Can proteins freely diffuse across the nuclear membrane without assistance?

The correct answer indicates that proteins cannot freely diffuse across the nuclear membrane primarily because they are too large. The nuclear membrane is composed of a double lipid bilayer that encapsulates the nucleus, and it contains nuclear pores that regulate the transport of molecules. While small molecules, ions, and some small proteins might passively diffuse through these nuclear pore complexes, larger proteins generally require assistance for transport.

Proteins are often more substantial in size due to their complex structures, which include folded polypeptide chains. As a result, many of them exceed the size limit for passive diffusion through the nuclear pores. Instead, larger proteins typically use specific transport mechanisms, such as nuclear localization signals (NLS) that allow them to be recognized and actively transported by nuclear transport receptors.

In summary, the inability of larger proteins to diffuse freely across the nuclear membrane is a key point in understanding how cellular compartmentalization and transport mechanisms function within eukaryotic cells.