Glucose is sometimes mistakenly thought to be a ketose, a carbohydrate containing a ketone compound group. In fact, glucose is an aldose, which is a carbohydrate containing an aldehyde compound group. Carbohydrates can either be ketoses or aldoses. However, the category the carbohydrate belongs to is dependant on its internal molecular structure.
Ketone Compound Groups
A ketone compound is any compound that contains a carbonyl group (CO) that is bonded to two other carbon atoms. These adjacent carbon atoms are known as the a-carbons. The resulting compound of the carbonyl group and the a-carbons thus takes the form: a-carbon1(CO)a-carbon2. Structurally, the carbonyl group of a ketone exists in the internal carbon atom, such as found in the compound acetone (CH3-CO-CH3). Acetone, otherwise know as propanone, is the simplest known ketone.
Aldehyde Compound Groups
An aldehyde compound is any compound that contains a terminal carbonyl group (CO), or a carbonyl group at the end of the carbon chain. Aldehyde compounds are comprised of a carbon atom that is bonded to a hydrogen atom, and double bonded to an oxygen atom. The resulting compound found in the aldehyde group thus takes the form: O=CH-. The aldehyde compound group is also commonly referred to as the formyl or methanoyl group.
Why Glucose in an Aldose
The carbohydrate glucose (C6H12O6), often referred to by the names “grape sugar,” “blood sugar” or “corn sugar,” is a simple sugar containing six atoms. These six atoms form a cyclic ring where each carbon is linked to an OH group, with the exception of one carbon atom that links to the outside of the ring. The carbon atom linking to the outside of the ring forms a carbonyl group at the end of the carbon chain, making it an aldehyde compound. Since the carbohydrate glucose contains an aldehyde compound, it is an aldose.
Fructose an Example of a Ketose
The carbohydrate fructose, as opposed to glucose, is an example of a ketose. Although it contains the same molecular formula as glucose (C6H12O6), fructose has a different molecular structure. In the cyclic ring formed by fructose, the OH group of the carbonyl group attaches to the second, internal carbon atom, bonding it to carbon atoms on either side. Since the carbonyl group for fructose is internal, the molecular structure of fructose contains a ketone compound, making fructose a ketose.
Is Fructose Better Than Glucose?
Neither glucose nor fructose can be deemed the "better" carbohydrate; they both have their advantages and disadvantages. Both are important for human consumption, and are also essential ingredients in high-fructose corn syrup, which is commonly used as a sugar substitute in many foods and beverages. Unlike glucose, too much fructose does not negatively affect blood sugar or insulin levels. This makes it a slightly better carbohydrate alternative than glucose for those with diabetes. However, fructose has been linked to an increase in visceral fat, which is the most dangerous type of fat, and is a primary cause in metabolic disorders.