Endomembrane system Endoplasmic reticulum – An organelle composed of an interconnected network of tubules, vesicles and cisternae. Smooth endoplasmic reticulum – A section of endoplasmic reticulum on which ribosomes are not attached is termed as smooth endoplasmic reticulum. It has functions in several metabolic processes, including synthesis of lipids, metabolism of carbohydrates and calcium concentration, drug detoxification, and attachment of receptors on cell membrane proteins.