Endoplasmic Reticulum: Rough ER vs. Smooth ER

A eukaryotic cell is sort of like a quaint little self-sustaining village, home to a variety of different organelles providing valuable services that benefit the entire town: a bakery, a mechanic, a grocery store and a mayor.

Cells have little stuff-doing structures called organelles that serve specific purposes just like the specialists in a community.

The endoplasmic reticulum, found in eukaryotic cells, is responsible for protein synthesis. ER is a network of tubes or flat sacs — kind of like a labyrinth of membranes — that serves as the factory of the cell, manufacturing and packaging proteins and lipids to send around the cell and even outside of it.

About half of the total membrane surface area in an animal cell is in the endoplasmic reticulum. The molecules the endoplasmic reticulum makes depends a lot on what kind of cell it is.

For instance, the endoplasmic reticulum in muscle cells store a lot of calcium ions because muscle cells need these to make muscles contract, and organs in the digestive system tend to have cells with an endoplasmic reticulum that manufactures a variety of different kinds of cholesterol.

There are two different regions to the endoplasmic reticulum: rough and smooth. You can find both in both plant and animal cells, and although they appear to be separate when you look at them under the microscope, they're really just different compartments of the same organelle.

The rough endoplasmic reticulum (RER), also known as granular endoplasmic reticulum, is an interconnected network of flattened, membrane-enclosed sacs known as cisternae.

It appears bumpy, hence the name, because it's studded with little molecules called ribosomes that assemble proteins out of polypeptide chains and package them up to be used by other organelles or membranes within the cell itself or even exported outside of it.

The ribosomes are not always attached to the cell membrane. During protein synthesis, they connect and disconnect when needed.

The rough endoplasmic reticulum provides quality control for these proteins and further organizes them for shipment out of the factory and to their final destination. In the case of improperly made proteins, the proteins will not move, staying within the endoplasmic reticulum.

When there are too many unfolded proteins in the ER, this can trigger the unfolded protein response (UPR). The process attempts to once again bring balance.

The smooth endoplasmic reticulum (SER) lacks ribosomes, so it appears more tubular and less bumpy under a microscope. Its job, much like that of the rough endoplasmic reticulum, is to manufacture and package molecules to the Golgi apparatus, which further processes them for transport to their final destinations.

The smooth endoplasmic reticulum also makes lipids and some steroid hormones, and in some types of cells, it metabolizes some sugars that attach to the outside of it. SER is scant in some cells, but more abundant in others, where it takes on a greater role.

Liver cells contain lots of smooth endoplasmic reticulum because the liver plays a big role in detoxification. So if you've had a few too many glasses of wine, it's the smooth endoplasmic reticulum in your liver that's helping move things along the next morning.

This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.