15.1 Carbohydrates Carbohydrates are a major source of energy from our diet. made from the elements carbon, hydrogen, and oxygen. also called saccharides, which means “sugars.”

Carbohydrates Carbohydrates, such as glucose, are produced by photosynthesis in plants. are synthesized in plants from C O2, H2O, and energy from the sun. are oxidized in living cells to produce C O2, H2O, and energy.

Types of Carbohydrates The types of carbohydrates are monosaccharides, the simplest carbohydrates; disaccharides, which consist of two monosaccharides; and polysaccharides, which contain many monosaccharides.

Monosaccharides Monosaccharides contain several hydroxyl groups attached to a chain of three to eight carbon atoms. that contain an aldehyde group are classified as aldoses. that contain a ketone group are classified as ketoses. have hydroxyl groups on all carbons except the carbonyl carbon.

Types of Monosaccharides Monosaccharides are also classified by the number of carbon atoms present. triose (three C atoms) tetrose (four C atoms) pentose (five C atoms) hexose (six C atoms) An aldopentose is a five-carbon saccharide with an aldehyde group. A ketohexose is a six-carbon saccharide with a ketone group.

Types of Monosaccharides

Study Check Identify each as aldo or keto and as tetrose, pentose, or hexose. ​ ​

Solution Identify each as aldo or keto and as tetrose, pentose, or hexose. A. aldohexose B. ketopentose​

Important Monosaccharides d-Glucose, d-galactose, and d-fructose are the most important monosaccharides. They are all hexoses with the molecular formula C6H12O6 and are isomers of each other.

Monosaccharides: D-Glucose is the most common hexose. is found in fruits, vegetables, corn syrup, and honey. is also known as dextrose and blood sugar in the body. is a building block of the disaccharides sucrose and lactose and of polysaccharides such as cellulose and glycogen.

Monosaccharides: D-Galactose d-galactose, an aldohexose with the formula C6H12O6, is obtained from the disaccharide lactose, found in milk. is important in the cellular membranes of the brain and nervous system. In a condition called galactosemia, the enzyme needed to convert d-galactose to d- glucose is missing. galactose accumulates in the blood and tissue, which can lead to cataracts, mental retardation, failure to thrive, and liver disease.

Monosaccharides: D-Fructose is a ketohexose with the formula C6H12O6. is the sweetest of the carbohydrates, twice as sweet as sucrose (table sugar). is obtained as one of the hydrolysis products of sucrose. High-fructose corn syrup (H F C S) is a sweetener that is produced by using an enzyme to break down sucrose to glucose and fructose.

Chemistry Link to Health: Hyperglycemia and Hypoglycemia In the body, glucose has a normal blood level of 70–90 mg/dL. a glucose tolerance test measures blood glucose for several hours after ingesting glucose.

Chemistry Link to Health: Hyperglycemia and Hypoglycemia Diabetes mellitus can cause hyperglycemia, which occurs when the pancreas is unable to produce sufficient quantities of insulin. allows glucose levels in the body fluids to rise as high as 350 mg/dL of plasma. Symptoms of diabetes include thirst and excessive urination. increased appetite and weight loss. In older adults, diabetes is sometimes a consequence of excessive weight gain.

Chemistry Link to Health: Hyperglycemia and Hypoglycemia When a person is hypoglycemic, the blood glucose level rises and then decreases rapidly to levels as low as 40 mg/dL. low blood sugar may occur as a result of an overproduction of insulin by the pancreas. symptoms may appear, such as dizziness, general weakness, and muscle tremors. a diet may be prescribed that consists of several small meals high in protein and low in carbohydrates.

Chemistry Link to Health: Glucose Testing Normally, blood glucose flows through the kidneys and is reabsorbed into the bloodstream. When the blood level exceeds about 160 mg of glucose/dL of blood, the kidneys cannot reabsorb all of the glucose, and it spills over into the urine, a condition known as glucosuria. The color of the dye o-methylaniline on a test strip determines the glucose level in urine.

15.3 Haworth Structures of Monosaccharides The most stable forms of pentose and hexose sugars are five- or six-atom rings. Haworth structures are produced from the reaction of a carbonyl group and a hydroxyl group in the same molecule.

Haworth Structures of Galactose is an aldohexose. differs from glucose only in the arrangement of the group on carbon 4. has a Haworth structure similar to that of glucose, except that the group on carbon 4 is drawn above the ring.

Haworth Structures of Fructose is a ketohexose. forms a five-atom ring structure with carbon 2 at the right corner of the ring. forms when the group on carbon 5 reacts with carbon 2 in the carbonyl group.

15.5 Disaccharides Lactose is a disaccharide found in milk and milk products. It contains the monosaccharides galactose and glucose.

