Endoplasmic Reticulum and the Unfolded Protein Response: Dynamics and Metabolic Integration - International Review of Cell and Molecular Biology - page 215

Figure 5.1 ER structure and general functions . Upper panel: The ER can be subdivided
into three well-defined domains, the sheet-like ER, the tubular ER and the nuclear enve-
lope. The first one is characterized as being rich in ribosomes, for which it received
the name rough ER. Since the tubular ER contains fewer ribosomes, it is commonly
called smooth ER. Both the sheet-like and tubular ER are highly dynamic and intercon-
vert between each other constantly. Most studies suggest that the tubular ER has the
ability to fuse, elongate and branch dynamically inside the cell. Lower panel: The ER
fulfills diverse functions in the cell, like calcium homeostasis through the use of a series
of channels, pumps and buffer proteins. It is essential for lipid and protein synthesis,
as well as the quality control and degradation of proteins. Together with the Golgi
apparatus, it takes part in the process of cell trafficking, which is important for the export
of products from reticulum toward the outside of the cell. Finally, the ER also regulates
the function of other organelles, such as mitochondria through dynamic interaction
zones called MAM.