Functions of carbohydrates

Carbohydrates, along with lipids, proteins, and nucleic acids, belong to 4 major macromolecules of life.

The main role of carbohydrates is to provide energy.

Living organisms use monosaccharides to fuel metabolic reactions. They produce ATP in a process of aerobic cellular respiration.

Stored form of carbohydrates is polysaccharides such as starch and glycogen.

Besides this base task - to serve as a source of energy, carbohydrates are irreplaceable for other cell functions.

For example, plants use cellulose for structural support. Peptidoglycan forms cell walls of bacteria.

To enhance the functionality of biological molecules, cells attach carbohydrates to proteins and lipids. Modified structures of lipids in cell membranes improve cell signaling and immune responses.

In the human body, carbohydrates function as part of hormones such as follicle-stimulating hormone and luteinizing hormone.

Monosaccharides - ribose and deoxyribose - are important parts of nucleotides, which are building blocks of DNA and RNA.

3 different types of carbohydrates

General formula of carbohydrates can be represented by the stoichiometric formula (CH2O)n, where n is the number of carbons in the molecule.

In carbohydrate molecules the ratio of carbon to hydrogen to oxygen is 1:2:1

The general formula of carbohydrates and the ratio of their atoms also explain the origin of the term “carbohydrate”: the components are carbon (“carbo”) and the components of water (hence, “hydrate”).

There are 3 different types of carbohydrates:

  1. Monosaccharides
  2. Oligosaccharides (dissaccharides)
  3. Polysaccharides

Monosaccharides - monomers of carbohydrates

Triose sugars – glyceraldehyde, dihydroxyacetoneMonosaccharides are the simplest form of 3 types of carbohydrates.

They (mono- = “one”; sacchar- = “sweet”) are simple sugars, the most common of which is glucose.

Most monosaccharide names end with the suffix -ose.

In monosaccharides, the number of carbons usually ranges from three to seven.

Depending on the number of carbons in the sugar, they also may be known as trioses (three carbons), pentoses (five carbons), and hexoses (six carbons).

The two simplest monosaccharides are:

  • dihydroxyacetone (a triose with a ketone group),
  • glyceraldehyde (a triose with an aldehyde group).

Structure of pentoses – ribose, deoxyribose, ribuloseThree common pentose sugars are:

  • ribose (a component of RNA),
  • deoxyribose (a sugar in DNA),
  • ribulose (used in photosynthesis).

Three common hexoses are:

  • glucose (source of energy for all cells),
  • galactose (milk sugar),
  • fructose (fruit sugar).

Although glucose, galactose, and fructose all have the same chemical formula (C6H12O6), they differ structurally and chemically (and are known as isomers) because of the different arrangement of functional groups around the asymmetric carbon; all of these monosaccharides have more than one asymmetric carbon.

Hexoses - molecular structure of glucose, fructose and galactose

Monosaccharides can exist as a linear chain or as ring-shaped molecules; in aqueous solutions they are usually found in ring forms.

Ring forms of monosaccharides serve as a monomer of carbohydrate polymers.

Ring structure of glucose

When a glucose molecule forms a six-membered ring, there is a 50 percent chance that the hydroxyl group at carbon one will end up below the plane of the ring.

Thus the ring structure of glucose can have two different arrangements of the hydroxyl group (-OH) around the anomeric carbon.

The anomeric carbon - carbon 1 that becomes asymmetric in the process of ring formation, stereocenter.

If the hydroxyl group is below carbon number 1 in the ring structure of glucose, it is said to be in the alpha (α) position, and if it is above the plane, it is said to be in the beta (β) position

alpha (α) position and beta (β) position of the anomeric carbon of the ring structure of glucose

a and b ring structure of glucose

Oligosaccharides (dissaccharides)

Oligosaccharides are second type of carbohydrates.

Usually oligosaccharides contain two or three simple sugars attached to one another by covalent bonds called glycosidic linkages.

Glycosidic bonds can be of the alpha or the beta type.

Examples of important disaсcharides are:

1) Maltose

Chemical structure of maltose is composed of two α - ring structures of glucose molecules held together by a 1-4 glycosidic linkage.

Maltose can be found in grains which is used in the production of beer.

2) Sucrose

Sucrose molecular structure consists of α - ring structure of glucose and α - ring structure of fructose with a 1-2 glycosidic linkage between them.

Sucrose is the most known as common table sugar.

3) Lactose

Molecular structure of lactose is composed of such monomers of carbohydrates as α - ring structure of glucose and α - ring structure of galactose.

Lactose is normally found in milk.

Oligosaccharides structure (disaccharides) – sucrose, lactose, maltose

Polysaccharides - complex carbohydrate polymers

Polysaccharides are the type of carbohydrate polymers that are made up of several hundred to several thousand monomers of carbohydrates - monosaccharides held together by glycosidic linkages.

Some polysaccharides are straight chains, and some are branched.

Starch, glycogen, cellulose, and chitin are primary examples of polysaccharides.


Starch is the stored form of carbohydrate polymers in plants and is made up of a mixture of amylose and amylopectin (both polymers of glucose).

Starch is made up of monomer of carbohydrates - glucose that are joined by α 1-4 or α 1-6 glycosidic bonds.

The numbers 1-4 and 1-6 refer to the carbon number of the two residues that have joined to form the bond.

Amylose is starch formed by unbranched chains of glucose monomers (only α 1-4 linkages), whereas amylopectin is a branched polysaccharide (α 1-6 linkages at the branch points).


Glycogen is the storage form of glucose in humans and other vertebrates and is made up of monomers of glucose.


Cellulose is the primary structural polysaccharide in all plants, and is a major component in cell walls. It is a straight chain polymer of β - ring structure of glucose that is held together by 1-4 glycosidic linkages.

Every other glucose monomer in cellulose is flipped over, and the monomers are packed tightly as extended long chains. This gives cellulose its rigidity and high tensile strength - which is so important to plant cells.

While the β 1-4 linkage cannot be broken down by human digestive enzymes, herbivores such as cows, koalas, buffalos, and horses are able, with the help of the specialized flora in their stomach, to digest plant material that is rich in cellulose and use it as a food source.

Carbohydrate polymers (polysaccharides) – cellulose, starch, glycogen, chitin


A cellulose-like polymer exists in the hard exoskeleton of insects, crustaceans. This polymer is known as chitin, which is a polysaccharide-containing nitrogen.

It is made of repeating units of N-acetyl-β-d-glucosamine, a modified monomer of carbohydrates - glucose.

Chitin is also a major component of fungal cell walls.

Fungi are neither animals nor plants and form a kingdom of their own in the domain Eukarya.

Carbohydrates - naming and classification

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