Carbohydrates: Structure, Properties, and Biological Significance

Carbohydrates

Introduction

Carbohydrates are a fundamental class of biomolecules, essential for energy storage, structural integrity, and cellular communication in living organisms. They are classified into monosaccharides, disaccharides, and polysaccharides based on their complexity and structure.

Chemical Properties of Monosaccharides

Oxidation of Monosaccharides

Monosaccharides in aqueous solution exist in equilibrium between their cyclic and open-chain forms. The open-chain form contains an aldehyde group (in aldoses) that can be oxidized to a carboxylic acid by oxidizing agents such as Benedict’s solution.

• Oxidation Reaction: The aldehyde group is converted to a carboxylic acid, producing a sugar acid.

• Reducing Sugars: Monosaccharides capable of reducing other substances (e.g., D-glucose) are termed reducing sugars.

• Naming: The suffix -ose is replaced with -onic acid for the oxidized product.

• Example: D-glucose oxidized by Cu2+ yields D-gluconic acid and Cu2O (brick red precipitate).

Oxidation: Fructose to Glucose

Fructose, a ketohexose, typically cannot be oxidized directly. However, in basic conditions (Benedict’s solution), it rearranges to form glucose, which can then be oxidized.

• Rearrangement: The ketone group on carbon 2 and hydroxyl group on carbon 1 of fructose rearrange to form an aldehyde group.

• Result: Fructose is converted to glucose, which is then oxidized.

Reduction of Monosaccharides

Reduction of the carbonyl group in monosaccharides produces sugar alcohols (alditols).

• Reaction: D-glucose is reduced to D-glucitol (D-sorbitol).

• Naming: The suffix -ose is replaced with -itol for sugar alcohols.

• Applications: Sugar alcohols are used as sweeteners in sugar-free products.

Examples of Sugar Alcohols

• D-sorbitol: From D-glucose

• D-xylitol: From D-xylose

• D-mannitol: From D-mannose

• Uses: Found in diet drinks, sugarless gum, and other sugar-free products.

Chemistry Link to Health: Testing for Glucose

Glucose in Urine (Glucosuria)

Normally, blood glucose is reabsorbed by the kidneys. When blood glucose exceeds ~160 mg/dL, excess glucose is excreted in urine, a condition called glucosuria.

• Clinical Test: Glucose in urine is detected using a paper strip containing 2-methylaniline and enzymes (glucose oxidase, peroxidase).

• Reaction: Glucose oxidase catalyzes the oxidation of glucose, producing hydrogen peroxide, which reacts with dye to produce a color change.

• Interpretation: The intensity of the color correlates with glucose concentration.

Table: Glucose Oxidase Test Results

Color	% (m/v)	mg/dL
Blue	0	0
Blue-green	0.10	100
Green	0.25	250
Green-brown	0.50	500
Brown	1.00	1000
Dark brown	2.00	2000

Disaccharides

Formation and Structure

Disaccharides are formed by the linkage of two monosaccharides via a dehydration reaction, producing a glycosidic bond.

• Common Disaccharides: Maltose, lactose, sucrose

• Formation: Glucose + glucose → maltose + H2O; Glucose + galactose → lactose + H2O; Glucose + fructose → sucrose + H2O

Maltose

Maltose (malt sugar) is composed of two α-D-glucose molecules linked by an α-1,4-glycosidic bond. It is obtained from starch hydrolysis and used in cereals, candies, and brewing.

Lactose

Lactose (milk sugar) is a disaccharide found in milk, composed of β-D-galactose and α- or β-D-glucose, linked by a β-1,4-glycosidic bond.

• Content: 6–8% in human milk, 4–5% in cow’s milk

Sucrose

Sucrose (table sugar) is obtained from sugar cane and beets. It consists of α-D-glucose and β-D-fructose linked by an α,β-1,2-glycosidic bond. Sucrose cannot form an open chain and is not a reducing sugar.

Sweetness of Sweeteners

Relative Sweetness

Sugars and artificial sweeteners differ in sweetness, measured relative to sucrose (assigned a value of 100).

Table: Relative Sweetness of Sugars and Artificial Sweeteners

Type	Sweetness Relative to Sucrose (=100)
Galactose	30
Glucose	75
Fructose	175
Lactose	16
Maltose	33
Sucrose	100
Sorbitol	60
Maltitol	80
Xylitol	100
Stevia	15,000
Aspartame	18,000
Saccharin	45,000
Sucralose	60,000
Neotame	1,000,000
Advantame	2,000,000

Artificial Sweeteners

• Sucralose: Marketed as Splenda, made from sucrose by replacing hydroxyl groups with chlorine atoms.

• Aspartame: Marketed as NutraSweet and Equal, composed of aspartate and a methyl ester of phenylalanine.

Disaccharide Example: Melibiose

Melibiose is a disaccharide 30 times sweeter than sucrose, linked by an α-1,6-glycosidic bond.

Polysaccharides

Structure and Types

Polysaccharides are polymers of monosaccharides, primarily glucose. Major types include amylose, amylopectin, glycogen, and cellulose.

• Amylose: Straight-chain polymer of α-D-glucose, connected by α-1,4-glycosidic bonds.

• Amylopectin: Branched-chain polymer, connected by α-1,4- and α-1,6-glycosidic bonds.

• Glycogen: Highly branched polymer, similar to amylopectin but with more frequent α-1,6 branches.

• Cellulose: Unbranched polymer of glucose, connected by β-1,4-glycosidic bonds; major structural unit in plants.

Amylose

Amylose makes up about 20% of starch and consists of 250–4000 α-D-glucose units in a continuous chain, coiled in a helical fashion.

Amylopectin

Amylopectin makes up 80% of starch, is branched, and contains both α-1,4 and α-1,6-glycosidic bonds. Starches hydrolyze easily to dextrins, maltose, and glucose.

Glycogen

Glycogen is stored in the liver and muscles of animals, hydrolyzed to maintain blood glucose levels. It is more highly branched than amylopectin, with α-1,6 branches every 10–15 glucose units.

Cellulose

Cellulose is the major structural unit of wood and plant fibers. It is insoluble in water, resistant to hydrolysis, and cannot be digested by humans due to the β-1,4-glycosidic bonds.

Learning Check: Polysaccharides and Glycosidic Bonds

• Amylose: α-1,4-glycosidic bonds

• Amylopectin: α-1,4 and α-1,6-glycosidic bonds

• Cellulose: β-1,4-glycosidic bonds

• Glycogen: α-1,4 and α-1,6-glycosidic bonds

Carbohydrates—Concept Map

A concept map summarizes the classification and relationships among monosaccharides, disaccharides, and polysaccharides, highlighting their structural features and biological roles.