Monosaccharide Definition

Monosaccharides are the simplest form of carbohydrates. The glycosidic bonds between monosaccharides can be used to form larger carbohydrates, known as oligosaccharides or polysaccharides. It is known as a disaccharide when an oligosaccharide contains only two monosaccharides. 

Polysaccharides are formed when more than 20 monosaccharides are combined with glycosidic bonds. There are thousands of monosaccharides in some polysaccharides, such as cellulose. Monosaccharides are monomers, or molecules that can combine with like molecules to form larger polymers.

Function of Monosaccharide

There are many functions that monosaccharides perform within cells. The primary function of monosaccharides is to produce and store energy. The energy released from glucose bonds is harvested by most organisms by breaking down the monosaccharide glucose. As a form of cellular structure, other monosaccharides form long fibers. 

For this purpose, plants create cellulose, while bacteria can create a similar cell wall from different polysaccharides. A complex matrix of polysaccharides surrounds even animal cells.

Monosaccharide Structure

The general formula of monosaccharides is (CH2O)n, which designates a central carbon molecule bonded to two hydrogens and one oxygen. The oxygen will also form a hydroxyl group when it bonds with a hydrogen. Multiple carbon molecules can bond together because carbon can form four bonds. A carbonyl group is formed when one of the carbons in the chain forms a double bond with an oxygen. 

A monosaccharide that has this carbonyl at the end belongs to the aldose family if it has this carbonyl at the end. Monosaccharides in the ketose family have carboxyl groups in the middle of the chain.

Nearly every life form uses glucose, one of nature’s most abundant monosaccharides. This simple monosaccharide is composed of 6 carbons. The first carbon is the carbonyl group. Because it is at the end of the molecule, glucose is in the aldose family. Typically, monosaccharides with more than 5 carbons exist as rings in solutions of water. The hydroxyl group on the fifth carbon will react with the first carbon. 

The hydroxyl group gives up its hydrogen atom when it forms a bond with the first carbon. The double-bonded oxygen on the first carbon bonds with new hydrogen when the second bond with the carbon is broken. This forms a fully connected and stable ring of carbons.

Examples of Monosaccharide


Glucose is an important monosaccharide that provides energy and structure to many organisms. Glucose molecules can be broken down in glycolysis, providing energy and precursors for cellular respiration. If a cell does not need any more energy at the moment, glucose can be stored by combining it with other monosaccharides. 

Plants store these long chains as starch, which can be disassembled and used as energy later. Animals store chains of glucose in the polysaccharide glyocogen, which can store a lot of energy.

Glucose can also be connected in long strings of monosaccharides to form polysaccharides that resemble fibers. Plants typically produce this as cellulose. Cellulose is one of the most abundant molecules on the planet, and if we could weigh all of it at once it would weigh millions of tons. 

Each plant uses cellulose to surround each cell, creating rigid cell walls that help the plants stand tall and remain turgid. Without the ability of monosaccharides to combine into these long chains, plants would be flat and squishy.


Although almost identical to glucose, fructose is a slightly different molecule. The formula ((CH2O)6) is the same, but the structure is much different. 

Notice that instead of the carbonyl group being at the end of the molecule, as in glucose, it is the second carbon down. This makes fructose a ketose, instead of an aldose. Like glucose, fructose still has 6 carbons, each with a hydroxyl group attached. However, because the double bonded oxygen in fructose exists in a different place, a slightly different shaped ring is formed. In nature, this makes a big difference in how the sugar is processed. 

Most reactions in cells are catalyzed by specific enzymes. Different shaped monosaccharides each need a specific enzyme to be broken down.

Fructose, because it is a monosaccharide, can be combined with other monosaccharides to form oligosaccharides. A very common disaccharide made by plants is sucrose. Sucrose is one fructose molecule connected to a glucose molecule through a glycosidic bond.


Galactose is a monosaccharide produced in many organisms, especially mammals. Mammals use galactose in milk, to give energy to their offspring. Galactose is combined with glucose to form the disaccharide lactose. The bonds in lactose hold a lot of energy, and special enzymes are created by newborn mammals to break these bonds apart. Once being weaned of their mother’s milk, the enzymes that break lactose down into glucose and galactose monosaccharides are lost.

Humans, being the only mammal species that consumes milk in adulthood, has developed some interesting enzyme functions. In populations that drink a lot of milk, most adults are able to digest lactose most of their lives. In populations that do not drink milk after being weaned, lactose-intolerance afflicts nearly the whole population. Although the monosaccharides could be broken down individually, the molecule lactose can no longer be digested. 

The symptoms of lactose-intolerance (abdominal cramps and diarrhea) are caused by toxins produced by bacteria in the gut digesting the excess lactose. The toxins and excess nutrients they create raised the total amount of solutes in the intestines, making them retain more water to keep a stable pH.

Related Biology Terms

  • Disaccharide – Two monosaccharides connected by a glycosidic bond.
  • Oligosaccharide – 3-20 monosaccharides connected by glycosidic bonds, typically used to move monosaccharides and store them for short times.
  • Polysaccharide – Many (20+) monosaccharides, usually connected in long chains, used for storage or structural support.
  • Carbohydrate – Sugars and starches, all made from monosaccharides.


What is a Monosaccharide?

A Monosaccharide is the simplest form of carbohydrate, often referred to as a “simple sugar.” It consists of a single sugar unit, which cannot be further broken down into smaller carbohydrates.

What are some examples of Monosaccharides?

Examples of Monosaccharides include glucose, fructose, and galactose. Glucose is the primary source of energy for cells and is commonly found in foods like fruits and honey. Fructose is found in fruits and is the sweetest naturally occurring sugar. Galactose is found in dairy products and is less sweet than glucose or fructose.

What is the molecular structure of Monosaccharides?

Monosaccharides have a general molecular formula of (CH2O)n, where n is typically between 3 and 7. They are characterized by a backbone of carbon atoms with hydroxyl (-OH) groups attached to each carbon, except one carbon that also has an oxygen atom to form a carbonyl group (either an aldehyde or a ketone).

How are Monosaccharides used in the body?

Monosaccharides serve as an important source of energy for the body. They are broken down during digestion into their simplest form (e.g., glucose) and absorbed into the bloodstream. Glucose is then utilized by cells for energy production through cellular respiration.

Are all Monosaccharides sweet in taste?

Not all Monosaccharides taste sweet. While glucose and fructose have a sweet taste, other Monosaccharides like galactose do not taste as sweet. The sweetness of Monosaccharides depends on their specific molecular structure and the way they interact with taste receptors on the tongue.

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