Which statement demonstrates the most accurate reasoning for maintaining a concentration gradient of an uncharged molecule across a membrane?

Which statement demonstrates the most accurate reasoning for maintaining a concentration gradient of an uncharged molecule across a membrane?

• A. Open channel proteins that will allow some of the molecules to diffuse across the plasma membrane into the cell
• B. Produce more of the molecule inside the cell
• C. The cell can use cytosolic enzymes to convert the molecule to a different molecule
• D. Turn on membrane proteins that pump the molecule from the surrounding environment to the inside of the cell

Answer: (C)

The core idea is that to keep an uncharged molecule at a higher outside concentration than inside, the cell must prevent accumulation inside or actively remove the molecule from the interior. If enzymes inside the cytosol convert that molecule to a different compound, the original molecule’s intracellular level drops. This creates a sink: diffusion can bring in more of the molecule from outside, but it is immediately transformed, so the inside remains relatively depleted. That ongoing conversion helps keep the gradient from dissipating. The other statements would tend to erase the gradient. Allowing open channels that let molecules diffuse into the cell would raise the inside concentration and reduce the difference. Making more of the molecule inside increases its intracellular concentration, also diminishing the gradient. Pumping the molecule from outside into the cell actively moves material in the wrong direction for maintaining a higher external concentration, further collapsing the gradient.

The core idea is that to keep an uncharged molecule at a higher outside concentration than inside, the cell must prevent accumulation inside or actively remove the molecule from the interior. If enzymes inside the cytosol convert that molecule to a different compound, the original molecule’s intracellular level drops. This creates a sink: diffusion can bring in more of the molecule from outside, but it is immediately transformed, so the inside remains relatively depleted. That ongoing conversion helps keep the gradient from dissipating.

The other statements would tend to erase the gradient. Allowing open channels that let molecules diffuse into the cell would raise the inside concentration and reduce the difference. Making more of the molecule inside increases its intracellular concentration, also diminishing the gradient. Pumping the molecule from outside into the cell actively moves material in the wrong direction for maintaining a higher external concentration, further collapsing the gradient.