Both companion cells and sieve tube elements are living to allow active process, have plasmodesmata to allow sucrose to pass between the cells
Companion cells have lots of mitochondria for active pumping of hydrogen ions across its membrane, they have a nucleus so they can control the activities of the sieve tube element as well as itself
Sieve tube element has limited cytoplasm, very few organelles and no nucleus, so there is no resistance for the sugar solution to pass through (ease of flow). End walls (sieve plates) have pores in to allow solution through