Final answer:
Proteins that are meant for the Golgi apparatus, endosomes, lysosomes, or the cell surface must first pass through the endoplasmic reticulum (ER). The ER, along with the Golgi apparatus, makes up a portion of the endomembrane system responsible for protein modification and transport.
Step-by-step explanation:
Proteins destined for the Golgi apparatus, endosomes, lysosomes, and the cell surface must pass through the endoplasmic reticulum (ER). After being synthesized on ribosomes attached to the rough ER, proteins enter the ER lumen where they can be modified and folded. Following this, they are packaged into vesicles that bud off and are transported to the Golgi apparatus. The Golgi apparatus further modifies, organizes, packages, and tags these proteins for delivery to their final destinations, including incorporation into cellular membranes or secretion out of the cell.
The endomembrane system, which includes the ER and Golgi apparatus, serves as a crucial assembly line where proteins are modified, tagged, packaged, and transported to their respective locations within the cell or to the exterior. This system is powered by the energy generated by mitochondria, another critical organelle often referred to as the power plant of the cell. Additionally, organelles like lysosomes and peroxisomes are formed as a result of this protein processing and serve various functions such as digestion of macromolecules and detoxification, respectively.