Answer:
Firstly they are too big to pass through the natural pores of the cell membrane the way water molecules do through aquaporins. Secondly, they would not pass through the hydrophobic layer of the cell membrane because the sodium ions are charged. This is because the hydrophobic layer does not interact positively with charged molecules/ions. Additionally, because sodium ions would be traveling up the concentration gradient into the cell, there is need for active transport. The channels proteins are able to harness energy from ATP to actively transport the sodium ions into the cell.
