Carbohydrates Definition, Examples, Structure, Types, and Functions

The term "carbohydrate" comes from the Greek word sakharon meaning "sugar". In the field of chemistry, carbohydrates comprise the most common type...

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This article writter by Sourav Bio on November 29, 2021

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Carbohydrates Definition, Examples, Structure, Types, and Functions
Carbohydrates Definition, Examples, Structure, Types, and Functions

Carbohydrates Definition

The term “carbohydrate” comes from the Greek word sakharon meaning “sugar”. In the field of chemistry, carbohydrates comprise the most common type of organic compounds with simple structures. A carbohydrate is an aldehyde , or one that contains other hydroxyl groups. The most basic carbohydrates are known as monosaccharides. They have the fundamental structure (C*H2O)n where 3 or more is the minimum.

Two monosaccharides join to make disaccharides. Monosaccharides and disaccharides are referred to as sugars, and usually have names that begin by the suffix “-ose. Two or more monosaccharides join into oligosaccharides or polysaccharides.

In the everyday context it is understood that the term “carbohydrate” refers to any food item with the highest amount of sugars or starches. In this case carbohydrates can refer to jelly, table sugar bread, cereals, bread and pasta, despite the fact that they may also contain other organic substances. For instance, cereals and pasta contain a certain levels of protein.

Carbohydrates can be described as ketones, polyhydroxyaldehydes, or compounds that create them via hydrolysis. The term “sugar” is used to describe carbohydrates that dissolve in water and sweet in taste.

Structure of Carbohydrates

  • Carbohydrates are made up of hydrogen, carbon and oxygen.
  • The most common empirical structure for carbohydrates can be described as (CH2O)n.
  • Organic compounds are organized into ketones or aldehydes with numerous hydroxyl groups that are derived from the carbon chain.
  • The fundamental building components for all carbohydrate are sugars that have simple structures, also known as monosaccharides.
  • Monosaccharides can be polyhydroxyaldehyde (aldose) or polyhydroxy ketone (ketose).
Structure of Carbohydrates
Structure of Carbohydrates | Source:×736.jpg

The carbohydrates are structurally represented in three forms:

  • Open chain structure: It’s the long straight-chain version of carbohydrates.
  • Hemi-acetal structure: here, the 1st carbon of glucose condenses to form the -OH five carbon create a ring-like structure.
  • Haworth structure: It’s that it has the pyranose rings structure.

Properties of Carbohydrates

Physical Properties of Carbohydrates 

  • Stereoisomerism –Compound shaving with the same structural formula however they differ in their spatial configuration. For example, glucose is composed of two isomers in relation to the carbon atom that is the penultimate. They are D-glucose and L-glucose.
  • Optical Activity – Optical Activity is the rotation of light plane-polarized creating (+) glucose and (-) glucose.
  • Diastereo isomers –It’s alters the configuration of the molecules in relation towards C2, or C4 within glucose. Example: Mannose, galactose.
  • Annomerism – Annomerism refers to the spatial arrangement with respect to the carbon atom that is first in aldoses and the second carbon atoms in ketoses.

Chemical Properties of Carbohydrates 

  • Osazone formation: Osazone is carbohydrate derivatives when sugars are reacting with a high amount of the phenylhydrazine. eg. Glucosazone
  • Benedict’s test: The reduction of sugars when heated with an alkali, is converted into powerful reducing species , known as enediols. If Benedict’s reagent and reduced sugars are heated together the solution transforms its color from orange-red to brick red.
  • Oxidation: Oxidation Monosaccharides reduce sugars when their carbonyl groups are oxidized to produce carboxylic acids. In Benedict’s test, D-glucose gets transformed into D-gluconic acid. Thus glucose is considered to be an reducing sugar.
  • Reduction to alcohols: C=O group in open-chain forms of carbohydrates are reduced to alcohols through sodium borohydride NaBH4 as well as catalytic hydrolysis (H2 Ni EtOH/H2O). The products are referred to by the name of “alditols”.

