Final answer:
When voltage and the electrochemical gradient work in the same direction, the movement of ions across a cell's membrane is accelerated, enhancing processes such as neuronal action potentials and ATP synthesis in mitochondria.
Step-by-step explanation:
When the voltage and the concentration (electrochemical) gradient work in the same direction, the movement of ions across a membrane is facilitated. For example, if a positive ion such as Na+ is outside a cell, and the interior of the cell is negatively charged, both the electrical gradient and concentration gradient would promote the ion's diffusion into the cell. This simultaneous action can greatly accelerate the ion's movement into the cell. In contrast, for ions like K+, which are more concentrated inside the cell, the electrical gradient may attract it inside, but the concentration gradient would tend to drive it out.
In biological systems, such concurrent gradients are essential for processes like the generation of an action potential in neurons and the synthesis of ATP in mitochondria via chemiosmosis. During ATP production, an electrochemical gradient of H+ ions, also known as protons, is established across the mitochondrial membrane. This gradient results from active transport, which creates both a voltage (due to separation of charges) and a concentration difference (due to uneven distribution of ions), and drives H+ ions back across the membrane through a specific protein complex, ATP synthase, resulting in the production of ATP.