Question

multiple-choice question
why are transport proteins needed in the cell membrane?
they make energy for the cell.
they help molecules that are too big or that are polar cross the membrane.

Explanation:

The cell membrane is selectively permeable; small nonpolar molecules can cross it passively, but large or polar molecules cannot diffuse through the hydrophobic lipid bilayer on their own. Transport proteins facilitate the movement of these otherwise unable molecules across the membrane. The first option is incorrect because energy production is the role of organelles like mitochondria, not transport proteins.

Answer:

They help molecules that are too big or that are polar cross the membrane.