What are Carbohydrates? Classification, Structures and Functions.

What are Carbohydrates?

carbohydrate is a combination of the “hydrates of carbon”. They are also known as “Saccharides” which is a derivation of the Greek word “Sakcharon” meaning sugar. The definition of carbohydrates in chemistry is as follows:
“Optically active polyhydroxy aldehydes or polyhydroxy ketones or substances which give these on hydrolysis are termed as carbohydrates”.

"carbohydrates or glucids are polyhydroxylated aldehydes or ketones, their derivatives and polymers

It is a group of organic compounds occurring in living tissues and foods in the form of starch, cellulose, and sugars. The ratio of oxygen and hydrogen in carbohydrates is the same as in water i.e. 2:1. It typically breaks down in the animal body to release energy.

Most of carbohydrates are present with a cyclic structure in nature, as a consequence of internal linkages between the carbonyl carbon (of the aldehyde or ketone group) with one of the hydroxyl groups in the same molecule.

Carbohydrates Formula

The general formula for carbohydrates is Cx(H2O)y.

Chemically, carbohydrates are defined as optically active polyhydroxy aldehydes or ketones or the compounds which produce units of such type on hydrolysis. Carbohydrates are also called saccharides which is a Greek word and it means sugar because almost all the carbohydrates have a sweet taste.

Sources of Carbohydrates

We know carbohydrates are an important part of any humans diet. Some common sources of carbohydrates are:

Potatoes
Maze
Milk
Popcorn
Bread

Classification of Carbohydrates

The different types of carbohydrates can be classified on the basis of their behavior on hydrolysis. They are mainly classified into three groups:

• Monosaccharides
• Disaccharides
• Polysaccharides    

Monosaccharides

Monosaccharide carbohydrates are those carbohydrates that cannot be hydrolyzed further to give simpler  thatis Monosaccharides are formed by a single molecule. It means that when hydrolyzed they can not release simpler molecules units of polyhydroxy aldehyde or ketone. If a monosaccharide contains an aldehyde group then it is called aldose and on the other hand, if it contains keto group then it is called as a ketose.  Examples of this group of carbohydrates are glucose, ribose and fructose, among others.

Structure of Carbohydrates – Glucose

Structure of Carbohydrates – Glucose
One of the most important monosaccharides is glucose. The two commonly used methods for the preparation of glucose are

From Sucrose: If sucrose is boiled with dilute acid in an alcoholic solution then we obtain glucose and fructose.
From Starch: We can obtain glucose by hydrolysis of starch and by boiling it with dilute H2SO4 at 393K under elevated pressure.
Glucose is also called aldohexose and dextrose and is abundant on earth.

Simple Carbohydrate - Glucose
Simple Carbohydrate – Glucose

Glucose is named as D (+)-glucose, D represents the configuration whereas (+) represents the dextrorotatory nature of the molecule.

The ring structure of glucose can explain many properties of glucose which cannot be figured by open chain structure.

The two cyclic structures differ in the configuration of the hydroxyl group at C1 called as anomeric carbon. Such isomers i.e. α and β form are known as anomers. The cyclic structure is also called pyranose structure due to its analogy with pyran. The cyclic structure of glucose is given below:

Cyclic Structure of Carbohydrates - Glucose


Structure of Carbohydrates – Fructose
It is an important ketohexose. The molecular formula of fructose is C6H12O6 and contains ketonic functional group at carbon number 2 and has six carbon atoms in a straight chain. The ring member of fructose is in analogy to the compound Furan and is named as furanose. The cyclic structure of fructose is shown below:

Examples of Carbohydrates

Here are a few examples of where you’ll find the most carbs:

Dairy Products – Yogurt, Milk, Ice cream
Fruits – Fruit juice or Whole fruit
Grains – Cereal, Bread, Wheat, Rice
Legumes – Plant-based proteins, Beans
Starchy Vegetables – Corn, Potatoes

Oligosaccharides:

 They are formed by 2-9 monomers linked through glycosidic linkages; in other words, when hydrolyzed these compounds release 2 to 9 monosaccharides (some texts say up to 20; in fact, oligosaccharides release “a few” monosaccharides).

