Smooth ER, also known as agranular endoplasmic reticulum, is primarily composed of a phospholipid bilayer membrane similar to the cell's plasma membrane. The phospholipid bilayer consists of a double layer of phospholipids, with the hydrophilic (water-loving) heads facing the aqueous environments on both sides of the membrane, and the hydrophobic (water-hating) tails facing inward, away from the water.

However, unlike the rough ER, which is studded with ribosomes on its cytoplasmic surface, the smooth ER lacks these ribosomes, giving it a smooth appearance under the microscope. Therefore, the main component of the smooth ER is its phospholipid bilayer membrane without ribosomes attached to it.