Carbohydrates

What are Carbohydrates?

Carbohydrates also called saccharides or carbs are the most abundant organic compounds in nature that provide energy for our bodies. Carbohydrates primarily contain carbon, hydrogen, and oxygen in their structure. A carbohydrate is a polyhydroxy aldehyde or ketone or compound produced by hydrolysis. Carbohydrates are mainly found in plant foods and occur in dairy products in the form of sugar called lactose. The term sugar is applied to carbohydrates soluble in water and sweet in taste.

The food products bread, pasta, beans, potatoes, rice, and cereals contain a high level of carbohydrates that participate in a wide range of biological functions in our bodies. Carbohydrates or polysaccharides are important components of our bodies that are involved mainly in the development of the immune system, health disease, blood clotting, and reproduction. It also provides energy to control several biological functions.

Benefits of Carbohydrates

Like proteins and fats, carbohydrates are one of the three main nutrients found in our daily foods and drinks. They are beneficial for our bodies and participate in a wide range of biological functions.

The most common biological functions of carbohydrates may include:

• Carbohydrates are the most abundant dietary source of energy for all organisms present on our earth. Our body breaks down carbohydrates into glucose which is the main source of energy for your body’s cells, tissues, and organs.
• The monosaccharide ribose is an essential component of adenosine triphosphate (ATP), flavin adenine dinucleotide (FAD), nicotinamide adenine dinucleotide (NAD), nicotinamide adenine dinucleotide phosphate (NADP), and the genetic molecule ribonucleic acid (RNA). Deoxyribose is an essential component of deoxyribonucleic acid (DNA).
• Carbohydrates are the precursors for many organic substances such as amino acids and perform numerous roles in living organisms.
• Carbohydrates such as glycoproteins and glycolipids are the components of the cell membrane. Therefore, they participate in various cellular functions such as cell growth, adhesion, and fertilization.
• Carbohydrates are the structural components of many organisms. These include the cellulose (fiber) of plants, the exoskeleton of some insects, and the cell wall of microorganisms.
• The storage form of carbohydrates or glycogen fulfills immediate demands of energy for our body.

Classification of Carbohydrates

The name carbohydrates or saccharides can be obtained from the Ancient Greek word sákkharon means ‘sugar’. They are a group of organic compounds that includes sugars, starch, and cellulose. Many saccharide forms of carbohydrates and their derivatives can play important biological roles in the development of immune health, fertilization processes, pathogenesis, blood clotting, and development.

Carbohydrates or saccharides are broadly classified into four major groups based on the number of sugar units.

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

Monosaccharides and oligosaccharides are sweet in taste, crystalline in nature, and soluble in water. Monosaccharides and disaccharides are the smallest carbohydrates that generally contain sugar units in their structure.

Polysaccharides are polymers of monosaccharide units with high molecular weight and form colloids with water. They are usually tasteless in nature.

Monosaccharides

Monosaccharides or simple sugars are the simplest groups of carbohydrates with the general molecular formula C_n(H₂O)_n. The monosaccharides are further divided into various categories based on the functional group and the number of carbon atoms.

Based on the functional group, they are mainly two types,

1. Aldoses: When the functional group in monosaccharides is an aldehyde, they are called aldoses. Glyceraldehyde and glucose are two common examples of aldoses.
2. Ketoes: When the functional group in monosaccharides is a ketone, they are called ketoes. Dihydroxyacetone and fructose are examples of ketoes.

Based on the number of carbon atoms, the monosaccharides are following types,

Monosaccharides	Empirical formula	Aldose	Ketose
Trioses	C3H6O3	Glyceraldehyde	dihydroxyacetone
Tetroses	C4H8O4	Erythrose	Erythrulose
Pentoses	C5H10O5	Ribose	Ribulose
Hexoses	C6H12O6	Glucose	Fructose
Heptoses	C7H14O7	Glucoheptose	Sedoheptulose

These terms with functional groups are used while naming carbohydrates. For example, glucose is an aldohexose while fructose is an aldoketose.

Disaccharides

A disaccharide contains two similar or dissimilar monosaccharide units held together by a glycosidic bond. They are crystalline and sweet in nature and soluble in water.

The disaccharides are two types:

1. Reducing disaccharides with a free aldehyde or keto group such as maltose and lactose.
2. Non-reducing disaccharides without a free aldehyde or keto group such as sucrose and trehalose.

Oligosaccharides

The name oligosaccharides is obtained from the Greek word oligo means few. An oligosaccharide is a saccharide polymer containing three to ten monosaccharides or simple sugar molecules.

Oligosaccharides are a type of carbohydrate that is found naturally in various plant foods such as fruits and vegetables. Therefore, it is possible to eat them regularly but they are relatively less abundant in the diet when compared to other more common carbohydrates.

