Which organelle functions in the transport of proteins and molecules across membranes?

The endoplasmic reticulum plays a crucial role in the transport of proteins and molecules across membranes within the cell. This organelle is composed of a series of interconnected tubular structures that serve as a site for the synthesis and processing of proteins.

The rough endoplasmic reticulum, which has ribosomes attached to its surface, is primarily responsible for the synthesis of proteins destined for secretion or for use in the cell membrane. Once these proteins are synthesized, they are packaged into vesicles that bud off from the endoplasmic reticulum and are transported to the Golgi apparatus for further processing and sorting.

Moreover, the smooth endoplasmic reticulum is involved in the synthesis of lipids and can also assist in the transport of various molecules. This intricate system emphasizes the endoplasmic reticulum's essential role in the cellular logistics that ensure proteins and other molecules reach their appropriate destinations in a timely manner.

In comparison, the Golgi apparatus primarily modifies, sorts, and packages proteins for secretion, rather than transporting them. The nucleus houses DNA and is involved in transcription, while the plasma membrane serves as a barrier that regulates what enters and exits the cell but does not actively transport proteins on its own. Thus, the endoplasmic reticulum is