Which cellular structure is characterized by a network of membrane-enclosed tubules that do not have ribosomes?

The cellular structure characterized by a network of membrane-enclosed tubules that do not have ribosomes is the smooth endoplasmic reticulum (Smooth ER). The Smooth ER is involved in various metabolic processes, including the synthesis of lipids, metabolism of carbohydrates, detoxification of drugs and poisons, and storage of calcium ions. The absence of ribosomes distinguishes it from the rough endoplasmic reticulum, which is studded with ribosomes and primarily involved in protein synthesis and modification.

Given its distinct structure and function, the smooth endoplasmic reticulum contributes significantly to cellular homeostasis and lipid supply, playing an essential role in maintaining the overall cellular environment. This is why the smooth endoplasmic reticulum is the correct answer to the question, as it specifically meets the criteria of being composed of a network of tubules without ribosomes.