Carbohydrates-Classification, Sources and Examples

Carbohydrates are all around us, in the rice we eat, in the milk we drink to the flowers we adore, and even in the protective shell of cockroaches. We have also heard of carbohydrates while talking about weight loss and sugar-free diets.

So what exactly are carbohydrates and what makes them so diverse and abundant? Let’s have a look.

What are Carbohydrates

Carbohydrates are organic molecules made up of carbon, oxygen, and hydrogen. They are one of the main sources of energy for our body and constitute the major portion of our diet.

Chemically they are poly-hydroxy aldehydes (-CHO functional group) or ketones (-C=O- functional group) or molecules that produce these upon hydrolysis.

They are mainly produced in plants and can be in the form of sugars, fibers, or starches.

They are capable of taking up various roles in nature including structural (cellulose in plants), energy storage (starch in plants glycogen in animals), coenzymes (ribose in RNA) and they even form the backbone of the genetic material.

They are also found commonly associated with other biomolecules in the body such as proteins and lipids to form functionally important molecules.

They generally have an empirical formula (Ratio of atoms in a molecule) of Cm(H2O)n where m and n may be the same or different.

However, not all molecules following this formula belong to the class of carbohydrates. There also might be some carbohydrates such as the deoxyribose in DNA that does not follow this rule.

Classification of Carbohydrates

On the basis of structure and degree of polymerization, carbohydrates are of two types: Simple Carbohydrates and Complex Carbohydrates.

Simple Carbohydrates

As the name suggests, simple carbohydrates have a simple structure and a low degree of polymerization. Simple carbohydrates have only one or two sugar molecules. Molecules of simple carbohydrates are digested and convert quickly with a rise in blood sugar level.

Simple carbs do not have vitamins, minerals, and fiber, that is why they are also known as empty calories. Dairy products, fruits, beer, candies, etc have high amounts of simple carbohydrates.

Monosaccharides, Disaccharides, and Oligosaccharides are the type of simple Carbohydrates.

Monosaccharides

• They are the simplest carbohydrates found and cannot be hydrolyzed further.
• Monosaccharides are generally colorless and water-soluble.
• Aldose is a monosaccharide with aldehyde group [ -CHO ].
• Ketose is a monosaccharide with a keto group [ =C=O ].

There are about 20 monosaccharides found to occur in nature and some of the important ones are-

Glucose – It is an aldohexose (six-carbon sugar with aldehyde functional group) and is the main source of energy during metabolism. Glucose acts as a monomer for a lot of larger carbohydrates such as starch and cellulose.

Fructose – It is a ketohexose (six-carbon sugar with ketone functional group) and is present in a lot of ripe fruits and honey. It forms a bond along with glucose to form the commonly used sugar –sucrose.

Ribose – Ribose naturally occurs in the form of d-Ribose. d-Ribose is a part of ribonucleotides, which in turn builds RNA. Ribose is an essential compound for coding, decoding, regulation, and expression of genes. Ribose is related to deoxyribose, which is found in DNA.

Galactose -Galactose is also a monosaccharide. When we combine Galactose with Glucose, through a condensation reaction, it results in lactose. Lactose is a disaccharide.

The hydrolysis of lactose takes place in the presence of catalysts, namely, lactase and β-galactosidase. The products of hydrolysis are galactose and glucose.

Oligosaccharides

• Oligosaccharides are the carbohydrates that yield 2 to 9 monosaccharides on hydrolysis.
• Further classification of Oligosaccharides depends on the number of monosaccharides they produce on hydrolysis. These are disaccharides, trisaccharides, tetrasaccharides, etc.
• Disaccharides are the most common type of oligosaccharides.

Disaccharides

• They are carbohydrates with two monomeric units joined together with a glycosidic bond and removal of a water molecule.
• If we hydrolyze disaccharides, we will get two molecules of monosaccharides. These monosaccharides can be similar or different.
• The most common disaccharide is Sucrose. Hydrolysis of Sucrose produces glucose and fructose. Cane sugar is an abundant source of sucrose.
• Some other important disaccharides are Maltose and Lactose.
• Hydrolysis of Maltose gives 2 glucose molecules. Similarly, Hydrolysis of Lactose gives one molecule of glucose and one molecule of Galactose. Lactose is abundantly available in milk.

Commonly known disaccharides are-

Sucrose – Glucose + Fructose; (Found in cane Sugar)
Maltose – Glucose + Glucose;
Lactose– Glucose + Galactose; (Found in milk)

Complex Carbohydrates

Complex carbohydrates have a complex structure and a high degree of polymerization. They have two or more sugar molecules. Molecules of complex carbohydrates digest and convert slowly.

