATP through hydrolysis (a chemical reaction with water) can then be used for biological work.
Only a few cells, such as liver and kidney cells, can produce their own glucose from amino acids, and only liver and muscle cells store glucose in the form of glycogen. Other body cells must obtain glucose from the bloodstream.
Under anaerobic conditions, lactate is formed from pyruvate. This reaction is important in the muscle when energy demands exceed oxygen supply. Glycolysis occurs in the cytosol (fluid portion) of a cell and has a dual role. It degrades monosaccharides to generate energy, and it provides glycerol for triglyceride synthesis. The Krebs cycle and the electron transport chain occur in the mitochondria . Most of the energy derived from carbohydrate, protein, and fat is produced via the Krebs cycle and the electron transport system.
Glycogenesis is the conversion of excess glucose to glycogen. Glycogenolysis is the conversion of glycogen to glucose (which could occur several hours after a meal or overnight) in the liver or, in the absence of glucose-6-phosphate in the muscle, to lactate. Gluconeogenesis is the formation of glucose from noncarbohydrate sources, such as certain amino acids and the glycerol fraction of fats when carbohydrate intake is limited. Liver is the main site for gluconeogenesis, except during starvation, when the kidney becomes important in the process. Disorders of carbohydrate metabolism include diabetes mellitus, lactose intolerance , and galactosemia .
Proteins contain carbon, hydrogen, oxygen, nitrogen , and sometimes other atoms. They form the cellular structural elements, are biochemical catalysts, and are important regulators of gene expression . Nitrogen is essential to the formation of twenty different amino acids, the building blocks of all body cells. Amino acids are characterized by the presence of a terminal carboxyl group and an amino group in the alpha position, and they are connected by peptide bonds.
Digestion breaks protein down to amino acids. If amino acids are in excess of the body's biological requirements, they are metabolized to glycogen or fat and subsequently used for energy metabolism. If amino acids are to be used for energy their carbon skeletons are converted to acetyl CoA, which enters the Krebs cycle for oxidation, producing ATP. The final products of protein catabolism include carbon dioxide, water, ATP, urea, and ammonia.
Vitamin B6 is involved in the metabolism (especially catabolism) of amino acids, as a cofactor in transamination reactions that transfer the nitrogen from one keto acid (an acid containing a keto group [-CO-] in addition to the acid group) to another. This is the last step in the synthesis of nonessential amino acids and the first step in amino acid catabolism. Transamination converts amino acids to L-glutamate, which undergoes oxidative deamination to form ammonia, used for the synthesis of urea. Urea is transferred through the blood to the kidneys and excreted in the urine.
The glucose-alanine cycle is the main pathway by which amino groups from muscle amino acids are transported to the liver for conversion to glucose. The liver is the main site of catabolism for all essential amino acids, except the branched-chain amino acids, which are catabolized mainly by muscle and the kidneys. Plasma amino-acid levels are affected by dietary carbohydrate through the action of insulin, which lowers plasma amino-acid levels (particularly the branched-chain amino acids) by promoting their entry into the muscle.
Body proteins are broken down when dietary supply of energy is inadequate during illness or prolonged starvation. The proteins in the liver are utilized in preference to those of other tissues such as the brain. The gluconeogenesis pathway is present only in liver cells and in certain kidney cells.
Disorders of amino acid metabolism include phenylketonuria , albinism, alkaptonuria, type 1 tyrosinaemia, nonketotic hyperglycinaemia, histidinaemia, homocystinuria, and maple syrup urine disease.
Fat (Lipid) Metabolism
Fats contain mostly carbon and hydrogen, some oxygen, and sometimes other atoms. The three main forms of fat found in food are glycerides (principally triacylglycerol [triglyceride], the form in which fat is stored for fuel), the phospholipids , and the sterols (principally cholesterol ). Fats provide 9 kilocalories per gram (kcal/g), compared with 4 kcal/g for carbohydrate and protein. Triacylglycerol, whether in the form of chylomicrons (microscopic lipid particles) or other lipoproteins , is not taken up directly by any tissue, but must be hydrolyzed outside the cell to fatty acids and glycerol, which can then enter the cell.
Fatty acids come from the diet , adipocytes (fat cells), carbohydrate, and some amino acids. After digestion, most of the fats are carried in the blood as chylomicrons. The main pathways of lipid metabolism are lipolysis, betaoxidation, ketosis , and lipogenesis.
Lipolysis (fat breakdown) and beta-oxidation occurs in the mitochondria. It is a cyclical process in which two carbons are removed from the fatty acid per cycle in the form of acetyl CoA, which proceeds through the Krebs cycle to produce ATP, CO2, and water.
