Carbohydrates are a very important part of our diet because they are one of our main sources of energy. The most basic unit of carbohydrates is monosaccharides, which is the main focus of our lesson.

What Are Monosaccharides?

Sometimes, we cannot resist indulging in something sweet. We put a dash of honey to go with our breakfast to sweeten things up, and in the summer, our trips to the ice cream shop become more frequent. Sometimes, when we bring our lunch, we add a piece of fruit to go with it so we can have it for a snack. These types of foods contain carbohydrates.

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Carbohydrates are biological molecules that contain carbon (C), hydrogen (H), and oxygen (O) atoms. Carbohydrates are very important because they provide energy and fuel for our bodies so that our brains can function properly and so that our muscles can work. Carbohydrates are our preferred source of energy. They can range from complex carbohydrates to simple carbohydrates. The simplest form of carbohydrates are monosaccharides.

Monosaccharides are the simplest units of carbohydrates and the simplest form of sugar. They are the building blocks of more complex carbohydrates such as disaccharides and polysaccharides. Physically, they are usually colorless, can dissolve in water, and have the appearance of a crystal-like substance. Molasses has a high content of the monosaccharide glucose. Cherries contain the monosaccharide fructose, and yogurt contains the monosaccharide galactose.

Structure of Monosaccharides

Now that we know what monosaccharides are, let’s take a look at how they’re structured.

Number of Carbon Atoms

We mentioned earlier that carbohydrates are composed of carbon, hydrogen, and oxygen atoms. Since monosaccharides are the simplest units of carbohydrates, then they are also made up of carbon, hydrogen, and oxygen atoms. Carbohydrates have this general formula:

The possible values for n are positive integers equal or greater than 3. If, for instance, n = 3, that means that there are three carbon atoms, and these monosaccharides are called trioses. If n = 4, there are four carbon atoms, so these monosaccharides are called tetroses. The following table shows the classification of monosaccharides based on the number of their carbon atoms, their general structure, and examples for each.

You may notice that for arabinose and glucose, their structures open up from a polygon-like structure to a chain. This is because biochemists usually use two types of drawings, called Fischer projections and Haworth projections, to draw sugar molecules. The Haworth projections are sugar molecules depicted in their cyclic forms, and Fischer projections are open chain forms as shown in the following image.

The carbon atoms are numbered in the Haworth and Fischer projections so that each edge in the Haworth projection is one carbon atom. Carbon 1 in the Haworth projection is also carbon 1 in the Fischer projection, and so forth.

Functional Groups

Monosaccharides are classified as well based on their functional groups. A functional group is categorized by atoms or bonds that are responsible for the chemical reactivity within a molecule.

If a monosaccharide contains a ketone group in an inner atom, then the monosaccharide is classified as a ketose. A ketone group is a carbon atom forming a double bond with oxygen and single bonds with two hydrocarbon groups. A hydrocarbon group is a group that contains carbon bonded with hydrogen.

If a monosaccharide contains an aldehyde group at an end carbon, meaning a carbon at the end of the chain in the Fischer projection, then the monosaccharide is classified as an aldose. An aldehyde group is a carbon atom forming a double bond with oxygen and a single bond with hydrogen.

D and L Projections

Sometimes, you may see that when we look up glucose in our textbooks, there is what we call D-glucose and L-glucose. Why are there two types of the monosaccharide glucose? This is because D-glucose and L-glucose are mirror images of each other. So, the ‘D’ and ‘L’ prefixes are used to differentiate monosaccharides that are mirror images of each other.

The ‘D’ and ‘L’ labels originate from Latin words. The ‘D’ label comes from the Latin word dexter, which means ‘on the right side,’ and the ‘L’ label comes from the Latin word laevus, which means ‘on the left side.’

How do we know which of the mirror images is ‘D’ and which is ‘L’? Let’s take a look at this example of the two mirror images of glucose.

Step 1: Number the carbon atoms starting from the top.

Step 2: Find the second highest number of carbon atoms for the mirror images of glucose. In this case, we’ll be looking at carbon number 5.

If the -OH group (also known as the hydroxyl group) is on the right side, then it is D-glucose. If it is on the left side, then it is L-glucose, so:

A good hint to remember is that the prefix ‘L’ means that the hydroxyl group is on the left.

Lesson Summary

Let’s review. Monosaccharides are the simplest unit of carbohydrates. They’re composed of carbon, hydrogen, and oxygen atoms, and they cannot be broken down further since they are already in their simplest form. Their general formula is (CH2O)n, where n is any number equal or greater than 3. If n = 3, then there are three carbon atoms, so these monosaccharide units are called trioses. If n = 4, there are four carbon atoms, and these are called tetroses, and if n = 5, the monosaccharides are classified as pentoses.

There are two ways that scientists draw monosaccharides. These are the Fischer projection, which is the open chain form of monosaccharides, and the Haworth projection, which is the cyclic form of monosaccharides. Monosaccharides are also classified as either aldoses or ketoses. Aldoses are monosaccharides with aldehyde functional groups, and ketoses are monosaccharides with ketone functional groups.

Monosaccharides generally have mirror images of each other, so these can be identified either by ‘D-‘ and ‘L-‘ prefixes. The ‘D-‘ prefix is used if the -OH or hydroxyl group is located on the right side of the second highest numbered carbon. The ‘L-‘ prefix is used if the hydroxyl group is located on the left side of the highest numbered carbon.

Monosaccharides: Key Terms

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Carbohydrates – biological molecules which provide energy for our brains and muscles

Monosaccharide – the simplest unit of carbohydrates and the simplest form of sugar

Haworth projections – drawings used by biochemists to depict sugar molecules in their cyclic forms

Fischer projections – drawings used by biochemists to depict sugar molecules in open chain forms

Functional group – a group categorized by atoms or bonds that are responsible for the chemical reactivity within the molecules

Ketose – a monosaccharide which contains a ketone group in an inner atom

Ketone group – a carbon atom forming a double bond with oxygen and single bonds with two hydrocarbon groups

Hydrocarbon group – a group that contains carbon bonded with hydrogen

Aldose – a monosaccharide which contains an aldehyde group at the end of the chain in a Fischer projection

Aldehyde group – a carbon atom forming a double bond with oxygen and a single bond with hydrogen

Learning Outcomes

At the end of this video, you should be able to:

  • Define carbohydrate and monosaccharide
  • Summarize the dietary value of carbohydrates
  • Distinguish between Fischer and Haworth projections
  • List different types of functional groups for monosaccharides