Monosaccharides

Monosaccharides are simple carbohydrates. They are also called simple sugars, since they are composed of one molecule of sugar (mono- meaning ‘one’, and -sacchar meaning ‘sugar’).

They are hydrophilic, meaning they are soluble in water. They are building blocks (monomers) of larger molecules of carbohydrates: polysaccharides (polymers).

Structure of monosaccharides

As explained in the article on carbohydrates, monosaccharides are organic molecules composed of carbon, hydrogen and oxygen atoms. They can be shown in their linear or ring structure. Have a look at figures 1 and 2, which show the two structures of the same molecule.

Fig. 1 - Linear structure of monosaccharide glucose

Fig. 2 - Ring structure of monosaccharide glucose

Types of monosaccharides

Monosaccharides are categorised according to how many atoms of carbon they contain. These are the three most common types:

• Trioses (3 Carbon atoms) are important in cellular respiration.

• Pentoses (5 Carbon atoms), for example, ribose in RNA and deoxyribose in DNA.

• Hexoses (6 Carbon atoms), like glucose, fructose, and galactose.

Figure 3 shows you the difference between trioses, pentoses, and hexoses. Notice how the linear structure gets lengthens as more carbon atoms are added.

Fig. 3 - Examples of triose, pentose, and hexose sugars. Observe the carbon backbone

Examples of monosaccharides

You will come across three monosaccharides classed as the most important in nutrition: glucose, galactose, and fructose. However, these are not the only ones. Deoxyribose and ribose are of great importance too, being the bases of DNA and RNA.

Glucose

Glucose is the most common monosaccharide. Along with oxygen, it is the product of photosynthesis. It is an important energy source for both animals and plants.

The molecular formula for glucose: $C_6{H}_{12}{O}_6$

This tells you that it has 6 carbon atoms, meaning glucose is a hexose.

Glucose has two isomers: α-glucose (alpha-glucose) and β-glucose (beta-glucose). They have the same molecular formula. However, they differ in structure. There is a slight difference in the arrangement of atoms in the molecule. Notice the position of the far-right hydrogen atom (H) and the hydroxyl group (OH) in figure 4.

Fig. 4 - Ring structures of α-glucose and β-glucose

Galactose and fructose

Galactose is found in milk, while fructose is found in fruit. They are similar to glucose in that all three share the same formula: ${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6$ However, similar to α-glucose and β-glucose, they differ in structure. In figure 5, notice how atoms and groups are arranged in all three sugars.

Fig. 5 -The linear form of the structures of glucose, galactose, and fructose

Deoxyribose and ribose

Another two very important monosaccharides are deoxyribose and ribose. They are found in DNA and RNA respectively, where they are bases of nucleotides, which carry genetic material.

Both deoxyribose and ribose are pentoses, meaning they consist of 5 carbon atoms. The formula for deoxyribose is ${\mathrm{C}}_5{\mathrm{H}}_{10}{\mathrm{O}}_4$ and ribose is ${\mathrm{C}}_5{\mathrm{H}}_{10}{\mathrm{O}}_5$. Notice the arrangement of the atoms in figure 6.

Fig. 6 - The ring structures of ribose and deoxyribose

Monosaccharides form disaccharides and polysaccharides

Monosaccharides are joined together in a process called condensation. The condensation reaction of certain monosaccharides results in the formation of disaccharides and polysaccharides. In disaccharides, two monosaccharides are bonded, while in polysaccharides, there are many (poly- stands for ‘many’).

A condensation reaction between two monosaccharides forms a covalent, glycosidic bond. There are different types of glycosidic bonds that form between monosaccharides. Depending on the placement of the bonds, there are, for example, 1,4-glycosidic bonds that form on the 1st carbon atom of one and the 4th carbon atom of the other monosaccharide. You will come across 1,6-glycosidic bonds in polysaccharides as well, where branches are formed in the chains.

Remember that there are two different glucose isomers: α-glucose and β-glucose. Depending on which isomer forms a certain disaccharide or polysaccharide, these bonds can be written as α-1,4-glycosidic bonds or β-1,4-glycosidic bonds. The same goes for 1,6 bonds.

Disaccharides and polysaccharides can be broken down into monosaccharides during hydrolysis. This is a reaction in which water molecules are added and the glycosidic bonds between monosaccharides are broken. This results in the breaking down of disaccharides and polysaccharides.