What are carbohydrates?

Carbohydrates are the organic compounds that are obtained in foods and living matters in the shape of sugars, cellulose, and starch. The general formula of carbohydrates is C_n(H₂O)₂. The ratio of H and O present in carbohydrates is identical to water.

Origin of carbohydrates

Carbohydrates act as a key portion of the human diet. Some important origins of carbohydrates are bread, milk, potatoes, popcorn, and maze. Carbohydrates could be seen in food in several forms which include sugar alcohols, total sugars, and dietary fiber.

Subdivisions of carbohydrates

 The carbohydrates are mainly categorized into two which are,

• Simple carbohydrates.
• Complex carbohydrates.

Simple Carbohydrates

Simple carbohydrates could be seen in natural sugars. The food products which have simple carbohydrates are cookies, candy, soft drinks, and other sweet snacks. Due to their less complexity, simple carbohydrates are easy to handle.

The simple carbohydrates could be categorized into two which are,

• Monosaccharides.
• Disaccharides.

Monosaccharides

Monosaccharides are the simplest form of sugars, and the monosaccharides could not be hydrolyzed into the simpler form. Some essential monosaccharides are,

• Glucose.
• Fructose.
• Galactose.
• Mannose.

Glucose

Due to the hydrolysis of lactose, cane sugars, starch, and maltose, glucose is created in our bodies. The boiling of sucrose with an alcoholic solution and dilute acid will give glucose & fructose. Glucose is also synthesized by the hydrolysis of starch & by boiling with dilute sulfuric acid at 393K under a huge quantity of pressure. Generally, glucose could be found in fruit juices. Glucose is named as the sugar of the body and it could be called blood sugar. Blood sugar (glucose) could be seen in honey and blood. The structure of glucose could be depicted in the shape of a chain or ring. The figure below depicts the structure of glucose.

The glucose is termed as D(+) glucose where + depicts the dextrorotatory nature of the compound and D depicts the configuration of the molecule.

Fructose

Fructose could be naturally present in apples, honey, and tomatoes. Fructose is the sweetest monosaccharide. The hydrolysis of sucrose will give the fructose. The figure below depicts the structure of fructose.

Galactose

Galactose is a component of glycolipid and glycoprotein. Galactose is created in the mammary glands & is hydrolyzed to forms the milk’s lactose. The figure below depicts the structure of galactose.

Mannose

Mannose is created by the hydrolysis of mannosans and plant gums. Mannose is a component of prosthetic polysaccharides of globulins, albumins, and mucoproteins. The figure below depicts the structure of the mannose.

Disaccharides

The general molecular for disaccharides is C_n(H₂O)₂. The disaccharides contain two monosaccharides which are connected through a glycosidic linkage.

Maltose

The interaction of amylase with starch will give the maltose a transitional product. Maltose is made up of 2 residues of glucose which are connected through 1,4 linkage. Maltose can be seen in most of the seeds and germinating tissues in a measurable amount. The figure below depicts the structure of maltose.

Lactose

Lactose is found in milk. The hydrolysis of lactose will form D-glucose and D-galactose. It contains free anomeric carbon in the residue of glucose. Hence, lactose acts as a reducing sugar. The figure below depicts the structure of lactose.

Sucrose

Sucrose is the disaccharide of glucose & fructose. During hydrolysis, sucrose will produce D-fructose & D-glucose. This is called inversion since it is lead by the net change in optical rotation. The other name of sucrose is table sugar & it is hugely available in plants. The figure shown below depicts the structure of sucrose.

Complex Carbohydrates

Complex carbohydrates are the essential origin of energy in the human body. The complex carbohydrates give sustained fuel for our body, which is important for daily activities, exercise, and even rest. The complex carbohydrates could be seen in food like whole grains, legumes & vegetables. Whole grains are a class of complex carbohydrates, and the examples of whole grains are brown rice, barley, buckwheat, and oatmeal, and oatmeal is considered as healthiest one. Peanuts and beans are examples of legumes.

Polysaccharides are the example of complex carbohydrates which have hundred and thousands of monosaccharides.

Polysaccharides

Polysaccharides are the polymer of alpha glucose that is made up of two components that are amylopectin and amylose. Polysaccharides are synthesized by the polymerization of a huge number of monosaccharides. Cellulose is a polysaccharide, and it is usually present in plants. Polysaccharides are used as storage for food. Consider starch, it is a kind of polysaccharide which acts as the storage polysaccharides for plants.

