Final answer:
Energy is needed for the sodium-potassium pump to move ions against their concentration gradients, an activity that requires ATP to supply the necessary energy.
Step-by-step explanation:
The sodium-potassium pump is an active transport mechanism that uses ATP (adenosine triphosphate) to move sodium ions (Na+) out of the cell and potassium ions (K+) into the cell against their respective concentration gradients. Energy is needed for this process because the pump is moving sodium and potassium ions from areas of lower concentration to areas of higher concentration, which is an 'uphill' movement contrary to the natural 'downhill' movement of diffusion. This requires energy, which is supplied by ATP as it is converted to ADP (adenosine diphosphate), thus powering the transport.
ATP provides the energy for the sodium-potassium pump by transferring a phosphate group to the carrier protein involved in the process. This transfer causes the protein to change its shape, enabling it to move the ions across the plasma membrane. The energy-expending phase of the pump is essential for maintaining the concentration gradients essential for a plethora of cellular functions, including nerve impulse transmission and muscle contraction.