What does the endoplasmic reticulum look like?

The endoplasmic reticulum (ER) looks like a large lace-like sac made up of a network of flat sacs, tubules, and empty spaces connected by a single phospholipid membrane. When observed under a microscope, the cross section of the folded membrane appears as flat discs stacked together.

The shape, structure, and appearance of the endoplasmic reticulum relate to its functions and may change if necessary. Some endoplasmic reticulum look rough while the others look smooth. The rough appearance of the ER is due to the presence of numerous ribosomes attached to the cytoplasmic surface of the outer membrane. These are known as rough ER. The smooth endoplasmic reticulum has a smooth outer surface because of the absence of ribosomes. 

The shape of the endoplasmic reticulum may change during certain cellular processes, such as mitosis.