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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 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.

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.

Carbohydrates in Science

The term”carbohydrate” or “hydrates” of carbon comes from its formula for the elemental element in which carbon is joined with oxygen and hydrogen in the same proportion as in water. Chemically, carbohydrates are ketones or polyhydroxy aldehydes, their basic derivatives or polymers.

Carbohydrates in grain are classified according to their chemical structure or digestibility in the case of humans as food or livestock for feed. Simple carbohydrates that are sweet and dissolve in water are referred to as sugars or disaccharides . the name ending for the majority of sugars is -ose. This is why we have names as sucrose for normal table sugars, glucose as the blood sugar, and maltose to refer to malt sugar.

Carbohydrates Formula

Carbohydrates are macromolecules that consist comprised of carbon (C) as well as hydrogen (H) and oxygen (O) and possess the standard Cx(H2O)y formula. Carbohydrates use the general formula Cx(H2O)y. Carbon’s hydrate is referred to as carbohydrates. They are composed of oxygen and hydrogen in the same proportions like water. It is worth noting that there are certain carbohydrate that don’t comply with the formula Cx(H2O)y such as 2-deoxyribose C5H10O4. However, the majority of them are in line with the formula Cx(H2O)y.

Carbohydrates are also referred to as sugars. They include partially methylated sugars as well as amino sugars and amino sugars that naturally occur and one nitro sugar that is natural is recognized. All carbohydrates are polyhydroxyaldehydes or ketones or other substances that release the same by hydrolysis.

Haworth projections depict the polysaccharides’ cyclic structure. Monosaccharides are composed of one of two groups: an aldehyde (aldose) or an ketose group (ketose) and a variety of OH groups. Straight chain sugars are cyclized in solution into ring-like structures with an linkage with ether. Glycosidic bonding occurs between monosaccharides, forming disaccharides and polysaccharides. Carbohydrates are utilized to provide energy sources and reserves.

Carbohydrate Sources

We all know that carbohydrates are an essential component of a human’s diet. Common sources of carbohydrates include:

  • Potatoes
  • Maze
  • Milk
  • Popcorn
  • Bread

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: https://open.oregonstate.education/app/uploads/sites/85/2019/10/animal-nutrition-23-1024×736.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

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

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: https://americangardener.net/wp-content/uploads/2020/11/images-1.png

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
Source: https://www.chemistrysteps.com/carbohydrates-structure-and-classification/

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: https://cdn-ccnli.nitrocdn.com/hPbgvskpJmIfLTeYLYGdGpfRfGKbrYYq/assets/static/optimized/rev-1466133/wp-content/uploads/2020/03/sucrose.jpg

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: http://www.nutrientsreview.com/wp-content/uploads/2014/08/Oligosaccharides.jpg

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: http://www.nutrientsreview.com/wp-content/uploads/2014/08/Polysaccharides-structure.jpg

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: https://biologydictionary.net/examples-of-carbohydrates/

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: https://biologydictionary.net/examples-of-carbohydrates/

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.

Common biological reactions involving carbohydrate.

Below are a few of the frequent biological reactions that involve carbohydrates.


  • In plants as well as other autotrophs that are photosynthetic the creation of sugars that are simple (e.g. glucose) occurs through photosynthesis.
  • The process makes use of carbon dioxide along with water, inorganic salts, as well as sunlight energy (from sunlight) that is captured by light-absorbing pigments such as chlorophyll, and other pigments to create water, glucose in addition to oxygen molecules.


  • The process in which complex carbohydrates are transformed in simpler compounds, for example, sugar, is referred to as saccharification.
  • It involves hydrolysis.
  • In humans as well as other animals, this is an the action of enzymes.
  • When you chew, complex glucose carbohydrates are broken into simpler forms via an action called salivary amylase.
  • Within the small intestinal tract it is where the process of digestion for complex carbohydrates continues.
  • Maltase sucrase, lactase, and maltase break down disaccharides to monosaccharide components.
  • The Glucosidases belong to a different category of enzymes which help to remove the terminal sugar from a polysaccharide made mostly by long-chains of glucose.


  • Monosaccharides that are digested from carbohydrates are absorbed by epithelial cells in the small intestinal.
  • The cells absorb them in the intestinal lumen via the sodium ion-glucose symport mechanism (via glucose transporters, or GluT).
  • They are proteins that facilitate the transportation of monosaccharides like glucose into cells.
  • They are then released into capillaries via facilited diffusion.
  • Tissue cells absorb them from bloodstreams through the GluTs.
  • Once inside the cell glucose is phosphorylated, allowing it within the cell.
  • The result is that glucose-6-phosphate could be utilized for any of the three metabolic pathways listed below: (1) glycolysis, to produce chemicals, (2) glycogenesis in which glucose is transported to the liver through the vena portae, where it is stored as glycogen in the cell or (3) the pentose phosphate pathway to create NADPH to synthesize lipids and pentoses for nucleic acids synthesizing.

Cellular respiration

  • The metabolization of glucose occurs within cells in the process known as cell respiration.
  • The most important steps or steps of cell respiration are (1) Glycolysis (2) Krebs cycle as well as (3) the process of oxidative phosphorylation.
  • In the beginning (i.e. glycolysis) it is a series of cytosol-based reactions leads to an enzymatic conversion of monosaccharides typically the sugar glucose into pyruvate as well as the simultaneous production of a small amount of biomolecules that are high in energy including ATP.
  • NADH, a molecule that carries electrons can also be produced.
  • In the absence of oxygen levels, the pyruvate produced by glycolysis is transformed to an organic substance that can be completely oxidized in the mitochondrion.
  • Electron carriers (e.g. NADH or FADH2) move electrons along in the transport chain for electrons.
  • Redox reactions are a series that is carried out along the chain. It ends with an electron-acceptor at the end i.e. molecular oxygen.
  • More ATP is created through an interconnected mechanism via chemiosmosis within the mitochondrial inner membrane.