Ketosis occurs when the rate of formation of ketones by the liver is greater than the ability of tissues to oxidize them. It occurs during prolonged starvation and when large amounts of fat are eaten in the absence of carbohydrate.
Lipogenesis occurs in the cytosol. The main sites of triglyceride synthesis are the liver, adipose tissue, and intestinal mucosa. The fatty acids are derived from the hydrolysis of fats, as well as from the synthesis of acetyl CoA through the oxidation of fats, glucose, and some amino acids. Lipogenesis from acetyl CoA also occurs in steps of two carbon atoms. NADPH produced by the pentose-phosphate shunt is required for this process. Phospholipids form the interior and exterior cell membranes and are essential for cell regulatory signals.
Cholesterol is either obtained from the diet or synthesized in a variety of tissues, including the liver, adrenal cortex, skin, intestine, testes, and aorta. High dietary cholesterol suppresses synthesis in the liver but not in other tissues.
Carbohydrate is converted to triglyceride utilizing glycerol phosphate and acetyl CoA obtained from glycolysis. Ketogenic amino acids, which are metabolized to acetyl CoA, may be used for synthesis of triglycerides. The fatty acids cannot fully prevent protein breakdown, because only the glycerol portion of the triglycerides can contribute to gluconeogenesis. Glycerol is only 5 percent of the triglyceride carbon.
Most of the major tissues (e.g., muscle, liver, kidney) are able to convert glucose, fatty acids, and amino acids to acetyl-CoA. However, brain and nervous tissue—in the fed state and in the early stages of starvation—depend almost exclusively on glucose. Not all tissues obtain the major part of their ATP requirements from the Krebs cycle. Red blood cells, tissues of the eye, and the kidney medulla gain most of their energy from the anaerobic conversion of glucose to lactate.
see also Carbohydrates; Fats; Nutrients; Protein.
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Metabolism refers to all of the chemical reactions that take place within an organism by which complex molecules are broken down to produce energy and by which energy is used to build up complex molecules. An example of a metabolic reaction is the one that takes place when a person eats a spoonful of sugar. Once inside the body, sugar molecules are broken down into simpler molecules with the release of energy. That energy is then used by the body for a variety of purposes, such as keeping the body warm and building up new molecules within the body.
All metabolic reactions can be broken down into one of two general categories: catabolic and anabolic reactions. Catabolism is the process by which large molecules are broken down into smaller ones with the release of energy. Anabolism is the process by which energy is used to build up complex molecules needed by the body to maintain itself and develop.
The process of digestion
One way to understand the process of metabolism is to follow the path of a typical nutrient as it passes through the body. A nutrient is any substance that helps an organism stay alive, remain healthy, and grow. Three large categories of nutrients are carbohydrates, proteins, and fats.
Words to Know
Anabolism: The process by which energy is used to build up complex molecules.
ATP (adenosine triphosphate): A molecule used by cells to store energy.
Carbohydrate: A compound consisting of carbon, hydrogen, and oxygen found in plants and used as a food by humans and other animals.
Catabolism: The process by which large molecules are broken down into smaller ones with the release of energy.
Chemical bond: A force of attraction between two atoms.
Enzyme: Chemical compounds that act as catalysts, increasing the rate at which reactions take place in a living organism.
Metabolic pool: The total amount of simple molecules formed by the breakdown of nutrients.
Nutrient: A substance that helps an organism stay alive, remain healthy, and grow.
Protein: Large molecules that are essential to the structure and functioning of all living cells.
Assume, for example, that a person has just eaten a piece of bread. An important nutrient in that bread is starch, a complex carbohydrate. As soon as the bread enters a person's mouth, digestion begins to occur. Enzymes in the mouth start to break down molecules of starch and convert them into smaller molecules of simpler substances: sugars. This process can be observed easily, since anyone who holds a piece of bread in his or her mouth for a period of time begins to recognize a sweet taste, the taste of the sugar formed from the breakdown of starch.
Digestion is a necessary first step for all foods. The molecules of which foods are made are too large to pass through the lining of the digestive system. Digestion results in the formation of smaller molecules that are able to pass through that lining and enter the person's bloodstream. Sugar molecules formed by the digestion of starch enter the bloodstream. Then they are carried to individual cells throughout a person's body.
The smaller molecules into which nutrients are broken down make up the metabolic pool. The metabolic pool consists of the simpler substances formed by the breakdown of nutrients. It includes simple sugars (formed by the breakdown of complex carbohydrates), glycerol and fatty acids (formed by the breakdown of lipids), and amino acids (formed by the breakdown of proteins). Cells use substances in the metabolic pool as building materials, just as a carpenter uses wood, nails, glue, staples, and other materials for the construction of a house. The difference is, of course, that cells construct body parts, not houses, from the materials with which they have to work.
