Final answer:
Galactose crosses the apical border using the Na+/glucose symport protein. Sodium moves down its gradient into the cell and carries glucose with it, allowing it to be transported across the basal membrane.
Step-by-step explanation:
The transporter that galactose uses to cross the apical border is the Na+/glucose symport protein. This protein facilitates the movement of both Na+ and glucose into the cell in the same direction, towards the interstitial space. Sodium moves down its electrochemical and concentration gradient into the cell and takes glucose with it. Once inside the cell, glucose diffuses across the basal membrane into the interstitial space.