Polysaccharides are composed of beta glucose units connected through the glycosidic linkage between the C-4  of the one glucose unit & the C-1 of the other glucose units. Glycans is the another name of a polysaccharide. On hydrolysis of polysaccharides with particular enzymes, will give the derivatives of monosaccharides. Based on the composition, the polysaccharides are categorized as follows:

• Homo glycans or homo polysaccharide.
• Hetero glycans or hetero polysaccharide.

Homoglycans

Homoglycans are complex carbohydrates that are produced by the polymerization of a single class of monosaccharide monomers.

Examples: Cellulose, starch, and glycogens are formed by a single class of monosaccharides called glucose.

Depending on the monosaccharide unit, the polysaccharide is named glycan that consists of glucose, fructon that is composed of fructose, xylan that is composed of xylose, araban that is composed of arabinose, and galactan composes galactose.

Heteroglycans

Heteroglycans are complex carbohydrates that are created by the type of condensation of either a single class of monosaccharide monomer or monosaccharide derivatives.

Examples: Some important examples of heteroglycan are arabanogalactans, chitin, agar, arabanoxylans, and peptidoglycan, etc.

Carbohydrate metabolism

Carbohydrate metabolism is the sequence of biochemical activities which is accountable for the interconversion, formation, and breakdown of carbohydrates in living organisms. Glucose is metabolized by almost all organisms. Carbohydrates are part of a large number of metabolic pathways across species. The carbohydrates are synthesized in plants from H₂O and CO₂ by the method of photosynthesis and the absorbed energy could be stored as starch & lipid.

Carbohydrate cetabolism

Carbohydrate cetabolism is also a metabolic reaction where cells undergo to break down huge molecules and extracting energy. The cetabolism of monosaccharides includes two main metabolic pathways that are glycolysis and citric acid cycle.

Uses of carbohydrates

Carbohydrates act as the energy source of our body and, it is used as fuel for heart muscles, brain, central nervous system, and kidneys.

Carbohydrates are useful in digestion. When we take food having carbohydrates, our digestive system will break down the digestible ones to sugar that enters into the blood. When the blood sugar level increases, the pancreas will form insulin, which is a hormone that brings about cells to hold blood sugar for storage and energy. The deficiency of carbohydrates will cause diseases like fatigue, nausea, headaches, bad breath, difficulties in concentration, weakness, mineral and vitamin deficiencies.

Context and Applications

This topic is important for both postgraduate and undergraduate courses, particularly for Bachelors and Masters in Chemistry, and Bachelors and Masters in Biochemistry.

Practice Problems

Question 1: The type of carbohydrate which could not be hydrolyzed further is called as

1. Polysaccharides
2. Disaccharides
3. Monosaccharides
4. Proteoglycan

Answer: Option 3 is correct.

Explanation: Among the given carbohydrates, the monosaccharide is the simplest kind of carbohydrate that could not be hydrolyzed further whereas the other carbohydrates such as polysaccharide, disaccharide, and proteoglycan can be hydrolyzed further.

Question 2: Among the following, which is considered as non-sugars?

1. Disaccharides
2. Polysaccharides
3. Monosaccharides
4. Oligosaccharides

Answer: Option 2 is correct.

Explanation: Polysaccharides are formed by the condensation of hundreds or thousands of monosaccharides. These polysaccharides are not sweet in taste. Hence they are termed as non-sugars.

Question 3: Among the following, which sugar is called invert sugar?

1. Fructose
2. Dextrose
3. Sucrose
4. Glucose

Answer: Option 3 is correct.

Explanation: Sucrose is called invert sugar. Because on hydrolysis of sucrose with hot dilute acid it forms an equimolar combination of D-fructose and D-glucose.

Question 4: Among the following biomolecules, which is considered as the staff of life?

1. Vitamins
2. Proteins
3. Carbohydrates
4. Lipids

Answer: Option 3 is correct.

Explanation: Carbohydrates are considered the staff of life. Because carbohydrates are the most abundant organic compounds present in living organisms.

Question 5: Among the following, the simplest carbohydrate is

1. Gulose
2. Glucose
3. Glyceraldehyde
4. Dihydroxy acetone

Answer: Option 3 is correct.

Explanation: Glyceraldehyde is the simplest carbohydrate because it is a triose monosaccharide with the chemical formula C₃H₆O₃.