In one active transport, we use the ATP energy molecule to move that molecule against the electrochemical gradient. In a second type of active transport, known as secondary active transport, we first use an ATP molecule to create an electric potential gradient, an electrochemical gradient, and then we use that electrochemical gradient to move our molecule across our semipermeable membrane. So, to summarize, let's take a look at the following diagram. Let's begin with passive diffusion. So once again, passive diffusion means the molecule is either nonpolar, as the case is with cholesterol, or it's very small and only slightly polar, as the case is with water. And these types of molecules can simply move directly through our phospholipid bilayer, as the case is with cholesterol and water, without the use of any type of protein, and down the electrochemical gradient.