According to the number of monosaccharides in the oligosaccharide, oligosaccharides can be dissacharides, trisaccharides, tetrasaccharides, etc. Disaccharides, formed by just 2 monosaccharides, are the most important subgroup of oligosaccharides. Disaccharides that appear in nature are lactose, or milk sugar (formed by galactose and glucose), and sucrose or table sugar (formed by fructose and glucose). Other important disaccharides are produced as result of starch digestion: maltose and isomaltose. These disaccharides are ,both of them, formed by two molecules of glucose, but linked in different ways. Cellobiose is a third dissacharide formed also by two molecules of glucose, but linked in such a way that animals can not break, unless animals have in the digestive system specific microorganisms that hydrolyze these linkages, as herbivors have (Cellobiose is formed as result of the digestion of cellulose).

Disaccharides

On hydrolysis, disaccharides yield two molecules of either same or different monosaccharide. The two monosaccharide units are joined by oxide linkage which is formed by the loss of water molecule and this linkage is called glycosidic linkage. Sucrose is one of the most common disaccharides which on hydrolysis gives glucose and fructose.

Maltose and Lactose (also known as milk sugar) are other two important disaccharides. In maltose, there are two α-D-glucose and in lactose, there are two β-D-glucose which are connected by oxide bond.

Polysaccharides

Polysaccharides contain long monosaccharide units joined together by glycosidic linkage. Most of them act as food storage for e.g. Starch. Starch is the main storage polysaccharide for plants. It is a polymer of α glucose and consists of two components-Amylose and Amylopectin.

Cellulose is also one of the polysaccharides that are mostly found in plants. It is composed of β-D- glucose units joined by a glycosidic linkage between C1 of one glucose unit and C4 of the next glucose unit.

Glycogen: These carbohydrates are stored mainly in the animal body. It is present in the liver, muscles, and brain. When the body needs glucose, enzymes break the glycogen.

Functions of carbohydrates

• They are used as material for energy storage and production.
• Starch and glycogen, respectively in plants and animals, are stored carbohydrates from which glucose can be mobilized for energy production. Glucose can supply energy both fueling ATP synthesis (ATP, the cell’s energy currency, has inside a phosphorylated sugar) and in the form of reducing power as NADPH.
• It should be noted that glucose, used as energy source, “burns” without yielding metabolic wastes, being turned in CO2 and water, and of course releasing energy.
• Monosaccharides supply 3.74 kcal/g, disaccharides 3.95 kcal/g, while starch 4.18 kcal/g; on average it is approached to 4 kcal/g.
• They exert a protein-saving action: if present in adequate amount in daily nourishment, the body does not utilize proteins for energy purpose, an anti-economic and “polluting” fuel because it will need to eliminate nitrogen (ammonia) and sulfur present in some aminoacids.
• Their presence is necessary for the normal lipid metabolism. More than 100 years ago Pasteur said: “Fats burn in the fire of carbohydrates“. This idea continues to receive confirmations from the recent scientific studies. Moreover, excess carbohydrates may be converted in fatty acids and triglycerides (processes that occur mostly in the liver).
• Glucose is indispensable for the maintenance of the integrity of nervous tissue (some central nervous system areas are able to use only glucose for energy production) and red blood cells.
• Two sugars, ribose and deoxyribose, are part of the bearing structure, respectively of the RNA and DNA and obviously find themselves in the nucleotide structure as well.
• They take part in detoxifying processes. For example, at hepatic level glucuronic acid, synthesized from glucose, combines with endogenous substances, as hormones, bilirubin etc., and exogenous substances, as chemical or bacterial toxins or drugs, making them atoxic, increasing their solubility and allowing their elimination.
• They are also found linked to many proteins and lipids. Within cells they act as signals that determine the metabolic fate or the intracellular localization of the molecules which are bound. On the cellular surface their presence is necessary for identification processes between cells that are involved e.g. in the recognition between spermatozoon and oocyte during fertilization, in the return of lymphocytes in the lymph nodes of provenance or still in the leukocyte adhesion to the lips of the lesion of a blood vessel.
• Two homopolysaccharides, cellulose (the most abundant polysaccharide in nature) and chitin (probably, next to cellulose, the second most abundant polysaccharide in nature), serve as structural elements, respectively, in plant cell walls and exoskeletons of nearly a million species of arthropods (e.g. insects, lobsters, and crabs).
• Heteropolysaccharides provide extracellular support for organisms of all kingdoms: in bacteria, the rigid layer of the cell wall is composed in part of a heteropolysaccharide contained two alternating monosaccharide units while in animals the extracellular space is occupied by several types of heteropolysaccharides, which form a matrix with numerous functions, as hold individual cells together and provide protection, support, and shape to cells, tissues, and organs.