Carbohydrates

Carbohydrates are one of three macronutrients that provide the body with energy (protein and fats being the other two). The chemical compounds in carbohydrates are found in both simple and complex forms, and in order for the body to use carbohydrates for energy, food must undergo digestion, absorption , and glycolysis . It is recommended that 55 to 60 percent of caloric intake come from carbohydrates.

Chemical Structure

Carbohydrates are a main source of energy for the body and are made of carbon, hydrogen, and oxygen . Chlorophyll in plants absorbs light energy from the sun. This energy is used in the process of photosynthesis, which allows green plants to take in carbon dioxide and release oxygen and allows for the production of carbohydrates. This process converts the sun's light energy into a form of chemical energy useful to humans. Plants transform carbon dioxide (CO₂) from the air, water (H₂O) from the ground, and energy from the sun into oxygen (O₂) and carbohydrates (C₆H₁₂O₆) (6 CO₂ + 6 H₂O + energy = C₆H₁₂O₆ + 6 O₂). Most carbohydrates have a ratio of 1:2:1 of carbon, hydrogen, and oxygen, respectively.

Humans and other animals obtain carbohydrates by eating foods that contain them. In order to use the energy contained in the carbohydrates, humans must metabolize , or break down, the structure of the molecule in a process that is opposite that of photosynthesis. It starts with the carbohydrate and oxygen and produces carbon dioxide, water, and energy. The body utilizes the energy and water and rids itself of the carbon dioxide.

Simple Carbohydrates

Simple carbohydrates, or simple sugars, are composed of monosaccharide or disaccharide units. Common monosaccharides (carbohydrates composed of single sugar units) include glucose , fructose, and galactose. Glucose is the most common type of sugar and the primary form of sugar that is stored in the body for energy. It sometimes is referred to as blood sugar or dextrose and is of particular importance to individuals who have diabetes or hypoglycemia . Fructose, the primary sugar found in fruits, also is found in honey and high-fructose corn syrup (in soft drinks) and is a major source of sugar in the diet of Americans. Galactose is less likely than glucose or fructose to be found in nature. Instead, it often combines with glucose to form the disaccharide lactose, often referred to as milk sugar. Both fructose and galactose are metabolized to glucose for use by the body.

Oligosaccharides are carbohydrates made of two to ten monosaccharides. Those composed of two sugars are specifically referred to as disaccharides, or double sugars. They contain two monosaccharides bound by either an alpha bond or a beta bond. Alpha bonds are digestible by the human body, whereas beta bonds are more difficult for the body to break down.

There are three particularly important disaccharides: sucrose , maltose, and lactose. Sucrose is formed when glucose and fructose are held together by an alpha bond. It is found in sugar cane or sugar beets and is refined to make granulated table sugar. Varying the degree of purification alters the

final product, but white, brown, and powdered sugars all are forms of sucrose. Maltose, or malt sugar, is composed of two glucose units linked by an alpha bond. It is produced from the chemical decomposition of starch, which occurs during the germination of seeds and the production of alcohol. Lactose is a combination of glucose and galactose. Because it contains a beta bond, it is hard for some individuals to digest in large quantities. Effective digestion requires sufficient amounts of the enzyme lactase.

Complex Carbohydrates

Complex carbohydrates, or polysaccharides, are composed of simple sugar units in long chains called polymers. Three polysaccharides are of particular importance in human nutrition : starch, glycogen , and dietary fiber .

Starch and glycogen are digestible forms of complex carbohydrates made of strands of glucose units linked by alpha bonds. Starch, often contained in seeds, is the form in which plants store energy, and there are two types: amylose and amylopectin. Starch represents the main type of digestible complex carbohydrate. Humans use an enzyme to break down the bonds linking glucose units, thereby releasing the sugar to be absorbed into the bloodstream. At that point, the body can distribute glucose to areas that need energy, or it can store the glucose in the form of glycogen.

Glycogen is the polysaccharide used to store energy in animals, including humans. Like starch, glycogen is made up of chains of glucose linked by alpha bonds; but glycogen chains are more highly branched than starch. It is this highly branched structure that allows the bonds to be more quickly broken down by enzymes in the body. The primary storage sites for glycogen in the human body are the liver and the muscles.

Another type of complex carbohydrate is dietary fiber. In general, dietary fiber is considered to be polysaccharides that have not been digested at the point of entry into the large intestine. Fiber contains sugars linked by bonds that cannot be broken down by human enzymes, and are therefore labeled as indigestible. Because of this, most fibers do not provide energy for the body. Fiber is derived from plant sources and contains polysaccharides such as cellulose , hemicellulose, pectin, gums, mucilages, and lignins.

The indigestible fibers cellulose, hemicellulose, and lignin make up the structural part of plants and are classified as insoluble fiber because they usually do not dissolve in water. Cellulose is a nonstarch carbohydrate polymer made of a straight chain of glucose molecules linked by beta bonds and can be found in whole-wheat flour, bran, and vegetables. Hemicellulose is a nonstarch carbohydrate polymer made of glucose, galactose, xylose, and other monosaccharides; it can be found in bran and whole grains. Lignin, a noncarbohydrate polymer containing alcohols and acids, is a woody fiber found in wheat bran and the seeds of fruits and vegetables.

In contrast, pectins, mucilages, and gums are classified as soluble fibers because they dissolve or swell in water. They are not broken down by human enzymes, but instead can be metabolized (or fermented) by bacteria present in the large intestine. Pectin is a fiber made of galacturonic acid and other monosaccharides. Because it absorbs water and forms a gel, it is often used in jams and jellies. Sources of pectin include citrus fruits, apples, strawberries, and carrots. Mucilages and gums are similar in structure. Mucilages are dietary fibers that contain galactose, manose, and other monosaccharides; and gums are dietary fibers that contain galactose, glucuronic acid, and other monosaccharides. Sources of gums include oats, legumes , guar, and barley.

