Mоlecules mоve аcrоss the selective bаrrier cаlled the plasma membrane according to their physical and chemical properties. If this process is driven by the potential energy stored in the form of a concentration gradient, it is called passive transport. Passive transport occurs without any additional energy supply by the cell. Molecules move from an area of higher concentration to an area of lower concentration in a process called diffusion. In the first virtual lab activity, we use an artificial membrane called dialysis tubing to mimic the cell membrane. Dialysis tubing is made of cellulose (polymer of glucose) and it acts as a semipermeable barrier much like a real cell membrane. It is selective for which molecules can pass through the dialysis membrane simply based on the size of the pores found in the wall of the membrane. Small molecules will be allowed to pass through while blocking larger molecules.