So we have certain ions which contain full, positive or negative charges and molecules which are very polar or large. These cannot move across the cell membrane because they cannot dissolve inside the hydrophobic nonpolar region of that membrane. Now, channels are essentially these pathogeways that allow these ions or molecules to move and bypass the non polar core of that membrane, so they can cross that cell membrane without ever interacting with that hydrophobic core of the membrane. So in this particular case, we saw that all the sodium ions can spontaneously move down their electrochemical gradient, just like the market can move spontaneously down its gravitational potential gradient from a high to a low. We have this barrier, the hydrophobic core, that prevents the movement of those sodium ions, just like my hand prevents the movement of this marker down its potential gradient. And channels essentially remove that hydrophobic core and allow those ions to move down that passageway that does not contain that hydrophobic core.