Hi, so, public transactions in Quorum Blockchain are exactly same as they are done in ethereum.They modify only the public state of the blockchain. Whereas the private transactions are done by specifying an extra feild privateFor along with other feilds. in which we give the constellation public keys of nodes for which the transaction should be privateFor. In private Transactions only pthe rivate state of the respective nodes for which the transaction was done is modified. And you cannot modify both public and private states in quorum in the same transaction.
You can refer the following link for more details