Substances that make up the metabolic pool are transported to individual cells by the bloodstream. They pass through cell membranes and enter the cell interior. Once inside a cell, a compound undergoes further metabolism, usually in a series of chemical reactions. For example, a sugar molecule is broken down inside a cell into carbon dioxide and water, with the release of energy. But that process does not occur in a single step. Instead, it takes about two dozen separate chemical reactions to convert the sugar molecule to its final products. Each chemical reaction involves a relatively modest change in the sugar molecule, the removal of a single oxygen atom or a single hydrogen atom, for example.
The purpose of these reactions is to release energy stored in the sugar molecule. To explain that process, one must know that a sugar molecule consists of carbon, hydrogen, and oxygen atoms held together by means of chemical bonds. A chemical bond is a force of attraction between two atoms. That force of attraction is a form of energy. A sugar molecule with two dozen chemical bonds can be thought of as containing two dozen tiny units of energy. Each time a chemical bond is broken, one unit of energy is set free.
Cells have evolved remarkable methods for capturing and storing the energy released in catabolic reactions. Those methods make use of very special chemical compounds, known as energy carriers. An example of such compounds is adenosine triphosphate, generally known as ATP. ATP is formed when a simpler compound, adenosine diphosphate (ADP), combines with a phosphate group. The following equation represents that change:
ADP + P → ATP
ADP will combine with a phosphate group, as shown here, only if energy is added to it. In cells, that energy comes from the catabolism of compounds in the metabolic pool, such as sugars, glycerol, and fatty acids. In other words:
catabolism: sugar → carbon dioxide + water + energy;
energy from catabolism + ADP + P → ATP
The ATP molecule formed in this way, then, has taken up the energy previously stored in the sugar molecule. Whenever a cell needs energy for some process, it can obtain it from an ATP molecule.
The reverse of the process shown above also takes place inside cells. That is, energy from an ATP molecule can be used to put simpler molecules together to make more complex molecules. For example, suppose that a cell needs to repair a break in its cell wall. To do so, it will need to produce new protein molecules. Those protein molecules can be made from amino acids in the metabolic pool. A protein molecule consists of hundreds or thousands of amino acid molecules joined to each other:
Amino acid 1 + amino acid 2 + amino acid 3 + (and so on) → a protein
The energy needed to form all the new chemical bonds needed to hold the amino acid units together comes from ATP molecules. In other words:
energy from ATP + many amino acids → protein molecule
The reactions by which a compound is metabolized differ for various nutrients. Also, energy carriers other than ATP may be involved. For example, the compound known as nicotinamide adenine dinucleotide phosphate (NADPH) is also involved in the catabolism and anabolism of various substances. The general outline shown above, however, applies to all metabolic reactions.
"Metabolism." UXL Encyclopedia of Science. 2002. Encyclopedia.com. (June 28, 2016). http://www.encyclopedia.com/doc/1G2-3438100422.html
"Metabolism." UXL Encyclopedia of Science. 2002. Retrieved June 28, 2016 from Encyclopedia.com: http://www.encyclopedia.com/doc/1G2-3438100422.html
Metabolism is the sum total of chemical changes that occur in living organisms and which are fundamental to life. All prokaryotic and eukaryotic cells are metabolically active. The sole exception is viruses , but even viruses require a metabolically active host for their replication.
Metabolism involves the use of compounds. Nutrients from the environment are used in two ways by microorganisms . They can be the building blocks of various components of the microorganism (assimilation or anabolism). Or, nutrients can be degraded to yield energy (dissimilation or catabolism). Some so-called amphibolic biochemical pathways can serve both purposes. The continual processes of breakdown and re-synthesis are in a balance that is referred to as turnover. Metabolism is an open system. That is, there are constant inputs and outputs. A chain of metabolic reactions is said to be in a steady state when the concentration of all intermediates remains constant, despite the net flow of material through the system. That means the concentration of intermediates remains constant, while a product is formed at the expense of the substrate.
Primary metabolism comprises those metabolic processes that are basically similar in all living cells and are necessary for cellular maintenance and survival. They include the fundamental processes of growth (e.g., the synthesis of biopolymers and the macromolecular structures of cells and organelles), energy production (glycolysis and the tricarboxylic acid cycle) and the turnover of cell constituents. Secondary metabolism refers to the production of substances, such as bile pigments from porphyrins in humans, which only occur in certain eukaryotic tissues and are distinct from the primary metabolic pathways.
