Question

Kidney cells contain a channel that allows intracellular ammonia to exit the cells. (a) Why did researchers originally believe that cells had no need for such a channel? (b) What is the free energy source for ammonia transport via the channel? (c) The same kidney cells also contain a proton pump that expels H⁺ from the cells. What is the free energy source for this pump, and how does its action prevent ammonia from moving back into the kidney cells?

Short answer

In solution (a), the cells do not need such a channel because ammonia is an uncharged molecule that can pass through the membrane.

Similarly, in solution (b), the free source for ammonia transport is passive mediated transport.

Then, in solution (c), the proton pump is an ATP-dependent pump, and it prevents ammonium into the cell because is an ionic polar molecule.

Step-by-step solution

In the solution (a), Cells no need for such a channel

Ammonia is tiny, non-charged molecule, and passes through the plasma membrane without difficulty through simple diffusion. For this reason, researchers originally thought cells do not need a channel for ammonia.

In the solution (b), Free energy source for ammonia transport via the channel

The ammonia concentration gradient fuels the ammonia transport.This means it is a passive-mediated transport.

In the solution (c), Free energy source for the pump

The proton pump is an active transport mechanism that uses ATP. When H⁺ is produced within the cell, ammonia NH₃is transformed into ammonium (NH₄⁺) which requires an antiporter to pass the membrane. Thus, the proton pumps cannot pass out in the intracellular zonesince it is an ionic molecule.

Hence, because ammonia may travel across the membrane, the cells do not require such a route. Passive-mediated transport is a free source of ammonia transfer via the channel. Because ammonium is an ionic molecule, the pump prevent it reentry into the cell.