CU or ALU? What communicates with memory?
The CU (Control Unit) is responsible for all data transport to and from the ALU. The ALU doesn't see buses, it sees two N-bit wide inputs, and ditto output. The CU must make sure that both inputs of the ALU are loaded (latches) with the required data. When the ALU has performed the operation the CU will route the output via internal (and possibly external) databus to register or RAM.