Metabolic control processes that occur inside cells include regulation of gene expression and metabolic feedback or feed-forward processes. The triggers of differential gene expression may be chemical, physical (e.g., bacterial cell density), or environmental (e.g., light). Differential gene expression is responsible for the regulation, at the molecular level, of differentiation and development, as well as the maintenance of numerous cellular "house-keeping" reactions, which are essential for the day-to-day functioning of a microorganism. In many metabolic pathways, the metabolites (substances produced or consumed by metabolism) themselves can act directly as signals in the control of their own breakdown and synthesis. Feedback control can be negative or positive. Negative feedback results in the inhibition by an end product, of the activity or synthesis of an enzyme or several enzymes in a reaction chain. The inhibition of the synthesis of enzymes is called enzyme repression. Inhibition of the activity of an enzyme by an end product is an allosteric effect and this type of feedback control is well known in many metabolic pathways (e.g., lactose operon ). In positive feedback, an endproduct activates an enzyme responsible for its own production.
Many reactions in metabolism are cyclic. A metabolic cycle is a catalytic series of reactions, in which the product of one bimolecular (involving two molecules) reaction is regenerated as follows: A + B → C + A. Thus, A acts catalytically and is required only in small amounts and A can be regarded as carrier of B. The catalytic function of A and other members of the metabolic cycle ensure economic conversion of B to C. B is the substrate of the metabolic cycle and C is the product. If intermediates are withdrawn from the metabolic cycle, e.g., for biosynthesis, the stationary concentrations of the metabolic cycle intermediates must be maintained by synthesis. Replenishment of depleted metabolic cycle intermediates is called anaplerosis. Anaplerosis may be served by a single reaction, which converts a common metabolite into an intermediate of the metabolic cycle. An example of this is pyruvate to oxaloacetate reaction in the tricarboxylic acid cycle. Alternatively, it may involve a metabolic sequence of reactions, i.e., an anaplerotic sequence. An example of this is the glycerate pathway which provides phosphoenol pyruvate for anaplerosis of the tricarboxylic acid cycle.
Prokaryotes exhibit a great diversity of metabolic options, even in a single organism. For example, Escherichia coli can produce energy by respiration or fermentation . Respiration can be under aerobic conditions (e.g., using O2 as the final electron acceptor) or anaerobically (e.g., using something other than oxygen as the final electron acceptor). Compounds like lactose or glucose can be used as the only source of carbon. Other bacteria have other metabolic capabilities including the use of sunlight for energy.
Some of these mechanisms are also utilized by eukaryotic cells. In addition, prokaryotes have a number of energy-generating mechanisms that do not exist in eukaryotic cells. Prokaryotic fermentation can be uniquely done via the phosphoketolase and Enter-Doudoroff pathways. Anaerobic respiration is unique to prokaryotes, as is the use of inorganic compounds as energy sources or as carbon sources during bacterial photosynthesis . Archaebacteria possess metabolic pathways that use H2 as the energy source with the production of methane, and a nonphotosynthetic metabolism that can convert light energy into chemical energy.
In bacteria, metabolic processes are coupled to the synthesis of adenosine triphosphate (ATP), the principle fuel source of the cell, through a series of membrane-bound proteins that constituent the electron transport system . The movement of protons from the inside to the outside of the membrane during the operation of the electron transport system can be used to drive many processes in a bacterium, such as the movement of the flagella used to power the bacterium along, and the synthesis of ATP in the process called oxidative phosphorylation.
The fermentative metabolism that is unique to some bacteria is evolutionarily ancient. This is consistent with the early appearance of bacteria on Earth, relative to eukaryotic organisms. But bacteria can also ferment sugars in the same way that brewing yeast (i.e., Saccharomyces cerevesiae ferment sugars to produce ethanol and carbon dioxide. This fermentation, via the so-called Embden Myerhoff pathway, can lead to different ends products in bacteria, such as lactic acid (e.g., Lactobacillus ), a mixture of acids (Enterobacteriacaeae, butanediol (e.g., Klebsiella, and propionic acid (e.g., Propionibacterium ).
See also Bacterial growth and division; Biochemistry
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Kurokawa (1972, 1977, 1992);
Jane Turner (1996)
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1. the sum of all the chemical and physical changes that take place within the body and enable its continued growth and functioning. Metabolism involves the breakdown of complex organic constituents of the body (see catabolism) and the building up of complex substances from simple ones (see anabolism). See also basal metabolism.
2. the sum of the biochemical changes undergone by a particular constituent of the body.
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me·tab·o·lism / məˈtabəˌlizəm/ • n. the chemical processes that occur within a living organism in order to maintain life. DERIVATIVES: met·a·bol·ic / ˈmetəˈbälik/ adj. met·a·bol·i·cal·ly / ˌmetəˈbälik(ə)lē/ adv.
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