glyc(o)- Also gluc(o)-.
[Greek glukus, sweet.]
The first substance named using glyco- was glycerine (now more usually glycerol, especially in scientific contexts), because it tasted sweet. Gluco- was first used in the name of the important sugar glucose.
In modern chemistry glyco- is more common than gluco-, and refers to the sugars, which in combination make up the carbohydrates and starches. So a glycoside is a compound produced from any of the simple sugars, a glycogen is a substance deposited in bodily tissues as a store of carbohydrates and a glycoprotein is a protein with carbohydrate groups attached.
Some compounds in gluco- relate to any sugar, such as glucocorticoid, one of a group involved in the metabolism of carbohydrates. However, most current words including it refer specifically to glucose: a glucoside is a glycoside that derives from glucose alone, a glucan is a complex sugar containing only glucose units, and glucagon is a hormone in the pancreas that promotes the breakdown of glycogen to glucose.