Carbohydrates are a type of macronutrient, meaning the body needs carbohydrates in large amounts. The National Institute of Medicine Food and Nutrition Board recommends that carbohydrates account for 45 to 65 percent of the total calories for an adult. Because the body can easily break down carbohydrates, they serve as the best source of energy for your cells. Three well-known compounds classified as carbohydrates -- fructose, galactose and glucose -- share many characteristics in common.
Classification
Carbohydrates can be classified even further into one of three different groups; sugars, starches or fiber. Fructose, galactose and glucose are all sugar molecules. Sugars can be further divided into two different groups, either monosaccharides or disaccharides. Monosaccharides consist of only one type of sugar molecule while disaccharides consist of two monosaccharides linked together. Fructose, galactose and glucose are all monosaccharides.
Molecular Structure and Shape
Carbohydrates all contain carbon, hydrogen and oxygen atoms and share the same general molecule formula of CH2O. The three common monosaccharides of fructose, galactose and glucose all contain six carbon atoms, 12 hydrogen atoms and six oxygen atoms, giving them the same molecular formula of C6H12O6. Although these three sugars contain the same molecules, the way they are arranged varies. Chemists classify fructose, galactose and glucose as structural isomers because they share the same molecule formula yet exhibit slightly different structures. The specific molecular makeup of fructose, galactose and glucose means that they also share their shape in common. All three sugars form a ring known as a hexose.
Use in Disaccharides
Each of these monosaccharides can combine with another monosaccharide to form a diasaccharide sugar molecule. Fructose and glucose bound together form the sugar sucrose, also known as ordinary table sugar. Combine one glucose molecule with one galactose molecule to form lactose, the sugar found in milk produced by mammals.
Sweetness
Fructose, galactose and glucose all taste sweet. Scientists measure the sweetness of sugars in relation to sucrose, which represents 100 on the sweetness scale. Fructose, found naturally in fruits, measures at 150, making it the sweetest sugar of this group. Glucose measures lower than sucrose with a sweetness factor of about 75. Galactose is the least sweet of these monosaccharides, measuring less than 50.
Use in the Body
Although the food sources of these three sugars differ, once they enter the body the cells utilize them in a similar way. Fructose, galactose and glucose cannot travel across cell membranes on their own. They require the help of specialized proteins known as hexose transporters found within cell membranes.
References
- Online Biology Textbook by Dr. John Kimball, 2011: Carbohydrates
- Georgia State University: Carbohydrates
- MedLinePlus: Carbohydrates
- National Institutes of Medicine Food and Nutrition Board: Daily Recommended Intakes
- Prince George's Community College: Molecules for Life -- Carbohydrates
- Colorado State University: Hexose Transporters