Digestion and Absorption

Carbohydrates must be digested and absorbed in order to transform them into energy that can be used by the body. Food preparation often aids in the digestion process. When starches are heated, they swell and become easier for the body to break down. In the mouth, the enzyme amylase, which is contained in saliva, mixes with food products and breaks some starches into smaller units. However, once the carbohydrates reach the acidic environment of the stomach, the amylase is inactivated. After the carbohydrates have passed through the stomach and into the small intestine, key digestive enzymes are secreted from the pancreas and the small intestine where most digestion and absorption occurs. Pancreatic amylase breaks starch into disaccharides and small polysaccharides, and enzymes from the cells of the small-intestinal wall break any remaining disaccharides into their monosaccharide components. Dietary fiber is not digested by the small intestine; instead, it passes to the colon unchanged.

Sugars such as galactose, glucose, and fructose that are found naturally in foods or are produced by the breakdown of polysaccharides enter into absorptive intestinal cells. After absorption, they are transported to the liver where galactose and fructose are converted to glucose and released into the bloodstream. The glucose may be sent directly to organs that need energy, it may be transformed into glycogen (in a process called glycogenesis) for storage in the liver or muscles, or it may be converted to and stored as fat.

Glycolysis

The molecular bonds in food products do not yield high amounts of energy when broken down. Therefore, the energy contained in food is released within cells and stored in the form of adenosine triphosphate (ATP), a high-energy compound created by cellular energy-production systems. Carbohydrates are metabolized and used to produce ATP molecules through a process called glycolysis.

Glycolysis breaks down glucose or glycogen into pyruvic acid through enzymatic reactions within the cytoplasm of the cells. The process results in the formation of three molecules of ATP (two, if the starting product was glucose). Without the presence of oxygen, pyruvic acid is changed to lactic acid , and the energy-production process ends. However, in the presence of oxygen, larger amounts of ATP can be produced. In that situation, pyruvic acid is transformed into a chemical compound called acetyle coenzyme A, a compound that begins a complex series of reactions in the Krebs Cycle and the electron transport system. The end result is a net gain of up to thirty-nine molecules of ATP from one molecule of glycogen (thirty-eight molecules of ATP if glucose was used). Thus, through certain systems, glucose can be used very efficiently in the production of energy for the body.

Recommended Intake

At times, carbohydrates have been incorrectly labeled as "fattening." Evidence actually supports the consumption of more, rather than less, starchy foods. Carbohydrates have four calories per gram, while dietary fats contribute nine per gram, so diets high in complex carbohydrates are likely to provide fewer calories than diets high in fat. Recommendations are for 55 to 60 percent of total calories to come from carbohydrates (approximately 275 to 300 grams for a 2,000-calorie diet). The majority of carbohydrate calories should come from complex rather than simple carbohydrates. Of total caloric intake, approximately 45 to 50 percent of calories should be from complex carbohydrates, and 10 percent or less from simple carbohydrates.

Low-Carb Diets

Low-carbohydrate diets, such as the Atkins and South Beach diets, are based on the proposition that it's not fat that makes you fat. Allowing dieters to eat steak, butter, eggs, bacon, and other high-fat foods, these diets instead outlaw starches and refined carbohydrates on the theory that they are metabolized so quickly that they lead to hunger and overeating. This theory, which was first popularized in the nineteenth century, came under scathing criticism from the medical establishment during the early 1970s when Dr. Robert Atkins published the phenomenally popular low-carb diet bearing his name. According to the American Medical Association (AMA), the Atkins diet was a "bizarre regimen" that advocated "an unlimited intake of saturated fats and cholesterol-rich foods" and therefore presented a considerable risk of heart disease. Most doctors recommended instead a diet low in fat and high in carbohydrates, with plenty of grains, fruits, and vegetables and limited red meat or dairy products. This became the received wisdom during the 1980s, at the same time that the U.S. waistline began to expand precipitously. As dieters found that weight loss was difficult to maintain on a low-fat diet, low-carb diets regained popularity—with as many as 30 million people trying a low-carb diet in 2003. Several small-scale studies began to suggest that a low-carb diet may indeed be effective and may not have the deleterious effects its detractors have claimed; other research found that any benefits of a low-carb diet are short-lived, and that the negative effects will take decades to become evident. The National Institutes of Health has pledged $2.5 million for a five-year study of the Atkins diet with 360 subjects. While the results of this and other large-scale studies are awaited, many researchers stress that the key issue in maintaining a healthy weight is the number of calories consumed, not the type of calories. The National Academy of Sciences recommends that adults obtain 45 to 65 percent of their calories from carbohydrates, 20 to 35 percent from fat, and 10 to 35 percent from protein.

—Paula Kepos

It is important to consume a minimum amount of carbohydrates to prevent ketosis , a condition resulting from the breakdown of fat for energy in the absence of carbohydrates. In this situation, products of fat breakdown, called ketone bodies, build up in the blood and alter normal pH balance. This can be particularly harmful to a fetus. To avoid ketosis, daily carbohydrate intake should include a minimum of 50 to 100 grams. In terms of dietary fiber, a minimum intake of 20 to 35 grams per day is recommended.

Exchange System

The exchange system is composed of lists that describe carbohydrate, fat, and protein content, as well as caloric content, for designated portions of specific foods. This system takes into account the presence of more than one type of nutrient in any given food. Exchange lists are especially useful for individuals who require careful diet planning, such as those who monitor intake of calories or certain nutrients. It is particularly useful for diabetics, for whom carbohydrate intake must be carefully controlled, and was originally developed for planning diabetic diets.

Diabetes, Carbohydrate-Modified Diets, and Carbohydrate Counting

Diabetes is a condition that alters the way the body handles carbohydrates. In terms of diet modifications, diabetics can control blood sugar levels by appropriately managing the carbohydrates, proteins, and fats in their meals. The amount of carbohydrates, not necessarily the source, is the primary issue. Blood glucose levels after a meal can be related to the process of food preparation, the amount of food eaten, fat intake, sugar absorption, and the combination of foods in the meal or snack.

One method of monitoring carbohydrate levels—carbohydrate counting—assigns a certain number of carbohydrate grams or exchanges to specific foods. Calculations are used to determine insulin need, resulting in better control of blood glucose levels with a larger variety of foods. Overall, diabetic diets can include moderate amounts of sugar, as long as they are carefully monitored.

see also Diabetes Mellitus; Fats; Nutrients; Protein; Weight Loss Diets.

