Kidney cells contain a channel that allows intracellular...

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 $\mathrm{H}^{+}$ 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?

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 $\mathrm{H}^{+}$ 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?

Key Concepts

Chemical Potential Gradient in Facilitated Diffusion

Transport of molecules through channels is often driven by the chemical potential gradient across the membrane. For ammonia, the free energy for its movement comes from the difference in concentration between the intracellular and extracellular environments. Even though the process is facilitated by a specific channel, no direct energy (such as ATP hydrolysis) is used; instead, the molecule moves down its own chemical potential gradient.

pH Gradient and Chemical Equilibrium of Ammonia

The action of the proton pump creates an extracellular acidic environment. This results in the protonation of free ammonia (NH?) to form ammonium (NH??), which is charged and cannot readily diffuse back through the membrane. This shift in chemical equilibrium, driven by the pH gradient, effectively traps ammonia on the outside, preventing its backflux into the cells.

ATP-Driven Proton Pump Mechanism

The proton pump in kidney cells uses energy derived from ATP hydrolysis to actively expel H? from the cell. This is an example of primary active transport, where the direct hydrolysis of ATP provides the energy necessary to move protons against their electrochemical gradient, thereby establishing and maintaining a pH gradient across the membrane.

Membrane Permeability and Passive Diffusion

Historically, it was believed that small, uncharged molecules like ammonia could freely diffuse across the lipid bilayer without the need for specific transport proteins. This concept of passive diffusion was based on the assumption that the plasma membrane is sufficiently permeable to ammonia due to its lipophilic nature, thus eliminating the need for a dedicated channel.

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