So before we discuss the different modes of cell transport, let's define what an electrochemical gradient is. So let's begin with a concentration gradient or a chemical concentration gradient. So let's recall a concept from physics. We know that in physics, according to Brownian motion, if we take a molecule and place it inside a fluid, the molecule will collide with the atoms or molecules of that fluid. And as a result, that molecule in the fluid will experience random and rapid motion. So let's conduct the following thought experiment.