[Electron Micrograph of a plant cell's endoplasmic
reticulum. Copyright Daniel Kunkel]
The endoplasmic reticulum (ER) is repsonible for the production of the
protein and lipid components of most of the cell's organelles. The ER contains a
great amount of folds - but the membrane forms a single sheet enclosing a single
closed sac. This internal space is called the ER lumen. The ER is additionally
responsible for moving proteins and other carbohydrates to the Golgi
apparatus, to the plasma membrane, to the lysosomes,
or wherever else needed.
There are two types of ER - rough, which is coated with ribosomes,
and smooth, which isn't. Rough ER is the site of protein
synthesis. The smooth ER is where the vesicles carrying newly synthesized
proteins (from the rough ER) are budded off.