What is used for mediated transport of polar molecules across the plasma membrane?

The correct answer highlights the role of protein carriers in the transport of polar molecules across the plasma membrane. The plasma membrane is primarily composed of a lipid bilayer, which is hydrophobic in nature. This characteristic creates a barrier that makes it challenging for polar molecules, which have difficulty interacting with the hydrophobic interior of the membrane, to pass through freely.

Protein carriers are specialized integral membrane proteins that facilitate the transport of these polar molecules. They can undergo conformational changes that allow them to bind to a polar molecule on one side of the membrane and, after a series of transformations, release it on the other side. This process is often referred to as facilitated diffusion when it occurs down the concentration gradient, or active transport if it requires energy to move molecules against their gradient.

Other options provided do not fulfill the specific role of transporting polar molecules. Lipid bilayers do not contain inherent mechanisms for transport and serve primarily as the structural framework of the membrane. Enzymes are biological catalysts that speed up reactions but do not directly transport molecules across membranes. Cholesterol, while it contributes to membrane fluidity and stability, does not play a direct role in the transport of polar substances.