Catherine N. Rasberry

Bibliography

Bounds, Laura E.; Agnor, Dottiedee; Darnell, Gayden S.; and Shea, Kirstin Brekken (2003). Health and Fitness: A Guide to a Healthy Lifestyle, 2nd edition. Dubuque, IA: Kendall/Hunt.

Duyff, Roberta Larson (2002). American Dietetic Association: Complete Food and Nutrition Guide, 2nd edition. Hoboken, NJ: John Wiley.

Mahan, L. Kathleen, and Escott-Stump, Sylvia (2000). Krause's Food, Nutrition, and Diet Therapy, 10th edition. Philadelphia: W. B. Saunders.

Robbins, Gwen; Powers, Debbie; and Burgess, Sharon (2002). A Wellness Way of Life, 5th edition. New York: McGraw-Hill.

Sizer, Frances, and Whitney, Eleanor (1997). Nutrition: Concepts and Controversies, 7th edition. Belmont, CA: Wadsworth Publishing.

Wardlaw, Gordon M., and Kessel, Margaret (2002). Perspectives in Nutrition, 5th edition. New York: McGraw-Hill.

Wilmore, Jack H., and Costill, David L. (1999). Physiology of Sport and Exercise, 2nd edition. Champaign, IL: Human Kinetics.

Internet Resources

American Diabetes Association. <http://www.diabetes.org>

American Dietetic Association. <http://www.eatright.org>

Carpi, Anthony. "Carbohydrates." Visionlearning. Available from <http://www.visionlearning.com>

Kennedy, Ron. "Carbohydrates in Nutrition." Doctor's Medical Library. Available from <http://www.medical-library.net/sites>

Northwestern University, Department of Preventive Medicine. "Nutrition Fact Sheets: Carbohydrates." Available from <http://www.feinberg.northwestern.edu/nutrition>

CARBOHYDRATES

CARBOHYDRATES. Plants manufacture and store carbohydrates as their main source of energy through photosynthesis. Once consumed, these organic compounds can be digested, absorbed, and metabolized, supplying humans or animals with energy. Carbohydrates provide roughly half of the total caloric intake of the average human diet. These calories may be used immediately for energy metabolism or may be transformed and stored as glycogen or fat to be used as an energy source as demanded. Dietary carbohydrates are comprised of a wide array of compounds ranging from the simple oneor two-unit sugars to the long chain starches, glycogen and cellulose. Carbohydrates can be classified as monosaccharides, di-and oligosaccharides, and polysaccharides.

Monosaccharides, often referred to as simple sugars, are the simplest form of carbohydrates and are seldom found free in nature. The three that can be absorbed by the human body include glucose, galactose, and fructose. Glucose is the most abundant of the monosaccharides and the most important nutritionally. It is the repeating monosaccharide unit in starch, glycogen, and cellulose, and is found in all edible disaccharides.

Oligosaccharides are short chains of monosaccharide units that are joined by glycosidic bonds. They generally have between two to ten units, with the disaccharides, those chains containing two units, being the most abundant. The most common disaccharides include:

Sucrose (from table, cane, and beet sugars), consisting of glucose and fructose

Lactose (from milk sugar), consisting of glucose and galactose

Maltose (from malt sugar), consisting of two glucose units

Polysaccharides are long chains of monosaccharide units. The major polysaccharides include the digestible forms (glycogen and starch) and nondigestible forms (cellulose, hemicellulose, lignin, pectin, and gums).

Starch is the most common digestible polysaccharide found in plants. It can be found in two forms—amylose and amylopectin. Amylose is a linear, unbranched molecule that is bound solely by a-1,4 glycosidic bonds. Amylopectin, which makes up the greatest percent of the total starch content, is branched with a-1,6 bonds at the branch points.

Glycogen is the major storage form of carbohydrates in animals, found primarily in the liver and skeletal muscle. When energy intake exceeds energy expenditure, excess calories from fat, protein, and carbohydrate can be used to form glycogen. It is made up of repeating glucose units and is highly branched. During times of fasting or in between meals, these chains can be broken down to single glucose units and used as an energy source for the body. Although found in animal tissue, animal products do not contain large amounts of glycogen because it is depleted at the time of slaughter due to stress hormones.

Cellulose is the major component of cell walls in plants. Just as starch and glycogen, it too is made up of repeating glucose molecules. However, the glycosidic bonds connecting the units are b-1,4. These bonds are resistant to mammalian digestive enzymes rendering cellulose, and other substances containing these bonds, indigestible. Thus, cellulose is not considered to be a significant source of energy for the body. However, as a fiber, it is important for intestinal bacteria.

Since cellulose is a major part of the plant cell wall, it also encases some of the starch, preventing the digestive enzymes from reaching it and decreasing the digestibility of some raw foods such as potatoes and grains. Cooking causes the granules to swell and also softens and ruptures the cellulose wall, allowing the starch to be digested.

Dietary Fiber

Fiber can be classified as soluble and insoluble. Soluble fiber, which includes pectin and gums, dissolves in water to form a gel in the digestive tract. This increases the time the food is in the small intestine, thus increasing the chance of nutrients being absorbed. It is believed that soluble fiber plays a role in lowering blood LDL cholesterol. This could be due to the binding and increased excretion of fat and bile acid (a derivative of cholesterol) or other mechanisms not yet understood. Bacteria in the bowel can use fiber as a food source. These bacteria can degrade the fiber and release some components that can then be absorbed and used by the body. The increased nutrition for the bacteria can increase microbial growth, which can then lead to increased stool bulk, with little of the fiber actually found in the stool.

Insoluble fiber, including cellulose, hemicellulose, and lignin (a noncarbohydrate component of the cell wall that is often included as dietary fiber), absorbs water, thereby increasing the bulk and volume of the stool. It helps to speed the movement through the intestinal tract, preventing constipation, and is prescribed in the treatment of irritable bowel syndrome. It has also been shown that insoluble fibers bind fat-soluble carcinogens and remove them from the gastrointestinal tract, helping to decrease cancer risk.

