Glucose

Glucose is known as blood sugar as it circulates in the blood and needs enzymes to initiate metabolism, dextrose, or grape sugar. The name "glucose" originated from a Greek word meaning "sweet wine." The molecular formula is C₂H₁₂O_6,  it has 4 chiral centers, and 16 stereoisomers. It can be absorbed directly into the bloodstream while digestion is occurring. In the human body most of the carbohydrates consumed morphs into glucose. The body either uses the glucose for immediate energy, stores it in muscle cells, or in the liver as glycogen for a later usage. As well as, glucose is one of the products of photosynthesis and it fuels cellular respiration. It is basically a monosaccharide found in plants.Without glucose it would be difficult for many organisms such as ourselves to function properly.

Because of glucose's stereochemisty there are many different forms, some examples are shown above. 

http://healthyeating.sfgate.com/difference-between-sucrose-glucose-fructose-8704.html

http://en.wikipedia.org/wiki/Glucose

http://images.tutorvista.com/cms/images/101/enantiomer.png