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 provid…
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.
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Q1: What is the chemical composition of carbohydrates?
Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen in a 1:2:1 ratio, represented by the formula (CH2O)n. This stoichiometric formula explains the term 'carbohydrate': carbon ('carbo') and water ('hydrate'). The hydrogen and oxygen atoms are typically present in the same proportions as in water molecules.
Q2: How do aldoses and ketoses differ in structure?
Aldoses and ketoses are monosaccharides classified by their functional groups. Aldoses contain an aldehyde group (R-CHO) at the end of the carbon chain, while ketoses contain a ketone group (RC(=O)R') typically at carbon 2. Both types usually have three to seven carbons, but their different functional groups affect their chemical properties and reactivity.
Q3: What are alpha and beta anomers in ring-form sugars?
Alpha and beta anomers are structural isomers that differ in the hydroxyl group orientation on the anomeric carbon (carbon 1) after ring formation. In the alpha form, the hydroxyl group and the CH2OH group are on opposite sides of the ring; in the beta form, they are on the same side. These different configurations affect how monosaccharides link together.
Q4: How are disaccharides formed from monosaccharides?
Disaccharides form when two monosaccharides undergo a condensation reaction, where a hydroxyl group from one sugar combines with a hydrogen from another, releasing water and forming a glycosidic bond. This covalent bond links the anomeric carbon of one monosaccharide to a hydroxyl group on the other, creating dimers like sucrose, which consists of glucose and fructose.
Q5: Why do different glycosidic linkages create different polysaccharides?
Polysaccharides are chains of glucose linked by glycosidic bonds, and variation in bond type and position produces distinct molecules. Glucose linked through β-1,4-glycosidic bonds forms cellulose, a water-insoluble structural polymer. Glucose linked through α-1,4-glycosidic bonds forms amylose, a water-soluble starch component. These differences in addition and condensation polymerization determine the polysaccharide's properties and function.
Q6: What is the structural difference between starch and glycogen?
Starch and glycogen are both glucose polymers but differ in branching and solubility. Starch, found in plants, comprises amylose and amylopectin linked by α-1,4-glycosidic bonds with minimal branching. Glycogen, the animal storage form, is highly branched with both α-1,4 and α-1,6-glycosidic bonds, making it more compact and readily mobilized when blood glucose levels decrease.
Q7: Why is cellulose insoluble in water while starch is soluble?
Cellulose and starch differ in their glycosidic linkage types, which affects their three-dimensional structure and water interactions. Cellulose uses β-1,4-glycosidic bonds, creating a linear, rigid structure that cannot dissolve in water. Starch uses α-1,4-glycosidic bonds, forming a more compact, helical structure that allows water molecules to interact with hydroxyl groups, making it water-soluble.