Refined and processed foods have not only most of the fiber removed, but along with it many of the vitamins, minerals, and phytochemicals (chemicals found in plants believed to contain protective properties) that contribute to the health benefits of whole grain foods. The federal government's Dietary Guidelines for Americans encourage individuals to include whole grain foods in their diet to ensure adequate fiber to promote proper bowel function, as well as to receive other added health benefits.

Digestion, Absorption, and Transportation

In order for carbohydrates to be absorbed by the intestinal mucosal cells, they must first be converted into monosaccharides. The digestive process begins in the mouth with salivary a-amylase that partially breaks down starch by hydrolyzing some of the a-1,4 bonds. However, the digestion that takes place here is of little significance since food remains in the mouth for only a brief period, although this may differ depending on chewing time. The enzyme continues to work for a short time in the stomach until the pH is lowered due to hydrochloric acid that inhibits the enzyme.

The bulk of carbohydrate digestion occurs in the small intestine by pancreatic a-amylase. The pH of the small intestines is increased due to the addition of bicarbonate and bile, allowing the enzyme activity to occur. Specific disaccharidases located on the intestinal mucosal cells help to further break down the carbohydrates into the monosaccharides: glucose, fructose, and galactose.

Once the carbohydrates have been broken down, the monosaccharides can be absorbed by the mucosal cells. Glucose and galactose enter by active transport, which requires energy as well as specific receptors and carriers. Fructose is absorbed by facilitated diffusion. Like active transport, facilitated diffusion requires a specific carrier, but instead of needing energy, it relies on the low levels of fructose inside the cell to "pull" the fructose inside. Once transported through the intestinal wall, the monosaccharides enter the blood through the capillaries and are carried to the portal circulation and then to the liver.

Metabolism of Carbohydrates

The liver is the major site of galactose and fructose metabolism, where they are taken up, converted to glucose derivatives, and either stored as liver glycogen or used for energy immediately when needed. Although glucose is metabolized extensively in the liver, unlike galactose and fructose, it is also passed into the blood supply to be used by other tissues. Tissues like skeletal muscle and adipose tissue depend on insulin for glucose uptake, whereas the brain and liver do not. This dependence on insulin becomes a problem for diabetics who either cannot make insulin (IDDM) or are resistent to insulin (NIDDM). For individuals left untreated, dietary carbohydrates cause glucose levels to rise, resulting in hyperglycemia, which will lead to serious consequences if steps are not taken to correct it.

Once in the tissues, the fate of glucose depends on the energy demands of the body. Glucose can be metabolized through the glycolysis pathway to pyruvate where it is either converted to lactate or completely oxidized to CO₂, H₂O, and energy. Liver and skeletal muscle can convert excess glucose to glycogen through a pathway known as glycogenesis. The glycogen is stored after meals to be used as an energy source when energy demands are higher than intake. At this time the glycogen is broken down into individual glucose units, a process known as glycogenolysis, and the glucose can be metabolized further. Excess carbohydrates also can be used as a substrate for fat synthesis.

Carbohydrates are an essential part of a healthy diet. They provide an easily available energy source, are an important vehicle for micronutrients and phytochemicals, help to maintain adequate blood glucose, and are important in maintaining the integrity and function of the gastrointestinal tract. Table 2 contains examples of foods that contain the various types of carbohydrates.

See also Digestion; Fiber, Dietary; Starch.

BIBLIOGRAPHY

Ettinger, Susan. "Macronutrients: Carbohydrates, Proteins and Lipids." In Krause's Food, Nutrition, and Diet Therapy, edited Kathleen L. Mahan and Sylvia Escott-Stump. 10th ed. Philadelphia, Pa.: W. B. Saunders, 2000.

FAO/WHO. Carbohydrates in Human Nutrition : Report of a Joint FAO/WHO Expert Consultation, Rome, 14–18 April 1997. Rome: World Health Organization, Food and Agriculture Organization of the United Nations, 1998.

Guthrie, Joanne, and Joan Morton. "Food Sources of Added Sweetners in the Diets of Americans." Journal of the American Dietetic Association 100 (2000): 43–48, 51.

Kiens, B., and E. A. Richter. "Types of Carbohydrates in an Ordinary Diet Affect Insulin Action and Muscle Substrates in Humans." American Journal of Clinical Nutrition. 63 (1996): 47–53.

Macdonald, I. A. "Carbohydrate as a Nutrient in Adults: Range of Acceptable Intakes." European Journal of Clinical Nutrition. 53 (1999): S101–S106.

Debra Coward McKenzie Rachel K. Johnson

Starch

Starch from plants makes up about half of our dietary carbohydrates. Starch molecules can aggregate to form granules that differ by size and shape depending on the source of the starch, for example, corn, potato, and manioc. Although there is no difference in the nutritional value between the starches since all cooked starches are broken down in the body into glucose molecules, they do differ by characteristics such as solubility, flavor, and thickening power. Because of these characteristics, starch is often removed from the source to use commercially. For example, the starch can be removed from tubers such as potatoes and manioc (also known as cassava) through a wet milling process, or in the case of manioc, through leaching and drying. The potato starch is often used as a thickener or instead of cornstarch in recipes, while manioc is best known as tapioca.

Carbohydrates

Carbohydrates are the most abundant natural organic compounds on Earth. The term "carbohydrate" derives from their general formula of C_n(H₂O)_n, first determined in the nineteenth century, and indicates that these compounds are hydrates of carbon. Carbohydrates are more specifically defined as polyhydroxy aldehydes or ketones and the products derived from them. Carbohydrates are synthesized via photosynthesis by plants, algae, and some bacteria. Animals feeding on these organisms then use the energy stored in these compounds.

Energy storage is not the only function of carbohydrates. They have a variety of functions in living organisms, including their contribution to the structure of cell walls and their vital role in communication at the site of cell membranes. Carbohydrates form part of the backbone of RNA and DNA molecules, and they are also found linked to proteins and lipids as glycoproteins and glycolipids. The three basic groups of carbohydrates based on size are: monosaccharides , oligosaccharides, and polysaccharides (saccharide from the Greek sakcharon, or "sugar"). The oligosaccharides and polysaccharides are composed of a few and many monosaccharides, respectively. Monosaccharides have two major groups: the aldoses and the ketoses.

