Final answer:
Sodium ion (Na+) concentration is greater outside enterocytes compared to the inside, maintained by the sodium-potassium pump using ATP for active transport.
Step-by-step explanation:
The concentration of sodium ions (Na+) is typically higher outside of enterocytes, and indeed most types of cells, relative to the inside. This gradient is maintained by the sodium-potassium pump, which actively transports sodium ions out of the cell and potassium ions into the cell. The pump uses ATP to move three sodium ions out for every two potassium ions it moves in, contributing to the net negative charge inside the cell. In the case of enterocytes, this gradient is crucial for the absorption process in the small intestine, where sodium ions are accumulated inside the cells via co-transport mechanisms before being pumped out to maintain the gradient.