Differentiate between the structure and functions of the Rough Endoplasmic Reticulum (RER) and Smooth Endoplasmic Reticulum (SER).

The Rough Endoplasmic Reticulum (RER) is studded with ribosomes, giving it a 'rough' appearance. Its primary function is protein synthesis (especially for secreted or membrane-bound proteins) and protein folding. The Smooth Endoplasmic Reticulum (SER) lacks ribosomes, hence its 'smooth' appearance. Its main functions include lipid synthesis, detoxification of drugs and poisons, and storage of calcium ions.