Carbohydrates are biomolecules composed of carbon, oxygen, and hydrogen. Their general formula is Cn(H2O)n. They are commonly known as sugars or sacchrides (Figure 2).

Figure 2. Healthy food rich in carbohydrates
Figure 2. Healthy food rich in carbohydrates

Carbohydrates play several roles in living organisms. In animals they acts as an energy source and help in energy transportation. In plants and insects, they also work as structural components.

Carbohydrates are categorized into four types, viz. monosacchrides, disaccharides, oligosaccharides, and polysaccharides.

Saccharides and their derivatives play key roles in growth, development, fertilization, blood clotting, and immunity. Carbohydrates are stored as glycogen in a cell.


Monosaccharides are the simplest form of carbohydrates and contain three to seven carbon atoms. They are aldehydes or ketones with two or more hydroxyl groups.


Monosacchrides cannot be dissociated or hydrolyzed into smaller units. Figures 6-10 show the chemical formulas for various monosaccharides.

They show optical activity in that they can rotate light in clockwise direction  (D for dextro) or in anticlockwise direction (L for laevo).

These two forms are called isomers and appear as mirror images of each other. Examples of monosacchrides are glucose, fructose, galactose, xylose, and ribose.

Examples of Monosacchrides

Glucose (Figure 3) is a major source of energy for a cell. Blood sugar means “glucose in the blood”.

Figure 3. D and L Structures of Glucose
Figure 3. D and L tructures of glucose

Fructose (Figure 4) is mainly found in vegetables and fruits.

Figure 4. D and L Structures of Fructose
Figure 4. D and L structures of fructose

Galactose (Figure 5) is present in milk and dairy products.

Figure 5. D and L Structures of Galactose
Figure 5. D and L structures of galactose

Xylose (Figure 6) is a monosaccharide of the aldopentose type consisting of five carbon atoms and is also used as a diagnostic agent to observe malabsorption in veterinary medicine.

Figure 6. D and L Structures of Xylose
Figure 6. D and L structures of xylose

Ribose (Figure 7) is a pentose sugar produced by the body. It is used as a medicine.

Figure 7. D and L Structures of Ribose
Figure 7. D and L structures of ribose

Glucose and fructose are the most commonly occuring monosaccharides. Table below shows difference between glucose and fructose.

Glucose Fructose
The most important monosaccharide is glucose, also known as blood sugar is the main energy source for the body Also known as fruit sugar and are commonly manufactured in the lab. It is also a source of energy. Often added to food and drinks to improve taste.
Unlike fructose, insulin is secreted primarily in response to elevated blood concentrations of glucose, and insulin facilitates the entry of glucose into cells. Does not stimulate substantial insulin release
Functional group in the structure of glucose is an aldehyde Functional group in the structure of glucose is a ketone

Even though fructose is many times sweeter than other sugars why sucrose is preferred over fructose for cooking?

Answer : Once fructose has been cooked though, it loses much of its sweetness. That is why sucrose, or granulated sugar, is recommended for baking instead of crystallized fructose.


Disaccharides are formed when two monosaccharide units combine by dehydration synthesis, which involves the elimination of a small molecule, such as water, from the functional groups.

Examples of Disaccharides

Examples of disaccharides are lactose (glucose + galactose), sucrose (glucose + fructose), and maltose (glucose +glucose).

Lactose (Figure 8) is the main sugar in milk and other dairy products.

Sucrose (Figure 9) is also known as ‘table sugar’. It is a common form of sugar found in sugarcane, some fruits and vegetables, and also in products, such as, cereal, ice cream, baked desserts and yoghurt.

Maltose (Figure 10) is referred to as ‘malt sugar’ and chemically consists of two glucose molecules. It is found in cereals containing barley and malt products, such as malted milkshakes, and beer.

Figure 8. Structure of Lactose
Figure 8. Structure of lactose
Figure 9. Structure of Sucrose
Figure 9. Structure of Sucrose
Figure 10. Structure of Maltose
Figure 10. Structure of maltose

Lactose Intolerance

Lactose intolerance is a condition, in which people have digestive problems such as bloating, diarrhea, and gas after drinking milk or eating milk products (Figure 11).

Figure 11.Lactose intolerance. Symptoms of the disease
Figure 11.Lactose intolerance. Symptoms of the disease

Lactase deficiency and lactose malabsorption may lead to lactose intolerance. Lactose intolerance should not be confused with milk allergy.

The latter is a reaction by the body’s immune system to one or more milk proteins. An allergic reaction to milk can be life threatening.

Milk allergy most commonly occurs in the first year of the child, while lactose intolerance occurs more often during adolescence or adulthood.


Oligosaccharides contain three to ten monosaccharide units. Examples of oligosaccharides are raffinose (Figure 12), maltotriose (trisacchrides), and stachyose (tetrasacchrides).

Figure 12. Structure of raffinose
Figure 12. Structure of raffinose


Polysaccharides contain more than ten monosaccharide units joined by the glycosidic bonds forming chain like structure.

The chain may be

  • branched, where molecules look like branches
  • unbranched, where the molecules are in a straight line

Examples of polysaccharides are glycogen, starch, cellulose, chitin. Glycogen (Figure 13) are stored in the liver and muscles by humans and animals.

Figure 13. Structure of Glycogen
Figure 13. Structure of glycogen

Starches are glucose polymers that are made up of amylose and amylopectin.

Cellulose (Figure 14) is an organic compound that is considered most abundant on earth and is one of the main structural constituents of plants.

This, is even excreted by some bacteria. Wood, paper, and cotton are mostly made of cellulose.

Figure 14. Structure of cellulose
Figure 14. Structure of cellulose

In addition to their roles in energy storage and  formation of cell structure, oligosaccharides and polysaccharides, are important in a variety of cell signaling processes.

For example, oligosaccharides are frequently linked to proteins, where they serve as markers to target proteins for transport to the cell surface or incorporate them into different subcellular organelles.

They also serve as markers on the surface of cells, playing important roles in cell recognition and the interactions between cells in tissues of multicellular organisms.