General, Organic, and Biological Chemistry Fourth Edition Karen Timberlake Chapter 15 Carbohydrates 15.1 Carbohydrates Lectures © 2013 Pearson Education, Inc.

Carbohydrates Glucose is a carbohydrate produced by photosynthesis in plants from CO2, H2O, and energy from the sun. oxidized in living cells (respiration) to produce CO2, H2O, and energy.

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

Carbohydrates Carbohydrates include table sugar, lactose, and cellulose. are made of carbon, hydrogen, and oxygen. make up about 65% of our diet.

Monosaccharides Monosaccharides consist of 3 to 6 carbon atoms (typically). a carbonyl group (aldehyde or ketone). several hydroxyl groups. There are two types of monosaccharide structures. Aldoses, in which the carbonyl group on the first carbon is an aldehyde. Ketoses, which contain a ketone carbonyl group on the second carbon.

Monosaccharides

Aldoses Aldoses are monosaccharides with an aldehyde group. with many hydroxyl (–OH) groups. Examples: aldotriose (3 C atoms) aldotetrose (4 C atoms) aldopentose (5 C atoms) aldohexose (6 C atoms)

Ketoses Ketoses are monosaccharides with a ketone group. with many hydroxyl (–OH) groups. Examples: ketotriose (3 C atoms) ketotetrose (4 C atoms) ketopentose (5 C atoms) ketohexose (6 C atoms)

Examples of Monosaccharides

Learning Check Identify each as aldo- or keto- and as tetrose, pentose, or hexose. A. B.

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

Fischer Projections A Fischer projection is used to represent carbohydrates. places the most oxidized group at the top. shows chiral carbons at the intersection of vertical and horizontal lines.

D and L Notations In a Fischer projection, the –OH group on the chiral carbon farthest from the carbonyl group determines if it is an L or D isomer. left is assigned the letter L for the L-isomer. right is assigned the letter D for the D-isomer.

Learning Check Identify each as the D or L isomer. A. B. C. __-Ribose __-Threose __-Fructose

Solution Identify each as the D or L isomer. A. B. C. __-Ribose __-Threose __-Fructose L L D

Important Monosaccharides The hexoses glucose, galactose, and fructose are the most important monosaccharides. The D stereoisomers are commonly found in nature and used in the cells of the body.

D-Glucose, C6H12O6 blood sugar is D-Glucose, also known as dextrose or blood sugar is an aldohexose found in fruits, corn syrup, and honey. found in polymers of starch, cellulose, and glycogen. Excess glucose is converted to fat and stored.

D-Galactose is an aldohexose with the formula C6H12O6. is obtained from lactose, a disaccharide. has a similar structure to glucose except for the –OH on C4. is important in cell membranes of the brain and nervous system.

D-Fructose is a ketohexose with the formula C6H12O6. is the sweetest carbohydrate. is found in fruit juices and honey. is converted to glucose in the body.

Learning Check Draw the Fischer projection of D-fructose.

Solution Draw the Fischer projection of D-fructose.

Haworth Structures Haworth structures are cyclic hemiacetals. form when the C═O group and the –OH are part of the same molecule. of hexoses form when the –OH on C5 reacts with a C═O group. of a D-isomer place the –CH2OH of C6 above the ring.

Guide to Drawing Haworth Structures

Drawing the Haworth Structures for Glucose Step 1 Turn the Fischer projection for D-Glucose clockwise by 90.

Drawing the Haworth Structures for Glucose Step 2 Fold the horizontal carbon chain into a hexagon and bond the O on carbon 5 to the carbonyl group to form the hemiacetal.

Drawing the Haworth Structures for Glucose Step 3 Complete the Haworth structure by drawing the –OH group on carbon 1 below the ring to give the α anomer, or above the ring to give the β anomer.

Mutarotation of -D-Glucose and β-D-Glucose The process of rings opening and closing is called mutarotation. Both anomers are converted to the open chain and back to a cyclic hemiacetal.

Haworth Structures of Fructose is a ketohexose. forms Haworth structures when the –OH on C5 bonds to the C═O on C2.

Learning Check Draw the Haworth structure of -D-galactose.

Solution Step 1 Turn the open-chain Fischer projection clockwise by 90.

Solution Step 2 Fold the horizontal carbon chain into a hexagon and bond the O on carbon 5 to the carbonyl group to form the hemiacetal.

Solution Step 3 Complete the Haworth structure by drawing the –OH group on carbon 1 below the ring to give the α anomer, or above the ring to give the β anomer.

Urine Test for Diabetes A symptom of diabetes mellitus is a high level of glucose in the urine. If the blood level exceeds about 160 mg of glucose/dL of blood, the kidneys cannot reabsorb it all, and glucose spills over into the urine. The color of a test strip determines the glucose level in urine.

Important Disaccharides A disaccharide consists of two monosaccharides linked together. When two monosaccharides combine in a dehydration reaction, the product is an acetal and water.

Maltose Maltose is a disaccharide also known as malt sugar. composed of two D-glucose molecules. obtained from the hydrolysis of starch. linked by an -1,4-glycosidic bond between the  –OH on C1 of the first glucose and –OH on C4 of the second glucose. used in cereals, candies, and brewing. found in both the - and - forms.

Formation of Maltose

Lactose Lactose is a disaccharide of -D-galactose and - or -D-glucose. contains a -1,4-glycosidic bond. is found in milk and milk products. is found in both the - and -forms.

Lactose

Sucrose Sucrose, or table sugar, is obtained from sugar cane and sugar beets. consists of -D-glucose and -D-fructose. has an ,-1,2-glycosidic bond. is hydrolyzed in the body by the enzyme sucrase to form glucose and fructose.

Sucrose

Sucralose Sucralose is made from sucrose by replacing some of the hydroxyl groups with chlorine atoms.

Learning Check Identify the monosaccharides in each of the following. 1. lactose A. -D-glucose B. -D-fructose C. -D-galactose 2. maltose A. -D-glucose B. -D-fructose C. -D-galactose 3. sucrose A. -D-glucose B. -D-fructose C. -D-galactose

Solution Identify the monosaccharides in each of the following. 1. lactose A. -D-glucose C. -D-galactose 2. maltose A. -D-glucose 3. sucrose A. -D-glucose B. -D-fructose

Important Polysaccharides Important polysaccharides include starches made of -D-glucose molecules, amylose, and amylopectin. glycogen (animal starch in muscle), which is made of -D-glucose. cellulose (plants and wood), which is made of -D-glucose.

Structures of Amylose and Amylopectin

Amylose Amylose is a polymer of -D-glucose molecules. linked by -1,4 glycosidic bonds. a continuous (unbranched) chain.

Amylopectin Amylopectin is a branched chain polymer of -D-glucose molecules. has -1,4-glycosidic bonds between the glucose units. has -1,6 bonds to branches.

Glycogen Animal starch or glycogen is stored in the liver and muscles of animals. hydrolyzed in our cells to maintain glucose and energy levels between meals. similar to amylopectin, but is more highly branched.

Cellulose Cellulose is a major structural unit of wood and plants. is a polysaccharide composed of unbranched glucose units in unbranched chains. has -1,4-glycosidic bonds. cannot be digested by humans because humans lack the enzyme to break -1,4-glycosidic bonds.

Cellulose The polysaccharide cellulose is composed of glucose units connected by β-1,4-glycosidic bonds.