What’s it made of?
Probably it evolved about two billion years ago out of the cell membrane around the nucleus of the cell, just like vacuoles evolved from the cell membrane around the outside of the cell.
All eukaryote cells have some endoplasmic reticulum, or the remnants of one they used to have.
What does “endoplasmic reticulum” mean?
It is connected to the nuclear membrane that wraps around a cell’s DNA. So there is a direct connection between the cell’s nucleus and the endoplasmic reticulum. The lipid of the endoplasmic reticulum makes a maze of little tubes or tunnels.
What does the endoplasmic reticulum do?
Inside this maze of tubes, RNA molecules drift out of the cell’s nucleus. They match up with protein and sugar molecules floating around in the tubes to make new proteins, amino acids, and long chains of sugars and other kinds of large hydrocarbon molecules.
In one-celled creatures
One-celled creatures like amoebae use these molecules to repair their cell membranes and to break up their food into smaller, more usable pieces, and to reproduce themselves. Sometimes they send these molecules out of the cell to digest their food outside the cell before bringing it in.
In plants and animals
Specialized cells inside multi-celled creatures like plants and animals also assemble more specialized molecules, that they can send out of the cell to do things in other parts of the body. For instance, animal cells produce insulin inside the endoplasmic reticulum. Plant cells produce long sugar molecules. They use these sugar molecules to stiffen their cell walls so they can stand upright.
Where do the molecules go?
After the RNA is done assembling the new molecules, the molecules float away down the tubes to the Golgi bodies.