Based on glycolysis by itself from glycolysis alone, there is a net ATP is just two (from the substrate level phosphorylation). With oxidative phosphorylation your net ATP is around 34. Therefore, the net ATP per glucose is around 36.2 If there is no oxygen or absence of oxygen, anaerobic catabolism takes place (e.g. through fermentation). It is an anaerobic process which produces ATP by glycolysis. Instead of transferring electrons within the chain of electron transportation NADH moves electrons to pyruvate, which restores NAD+ to the NAD+ that powers glycolysis.2 The total amount of ATP that is generated per glucose during fermentation is just about two.


  • The process of gluconeogenesis is similar to the opposite of glycolysis, in the manner that glucose is converted into pyruvate. In the process of gluconeogenesis pyruvate is transformed to glucose.
  • The essence of gluconeogenesis is an enzyme that produces glucose. is produced from non-carbohydrate precursors e.g. lactate, glycerol, pyruvate and glucogenic amino acid.
  • In humans as well as many vertebrates, gluconeogenesis take place mainly within liver cells.
  • It usually occurs during periods of fasting, diets with low carbohydrate or during intense exercise.
  • Cytologically, the process is initiated in mitochondria and is completed in the lumen of the endoplasmic-reticulum.
  • Glucose formed from hydrolyzing glucose-6-phosphate by the enzyme glucose-6-phosphatase is shuttled from the endoplasmic reticulum into the cytoplasm.


  • Glycogenesis is a metabolic process that produces glycogen from glucose, which is stored primarily in muscle and liver cells in response to excessive levels of glucose in bloodstream.
  • Shorter polymers of glucose specifically exogenous glucose are transformed into long polymers that are stored in cells, particularly in the liver and muscle.
  • In the event that the body needs the energy of metabolism glycogen is degraded into glucose subunits by glycogenolysis. This is why glycogenesis is the reverse process of glycogenolysis.


  • Glycogenolysis is the process that involves breaking down glycogen stored in the liver, so that glucose can be made to aid to fuel metabolism.
  • The glycogen stored in the liver is broken to glucose precursors.
  • The glucose molecules gets separated from glycogen, and converted into glucose-1-phosphate. It then, transforms into glucose-6-phosphate which is able to be used in glycolysis.

Pentose phosphate pathway

  • The Pentose-Phosphate pathway is an enzyme that converts glucose into where five carbon-based sugars (pentoses) and NADPH are produced within the cytosol.
  • The pentose phosphate pathway functions as an alternative route for the breakdown of glucose.
  • For animals, this is found in the adrenal cortex, the liver and adipose tissue, testis, etc.
  • This is the primary metabolic pathway that neutrophils follow.
  • Therefore, congenital insufficiency of the pathway results in an increased sensitivity to infections.
  • In plants, a part of the pathway is responsible for the creation of hexoses out of carbon dioxide that is produced during photosynthesis.

Leloir pathway (Galactose metabolism)

  • In this pathway of metabolism it is the case that galactose undergoes glycolysis first being phosphorylated by the enzyme galactokinase. It is then transformed into glucose-6-phosphate.
  • Galactose is the result of lactose (milk sugar made up by a glucose-containing molecule as well as an molecule of galactose).

The Fructose 1-phosphate pathway

  • In this pathway of metabolism, fructose is the primary ingredient, and not glucose, enters glycolysis.
  • However, fructose must take additional steps to be taken before entering glycolysis.
  • It is found in animals and can be found within the muscle, fat tissue, and the kidney.


  • The proper carbohydrate metabolism is crucial to ensure proper digestion and catabolism of carbohydrates in the human body.
  • The steady levels of glucose within the body is known as glucoregulation.
  • Hormones, like insulin and glucagon, both of which originate from the pancreas regulate the metabolism of glucose.
  • Blood sugar is levels of glucose that circulates within the body. If you have a low concentration of glucose in the blood, insulin gets released.
  • In contrast, a high blood glucose level can trigger an increase in insulin. Insulin regulates the metabolism carbohydrate (as and fats) by encouraging the absorption of glucose in the bloodstream into the muscles of the skeletal and fat tissues, where it is stored as glycogen to subsequent use as glycogenolysis.
  • Then, glucagon is responsible for stimulating the sugar production.
  • Particularly, this causes glycogen stored inside the liver convert into glucose, which will then be released into the bloodstream.
  • Unbalanced carbohydrate metabolism could cause certain metabolic disorders or conditions, e.g. lactose intolerance, diabetes mellitus galactosemia, glycogen storage disorder as well as fructose malabsorption.

Dehydration Synthesis

  • Monosaccharide is a form of carbohydrates that joins with glycosidic bonds an process known as dehydration synthesizing.
  • When forming disaccharide, such as the union of two monosaccharides leads to an release of water, which is a byproduct.
  • In the same way, polysaccharides form from a chain of monosaccharide units through a further dehydration.
  • The glycogen and starch that are made serve as energy-rich molecule.
  • Complex carbohydrates are broken down to simpler form (e.g. glucose) as the body needs more energy.
  • This process is known as saccharification.



MN Editors. (November 29, 2021).Carbohydrates Definition, Examples, Structure, Types, and Functions. Retrieved from https://microbiologynote.com/carbohydrates-structure-types-and-functions/


MN Editors. "Carbohydrates Definition, Examples, Structure, Types, and Functions." Microbiology Note, Microbiologynote.com, November 29, 2021.


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