Properties of Monosaccharides

  • The majority of monosaccharides are sweet tasting (fructose is the most sweet; it is 73 percent more sweeter than sucrose).
  • The solids are formed when they are at room temperatures.
  • They are highly easily soluble in water. Despite their molecular weights being high they have many OH groups makes the monosaccharides more water-soluble than other molecules with similar MW.
  • Glucose dissolves in small amounts of water, resulting in syrup (1 1 ml = g H2O).

Types of Carbohydrates

These carbohydrates can be further divided into complex and simple, that is mostly determined by their chemical structure as well as the extent of polymerization.

Simple Carbohydrates (Monosaccharides, Disaccharides and Oligosaccharides)

Simple carbohydrates comprise two or three sugar molecules. In simple carbohydrates, the molecules are digested quickly and converted into sugars, leading to a rapid increase in level of blood sugar. They are found in a variety of beer, milk products fruits and candies, refined sugars and more. They are referred to as empty calories since they are not a source of nutrients, fiber, or vitamins.

Producing plants, they produce glucose (C6H12O6) by using basic substances like carbon dioxide, water and, in the presence of sunlight. Photosynthesis transforms solar energy into chemical energy. Plants are the food source for consumers and take advantage of the energy stored in bonds of the compounds produced by plants.

1. Monosaccharides

Monosaccharides (Greek Mono-one) are the most basic group of carbohydrates. They are commonly called simple sugars. They possess the general formula Cn(H2O)n They are unable to be further hydrolyzed.

Classification of Monosaccharides

A. Classification Based on the presence of functional Group

Monosaccharides are classified into distinct categories according to the functional group as well as the amount of carbon atoms

  • Aldoses: If the functional monosaccharide group is an aldehyde(-CHO) They are known as the aldoses e.g. glyceraldehyde, glucose.
  • Ketoses: If the functional group has a keto (CO) group they are called ketoses e.g. dihydroxyacetone, fructose.
Aldose and Ketose
Aldose and Ketose | Source:

B. Classification Based on the number of carbon atoms

Monosaccharides are further classified according to the carbon atom count:

  • Trioses (C3H6O3): The trioses contain three carbon atoms per molecule. Example: Glyceraldehyde
  • Tetroses (C4H6O4): Monosaccharides contain four carbon atoms in each molecule. Example: Erythrose.
  • Pentoses,
  • Hexoses
  • Heptoses
Classification of Carbohydrates Based on the number of carbon atoms

2. Disaccharides

Two monosaccharides are combined to make disaccharides. Examples of carbohydrates that have two monomers are Sucrose Lactose, Maltose, Lactose and many more.

Structure of Disaccharides
Structure of Disaccharides | Source:

3. Oligosaccharides

Carbohydrates derived from two or more monomers are referred to as the oligosaccharides. According to this definition trioses, pentoses, and Hexoses, pentoses and trioses are all oligosacchari.

Structure of Oligosaccharides
Structure of Oligosaccharides | Source:

Complex Carbohydrates (Polysaccharides)

Complex carbohydrates contain several sugar molecules. This is why they are often referred to as starchy food. Complex carbohydrates contain molecules that are digested, and then converted slowly in comparison to simple carbohydrates. They are plentifully found in beans, lentils potato, peanuts peas, whole-grain bread, corn cereals, and other cereals.


Polysaccharides are complex carbohydrate molecules formed by polymerization of many monomers. Examples of polysaccharides are glycogen, starch and many others. with extensive branching. They are homopolymers made up of glucose units only.

  • Starch is comprised of two elements: amylopectin and Amylose. Amylose is the linear chain while amylopectin is a chain that has a lot of branches.
  • Glycogen is also known as animal starch. It has a structure that is similar to starch but with greater branches.
  • Cellulose is an essential carbohydrate that is the primary structural component of the cell wall. The fibrous polysaccharide having great tensile strength. In contrast to starch or glycogen, cellulose is an elongated polymer.
Structure of Polysaccharides
Structure of Polysaccharides | Source:

Functions of Carbohydrates

Carbohydrates are the most widely distributed molecules within animal and plant tissues. In arthropods and plants carbohydrates are found in the skeletal structures are also used as food reserves in animals and plants. They are a vital source of energy that are required for various metabolic functions and are derived from an oxidation process.