Aldoses

The simplest of the aldoses is glyceraldehyde, which is a triose, or three-carbon sugar. Glyceraldehyde has one chiral carbon and therefore two stereoisomers, designated D and L. In nature, only D sugars occur in abundance. Other aldoses can be derived from glyceraldehyde via insertion of additional hydroxy carbons between the carbonyl carbon and the molecule's other carbons. In this way, tetroses, pentoses, and hexoses are formed. Although glyceraldehyde and the tetroses can occur only as simple linear structures,

the pentoses and the hexoses can also form rings. The ring formation has important effects on the properties of these molecules.

Glucose , an aldohexose, is the most common of the monosaccharides. In various combinations and permutations, it forms starch, cellulose, sucrose (table sugar), and lactose (milk sugar), among other things. When metabolized via the glycolytic pathway, it is the major energy source for many living things. Most commonly, glucose forms a ring, its fifth hydroxyl group reacting with the aldehyde carbonyl group to form a hemiacetal (see Figure 2). As a result of this reaction, the sugar forms a six-membered ring and the carbonyl carbon becomes chiral. The two new stereoisomers of glucose that revolve on the aldehyde carbon are designated α and β and are considered anomers of one another. The now chiral carbon is called the "anomeric" carbon. These six-membered ring structures are called pyranoses, as they resemble the compound pyran. Thus, in its ring forms, glucose is properly designated α -D-glucopyranose, or β -D-glucopyranose.

These pyranose rings are not flat and can assume several different conformations. In general, the most stable conformation is the "chair" conformation, in which the bulky atomic groups (e.g., the hydroxyl and hydroxymethyl groups) are equatorial or within the plane of the ring (see Figure 3). The hydrogens would then be axial or perpendicular to the plane of the ring. Only in β -D-glucopyranose are all of the bulky groups equatorial; thus, β -D-glucopyranose is more stable than α -D-glucopyranose, in which the C-1 hydroxyl group is axial to the ring. The α and β forms of

glucose are freely interconvertible in solution. At equilibrium, a solution of glucose contains about two-thirds α -D-glucopyranose, one-third α -D-glucopyranose, and small amounts of the linear and five-membered ring forms of glucose.

Other widely occurring aldoses include mannose, galactose , and ribose (see Figure 1). Mannose and galactose are, like glucose, aldohexoses and can form six-membered rings. Mannose is an important part of the complex sugars, or oligosaccharides, that attach to proteins in the formation of glycoproteins. Galactose combines with glucose to form lactose or milk sugar. Structurally, each of these sugars differs from glucose only in the stereo-chemistry that revolves on one carbon: mannose on C-2 and galactose on C-4. Sugars that differ from one another only in respect to the stereo-chemistry at one carbon are considered epimers of each other. Thus mannose is the C-2 epimer of glucose, and galactose the C-4 epimer. Mannose and galactose are not epimers; they differ from each other in respect to the stereochemistry revolving around two carbons.

Ribose is an aldopentose. It composes the carbohydrate portion of the ribonucleotides that form a cell's RNA. Ribose, like the aldohexoses, can form a ring. However, ribose forms a five-membered ring, called a "furanose" because of its similarity to the compound furan. Once again, there are two possible forms this ring can have: α -D-ribofuranose and β -D-ribofuranose. RNA contains β -D-ribofuranose.

Ketoses

There are fewer ketoses than there are aldoses because ketoses have one less chiral carbon. The most prevalent of the ketoses are dihydroxyacetone, ribulose, xylulose, and fructose (see Figure 4). All four of these sugars are important intermediates in metabolism . Fructose is, along with glucose, part of sucrose or table sugar. Fructose is a ketohexose, and the only one of the four ketoses that can assume a ring structure. Like ribose, fructose forms a five-membered (or furanose) ring and has α and β anomers.

Monosaccharide Derivatives

Monosaccharides undergo incorporation into oligo- and polysaccharides. Individual monosaccharides can also undergo a variety of transformations. One important modification of monosaccharides is the formation of deoxy sugars. The most biologically significant of the deoxy sugars is β -D-2-deoxyribose (see Figure 5), in which the C-2 hydroxyl group of ribose has been replaced with hydrogen. This deoxy sugar is the sugar component of DNA.

In the amino sugars, an amino group replaces one or more of the hydroxyl groups. The most common of these sugars are D-glucosamine and D-galactosamine, both of which have an amino group in place of the hydroxyl group on the second carbon. Often, these amino groups are acetylated to give N-acetyl sugars, such as N-acetylglucosamine and N-acetylgalactosamine. These sugars are important components of larger polysaccharides. Other important amino sugars are muramic acid and N-acetylneuraminic acid, which are components of the oligosaccharides of glycoproteins and glycolipids, and of bacterial cell walls. Muramic acid and N-acetylneuraminic acid are glucosamines, which have been linked at either C-3 or C-1 to three-carbon acids. Muramic acid is formed via an ether linkage between the C-3 of glucosamine and the hydroxyl group of lactic acid. N-acetyl-D-neuraminic acid results from the formation of a C–C bond between the C-1 of N-acetyl-D-mannosamine and the C-3 of phosphoenolpyruvate. This and other derivatives of neuraminic acid are collectively called sialic acids and are widely found in bacteria and animals.

Finally, monosaccharides often form ester linkages with phosphate and sulfate ions. In fact it is rare to find free monosaccharide in cells. Glucose is the only unmodified monosaccharide that exists in substantial quantities in living things, in which it exists primarily extracellularly.

see also Chirality; Glycolysis.

Stephanie E. Dew

Bibliography

Nelson, David L., and Cox, Michael M. (2000). Lehninger Principles of Biochemistry, 3rd edition. New York: Worth Publishers.

Robyt, John F. (1998). Essentials of Carbohydrate Chemistry. New York: Springer.

Voet, Donald; Voet, Judith G.; and Pratt, Charlotte (1999). Fundamentals of Biochemistry. New York: Wiley.

