Index
They are the most basic form of carbohydrates.
Monosaccharides can combine through glycosidic linkages to form more significant carbohydrates, known as oligosaccharides or polysaccharides.
An oligosaccharide with only two monosaccharides is known as a disaccharide. When more than 20 monosaccharides are combined with glycosidic bonds, an oligosaccharide becomes a polysaccharide. Some polysaccharides, like cellulose, contain thousands of monosaccharides.
A monosaccharide is a monomer or molecule that can combine with similar molecules to create a larger polymer.
Monosaccharide function
Monosaccharides have many functions within cells. First, monosaccharides are used to produce and store energy. Most organisms create energy by breaking down monosaccharide glucose and harvesting the energy released from the bonds.
Other monosaccharides form long fibers, which can be used as a form of cell structure. Plants create cellulose, while some bacteria can produce a similar cell wall from slightly different polysaccharides.
Even animal cells surround themselves with a complex matrix of polysaccharides; all made up of smaller monosaccharides.
Structure of monosaccharides
All monosaccharides have the same general formula of (CH 2 O) n, which designates a central carbon molecule attached to two hydrogens and one oxygen. Oxygen will also bond with hydrogen, creating a hydroxyl group.
Because carbon can form 4 bonds, several of these carbon molecules can bond with each other. One of the carbons in the chain will form a double bond with oxygen, called a carbonyl group.
If this carbonyl occurs at the end of the chain, the monosaccharide is in the aldose family. If the carboxyl group is in the middle of the chain, the monosaccharide is in the ketose family.
Glucose is one of the most common monosaccharides in nature, used in almost all life forms. This simple monosaccharide comprises six carbons, each labeled in the image. 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 five carbons exist as rings in water solutions. The hydroxyl group on the fifth carbon will react with the first carbon.
The hydroxyl group leaves its hydrogen atom when it forms a bond with the first carbon. When the second carbon bond is broken, the oxygen double bond in the first carbon bonds with new hydrogen. This forms a fully connected and stable carbon ring.
Examples of monosaccharides
Glucose
Glucose is an essential monosaccharide as it provides energy and structure to many organisms. Glucose molecules can be broken down in glycolysis, giving power and precursors for cellular respiration.
If a cell does not need more energy at this time, glucose can be stored by combining it with other monosaccharides. Plants store these long chains as starch, which can be disassembled and used for energy later.
Animals store chains of glucose in the glycogenic polysaccharide, which can store a lot of energy.
Glucose can also connect to long chains 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 everything at once, it would weigh millions of tons.
Each plant uses cellulose to surround each cell, creating rigid cell walls that help plants stand firm and erect. Without the ability of monosaccharides to combine into these long chains, plants would be flat and fluffy.
Fructose
Although it is almost identical to glucose, Fructose is a slightly different molecule. The formula ((CH 2 O) 6 ) is the same, but the structure is very different.
Note that instead of the carbonyl group being at the end of the molecule, as in glucose, it is the second diminished carbon. This makes fructose ketosis rather than aldose. Like glucose, Fructose still has six carbons, each with a hydroxyl group.
However, because the oxygen double bond in Fructose exists in a different place, a slightly differently shaped ring is formed. In nature, this makes a big difference in how sugar is processed.
Specific enzymes catalyze most reactions in cells. Monosaccharides of different forms need a particular enzyme to break down.
Because it is a monosaccharide, Fructose can combine with other monosaccharides to form oligosaccharides. A very common disaccharide made by plants is sucrose. Sucrose is a fructose molecule connected to a glucose molecule through a glycosidic bond.
Galactose
Galactose is a monosaccharide produced in many organisms, especially mammals. Mammals use galactose in milk to power their offspring. Galactose combines with glucose to form the disaccharide lactose.
The bonds in lactose contain a lot of energy, and newborn mammals create special enzymes to break these bonds. Once weaned from breast milk, the enzymes break down lactose into glucose, and galactose monosaccharides are lost.
Humans have developed some exciting enzyme functions as the only mammal species that consumes milk in adulthood. In populations that drink a lot of milk, most adults can digest lactose for most of their lives.
In populations that do not drink milk after weaning, lactose intolerance affects almost everyone. Although monosaccharides could be broken down individually, the lactose molecule can no longer be digested.
The symptoms of lactose intolerance (abdominal cramps and diarrhea) are caused by toxins produced by bacteria in the intestine that digest excess lactose.
