Monosaccharides are the simplest type of carbohydrate, containing only one sugar molecule. In fact the name itself is composed of the Greek monos, meaning single and sacchar, meaning sugar. Generally speaking monosaccharide molecular formulas are multiples of CH2O. An excellent example of this would be the simple sugar glucose, with the formula C6H12O6. This is simply CH2O times six.
Glucose is probably the single most important monosaccharide in biology. Most organisms, all the way from bacteria to humans, utilize this sugar as an energy source. It is transported throughout our bodies via our bloodstream. An important point to remember is that glucose has two isomers, called fructose and galactose.
The isomer fructose is actually found mostly in plants where it functions as, you guessed it, a source of energy. We consume much of our fructose in this form of fruit, berries and root vegetables. Galactose is produced in the body where it serves multiple purposes and when combined with glucose, creates lactose which is found in milk.
As far as structure, sugars like glucose are composed of a carbonyl group (>C=O) and many hydroxyl groups (-OH). We can classify a monosaccharide as an aldose or a ketose, depending on the location of carbonyl group. We can further categorize simple sugars based on the size of their carbon skeleton. Those with three carbons are knowns as trioses, while pentoses have five. Finally there are hexoses with six carbons.
Simple sugars are clearly an invaluable source of energy for most organisms, where they are utilized during processes like cellular respiration. Sugar molecules also serve as raw materials in other organic molecules such as amino acids. Eventually many will be incorporated into disaccharides and polysaccharides, both of which are carbohydrates you’ll need to understand as well.
Silvia S. Mader ((2010) Essentials of Biology (2nd edition). McGraw-Hill NY