Describe the Endoplasmic Reticulum.

(N/A) $ \Rightarrow $ The network of tiny tubular structures scattered in the cytoplasm of eukaryotic cells is called the Endoplasmic Reticulum $(ER)$.
$ \Rightarrow $ The $ER$ divides the intracellular space into two distinct compartments, i.e., luminal (inside $ER$) and extra-luminal (cytoplasm) compartments.
$ \Rightarrow $ The $ER$ often shows ribosomes attached to its outer surface.
$ \Rightarrow $ The endoplasmic reticulum bearing ribosomes on its surface is called rough endoplasmic reticulum $(RER)$.
$ \Rightarrow $ In the absence of ribosomes, they appear smooth and are called smooth endoplasmic reticulum $(SER)$.
$ \Rightarrow $ $RER$ is frequently observed in cells actively involved in protein synthesis and secretion.
$ \Rightarrow $ They are extensive and continuous with the outer membrane of the nucleus.
$ \Rightarrow $ The smooth endoplasmic reticulum is the major site for the synthesis of lipids. In animal cells, lipid-like steroidal hormones are synthesized in the $SER$.