Question

why do the polar molecules need a carrier protein to go into the cell membrane

Answer 1

Polar molecules need a carrier protein to go into the cell membrane as the cell membrane is impermeable to charged and polar molecules. The cell membrane is a phospholipid bilayer and consists of hydrophobic fatty acid side chains which span the lipid bilayer whereas polar head groups of fatty acids are present at exterior. The presence of fatty acid side chains does not allow passage of polar molecules through the cell membrane and they have to be actively transported by carrier molecules.