If you're talking about the signature verification when someone sends a transaction, the public key is sent in the scriptsig of the transaction so nodes can verify the signature. The scriptsig contains both the signature and the public key. If it doesn't verify, the transaction is invalid.
Ok. I think I get it: the public key of txin bitcoin address is made public when you announce a transaction.
Is it also possible to use the public key directly as address instead of the bitcoin address? I ask because this may be useful if you want the sender to attach a private message to the receiver.