In the following graph, the magnitude of concentration difference across the nuclear pore complexes (NPCs) is plotted for four molecules (A to D) as a function of time, starting from an arbitrarily chosen initial concentration difference. Indicate which curve corresponds to each of the following molecules. Your answer would be a four-letter string composed of letters A to D only, e.g. ABCD.

( ) A large protein that is being actively transported across the NPC
( ) A small water-soluble molecule
( ) A small protein composed of a few dozen residues
( ) A large protein that is NOT actively transported into or out of the nucleus

Question

In the following graph, the magnitude of concentration difference across the nuclear pore complexes (NPCs) is plotted for four molecules (A to D) as a function of time, starting from an arbitrarily chosen initial concentration difference. Indicate which curve corresponds to each of the following molecules. Your answer would be a four-letter string composed of letters A to D only, e.g. ABCD.
  
( )	A large protein that is being actively transported across the NPC
( )	A small water-soluble molecule
( )	A small protein composed of a few dozen residues
( )	A large protein that is NOT actively transported into or out of the nucleus

Quizplus · Accepted Answer

While small molecules and proteins of up to about 5 kD in mass can freely diffuse through the nuclear pores, diffusion of larger proteins becomes slower with increasing size, with the NPC acting as a diffusion barrier for large proteins that are not actively transported. Unlike passive diffusion, active transport across NPCs can increase the concentration gradient.

In the Following Graph, the Magnitude of Concentration Difference Across