Biology · CELL THE UNIT OF LIFE

According to the fluid mosaic model of Singer and Nicolson, which of the following physiological principles correctly explains why polar molecules require carrier proteins to cross the plasma membrane?

1. A
   Polar molecules cannot pass directly through the nonpolar hydrophobic lipid bilayer and thus require carrier proteins to facilitate their passage.
2. B
   The quasi-fluid nature of membrane lipids completely prevents the lateral diffusion of integral proteins.
3. C
   Polar molecules undergo simple diffusion only when moving against a steep concentration gradient.
4. D
   The plasma membrane is completely impermeable to water molecules, which move exclusively via \( \text{Na}^+/\text{K}^+ \) ATPases.