Definition of the Smooth ER. The smooth endoplasmic reticulum, or smooth ER, is an organelle found in both animal cells and plant cells. An organelle is a sub-unit within a cell that has a specialized function.The main function of the smooth ER is to make cellular products like hormones and lipids.he interior of the smooth ER is called the lumen, which is enclosed by a phospholipid membrane, just like the membrane that encloses the entire cell. The number of smooth ER units in a cell depends on the type of cell and what its manufacturing needs are.