Cellular functions (such as proliferation, apoptosis, migration, differentiation, tissue formation etc) are controlled by extracellular ligands. Binding of the ligands (e.g. growth factors) to their receptors at the plasma membrane initiates signal transduction. Subsequent internalisation of the ligand/receptor complexes plays crucial role in regulating the signalling output.

The ER is the major intracellular reservoir of calcium and is important in the biogenesis of peroxisomes and autophagosomes. Moreover, it plays a key role in metabolic processes, such as gluconeogenesis, lipid synthesis and protein folding of the secretory pathway.