Final answer:
When an ion's concentration gradient is counteracted by the voltage gradient across a membrane, it can lead to no net movement of the ion, a situation known as the Nernst equilibrium. This is a component of the electrochemical gradient, critical in cellular processes such as nerve impulse transmission.
Step-by-step explanation:
If an ion's concentration gradient opposes its voltage gradient, it is possible that its flux across the membrane is zero. This situation involves the concept of the electrochemical gradient, which encompasses both the chemical and electrical forces acting on an ion. In the context of a neuron at rest, ion channels embedded in the semipermeable cell membrane are selectively permeable to certain ions. This semipermeability allows potassium (K+) and chloride (Cl-) ions to diffuse across the membrane, while sodium (Na+) ions are initially barred.
Diffusion, driven by the concentration gradient, allows ions to move from areas of high concentration to low concentration. Simultaneously, the electrical gradient (membrane potential) can either support or counteract this movement. For example, the membrane potential typically favors the influx of cations into the negatively charged interior of the cell and the efflux of anions from the cell.
However, at a certain point, the build-up of charge from ions crossing the membrane generates a Coulomb force which creates a balance between the concentration and electrical gradients. This balance can reach an equilibrium where no net flux of a particular ion type occurs across the membrane despite opposing concentration and voltage gradients, known as the Nernst equilibrium. Essentially, if the electrical force pulling ions in one direction is equal in magnitude but opposite in direction to the chemical force (concentration gradient), the ion flux can be zero.