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Sugar Molecule: Structure & Formula

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  • 0:01 What Are Sugars?
  • 0:44 Monosaccharides
  • 1:25 Disaccharides
  • 3:29 Polysaccharides and…
  • 4:49 Lesson Summary
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Lesson Transcript
Instructor: Shannon Compton

Shannon teaches Microbiology and has a Master's and a PhD in Biomedical Science. She also researches cancer and neurodegenerative diseases.

Sugar molecules are also known as carbohydrates. You have likely heard of simple and complex carbohydrates, but what is the difference? This lesson discusses the basic structure of sugars and how they combine to form complex carbohydrates.

What are Sugars?

Sugars are members of the saccharide family of carbohydrates. Saccharide comes from the Greek sakkharon meaning 'sugar'. Sugars are organic compounds that contain only carbon, hydrogen, and oxygen.

There are four basic types of saccharides: monosaccharides, disaccharides, oligosaccharides, and polysaccharides. Monosaccharides and disaccharides are considered simple sugars as they are the simplest of the saccharides. Oligosaccharides and polysaccharides are the complex carbohydrates because their structures contain either hundreds of sugars or other molecules.

Monosaccharides

The monosaccharides are the simplest saccharides because they cannot be broken down into smaller sugars. The general formula of a monosaccharide is (CH2O)n, where n is three or more. Figure 1 shows an example the simplest monosaccharide, dihydroxyacetone (DHA), which is a 3-carbon sugar.

The molecular structure of DHA
Molecular structure of DHA

The most well-known monosaccharides are probably glucose and fructose. Glucose is the molecule our bodies can use to make ATP, which is the energy molecule of our cells. Fructose is found in honey and fruits, while glucose is found in most foods.

Disaccharides

When you think of a sugar, you probably think of the white, sweet stuff your mom didn't like you to put all over your breakfast cereal. This is sucrose, a disaccharide that is composed of a glucose molecule and a fructose molecule. Disaccharides consist of two monosaccharides linked together through an oxygen bond. Figure 2 shows glucose, fructose, and the combination of the two, sucrose.

The molecular structures of glucose, fructose, and sucrose
Molecular structures of glucose, fructose, and sucrose

The reaction that makes a disaccharide from a monosaccharide has two names; 1) a condensation reaction or 2) dehydration synthesis. Either name for the process is correct. The name of the reaction suggest what happens. That is, when two monosaccharides condense to form a disaccharide, it earns the name condensation reaction. When they condense, a water molecule is lost. Loss of water is dehydration while formation of something new is synthesis, thus the alternative name of dehydration synthesis.

The red box in figure 2 outlines the water molecule that is lost. Two -OH groups, one from each sugar, combine to form water. The water is lost to the environment. The remaining oxygen molecule forms the bridge between the two sugars.

All organisms use sugars to make ATP for energy. Both fructose and glucose can be used to make ATP; however, fructose yields less ATP than glucose. Disaccharides are not directly useful for making ATP. They must be broken down into glucose and fructose.

The process we use to break down disaccharides is called hydrolysis. It is the opposite of the formation reaction. Specifically, in hydrolysis a water molecule is added to sucrose. This breaks the bond between glucose and fructose. We can directly use glucose to make ATP or store it as glycogen. Glycogen is simply several glucose molecules attached together. It looks much like a net with chains of glucose molecules wrapped around a protein center (figure 3).

The molecular structure of glycogen
Image of glycogen

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