What Does It Mean for a Molecule to Be in an 'Activated Form'?

[Noesis]

What exactly does it mean for a molecule to be in an 'activated form?'

How exactly does this correspond to its energy levels?

By this I mean how UDP-glucose is an activated form of glucose, ATP is an activated form of orthophosphate, and acetyl CoA is an activated form of acetate. (I pull this sentence from a textbook).

All I can gather is that the molecule is better able to elicit a reaction. But this doesn't seem like a satisfactory explanation. Hopefully someone can elaborate on a much deeper level.

Thank you very much.

 

[Ygggdrasil]

At its heart, chemistry is about breaking bonds to form new bonds. In general, this process will occur when the bonds that are broken are weaker than the bonds that are being formed. These activated molecules, have a bond in a key position that is fairly unstable (i.e. easy to break). This is normally achieved by attaching the molecule in question to a good "leaving group" (the bond connecting the leaving group to the rest of the molecule is weak so the group can leave the molecule easily). For example, in acetyl CoA, the carbonyl OH group of acetic acid is replaced with a good leaving group, coenzyme A, via a weak thioester bond. Because the thioester bond is easily broken, it is easier for most other molecules to attach to the acetyl moiety via displacement of the thioester than it would have been to attach via displacement the OH from acetic acid.