Final answer:
Charged particles move according to the electrochemical gradient present across a cell membrane, which entails both voltage and concentration gradients. The direction of ion movement depends on which gradient is dominant.
Step-by-step explanation:
When voltage and concentration gradients work in the opposite direction, charged particles will move according to the dominance of one gradient over the other. If the concentration gradient is stronger, ions will move down their concentration gradient. If the electrical (voltage) gradient is stronger, ions will move towards the area of opposite charge. In living cells, the movement of ions like Na+ and K+ across the cell membrane is influenced by the electrochemical gradient, a combination of both the concentration gradient and the electrical gradient. The direction of ion movement is essential for processes such as nerve impulse transmission and muscle contraction and is also important in the context of active transport mechanisms that move substances against these gradients utilizing energy from ATP.