Final answer:
The term for the difference in ion concentration across a cell membrane is the electrochemical gradient, which affects ion movement and membrane potential.
Step-by-step explanation:
A reflection of the difference in the concentration of ions across a cell membrane at any given time is known as the electrochemical gradient. This gradient is a combination of both the electrical gradient, which is the difference in charge across the plasma membrane, and the concentration gradient, which is the difference in the concentration of ions across the membrane. The cell expends energy to maintain these gradients, which are essential for processes like electrical signaling and ion transport.
Within the cell membrane, diffusion tends to move ions like K+ (potassium) and Cl- (chloride) until they are stopped by the Coulomb force. This movement establishes a voltage across the cell membrane due to a layer of positive charge outside and a layer of negative charge inside. Although diffusion works to equalize ion concentrations, ions are differentially distributed because the membrane is selectively permeable, often being impermeable to ions like Na+ (sodium).