Biochemically, glucose, sucrose, and lactose are all sugars. Unlike glucose and sucrose, lactose doesn't taste particularly sweet, but all three molecules affect the body in similar ways, and all three can be used to provide for cellular energy requirements. Glucose is a very common sugar found in table sugar and starch, while sucrose is table sugar, and lactose is milk sugar.
Significance
Glucose, sucrose, and lactose fall into the biochemical class of carbohydrates. Carbohydrates are all composed of the elements carbon, hydrogen and oxygen, note Drs. Reginald Garrett and Charles Grisham in their book "Biochemistry," and they're made up of one or more saccharide, or sugar, molecules. Glucose is a monosaccharide, and consists of a single sugar molecule, while lactose and sucrose are both disaccharides. Each contains a glucose unit, and sucrose contains a fructose unit, while lactose contains a galactose unit.
Function
In the body, glucose, sucrose and lactose can all be used to provide for cellular energy needs. The intestine can only absorb monosaccharides, however, so sucrose and lactose must be broken into their constituent sugar units before being absorbed, explains Dr. Lauralee Sherwood in her book "Human Physiology." The sugars, once absorbed into the bloodstream, are taken up by cells and metabolized for energy through a pathway called glycolysis, meaning "sugar splitting."
Features
Structurally, glucose is quite similar to the other constituent molecules of sucrose and lactose. Fructose and galactose are both monosaccharides like glucose, and all three share the same chemical formula--C6H12O6. There are structural differences among the molecules, however, that are significant enough to require that different enzymes be involved in metabolizing the sugar units. Further, enzymes are required to split apart the sugar units in lactose and sucrose prior to absorption of the monosaccharide components of those molecules.
Considerations
One major difference between lactose and the other two sugars is that while all humans have the ability to split sucrose in the digestive tract and absorb its components, many individuals lack the lactase enzyme that breaks lactose into glucose and galactose. The result, note Drs. Garrett and Grisham, is that these individuals are lactose intolerant. The lactose moves undigested into the lower intestine, and is there metabolized by bacteria, resulting in cramping and gas.
Expert Insight
While glucose, sucrose and lactose are all chemically classified as sugars, not all sugars bind to sweetness receptors on the tongue in the same way. Sugars with greater affinity for sweetness receptors bind to receptors harder, note Drs. Garrett and Grisham, and consequently taste sweeter. Sucrose is the sweetest of the three sugars, and is approximately one-third sweeter than glucose. Lactose is the least sweet of the three, and is only about 15 percent as sweet as table sugar.
References
- "Biochemistry"; Reginald Garrett, Ph.D. and Charles Grisham, Ph.D.; 2007
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
