Final answer:
The diffusion of ions across a cell membrane is driven by a chemical force, the ion concentration gradient, and an electrical force, the membrane potential. These combined forces create an electrochemical gradient. For Na+ and K+ ions, the driving forces differ due to different ion concentration gradients and membrane permeability, but both contribute to the membrane potential.
Step-by-step explanation:
The two forces that drive the diffusion of ions across the cell membrane are the chemical force, which is the ion concentration gradient, and the electrical force, which is the effect of the membrane potential on the ion movement. The combination of these forces is referred to as the electrochemical gradient.
The membrane potential is generated due to the separation of charges across the membrane: the inside of the cell is negative compared to the outside because of the differential concentrations of ions, particularly potassium (K+) and sodium (Na+). The repulsion of like charges and the attraction of unlike charges due to the Coulomb force can halt the diffusion, creating a voltage across the membrane often called the membrane potential.
For Na+ and K+ ions, the driving forces across the membrane differ because they have different concentration gradients and the membrane’s permeability to these ions varies. However, the movement of both ions contributes to the creation of the membrane potential and influences the electrochemical gradient.