This is a process by which substances are transported across cell membranes by means of protein carrier molecules.

Facilitate diffusion is the process in which substances are transported across the plasma membrane along the concentration gradient with the help of carrier molecules.

In active transport also carrier proteins are used but they are used to transport the molecules against the concentration gradient.  

Carrier proteins bind the molecules to be transported non-covalently and after binding, it undergoes a conformational change which helps in the movement of the molecule through the protein inside the cytoplasm of the cell.

Charged and polar molecules are mainly passed through facilitated diffusion like amino acids, carbohydrates, ions. The rate of transfer of molecules across the plasma membrane is higher in facilitated diffusion than simple diffusion.

c. heat from the poles back down towards the equator.