They are readily available in potatoes, beans, peanuts, cereals, whole wheat products, etc. They also go by the name of starchy food.

Polysaccharides are the form of complex carbohydrates.

Polysaccharides

• They are carbohydrates with many monomeric units joined together with a glycosidic bond. They are mostly insoluble in water and are high molecular weight carbohydrates.
• Polysaccharides yield a large number of monosaccharides on hydrolysis.
• Although Polysaccharides have long chains of sugar still they are called non-sugar because they are not sweet in taste.
• Polysaccharides assume the shape of threads and monosaccharides act as the building blocks.
• The most common examples of polysaccharides are Starch, Cellulose, Glycogen, and Gums.

Starch

• It is formed by the condensation of amylose and amylopectin and is a polymer of branched glucose units.
• It is the main storage molecule in plants.
• Starch is the most common carbohydrate in nature.
• Starch predominantly acts as food storage or structural material.
• Cereals, roots, tubers, etc. are good sources of starch. It is very essential for human beings.
• Starch has two primary components – Amylose and Amylopectin.
• Amylose makes about 15-20% of starch. Amylose is a water-soluble constituent of starch.
• Amylopectin makes about 80-85% of the starch. Amylopectin is insoluble in water.

Cellulose

• The cell wall of plants is made of cellulose, a long unbranched chain of glucose subunits.
• Cellulose is a homopolymer.
• Cellulose is constituent of only plants. It is not available anywhere else.

Glycogen

• Glycogen is the main storage molecule in animals and is also a polymer of glucose units.
• The structure of Glycogen is similar to the amylopectin with some extra branches.
• Glycogen is present in the liver, muscles, and brain of the animals.
• It is also known as animal starch.
• Glycogen breaks down to glucose and glucose produces energy.
• It is also present in yeast and fungi.

While the ones mentioned above have a single monomeric unit (Homopolysaccharides), there are polysaccharides with multiple monomeric units and are known as Heteropolysaccharide.

Functions of Carbohydrates

• Carbohydrates are the basic components of the food.
• The main function of carbohydrates is to provide energy.
• Carbohydrates help in preventing ketosis.
• Carbohydrates also help in the metabolism of fats.
• They slow down the breakdown of proteins for energy as they are the primary source of energy.

Carbohydrates in Health

The complex carbohydrates consumed are digested by various enzymes in our digestive tract to release monosaccharides into the blood (especially glucose). In the presence of a hormone known as insulin, the glucose present in the blood is taken up by various cells in our body that store it as glycogen.

When needed, these glycogen molecules are broken down into glucose that enters the carbon metabolism cycle providing ATPs in the end. The ATPs act as energy carriers in the cell and provide energy for various functions that occur in our bodies.

Despite the common misconception that a diet low in carbohydrates may lead to weight loss, there is no clinical evidence to prove the same. While restriction in calorie intake might slow down fat accumulation, the change in the ratio of macromolecules consumed might not.

The effects of a low carbohydrate diet on body and health are not well studied and might have an effect on cardiovascular health or gastrointestinal health.

Glycemic index is a measure of the time the various carbohydrate-rich food takes to increase the blood glucose levels. While high GI food such as flour and potatoes can release large amounts of glucose relatively fast, low GI foods like rice increase blood glucose levels slowly.

Eating too many GI rich foods will cause sudden spikes in blood glucose levels and might lead to the onset of obesity, type2 diabetes, and heart diseases.

Sources of Carbohydrates

There are abundant sources of carbohydrates present in nature. They are almost everywhere. Some of the common sources are:

• Dairy products are the source of galactose and lactose.
• Sucrose is present in honey and sugar.
• Cereals, Potatoes, pasta, etc. are the rich source of Maltose.
• Fructose is present in many fruits.

Refining and processing of the products decrease their nutrition value and they are labelled as “enriched”. The best way to consume carbohydrates, vitamins, etc. is to consume them in their natural form.

Carbohydrate Foods

As the saying goes, “Healthy mind resides in a healthy body”. To achieve a healthy body, we must eat healthy food and a balanced diet. Too much of everything is bad. The same goes for carbohydrates. A high amount of sugar increases calories abnormally.

High calories result in obesity and low calories result in malnutrition. It is essential to know the difference between good and bad carbohydrates.

Conclusion

In this article, we learned about carbohydrates. We learned about their classifications into simple and complex carbohydrates. We then learned about monosaccharides, oligosaccharides, disaccharides in simple, and polysaccharides with some specific carbohydrates in the complex.

We also learned about the functions and sources of carbohydrates, carbohydrates in health, and carbohydrate foods. Try recalling each one of them.