What is the best reason why oxygen (o2) can cross the plasma membrane by simple diffusion, but glucose must be transported across the barrier by facilitated diffusion? what is the best reason why oxygen (o2) can cross the plasma membrane by simple diffusion, but glucose must be transported across the barrier by facilitated diffusion? oxygen can be burned/consumed as it crosses the membrane, providing the energy needed for its passage. oxygen is a nonpolar molecule, easily passing through the nonpolar middle area of the plasma membrane. glucose is nonpolar, making it very difficult for it to cross the polar portions of the plasma membrane structure. oxygen is a much smaller molecule, making it easier to pass through the molecules of the plasma membrane.