Carbohydrates – An Introduction

Definition

Carbohydrates may be defined chemically as an aldehyde or ketone derivatives of polyhydroxy (more than one hydroxy group) alcohol or as compounds that yield these derivatives on hydrolysis.

 

Structure

Carbohydrates may be aldehydes or ketones

Aldehyde carbohydrates

Glyceraldehyde

Ribose

Glucose

 

Ketonic carbohydrates

Dihydroxyacetone

Ribulose

Fructose

Classification

Carbohydrates are classified into three groups:

1. Monosaccharides
2. Oligosaccharides
3. Polysaccharides

Monosaccharides (Greek: Mono = one)

All these are simple sugars. They are sweet in nature and therefore are easily soluble in water. They also cannot be hydrolyzed into sugars. Chemically they’re either polyhydroxy aldehydes or ketones. All carbon atoms in a monosaccharide except one, possess a hydroxyl group. The rest of the carbon molecule is a part of an aldehyde group or a keto group. The glucose with aldehyde functional group is called aldo-sugar; as well as the keto functional group as keto-sugar. They are subdivided into two classes as follows:

1. Depending upon the number of carbon atoms they

possess, e.g.

– Trioses– Tetroses – Pentoses– Hexoses– Heptoses.

2. Depending upon the functional aldehyde (CHO) or

ketone (C=O) group present:

• – Aldoses
• – Ketoses

(as mentioned above in the subheading “structure”)

Trioses are intermediate in respiration and photosynthesis. Tetroses are rare in nature and mostly found in some bacteria. Pentoses and hexoses are the most common. From the biological point of view the most important hexose is glucose. It is an aldose sugar.

Glucose

Physiologically and biomedically, glucose is the most essential monosaccharide. In free state, sugar is found in every fruit, being abundant in dates, seeds, figs, and grapes.

Our blood glucose generally comprises 0.08% sugar levels. In combined form, it is found in many disaccharides and polysaccharides. Starch, cellulose, and glycogen yield glucose on complete hydrolysis.

Glucose is naturally produced in green plants which take carbon dioxide from the air and water from the soil to synthesize glucose by the process of photosynthesis.

The energy is stored in the glucose molecules as chemical energy and becomes available in all organisms when it is oxidized in the body.

Structure of glucose

 

Oligosaccharides (Greek: oligo = few)

Oligosaccharides consist of a short chain of monosaccharide units (2 to 10 units), joined together by a characteristic bond called glycosidic bond which, on hydrolysis, gives two to ten molecules of simple sugar (monosaccharide) units. Oligosaccharides are subdivided into different groups based on the number of monosaccharide units present.

The disaccharides which have two monosaccharide units are the most abundant in nature. Oligosaccharides with more than three subunits are usually found in glycoproteins; such as blood group antigens.

Disaccharides

• Disaccharides consist of two monosaccharide units.
• They are crystalline, water soluble, and sweet to taste.
• They are subclassified on the basis of the presence or absence of free reducing (aldehyde or ketone) groups

1. Reducing disaccharides with free aldehyde or keto group, e.g. maltose, lactose
2. Non-reducing disaccharides with no free aldehyde or keto group, e.g. sucrose.

Maltose

• Maltose contains two glucose molecules, joined by the glycosidic linkage between C-1 (the anomeric carbon) of one glucose residue and C-4 of the other, leaving one free anomeric carbon of the second glucose residue, which can act as a reducing agent. Thus, maltose is a reducing disaccharide.
• The numerical description like (1 → 4) of the glycosidic bond represents the number of carbon atoms that connect the two sugars.
• Maltose is produced as an intermediate product in the digestion of starch and glycogen by the action of the enzyme α-amylase.

 

Anomeric carbon The carbon derived from carbonyl compound as an aldehyde or ketone functional group is called anomeric carbon.

 

Sucrose (Common Table Sugar)

• Sucrose is a disaccharide of glucose and fructose. It is formed by plants but not by human beings. Sucrose is an intermediate product of photosynthesis. It is the commonly used table sugar.
• The anomeric carbon of both glucose and fructose are involved in the glycosidic bond. Sucrose is, therefore, a non-reducing sugar.
• Sucrose is hydrolyzed to fructose and glucose by an enzyme sucrase which is also called invertase.

 

POLYSACCHARIDES (GLYCANS)

Carbohydrates composed of ten or more monosaccharide units or their derivatives (such as amino sugars and uronic acids) are generally classified as polysaccharides. Polysaccharides are colloidal in size. In polysaccharides, monosaccharide units are joined together by glycosidic linkages. Another term for polysaccharides is “glycans”.

Polysaccharides are subclassified into two groups:

1. Homopolysaccharides (Homoglycans): When a polysaccharide is made up of several units of one and the same type of monosaccharide unit only, it is called homopolysaccharide.
2. Heteropolysaccharides (Heteroglycans): They contain two or more different types of monosaccharide units or their derivatives.

Homopolysaccharides or Homoglycans

Starch

It is the storage form of glucose in plants, e.g. in potato, in grains and seeds, and in many fruits. Starch is composed of two constituents viz. amylose and amylopectin.

Amylose

Amylose is a linear polymer of D-glucose units joined by α-1 → 4 glycosidic linkages.

Amylopectin

Amylopectin is structurally identical to those of amylose (α-1→ 4 glycosidic linkages) but with side chains joining them by α-1→ 6 linkages. Thus, amylopectin is a branched polymer having both α-(1 → 4) and α-(1 → 6) linkages. The branch points in amylopectin are created by α-1 → 6 bonds and occur at an interval of 20 to 30 units of glucose.

Heteropolysaccharides or Heteroglycans

Glycosaminoglycans (GAGs) or

Mucopolysaccharides

A GAG is an unbranched heteropolysaccharide, made up of repeating disaccharides. GAG is a polymer of [uronic acid-amino sugar] n.

This polymer is attached covalently to extracellular proteins called core protein to form proteoglycans. One component of which is always an amino sugar (hence the name glycosaminoglycans).

The other component of the repeating disaccharide (except in the case of keratan sulfate) is uronic acid, either L-glucuronic acid or its epimer L-iduronic acid.

They are major components of the extracellular matrix or ground substance. They act as “molecular sieves”, determining which substances enter and leave cells.

Sources of carbohydrates

Carbohydrates are available in a broad range of nutritious, healthy, and noxious foods including bread, legumes, milk, popcorn, potatoes, cookies, soft beverages, corn, and cherry pie.

They also come in many different forms. The most typical and abundant kinds are sugars, fibers, and starches.

Naturally, they can be obtained from sugarcane, vegetables like potatoes, grains, wheat, oats, barley, rice, and fruits. Dates are enriched with carbohydrates(glucose).

Functions of carbohydrates

• Foods high in carbohydrates are essential for healthy diet. Carbohydrates provide the body with glucose, which is converted to energy (ATP) used to support body functions, metabolism, and physical activity.
• The storage form of energy, e.g. glycogen in animal tissue and starch in plants
• Serve as a structural component, e.g. glycosaminoglycans humans, cellulose in plants, and chitin in insects
• Non-digestible carbohydrates like cellulose, serve as dietary fibers
• A constituent of nucleic acids RNA and DNA, e.g. ribose and deoxyribose sugar
• Play a role in lubrication, cellular intercommunication, and immunity
• Carbohydrates are also involved in detoxification, e.g. glucuronic acid.