Other Resources

American Chemical Society. Division of Carbohydrate Chemistry. Information available from <http://membership.acs.org/C/>.

International Union of Pure and Applied Chemistry. Information available from <http://www.chem.qmw.ac.uk/iupac/>.

carbohydrates are one of the major classes of biological molecules, along with proteins, lipids (fats), and nucleic acids. Carbohydrates range from small molecules — mono,- di-, or tri-saccharides — to large molecules called polysaccharides. As these names imply, carbohydrates are made up of sugars.

The most abundant organic compound in the biosphere is a carbohydrate — cellulose, a substance which gives strength and integrity to plant cell walls. It consists of linked linear chains of glucose molecules. This configuration allows arrays of long, parallel straight fibrils to form, giving cellulose its characteristic properties. Cellulose is difficult to break down, and only some bacteria, fungi, and protozoa secrete the enzymes (cellulases) that can do so. So mammals are unable to digest cellulose, except some ruminants that have cellulase-secreting bacteria in their rumens. (If it were not for these bacterial cellulases, the disposal of 10¹⁵ kg of plant waste per year world-wide would present an enormous pollution problem.) Nevertheless, in man, cellulose is an important component of the diet as ‘roughage’.

glycogen, another polysaccharide made of linked chains of glucose molecules, is used by mammals as a way of storing energy in the cells of most tissues, but notably in liver cells as a store for the whole body, and in muscles for their own use. Here the type of linkage results in the formation of an open helix, readily broken down by the relevant enzymes (glycogenases) when the sudden need for energy arises.

Carbohydrate-rich foods are starchy ones, such as the staples bread, potatoes, and pasta. Large quantities of carbohydrates are ingested as common sugar and in confectionary. Common dietary carbohydrates are sucrose, lactose, and mannose, all disaccharides (formed from two simple sugars), which are broken down by digestive processes to monosaccharides and used to derive energy.

The conversion of monosaccharides to energy in the form of adenosine triphosphate (ATP) follows a common pathway — used also for the conversion of fats and, to a lesser extent, proteins into energy — which involves the utilization of oxygen. The end result of this metabolism is that one molecule of glucose generates 38 molecules of ATP. The respiratory quotient (RQ) for carbohydrates is 1, where the RQ is the ratio of the number of CO₂ molecules formed to the number of molecules of oxygen consumed in oxidizing (burning) one sugar molecule. As carbohydrates have the formula (CH₂O)_n, hydrogen and oxygen are in the correct ratio to form water. (When fats are oxidized, extra oxygen is needed to form water, such that the RQ for fats is around 0.7.)

In an ideally balanced diet, about two-thirds of the energy supply should be from carbohydrates. But unlike proteins and fats, which must provide certain essential components, no particular dietary carbohydrates are necessary for health. This may seem paradoxical, in that the brain crucially needs a constant supply of glucose — but this can, if necessary, be made internally from proteins. Excess of dietary carbohydrate, when the glycogen stores are filled, is converted and stored as fat, and this can be released as fatty acids when needed for energy production. In starvation, the mobilization of fatty acids from body fat results in their uptake by the liver and production from them of ketone bodies, an alternative energy source. At this stage acetone can be smelt (like pear drops) on the breath. The same happens when carbohydrate starvation occurs at the cellular level in diabetes because glucose entry into cells is impaired.

Alan W. Cuthbert

Carbohydrate

Carbohydrates are compounds composed of carbon, hydrogen, and oxygen. The carbohydrate group includes sugars, starches, and cellulose. Sugars and starches provide organisms with energy for cell functions, and cellulose is a fibrous substance making up plant cell walls.

The role of plants

Carbohydrates are produced by green plants through a process known as photosynthesis. In photosynthesis, a pigment called chlorophyll (pronounced KLOR-uh-fill) in the leaves of plants absorbs light energy from the Sun. Plants use this light energy to convert water and carbon dioxide from the environment into glucose and oxygen. Some glucose is used to form the more complex carbohydrate cellulose, the main structural component of plant cell walls. Some is used to provide immediate energy to plant cells. The rest is changed to a different chemical form, usually starch, and stored in seeds, roots, or fruits for later use.

The carbohydrates produced by plants are an important source of energy for animals. When animals eat plants, energy stored in carbohydrates is released in the process of respiration, a chemical reaction between glucose and oxygen to produce energy (for cell work), carbon dioxide, and water. Glucose is also used by animal cells in the production of other substances needed for growth.

Types of carbohydrates

Carbohydrates are usually divided into three categories: monosaccharides, having one sugar unit; disaccharides, having two sugar units; and polysaccharides, having many sugar units. The arrangement of atoms in different carbohydrate molecules gives each its specific properties.

Among the most important monosaccharides are glucose (found in plants and animals), fructose (a fruit sugar), and galactose (formed from the milk sugar lactose). Glucose is especially important in vital life processes. Each of these simple sugars is made up of the same number and types of atoms, arranged differently. All three are readily absorbed into the bloodstream from the digestive tract.

Disaccharides are carbohydrates composed of two subunits of simple sugars. They include sucrose (ordinary table sugar), maltose (formed from starch), and lactose (found in milk and the only sugar not occurring in plants). Enzymes in the digestive tract of humans and animals split disaccharides into the more easily absorbed monosaccharides. For example, sucrose is split into the simple sugars glucose and fructose during digestion.

Polysaccharides are the most abundant carbohydrates. A polysaccharide molecule can contain thousands of glucose units. These highly complex carbohydrates include starches, cellulose, and glycogen. Starch is the usual form in which carbohydrates are stored as energy by plants. Plants can split starch into its simpler glucose units for use as energy when needed.

Cellulose is known as a structural carbohydrate because of the fibers formed by its molecules. It is the major component of plant cell walls and comprises over half of the carbon found in plants. Humans and most animals cannot digest cellulose for food but use it as a fiber (often termed roughage) to help in the elimination of waste by the intestine. Some grazing animals such as cows and sheep have microorganisms in their digestive tracts that partially digest cellulose, allowing these animals to use cellulose as food.