Disaccharides A disaccharide consists of two monosaccharides linked together. is formed when two monosaccharides combine in a dehydration reaction. Monosaccharides Blank Disaccharide glucose + glucose maltose + H2O glucose + galactose lactose + H2O glucose + fructose sucrose + H2O The most common disaccharides are maltose, lactose, and sucrose.

Maltose Maltose is a disaccharide also known as malt sugar. composed of two D-glucose molecules. obtained from the hydrolysis of starch. used in cereals, candies, and brewing.

Formation of Maltose Maltose is linked by an glycosidic bond formed from the on carbon 1 of the first glucose and the on carbon 4 of the second glucose.

Lactose, Milk Sugar Lactose is a disaccharide found in milk and milk products. makes up 6–8% of human milk and about 4–5% of cow’s milk.

Lactose Lactose is a disaccharide of

Sucrose, Table Sugar Sucrose, or table sugar, consists of

Sucrose Sucrose cannot form an open chain or be oxidized. react with Benedict’s reagent and is not a reducing sugar. The sugar we use to sweeten our cereal, coffee, or tea is sucrose. Most of the sucrose for table sugar comes from sugar cane (20% by mass) or sugar beets (15% by mass).

Chemistry Link to Health: Sweetness of Sweeteners Sugars and artificial sweeteners differ in sweetness. are compared to sucrose (table sugar), which is assigned a value of 100. Table 15.2 Relative Sweetness of Sugars and Artificial Sweeteners blank Sweetness Relative to Sucrose (100) Monosaccharides Galactose 30 Glucose 75 Fructose 175

Chemistry Link to Health: Sweetness of Sweeteners [Table 15.2 Continued] Disaccharides blank Lactose 16 Maltose 33 Sucrose 100 Sugar Alcohols Sorbitol 60 Maltitol 80 Xylitol Artificial Sweeteners (Noncarbohydrate) Aspartame 18000 Saccharin 45000 Sucralose 60000 Neotame 1000000

Chemistry Link to Health: Sweetness of Sweeteners Sucralose is marketed as Splenda. made from sucrose by replacing some of the hydroxyl groups with chlorine atoms.

Chemistry Link to Health: Sweetness of Sweeteners Aspartame, which has some caloric value, is marketed as NutraSweet or Equal. a noncarbohydrate sweetener made from aspartic acid and a methyl ester of the amino acid phenylalanine. so sweet that a very small quantity is needed.

Chemistry Link to Health: Sweetness of Sweeteners Neotame, another artificial sweetener, is a modification of the aspartame structure. differs by the addition of a large alkyl group to the amine group, which prevents enzymes from breaking the amide bond between aspartic acid and phenylalanine, so phenylalanine is not produced. is very sweet; only small amounts of Neotame are needed because it is about 10000 times sweeter than sucrose

Chemistry Link to Health: Sweetness of Sweeteners Saccharin, another artificial sweetener, is marketed as Sweet’N Low. has been used as a noncarbohydrate artificial sweetener for more than 60 years. has been banned in Canada as an artificial sweetener because studies indicate that it may cause bladder tumors. is still approved by the FDA for use in the United States.

Polysaccharides Polysaccharides are formed when many monosaccharides are joined together. include amylose, amylopectin, cellulose, and glycogen, which are polymers of D-glucose and differ by branching and types of glycosidic bonds.

Starch Starch is a storage form of glucose in plants, found as insoluble granules in rice, wheat, potatoes, beans, and cereals. composed of two kinds of polysaccharides: amylose and amylopectin.

Starch: Amylose Amylose, which makes up about 20% of starch, consists of 250 to 4000 α-D-glucose molecules connected by in a continuous chain. Polymer chains of amylose are coiled in a helical fashion.

Starch: Amylopectin Amylopectin makes up as much as 80% of starch. is a branched-chain polysaccharide. contains glucose molecules connected by

Starch: Amylopectin In our bodies, these complex carbohydrates are digested by the enzymes amylase in saliva and maltase in the intestines. provide about 50% of our nutritional calories from the glucose obtained in digestion.

Animal Starch: Glycogen Glycogen is a polymer of glucose that is stored in the liver and muscle of animals. hydrolyzed in our cells at a rate that maintains the blood level of glucose and provides energy between meals. similar to amylopectin but more highly branched. The glucose units in glycogen are joined by with branches attached by that occur every 10–15 glucose units.

Cellulose Cellulose, the major structural unit of wood and plants, is a polysaccharide of glucose units in unbranched chains with cannot form hydrogen bonds with water, making it insoluble in water. gives a rigid structure to the cell walls in wood and fiber. is more resistant to hydrolysis than are the starches. cannot be digested by humans because humans cannot break down

Cellulose The polysaccharide cellulose is composed of glucose units connected by