Oligosaccharides with more than 3 monosaccharide units are not usually digested by human enzymes. These types of carbohydrates cannot be broken down by the human digestive tract. They travel through your gut to the colon and feed or support the growth of beneficial bacteria.

Raffinose is an example of a trisaccharide found in vegetables and whole grains. This type of carbohydrate contains galactose, glucose, and fructose. It is a type of carbohydrate that is considered a good source of prebiotic fiber (bacterial food).

Polysaccharides

The name polysaccharides is obtained from the Greek word poly means many. An oligosaccharide is a saccharide polymer containing three to ten monosaccharides or simple sugar molecules. Polysaccharides are long chains of carbohydrate molecules that contain several smaller monosaccharide molecules.

These complex types of bio-macromolecules provide energy in animal cells and form a structural component of a plant cell. Carbohydrates may play important biological roles in living organisms. Polysaccharides such as starch and glycogen can store energy for animals while cellulose is the main structural component of plants.

Examples of Carbohydrates

Foods that contain high levels of carbohydrates are an important part of our healthy diet because they are the most abundant dietary source of energy for all organisms. They have come in a variety of forms but the most common and abundant forms of carbohydrates are sugars, fibers, and starches.

Examples of Monosaccharides

Some important examples of carbohydrates or saccharides with their occurrence and biochemical importance are listed below in the table,

Carbohydrate		Occurrence	Biochemical importance
Trioses	Glyceraldehyde	Phosphate form is found in cells	Glyceraldehyde 3-phosphate is an intermediate in glycolysis.
	Dihydroxyacetone		It’s 1-phosphate functioning as an intermediate in glycolysis
Tetroses	D-Erythrose	Widespread	Its 4-phosphate is an intermediate in the HMP pathway of carbohydrate metabolism
Pentoses	D-Ribose	Found widely as a constituent of RNA and nucleotides	For the structure of RNA, coenzymes (ATP, NAD, and NADP)
	D-deoxyribose	It is found in DNA	For the structure of DNA
	D-Ribulose	Produced during metabolism	It is an important metabolite in hexose monophosphate shunt
	D-Xylose	It is found in glycoproteins and gums	Involved in the formation of glycoproteins
	L-Xylulose	It is an intermediate in the uronic acid pathway	Excreted in urine in essential pentosuria
	D-Lyxose	Heart muscle	It is a constituent of lyxoflavin in the heart muscle.
Hexoses	D-Glucose	It is a constituent of starch, glycogen, cellulose, maltose, lactose, and sucrose.	Sugar fuel of life and structural unit of cellulose in plants.
	D-Galactose	It is a constituent of lactose (milk sugar)	Converted to glucose, failure leads to galactosemia
	D-Mannose	These types of carbohydrates are found in plant polysaccharides and animal glycoproteins	For the structure of polysaccharides
	D-Fructose	It is a constituent of sucrose and inulin	Its phosphate forms are intermediate of glycolysis
Heptoses	D-Sedoheptulose	Found in plants	Its 7-phosphate is an intermediate in hexose monophosphate shunt and photosynthesis

Examples of Disaccharides

Disaccharide		Occurrence	Biochemical importance
Sucrose		It is a constituent of cane sugar and beet sugar, pineapple	The most commonly used table sugar supplies calories
Lactose		Milk sugar	Exclusive carbohydrate source for breastfed infants. Lactase deficiency leads to flatulence
Maltose		The product of starch hydrolysis occurs in germinating seeds	An important intermediate in the digestion of starch

Nutrition Sources of Carbohydrates

Carbohydrates are important types of food nutrients found in a wide variety of natural and processed foods. Like vitamins and minerals, carbohydrates are nutrients that cannot be produced by the human body. Therefore, we should take them through our daily diet.

There are both healthy and unhealthy sources of carbohydrates or polysaccharides found in our daily food items.

1. Healthy sources: Animal and plant-based products such as fresh fruits, vegetables, corn, potatoes, milk, and milk products are the main healthy sources of carbohydrates.
2. Unhealthy sources: Unhealthy sources may include soda, white bread, artificial sugar, pastries, etc.

They can be found in different forms such as sugars, starch, and fibers.

1. Sources of Sugars: Sugars appear in the human diet mainly in the form of table sugar (sugarcane or sugar beets), lactose (milk), glucose, and fructose. These types of carbohydrates occur naturally in honey, many fruits, and some vegetables. Carbohydrates containing items such as table sugar, milk, or honey are added to drinks and many prepared foods such as jam, biscuits, and cakes.
2. Sources of Starch: Starch is a carbohydrate or polysaccharide that is found abundantly in plants based foods such as cereals (wheat, maize, rice), potatoes, and processed food based on cereal flour, such as bread, pizza, or pasta.
3. Sources of Fibers: Cellulose is a carbohydrate that is one of the main components of insoluble dietary fiber. These types of carbohydrates are not digestible by humans but help to maintain a healthy digestive system by facilitating bowel movements.