21.5: Chemistry of Carbohydrates

Carbohydrates are an essential part of the diet in humans and animals. Grains, fruits, and vegetables are natural sources of carbohydrates that provide energy to the body, particularly through glucose, a simple sugar that is a component of starch and an ingredient in many staple foods. The stoichiometric formula (CH₂O)_n, where n is the number of carbons in the molecule represents carbohydrates. In other words, the ratio of carbon to hydrogen to oxygen is 1:2:1 in carbohydrate molecules. This formula also explains the origin of the term “carbohydrate”: the components are carbon (“carbo”) and water (“hydrate”). Carbohydrates can be classified into simple and complex. Monosaccharides and disaccharides are simple carbohydrates. Polysaccharides are complex carbohydrates.

Monosaccharides

Monosaccharides are simple sugars, the most common of which is glucose. In monosaccharides, the number of carbons usually ranges from three to seven. If the sugar has an aldehyde group (the functional group with the structure R-CHO), it is an aldose, and if it has a ketone group (the functional group with the structure RC(=O)R'), it is a ketose. Depending on the number of carbons in the sugar, they can be trioses (three carbons), pentoses (five carbons), and/or hexoses (six carbons).

Galactose and fructose are other common monosaccharides. Glucose, galactose, and fructose are isomeric monosaccharides (hexoses), meaning they have the same chemical formula but have slightly different structures. Glucose and galactose are aldoses, and fructose is a ketose.

Monosaccharides can exist as a linear chain or as ring-shaped molecules. In aqueous solutions, they are usually in ring forms. Glucose in a ring form can have two different hydroxyl group arrangements (OH) around the anomeric carbon (carbon 1 that becomes asymmetric in the ring formation process). If the hydroxyl group is below carbon number 1 in the sugar, it is in the alpha (α) position, and if it is above the plane, it is in the beta (β) position.

Disaccharides

Disaccharides form when two monosaccharides undergo a dehydration reaction (or a condensation reaction or dehydration synthesis). During this process, one monosaccharide's hydroxyl group combines with another monosaccharide's hydrogen, releasing a water molecule and forming a covalent bond. This is called a glycosidic bond. Glycosidic bonds (or glycosidic linkages) can be alpha or beta type. An alpha bond is formed when the OH group on the carbon-1 of the first glucose is below the ring plane, and a beta bond is formed when the OH group on the carbon-1 is above the ring plane. The most common disaccharide is sucrose, or table sugar, which is composed of glucose and fructose monomers.

Polysaccharides

A long chain of monosaccharides linked by glycosidic bonds is a polysaccharide. The chain may be branched or unbranched, and it may contain different types of monosaccharides. The molecular weight may be 100,000 daltons or more depending on the number of joined monomers. Starch, glycogen, cellulose, and chitin are examples of polysaccharides.

Plants store starch in the form of sugars. In plants, an amylose and amylopectin mixture (both glucose polymers) comprise these sugars. Starch comprises glucose monomers that are joined by α 1-4 or α 1-6 glycosidic bonds. The numbers 1-4 and 1-6 refer to the carbon number of the two residues that have joined to form the bond.

Glycogen is the storage form of glucose in humans and other vertebrates and is composed of monomers of glucose. Glycogen is the animal equivalent of starch and is a highly branched molecule usually stored in liver and muscle cells. Whenever blood glucose levels decrease, glycogen breaks down to release glucose.

Cellulose is the most abundant natural biopolymer. Cellulose mostly comprises a plant's cell wall. This provides the cell structural support. Wood and paper are mostly cellulosic in nature. Glucose monomers comprise cellulose that β 1-4 glycosidic bonds link

This text is adapted from Openstax, Biology 2e, Chapter 3.4: Carbohydrates.

Carbohydrates are compounds composed of carbon, hydrogen, and oxygen. These molecules get their name from the empirical formula of many monosaccharides that have two atoms of hydrogen and one atom of oxygen for every carbon.

Simple carbohydrates are monomers called monosaccharides and dimers called disaccharides. Complex carbohydrates are polymers called polysaccharides.

A monosaccharide can be classified as an aldose or a ketose by its functional group. Those having an aldehyde group are called aldoses and those having a ketone group are called ketoses.

Monosaccharides usually contain three to seven carbon atoms in their chains. Each of the carbons can be numbered starting with the number one from the end closest to the carbonyl carbon.

Sugars can form non-planar ring structures and in nature mostly exist in this cyclic form. Ring structures form when the carbonyl group reacts with a hydroxyl group at the opposite end of the molecule through a condensation reaction. The carbon attached to the reactive aldehyde or ketone is also known as the anomeric carbon.

These carbon-oxygen ring structures can exist as two anomers: alpha and beta. Anomers are structural isomers that differ in the configuration at the carbonyl carbon. In the alpha form, the hydroxyl group on the anomeric carbon and the CH₂OH group on the last carbon within the ring are located on opposite sides of the ring, and in the beta form, they are located on the same side.

A similar naming convention is used for bonds linking the monosaccharides to other molecules. 

Bonds formed below the ring structure are known as alpha linkages, whereas those above the ring are known as beta linkages. The numbers in the name of glycosidic linkages indicate the carbon numbers from the monosaccharides that are involved in the linkages.

When two monosaccharides link together through glycosidic linkages, they form disaccharides.

Glycosidic linkages are covalent bonds that are formed when the hydroxyl group of one monosaccharide reacts with the anomeric carbon of another monosaccharide with the elimination of a water molecule.

Polysaccharides or complex carbohydrates are a chain of monosaccharides held together by covalent bonds. Variation in the glycosidic linkages present in these molecules can result in different types of polysaccharides from the same monosaccharide.

Glucose molecules linked through β-1,4-glycosidic bonds form cellulose, a water insoluble polysaccharide. Glucose molecules linked through α-1,4-glycosidic bonds form amylose, a water-soluble polymer and a component of starch. Starch and cellulose are polysaccharides found in plants.

Glycogen is a water-insoluble molecule made of glucose linked through α-1,4-glycosidic bonds and intermittent branching thorough α-1,6-glycosidic bonds. Glycogen serves as a storage polysaccharide in animals.