Smooth endoplasmic reticulum is found in a variety of cell types and it serves different functions in each. It consists of tubules and vesicles that branch forming a network. In some cells there are dilated areas like the sacs of rough endoplasmic reticulum. The network of smooth endoplasmic reticulum allows increased surface area for the action or storage of key enzymes and the products of these enzymes. In the case of smooth endoplasmic reticulum in muscle cells, the vesicles and tubules serve as a store of calcium which is released as one step in the contraction process.
Calcium pumps serve to move the calcium.
This electron micrograph shows an area of smooth endoplasmic reticulum consisting of tortuous tubules and vesicles. This field is from a liver cell. After the side chain of cholesterol is cleaved in the mitochondria, the product is passed to the smooth endoplasmic reticulum and further modified. Then, it is passed back to mitochondria for final modifications.