Each bitcoin is owned by the public key that it was last sent to. You can't send it from a different key - it won't be accepted by the network. The public key

Each bitcoin is owned by the public key that it was last sent to. You can't send it from a different key - it won't be accepted by the network. The public key continues to own it until a transaction signed by that public key is accepted by the network. Read the whitepaper.