Lactose, glucose, and galactose are all carbohydrates. Biochemically, they're similar in that they're made up of carbon, hydrogen and oxygen in approximately 1-2-1 ratios. All three are sugars, meaning that they taste sweet, and all can provide energy to cells. Lactose is milk sugar, and is made up of the two smaller sugars glucose and galactose.
Function
Lactose, glucose and galactose all have roles as sources of nutrition for cells. Of the three, glucose is the most ubiquitous and is found in many places in nature. Both glucose and galactose are monosaccharides, explain Mary Campbell and Shawn Farrell in their book "Biochemistry," meaning that they're single sugar units. Lactose is a disaccharide and is made up of two sugar units -- a glucose and a galactose -- chemically linked together.
Absorption
While glucose and galactose can be used for energy by cells, the digestive system can't absorb lactose. As such, it breaks lactose down into its monosaccharide components with the help of the enzyme lactase, explains Lauralee Sherwood in her book "Human Physiology." Glucose and galactose units obtained from lactose can then be absorbed into the bloodstream and taken up by cells, in the same manner as glucose and galactose.
Utilization
With regard to utilization of glucose and galactose by cells -- cells don't utilize lactose directly, since it must be broken into components before it's absorbed -- there are marked similarities in the metabolic pathways through which the sugars are burned for energy. In their book "Biochemistry," Reginald Garrett and Charles Grisham explain that galactose enters glycolysis -- the first series of reactions responsible for obtaining energy from molecules -- through a mechanism similar to that of glucose, but requires the intervention of a few extra enzymes. Cells easily use either glucose or galactose for energy.
Considerations
While cells can use both glucose and galactose for energy, and while lactose is theoretically a good source of both sugars, not all humans possess the lactase enzyme necessary to split glucose and galactose from one another. As such, in these individuals, lactose can pass undigested into the lower intestinal tract, explains Sherwood, where it is digested by intestinal bacteria. The bacteria produce large amounts of gas as a result, leading to the painful bloating and cramping associated with lactose intolerance.
Expert Insight
Though glucose, galactose and lactose are all colloquially referred to as sugars, explain Campbell and Farrell, they don't all activate the sweetness receptors of the tongue equally. Sucrose, or table sugar, is the most familiar of all sugars. Relative to sucrose -- arbitrarily rated 100 on a linear scale -- glucose is slightly less sweet, with a rating of about 80. Galactose is significantly less sweet than table sugar, with a rating of about 30 to 40 on a sweetness scale. Lactose is the least sweet of the three -- the reason that milk doesn't taste very sugary -- and ranks about 20.
References
- "Biochemistry"; Mary Campbell, Ph.D. and Shawn Farrell, Ph.D.; 2005
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
- "Biochemistry"; Reginald Garrett, Ph.D. and Charles Grisham, Ph.D.; 2007
