The endomembrane system is composed of the different membranes that are suspended in the cytoplasm within a eukaryotic cell. These membranes divide the cell into functional and structural compartments, or organelles. In eukaryotes the organelles of the endomembrane system include: the nuclear membrane, the endoplasmic reticulum, the Golgi apparatus, lysosomes, vesicles, endosomes and the cell membrane, among others. The system is defined more accurately as the set of membranes that form a single functional and developmental unit, either being connected directly, or exchanging material through vesicle transport. Importantly, the endomembrane system does not include the membranes of chloroplasts or mitochondria, but might have evolved from the latter (see below: Evolution).
The nuclear membrane contains two lipid bilayers that encompass the contents of the nucleus. The endoplasmic reticulum (ER) is a synthesis and transport organelle that branches into the cytoplasm in plant and animal cells. The Golgi apparatus is a series of multiple compartments where molecules are packaged for delivery to other cell components or for secretion from the cell. Vacuoles, which are found in both plant and animal cells (though much bigger in plant cells), are responsible for maintaining the shape and structure of the cell as well as storing waste products. A vesicle is a relatively small, membrane-enclosed sac that stores or transports substances. The cell membrane is a protective barrier that regulates what enters and leaves the cell. There is also an organelle known as the Spitzenkörper that is only found in fungi, and is connected with hyphal tip growth.
In prokaryotes endomembranes are rare, although in many photosynthetic bacteria the plasma membrane is highly folded and most of the cell cytoplasm is filled with layers of light-gathering membrane. These light-gathering membranes may even form enclosed structures called chlorosomes in green sulfur bacteria.
Rough Endoplasmic Reticulum
The part of the endoplasmic reticulum that contains ribosomes is named the rough endoplasmic reticulum. In the rough ER, or simply RER, there are membranous compartments and sacs studded with ribosomes. Ribosomes attached to this membrane make it look like folds of bubble wrap, perfect for wrapping presents.
Smooth Endoplasmic Reticulum
There are two parts to the structure known as the endoplasmic reticulum. Both have a membranous tubular structure and are located near the nucleus of the cell. Notice that the membrane folds, providing plenty of surface space for work to be done, like multiple benches for Santa's elves. This also creates compartments without any open ends. The lumen is the inside compartment of the endoplasmic reticulum. The lumen is completely separated from the cytoplasm that surrounds it.
You can tell apart the two types of endoplasmic reticulum in a cell image because one is studded with small ribosomes and one is not. The one without ribosomes is known as the smooth endoplasmic reticulum, because without ribosomes, it really looks like a membranous compartment of smooth tubes. This can also be referred to as the smooth ER, or simply the SER.
The smooth ER is the site of lipid and steroid synthesis. In the lumen of the smooth endoplasmic reticulum, proteins and small molecules can be chemically modified. The smooth endoplasmic reticulum is also the detoxifying space of the cell. The liver is the detox organ of your body. Therefore, your liver cells can have smoother endoplasmic reticulum than your heart cells, for example, as it uses this cellular component to detoxify drugs and alcohol.