Some of their major functions include:

  • Living organisms make use of carbohydrates for energy that can be used to fuel cell reactions. They are the biggest food source of energy (4kcal/gram) in all living things.
  • Carbohydrates in addition to being the main energy source in a variety of animals, are a quick source of energy. The breakdown of glucose is accomplished by Kreb’s and glycolysis cycles to create ATP.
  • serve as energy stores as well as fuels and metabolic intermediates. Glycogen is stored in animals and starch plants.
  • Storage carbohydrates are an energy source, not proteins.
  • They are structures and protectants such as in the cell walls of microorganisms as well as plants. Structural elements found in wall of the cells in microorganisms (peptidoglycan also known as murein) and the plants (cellulose) as well as mammals (chitin).
  • Carbohydrates are intermediates in biosynthesis of proteins and fats.
  • Carbohydrates help regulate the function of nerve tissue and are the primary energy source for the brain.
  • Carbohydrates are linked to proteins and lipids, forming receptor molecules, antigens on the surface as well as vitamins and antibiotics.
  • Creation of the structure of the RNA and DNA (ribonucleic acid and deoxyribonucleic acids).
  • They are connected to numerous proteins and the lipids. These linked carbohydrates are crucial for cell-cell communication, as well as interactions between cells as well as other components of the cell’s environment.
  • In animals, they’re an important component of connective tissue.
  • Carbohydrates with high fiber content can help prevent constipation.
  • They also aid in the regulation of the immune system.

Examples of Carbohydrates

A. Examples of Polysaccharides

1. Starch

Starch is made by green plants and composed of glucose that is created by photosynthesis. It is utilized for plant use as a storage molecule. It is stored inside the chloroplasts (where it is stored in crystals) as well within tubers (e.g. potatoes) or in the roots of some plant species (like the cassava).

Starches can be found in a variety of foods, including grains , and grain-based products (like pasta, bread wheat, oats, and wheat) specific vegetables (such as squash, potatoes and corn) and legumes (like beans, peas and legumes).

2. Glycogen

While starch is utilized for energy storage in plant life, glycogen can be utilized for energy storage in more advanced creatures (including humans) as well as various microorganisms (such as fungi and bacteria). In humans, it’s most often found in the liver as well as muscles.

3. Cellulose

Cellulose (AKA fiber) is a structural substance found in cell walls of plant cells. It is extremely rigid and is used to keep the shape of cells and to protect their contents.

A variety of animals (like horses, cows and Koalas) are able to digest cellulose however humans do not have the enzyme needed to accomplish this. But cellulose is vital to ensure healthy digestion for humans as it assists food items move in into the intestinal tract. Cellulose in our diet is known as fiber).

B. Examples of Monosaccharides

Glucose, fructose and galactose are monosaccharides
Glucose, fructose and galactose are monosaccharides | Source:

1. Glucose

The most well-known kind of monosaccharide found in the natural world and can be present in rice, bread pasta, potatoes, fruits, vegetables, as well as refined sugar.

2. Fructose

Fructose is the “fruit sugar’ present in all fruits, vegetables honey, table sugar.

3. Galactose

Galactose is a natural sugar that can be typically found in association with other sugars, such as in lactose (the sugar in milk).

Disaccharides are yet another kind of basic carbohydrate. Disaccharide is a term that refers to two sugars, so, they are composed of two monosaccharides that are joined through a glycosidic connection. Glycosidic bonds are formed between sugars due to an chemical reaction referred to as the condensation reaction (AKA Dehydration Reaction).

C. Examples of Disaccharides

Disaccharides are formed by condensation reactions
Disaccharides are formed by condensation reactions | Source:

1. Lactose

Lactose can be found in milk and is comprised from one glucose molecule linked to one molecule galactose.

2. Sucrose

Sucrose is a used energy storage molecule in green plants. It’s made up from one molecule of fructose that is bound to a single glucose molecule. It is extracted from plant material to be used as table sugar. It can be present in cake, candy and various sweetened foods.

3. Maltose

Maltose (or malt sugar) is a natural ingredient found in barley, wheat cornmeal, barley, as well as other grains. It is also present in certain fruits, like peaches and peaches. Maltose is comprised from two glucose molecules that have been joined together.

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