Bit-commitment based on public-key encryption

[Dragonfall]

Can any public-key cryptosystem be turned into a bit-commitment scheme? For example, if I encrypt a bit using my public key and send it to Bob, how can I cheat?

[Dragonfall]

Also, from Wikipedia:

A commitment scheme can either be perfectly binding (it is impossible for Alice to alter her commitment after she has made it, even if she has unbounded computational resources) or perfectly concealing (it is impossible for Bob to find out the commitment without Alice revealing it, even if he has unbounded computational resources) but not both.

Why not?