The endomembrane system (endo = within) is a group of membranes and organelles (see Figure 1) in eukaryotic cells that work together to modify, package, and transport lipids and proteins. It includes the nuclear envelope, lysosomes, and vesicles, the rough and smooth endoplasmic reticulum and Golgi apparatus, which we will cover shortly. Although not technically within the cell, the plasma membrane is included in the endomembrane system because, as you will see, it interacts with the other endomembranous organelles. None of the organelles that make up the endomembrane system are found in prokaryotes with the exception of the plasma membrane. Although ribosomes are found on the rough endoplasmic reticulum, they are not technically a member of the endomembrane system because they are not made of membrane. Also, remember that ribosomes can be found free in the cytoplasm, so are not always located on the rough ER.
Text adapted from: OpenStax, Concepts of Biology. OpenStax CNX. May 18, 2016 http://firstname.lastname@example.org