Glycogen is the form in which glucose is stored in the liver and muscles of animals for energy needs. Muscle glycogen is used primarily to fuel muscle contractions (such as during exercise). Liver glycogen is used to restore glucose to the blood when the sugar level is low (such as between meals).

[See also Photosynthesis ]

Carbohydrates

Carbohydrates are one of four major classes of biological molecules, along with nucleic acids, lipids , and proteins . They are the most abundant biological molecules, and are an important nutritional component of many foods. Carbohydrates are usually composed solely of carbon, hydrogen, and oxygen, although some also contain nitrogen, sulfur, or phosphorus.

Carbohydrates are classified according to size. The smallest carbohydrates are called monosaccharides (mono means "one"; saccharide means "sugar"). As the name implies, these are single sugar molecules. The most common monosaccharides, such as fructose and glucose , have six carbon atoms, but monosaccharides can have as few as three or as many as seven. Monosaccharides with five or more carbons usually have a ring-shaped structure when they are in a solution.

Oligosaccharides (oligo means "few") are more complex carbohydrates composed of chains of two or a few (up to about twenty) simple sugars joined with a type of covalent bond called a glycosidic bond. The longer oligosaccharides may be linear or branched. The most common oligosaccharides, composed of only two sugars, are called disaccharides (di means "two"). The most common disaccharide, sucrose, or cane sugar, consists of a glucose molecule bonded to a fructose molecule. Other important disaccharides are maltose (two glucoses joined together) and lactose, or milk sugar (glucose joined to galactose). Longer oligosaccharides are usually bound to other molecules, such as lipids or proteins, to form glycolipids and glycoproteins, respectively ( glyco means "sweet"), rather than being free in solution. These kinds of molecules are important in cell recognition, signaling, and adhesion , and are commonly found on the outer surface of cell membranes.

Polysaccharides (poly, means "many") are important energy-storage and structural molecules. They are formed of long chains of sugars, most commonly glucose. Like oligosaccharides, they may be linear or branched. Important polysaccharides are starch, glycogen (animal starch), cellulose , and chitin . Starch and glycogen are similar energy-storage molecules found in plants and animals, respectively. Both are made of glucose molecules that are bonded in the same manner; however, glycogen has a higher degree of branching compared to starch.

Cellulose is also made of glucose, but the individual glucose units are linked differently, resulting in a long, fibrous structure that is not soluble in water. Cellulose is the main structural component of most plant and some protozoan and bacterial cell walls. Wood is largely cellulose, and paper is an almost-pure sheet of cellulose prepared from wood. Cotton is also nearly pure cellulose.

Chitin is similar to cellulose, but its sugar subunits are a modified form of glucose called N -acetyl glucosamine. Chitin is the main structural component of fungal cell walls and of animal exoskeletons , such as the shells of insects and crustaceans. Other important structural polysaccharides form the matrix of cartilage and other connective tissues of animals. Carbohydrates are at the center of cellular metabolic pathways. The most fundamental process, glycolysis , uses glucose to produce energy for cellular needs.

see also Cell Wall; Extracellular Matrix; Glycolysis and Fermentation

David W. Tapley

Bibliography

Sharon, N. "Carbohydrates." Scientific American 243 (1980): 90–102.

carbohydrate Carbohydrates are the major source of metabolic energy, the sugars and starches. Chemically they are composed of carbon, hydrogen, and oxygen in the ratio C_n : H_2n : O_n. The basic carbohydrates are the monosaccharide sugars, of which glucose, fructose, and galactose are the most important nutritionally.

Disaccharides are composed of two monosaccharides: nutritionally the important disaccharides are sucrose (a dimer of glucose + fructose), lactose (a dimer of glucose + galactose) and maltose (a dimer of two glucose units).

A number of oligosaccharides occur in foods, consisting of 3–5 monosaccharide units; in general these are not digested, and should be considered among the unavailable carbohydrates.

Larger polymers of carbohydrates are known as polysaccharides or complex carbohydrates. Nutritionally two classes of polysaccharides can be distinguished: (1)starches, which are polymers of glucose units, either as a straight chain (amylose) or with a branched structure (amylopectin) and are digested; and(2)a variety of other polysaccharides which are collectively known as non‐starch polysaccharides (NSP), and are not digested by human digestive enzymes.

The reserve of carbohydrate in the liver and muscles is glycogen, a glucose polymer with a similar structure to the amylopectin form of starch, but more branched.

Carbohydrates form the major part of the diet, providing between 50 and 70% of the energy intake, largely from starch and sucrose. The metabolic energy yield of carbohydrates is 4 kcal (17 kJ)/g. More precisely, monosaccharides yield 3.74 kcal (15.7 kJ), disaccharides 3.95 kcal (16.6 kJ), and starch 4.18 kcal (17.6 kJ)/g. Glycerol is a 3‐carbon sugar alcohol, and classified as a carbohydrate; it yields 4.32 kcal (18.1 kJ)/g.

See also sugar; sugar alcohols.

carbohydrate any member of a large class of chemical compounds that includes sugars, starches, cellulose, and related compounds. These compounds are produced naturally by green plants from carbon dioxide and water (see photosynthesis ). Carbohydrates are important as foods; they supply energy and are used in the production of fats. They are also used in various forms in industry and commerce. There are three main classes of carbohydrates. Monosaccharides are the simple sugars, e.g., fructose and glucose ; they have the general formula (CH ₂ O) _n , in which n is an integer larger than 2. Disaccharides include lactose , maltose , and sucrose . Upon hydrolysis, a disaccharide molecule yields two monosaccharide molecules. Most disaccharides have the general formula C _n (H ₂ O) _n-1 , with n larger than 5. Polysaccharides include such substances as cellulose , dextrin , glycogen , and starch ; they are polymeric compounds made up of the simple sugars and can be hydrolyzed to yield simple sugars. The disaccharides are sometimes grouped with the simpler polysaccharides (usually those made up of three or four simple sugar units) to form a class of carbohydrates called the oligosaccharides.

carbohydrate One of a group of organic compounds based on the general formula C_x(H₂O)_y. The simplest carbohydrates are the sugars (saccharides), including glucose and sucrose. Polysaccharides are carbohydrates of much greater molecular weight and complexity; examples are starch, glycogen, and cellulose. Carbohydrates perform many vital roles in living organisms. Sugars, notably glucose, and their derivatives are essential intermediates in the conversion of food to energy. Starch and other polysaccharides serve as energy stores in plants, particularly in seeds, tubers, etc., which provide a major energy source for animals, including humans. Cellulose, lignin, and others form the supporting cell walls and woody tissue of plants. Chitin is a structural polysaccharide found in the body shells of many invertebrate animals. Carbohydrates also occur in the surface coat of animal cells and in bacterial cell walls.

