Both glucose and galactose are monosaccharides, meaning they're carbohydrate molecules each made up of a single sugar ring. Chemically, they appear very similar, but they're found in different food sources and the body processes them slightly differently. Of the two, glucose is by far the more common molecule -- it's one of the most common molecules in nature.
Glucose Chemistry
Glucose has the chemical formula C6H12O6 and is made up of a ring structure consisting of five atoms of carbon and one atom of oxygen. There are many arms coming off the central ring; these consist of oxygen, hydrogen and -- in the case of one of the arms -- the sixth atom of carbon. Glucose is one of the most ubiquitous molecules in nature and is part of many larger molecules, including many sugars, starch and fiber.
Galactose Chemistry
Like glucose, galactose has the chemical formula C6H12O6 and consists of a central ring made of five atoms of carbon and one atom of oxygen. Also like glucose, galactose has many arms coming off the central ring that account for the remaining atoms in the chemical formula. The spatial orientation of these arms is different from that of glucose's arms, which is what differentiates galactose from glucose. Overall, the two molecules are visually similar, but have different absolute shapes.
Sources
Galactose isn't as common in nature as is glucose. You ingest glucose any time you eat table sugar, which contains glucose linked to another monosaccharide called fructose. Starch, or amylose, consists entirely of long chains of glucose molecules bonded together, explain Drs. Reginald Garrett and Charles Grisham in their book "Biochemistry." Even though you can't digest fiber, it's also made of glucose. The only significant source of galactose in the human diet is milk sugar, or lactose, which consists of a glucose ring linked to a galactose ring.
Digestion and Absorption
Regardless of the source, your intestine has to split both glucose and galactose apart from any other sugar rings to which they are bonded before you can absorb the monosaccharides. When you eat starch, enzymes in your digestive tract separate all the glucose rings from one another and you absorb them individually, explains Dr. Lauralee Sherwood in her book "Human Physiology." When you consume a source of galactose -- as in milk, for instance -- you break the galactose apart from the glucose to which it is bonded and absorb them separately.
Utilization
Your cells can use either glucose or galactose for energy. To burn glucose, the cells start by splitting glucose into two molecules called pyruvate through a process called glycolysis. Doing this requires converting glucose first to a molecule called glucose-6-phosphate and then further reacting the glucose-6-phosphate. You can also process galactose through glycolysis, but it takes a longer pathway to get the reaction started. Your cells convert galactose to galactose-1-phosphate, which they convert to glucose-1-phosphate, which they then convert to glucose-6-phospate, explain Garrett and Grisham. As such, it takes three steps to start glycolysis for galactose, where it only takes one step for glucose.
References
- "Biochemistry"; Reginald Garrett, Ph.D., and Charles Grisham, Ph.D.; 2007
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
