carbohydrate | Definition, Classification, & Examples

Carbohydrates are biological molecules composed mainly of carbon, hydrogen and oxygen. They are the most abundant organic compounds in the biosphere. Product of photosynthesis, where solar energy is converted into chemical energy, carbohydrates are the main energy source of heterotrophic organisms.

Carbohydrates are known as carbohydrates , because the general chemical structure looks like a carbon with a water molecule C n (H 2 O) n . They are also called saccharides or sugars . The word “saccharide” derives from the Greek sákcharon meaning “sugar”.

Basic structure of carbohydrates

Like all biomolecules, carbohydrates have three basic elements: carbon (C), hydrogen (H) and oxygen (O). There are two chemical groups that characterize carbohydrates: the carbonyl group (-C = O) and the hydroxyl group (-OH). Carbonyl is found in aldehydes and ketones; the hydroxyl is found in alcohols.

A carbohydrate can then be an aldehyde or a ketone:

  • if the carbonyl group is at one end it is an aldehyde ;
  • if the carbonyl group is in the intermediate carbons it is a ketone .

Additionally, a carbohydrate has several hydroxyl groups, so they can be considered as polyalcohols or polyhydroxy. Structurally, the carbohydrates are polyhydroxy aldehydes or polyhydroxy ketones and their derivatives.

Types of Carbohydrates

Carbohydrates are divided into simple and complex, depending on the number of units that compose them.

Simple carbohydrates

The monosaccharides are the simplest carbohydrates. The best known examples are glucose, fructose, ribose and galactose. Dihydroxyacetone and glyceraldehyde are the two simplest monosaccharides, each having three carbon atoms in its main chain.

Glucose, galactose and fructose have the same chemical formula C 6 H 12 O 6 but are different in the way their atoms are spatially arranged. These types of molecules are known as isomers .

Complex carbohydrates

Carbohydrates with more than one unit of monosaccharides are known as complex carbohydrates.


The disaccharides are constituted by two monosaccharides (or monomers) as if they were “holding hands”. The most common disaccharides are:

  • lactose: combination of galactose and glucose found in milk.
  • Sucrose: combination of glucose and fructose found in commonly used sugar.
  • Maltose: combination of glucose and glucose found in malt.
  • Cellobiose: combination of glucose and glucose found in cellulose.


Polysaccharides are chains of more than ten monosaccharides. The best known are starch, glycogen and cellulose.

The starch is the storage polysaccharide of glucose in plants. It consists of two types of polymers: amylose and amylopectin. Amylopectin consists of glucoses linked linearly and in branches. Amylose is characterized by having the glucose in a linear form.

The glycogen is the storage polysaccharide of glucose in animals. It is characterized by many ramifications.

The cellulose is the structural polysaccharide of plants. It is found in the cell walls of plants, in the trunk, branches and in all the rigid parts. It is composed of glucose linked in chains, which form microfibers connected to each other by hydrogen bonds.

The chitin is a polysaccharide consisting of units acetylglucosamine. Chitin is the main component of the exoskeleton of infinity of insects and crustaceans.

Classification of carbohydrates

Monosaccharides can be classified according to different categories: functional group, carbon number

Place of the carbonyl group

  • Aldosa: is the monosaccharide with the carbonyl group at the end of the molecule, so it is an aldehyde. Example: glyceraldehyde, glucose,
  • Cetosa: is the monosaccharide where the carbonyl group is one of the intermediate groups of the molecule, so it is a ketone. Example: dihydroacetone and fructose.

Number of carbons in the molecule

Carbon number Name of monosaccharide Example
3 triose glyceraldehyde, dihydroxyacetone
4 tetrosa eritrulosa, treosa
5 pentose ribose, arabinose, xylose
6 hexosa glucose, fructose, galactose
7 heptosa Sedoheptulosa

Function of carbohydrates

Carbohydrates are the compounds with greater distribution in the biosphere and fulfill several functions:

Storage function

Plants store their energy reserves in the form of carbohydrates in fruits and roots. Mammalian animals store glucose in the form of glycogen in the liver and muscles.

Structural function

Ribose and deoxyribose are carbohydrates that form a structural part of nucleic acids, biological macromolecules of great importance in the genetic transmission and synthesis of proteins.

Pectins are also structural polysaccharides found in the cell walls of plants. They are composed of galacturonic acid polymers.

Energy source

The glucose is one of the most common carbohydrate and an important source of energy. During cellular respiration, the energy that is released from glucose is used to make adenosine triphosphate (ATP), which is the energetic currency for most cellular reactions.

Nutritional function

Carbohydrates are an essential part of the diet: grains, fruits and vegetables are natural sources of carbohydrates.

Carbohydrates also have insoluble elements known as fiber. Fiber promotes intestinal movement, regulates the absorption of glucose and helps remove cholesterol in the diet.

Lubricant function

Synovial fluid, fluid that allows movement in the joints, is composed of hyaluronic acid. This glycosaminoglycan is formed by repeats of the disaccharides of glucuronic acid and N-acetyl-glucosamine.

Foods with carbohydrates

Fruits, grains, and vegetables are all sources of carbohydrates. There are carbohydrates in all the foods we eat, but some foods are especially rich in some types of sugars. Let’s see some examples.


Honey is composed mainly of carbohydrates, of which 75% are monosaccharides and the rest are disaccharides and other sugars. The sugars present in honey are responsible for the viscosity and energy value.

Fructose is the largest constituent (38%) of honey, followed by glucose (31%) and maltose (7%).


The Beta vulgaris beet is one of the most industrially used foods to obtain the sugar for daily consumption (sucrose).

Mammals are particularly characterized because they feed their young with milk, a nutritious liquid produced in the mammary glands.

Lactose is the sugar present in milk. It is a disaccharide made up of galactose and glucose. In the digestive system there is the enzyme lactase, whose function is to break the disaccharide and release glucose and galactose to be absorbed in the intestine. The lactose intolerance occurs when this enzyme stops working.

Sweet potato, sweet potato or sweet potato

The sweet potato, sweet potato, kumar or sweet potato ( Ipomoea batatas ) is a plant widely used in Latin America and Africa as food. The leaves and roots are edible. Its carbohydrate content varies depending on the variety, but it is between 8 and 12%, mostly as fiber in the form of cellulose, starch and pectin.

The sweet potato is also very rich in beta carotene, the precursor of vitamin A and anthocyanins, pigments that give color to vegetables and are associated as antioxidants.


The chickpea Cicer arietinum is a legume, source of carbohydrates such as starch, fiber, glucose and sucrose. It is very versatile in the kitchen, especially Asian cuisine, where they prepare chickpea flour, hummus or chickpea pate and falafel or fried chickpea balls.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Back to top button