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Monosaccharides

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.

Monosaccharides, Linear structure of monosaccharide glucose, StudySmarterFig. 1 - Linear structure of monosaccharide glucose

Monosaccharides, Ring structure of monosaccharide glucose, StudySmarterFig. 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.

Monosaccharides, examples of triose pentose and hexose sugars structure, StudySmarterFig. 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:

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.

Monosaccharides, Ring structures of α-glucose (alpha-glucose) and β-glucose (beta-glucose), StudySmarterFig. 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: 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.

Monosaccharides, linear structures of glucose galactose and fructose, StudySmarterFig. 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 and ribose is . Notice the arrangement of the atoms in figure 6.

Monosaccharides, Ring structure of ribose and deoxyribose, StudySmarterFig. 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.

You may notice that we have used certain next to monosaccharides instead of all monosaccharides in the text above. This is because deoxyribose and ribose do not attach to one another to form disaccharides and polysaccharides, even though they are monosaccharides. Instead, they are the sugar bases of nucleotides, monomers of DNA and RNA. During the condensation reaction of nucleotides to the 3’ carbon atom on deoxyribose or ribose, a phosphate group is attached and two nucleotides are joined together. You can learn more about this in the article on nucleic acids.

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.

Monosaccharides - Key takeaways

  • Monosaccharides are simple carbohydrates composed of one molecule of sugar and are the building blocks (monomers) of larger molecules of carbohydrates: polysaccharides (polymers).

  • Monosaccharides are categorised according to how many atoms of carbon they contain: trioses (3 carbon atoms), pentoses (5 carbon atoms), for example, ribose in RNA and deoxyribose in DNA, and hexoses (6 carbon atoms), like glucose, fructose, and galactose.

  • Glucose is the most common monosaccharide. It has two isomers: α-glucose (alpha-glucose) and β-glucose (beta-glucose). They have the same molecular formula C6H12O6. However, they differ in the arrangement of atoms in the molecule: the position of the far-right hydrogen atom (H) and the hydroxyl group (OH).

  • Another two very important monosaccharides are deoxyribose and ribose. They are found in DNA and RNA.

  • Monosaccharides join together in a process called condensation. The covalent glycosidic bond forms between monosaccharides, with molecules of water removed.

Frequently Asked Questions about Monosaccharides

A monosaccharide is a simple carbohydrate. It is an organic biological molecule composed of one molecule of sugar. Monosaccharides are building blocks (monomers) of larger molecules of carbohydrates (polymers).

Yes, glucose is a monosaccharide and the most common of them all.

No, lactose is a disaccharide composed of two monosaccharides: glucose and galactose. 

The three monosaccharides are glucose, fructose, and galactose.

The monosaccharide that makes up starch is glucose.

Final Monosaccharides Quiz

Question

What is the definition of monosaccharides?

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Answer

Monosaccharides are simple carbohydrates composed of one molecule of sugar.

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Are monosaccharides soluble in water?

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Monosaccharides are hydrophilic, meaning they are soluble in water. 

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What is the role of monosaccharides in terms of the relationship between monomers and polymers?

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Answer

Monosaccharides are building blocks (monomers) of larger molecules of carbohydrates (polymers called polysaccharides).

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Monosaccharides can be shown in their linear and ring structures. Draw the ring structure of glucose on a piece of paper or on your gadget.

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Answer

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What are the three different types of monosaccharides?

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  1. Trioses (3 carbon atoms)
  2. Pentoses (5 carbon atoms)
  3. Hexoses (6 carbon atoms)

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What type of monosaccharides are ribose and deoxyribose?

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Hexoses.

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What are three examples of hexose sugars?

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Glucose, galactose, and fructose. 

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What are the five most important monosaccharides? 

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The five most important monosaccharides are glucose, galactose, fructose, deoxyribose, and ribose.

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Which biological macromolecules have deoxyribose and ribose in their bases?

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Answer

DNA and RNA.

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Glucose has two isomers. What are they?

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α-glucose (alpha-glucose) and β-glucose (beta-glucose)

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α-glucose and β-glucose have the same molecular formula. What about their structure?

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They differ in structure. There is a slight difference in the arrangement of atoms in the molecule: the position of the far-right hydrogen atom (H) and the hydroxyl group (OH). 


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How are galactose and fructose similar to each other and to glucose?

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Galactose, fructose, and glucose all have the same molecular formula , but different arrangements of atoms in their structures.

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What is the reaction of monosaccharides joining together to form disaccharides and polysaccharides called?

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Condensation reaction.

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How many monosaccharides join together to form disaccharides, and how many for polysaccharides?

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In disaccharides, two monosaccharides bond together, while in polysaccharides, there are many (poly- stands for ‘many’).

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What is the covalent bond between monosaccharides called? 

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Glycosidic bond. 

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What happens during hydrolysis of disaccharides and polysaccharides?

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During hydrolysis, disaccharides and polysaccharides break down into monosaccharides. It is a reaction in which a water molecule is added and glycosidic bonds between monosaccharides are broken.

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Which of the following is NOT a monosaccharide?

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Glycogen.

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Which of the following is a monosaccharide?

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Glucose.

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Many ____ molecules join together to form a molecule of cellulose.

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Glucose 

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Which type of reaction is required to form a molecule of cellulose?

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Hydrolysis.

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Which type of reaction is required to form a molecule of glucose from glycogen?

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Reduction

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A ____ reaction is required to convert monosaccharides into a disaccharide.

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Condensation

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Which type of bond is used to join two monosaccharides together?

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Glycosidic

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Which type of bond is broken when we break cellulose down into glucose?

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Glycosidic

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What is the smallest type of carbohydrate?

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Monosaccharide

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How many monosaccharides join together to form a disaccharide?

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1

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True or False, monosaccharides are formed when we join many disaccharides together.

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True

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What type of reaction is required to break a disaccharides down into two monosaccharides?

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Hydrolysis

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Which monomer forms the basis of cellulose?

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Beta glucose

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Which monomer forms the basis of glycogen?

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Answer

Alpha glucose

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