Which organelle in the cell functions similarly to the circulatory system by transporting substances?

The endoplasmic reticulum is indeed the correct answer as it is responsible for transporting proteins and other substances within the cell, much like how the circulatory system transports nutrients and waste throughout the body. There are two types of endoplasmic reticulum: smooth and rough. The rough endoplasmic reticulum is studded with ribosomes, which synthesize proteins, while the smooth endoplasmic reticulum is involved in lipid synthesis and detoxification processes. These structures form a network of membranes that facilitate the movement of proteins and lipids to different destinations within the cell or for export outside the cell, effectively functioning as an internal transport system, akin to the role of blood vessels in the circulatory system.

The other organelles mentioned have distinct roles. The nucleus serves as the control center of the cell, housing genetic material and coordinating activities like growth and reproduction but does not transport substances. The Golgi apparatus modifies, sorts, and packages proteins and lipids for distribution, but it is more like a processing center rather than a transport system. Mitochondria are the energy powerhouses of the cell, generating ATP through cellular respiration, but they do not perform transport functions across the cell.