Difference in concentration of ions across a membrane.

• A. Membrane Potential
• B. Concentration Gradient
• C. Equilibrium
• D. Osmotic Pressure

Answer: (B)

The difference in concentration of ions across a membrane is the concentration gradient. This gradient represents the chemical potential that drives passive movement of ions from where they are more concentrated to where they are less concentrated. In biological membranes, channels and transporters respond to this gradient, letting ions diffuse down their concentration gradient and, in many cases, contribute to building membrane potential as a separate electrical phenomenon.

Membrane potential is about the voltage created by unequal charge distribution across the membrane, not merely the amount of solute difference. Equilibrium means the concentrations are equal on both sides and net movement stops. Osmotic pressure describes the force required to prevent water movement due to solute concentration differences, focusing on water flow rather than the sheer gradient of ions. So the phrase specifically identifies the concentration gradient as the driving factor for ion movement across the membrane.