 Welcome to passive transport, simple diffusion. Passive transport is the movement of molecules across cell membranes without using any energy. When cells use simple diffusion, molecules naturally move from an area of high concentration to one of low concentration through a permeable membrane. The difference between the two sides is called the concentration gradient. As passive transport doesn't require any energy, the speed at which each side reaches equilibrium is entirely dependent on the porosity of the membrane. The more porous the membrane, the faster the molecules diffuse. Simple diffusion moves molecules from high to low concentration until both sides are uniform or have reached an equilibrium. Oxygen is the one molecule that moves into cells using simple diffusion. When the oxygen concentration outside of the cell is high, the molecules are able to quickly cross the phospholipid bilayer and enter the cell. The cell doesn't have to expend any energy to get the oxygen in, which is good because all cells need oxygen to survive. Let's look at this in action. Note the difference in the high concentration of molecules inside A versus side B. The molecules and water are permeable to the membrane inside of this beaker. The molecules move randomly using kinetic energy. They may collide into the walls in each other or they may move through an opening in the permeable membrane. Because there are more molecules moving on side A, there are also more collisions, so more particles move from A to B than from B to A. The amount of materials moving from side to side is called net diffusion. Net diffusion moves down the concentration gradient from areas of high concentration to areas of low concentration. The particles continue to move back and forth between side A and B until they come to equilibrium. That is, they move until the number of particles on each side is equal. You've reviewed passive transport simple diffusion.