Final answer:
Proteins are typically not in their fully folded state when being transported across cell membranes, in contrast to transport through the NPC. They may unfold to expose signal peptides or undergo modifications for transport, aided by carrier proteins that facilitate passive transport via conformational changes.
Step-by-step explanation:
Transportation of Proteins Across Membranes
During the transportation of proteins across a cell membrane, proteins are generally not in their fully folded state. This is in contrast to proteins transported through the nuclear pore complex (NPC), which are typically in their folded state. In the case of membrane transport, proteins may need to unfold partially or fully to translocate across membranes. For instance, post-translational transport into mitochondria involves proteins that have already folded in the cytoplasm but must then partially unfold to expose a signal peptide for recognition.
The process of protein folding is vital for the biological function of proteins. This folding occurs after translation and involves the protein assuming a three-dimensional shape that is necessary for its function. If the protein is destined for areas within the cell or needs to be secreted, it might undergo further modifications in the Golgi apparatus before reaching its final destination.
Carrier proteins play a crucial role in the transport of molecules across the cell membrane, including proteins. They bind to the molecules, undergo a conformational change to shuttle the molecules across, and release them on the other side, all without the use of energy, which categorized this as a form of passive transport.