Modularity is the way of separation from the rest, which is also called "abstracting out". Cells are separated pieces of a "primordial soup" - a separated (boxed) pieces of an ocean, if you wish.
While individual proteins are data-structures and enzymes are procedures, organelles are modules, cells, (even capsids of a virus) are modules.
If a procedure is a "black-box", a module is a box for black boxes, which only certain parts (particularly chosen interfaces) are exported, that is visible from outside.
This abstraction corresponds to "portals" and "pumps" on a cell's membrane. These are exported interfaces of a cell.