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

Carbohydrates are known as carbohydrates , as 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 sakcharon which means “sugar”.

Basic structure of carbohydrates

Structure of the simplest carbohydrates: glyceraldehyde and dihydroxyacetone.

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; 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 polyols or polyhydroxy. Structurally, 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 make them up.

Simple carbohydrates

Glucose and galactose are monosaccharides present in milk.

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 they are different in the way their atoms are arranged spatially. These types of molecules are known as isomers .

Complex carbohydrates

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


Different disaccharides present in nature.

Disaccharides are made up of two monosaccharides (or monomers) as if they were “holding hands.” The most common disaccharides are:

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

The link between the sugars is called the glycosidic or glycosidic bond . They occur when a hydroxyl of a sugar reacts with carbon and a molecule of water is released. They can be of two types: alpha or beta.


Starch, glycogen, and cellulose are polysaccharides of glucose.

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 is made up of two types of polymers: amylose and amylopectin. Amylopectin consists of branched and linearly linked glucoses. Amylose is characterized by having 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 rigid parts. It is made up of glucose linked together 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 countless insects and crustaceans.

Carbohydrate classification

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

Carbonyl group site

  • Aldose: it is the monosaccharide with the carbonyl group at the end of the molecule, making it an aldehyde. Example: glyceraldehyde, glucose,
  • Ketose: is the monosaccharide where the carbonyl group is one of the intermediate groups of the molecule, making it a ketone. Example: dihydrocylacetone and fructose.

Number of carbons in the molecule

Number of carbons Monosaccharide name Example
3 triosa glyceraldehyde, dihydroxyacetone
4 tetrosa erythrulose, treosa
5 pentose ribose, arabinose, xylose
6 hexose glucose, fructose, galactose
7 heptosa Sedoheptulose

Function of carbohydrates

Carbohydrates are the compounds with the greatest 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 are a structural part of nucleic acids, biological macromolecules of great importance in genetic transmission and protein synthesis.

Arthropods have an outer skeleton (exoskeleton) that is made of chitin. Chitin is a polysaccharide of N-acetyl-β-d-glucosamine, a modified sugar. Chitin is also a component of cell walls in fungi.

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

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 energy 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. The fiber favors the intestinal movement, regulates the absorption of glucose and helps to remove cholesterol in the diet.

Lubricating function

Synovial fluid, fluid that allows movement in the joints, is made of hyaluronic acid. This glycosaminoglycan is formed by repeats of the glucuronic acid disaccharides 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 one of the foods with the highest energy value in the world.

Honey is mainly composed 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 highest constituent (38%) of honey, followed by glucose (31%) and maltose (7%).


Beetroot is an important source of sucrose.

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


Milk is food for the young of mammals.

Mammals are particularly characterized by feeding 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 so that they are absorbed in the intestine. The lactose intolerance occurs when this enzyme stops working.

Sweet potato, sweet potato or sweet potato

Sweet potato root Ipomoea batatas is a food with great nutritional value.

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 is between 8 and 12%, most of it as fiber in the form of cellulose, starch and pectin.

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 Cicer arietinum chickpea is a legume of great worldwide appreciation.

Cicer arietinum chickpea is a legume, a source of carbohydrates such as starch, fiber, glucose and sucrose. It is very versatile in the kitchen, especially Asian cuisine, where chickpea flours, hummus or chickpea paté and falafel or fried chickpea balls are prepared.


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