Carbohydrates

Carbohydrates are a diverse group of compounds composed of the elements carbon (C), hydrogen (H), and oxygen (O) with the empirical formula [CH₂ O]_n, where n represents the number of CH₂ O units in the compound. The ultimate source of all carbohydrates is photosynthesis, in which the energy of sunlight is used to chemically fix atmospheric CO₂ into carbohydrate. Carbohydrates constitute as much as 80 percent of the dry weight of a plant. This is largely due to the presence of cell walls made of complex carbohydrates surrounding each plant cell.

Sugars, also called saccharides, are carbohydrates. Common sugars occurring in nature have from three (glyceraldehyde) to seven (sedoheptulose) carbon atoms bonded together to form the molecule's backbone. Sugars can be classified chemically as either aldehydes or ketones, and contain OH (hydroxyl ) groups attached to their carbon backbones. Glucose (an aldehyde sugar) and fructose (a ketose sugar) are two examples of common six-carbon sugars that occur widely in plants. These molecules form ring structures, with glucose forming a six-member ring, and fructose a five-member ring.

Individual sugars are called monosaccharides. Two sugars linked together chemically are called disaccharides; three are trisaccharides; four are tetrasaccharides, and so on. A generic term for several sugars linked together is oligosaccharide. Molecules with many sugars linked together to form a polymer are called polysaccharides. Sucrose (table sugar) is the most abundant disaccharide and is comprised of glucose and fructose. A large amount of sucrose is formed in plant leaves from photosynthetic products. Sucrose is translocated over long distances in plants in specialized conductive tissue known as phloem. Sucrose can be broken down, and its components (glucose and fructose) metabolized for energy and as the initial raw material for building cellular components in plant cells distant from photosynthetic leaves. Sucrose also can be stored. High concentrations are found in storage organs such as fruits, sugarcane stems, and enlarged roots of sugar beets. The latter two are commercial sources from which sucrose is refined.

Two important plant polysaccharides, cellulose and starch, are composed exclusively of glucose units bonded together. Cellulose is the most abundant polysaccharide in plant cell walls, and thus, the most abundant polysaccharide on Earth. In cell walls, cellulose occurs along with other complex polysaccharides, each of which is composed of more than one type of sugar. Because of the way glucose is linked in cellulose, individual chains, hundreds of glucose molecules long, are able to bond together by hydrogen bonding in a crystalline arrangement to form a cable-like structure known as a microfibril. These microfibrils are interwoven and give cellulose its strength in plant cell walls and in cotton fabric and paper.

Starch and cellulose differ in the way the glucose molecules are bonded together. Starch is a storage compound in plants, and is broken down as needed into glucose for metabolic use.

Carbohydrates are important in human nutrition, often constituting the major source of calories in the diet. Glucose, fructose, sucrose, and starch are readily digested by humans. Cellulose and the other complex carbohydrates of the plant cell wall are not readily digested, but constitute useful dietary fiber.

Cellulase, an enzyme that can degrade cellulose, is made by many different fungi and some microorganisms. This enzyme allows these organisms to decompose plant material, an important step in the recycling of materials in food webs.

see also Photosynthesis, Carbon Fixation and; Sugar.

D. Mason Pharr

John D. Williamson

Bibliography

Taiz, Lincoln, and Eduardo Zeiger, eds. Plant Physiology, 2nd ed. Sunderland, MA:Sinauer Associates, 1998.

Treatment of denim with cellulase has largely replaced pumice stone in the production of "stone-washed" jeans.

carbohydrate Organic compound of carbon, hydrogen and oxygen that is a constituent of many foodstuffs. The simplest carbohydrates are sugars. Glucose and fructose are monosaccharides, naturally occurring sugars; they have the same formula (C₆H₁₂O₆) but different structures. One molecule of each combines with the loss of water to make sucrose (C₁₂H₂₂O₁₁), a disaccharide. Starch and cellulose are polysaccharides, carbohydrates consisting of hundreds of glucose molecules linked together. The popular Atkins diet is based on not eating foods with carbohydrates, such as bread and potatoes. See also saccharide

carbohydrate (kar-boh-hy-drayt) n. any one of a large group of compounds, including the sugars and starch, that contain carbon, hydrogen, and oxygen and have the general formula C_x (H₂O)_y. Carbohydrates are important as a source of energy: they are manufactured by plants and obtained by animals and humans from the diet, being one of the three main constituents of food. See also disaccharide, monosaccharide, polysaccharide.

carbohydrate A generic term for molecules that are aldehyde or ketone derivatives of polyhydroxyl alcohols and that have a basic empirical formula of C_x(H₂O)_y, where x and y are variable numbers. They are normally classified as mono-, oligo-, or poly-saccharides, depending upon whether they are single sugars, short-chain molecules, or large polymers respectively.

carbohydrate A generic term for molecules that have a basic empirical formula of C_x(H₂O)_y and that are aldehyde or ketone derivatives of polyhydroxyl alcohols. They are normally classified as mono-, oligo-, or poly-saccharides, depending upon whether they are single sugars, short-chain molecules, or large polymers respectively.

car·bo·hy·drate / ˌkärbəˈhīˌdrāt/ • n. Biochem. any of a large group of organic compounds occurring in foods and living tissues and including sugars, starch, and cellulose. They contain hydrogen and oxygen in the same ratio as water (2:1).

carbohydrate A generic term for molecules that have a basic empirical formula of C_x(H₂O)_y, where x and y are variable numbers. They are normally classified as mono-, oligo-, or poly-saccharides, depending upon whether they are single sugars, short-chain molecules, or large polymers respectively.

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