The lipid bilayer of a cell’s membrane is impassable for many molecules, including hydrophilic ("water-loving") ones. Press the buttons on the left side of the lab to release two different hydrophilic molecules on the inside and the outside of the cell. Press these buttons multiple times to release more molecules.

Drag and drop the two kinds of uniporter proteins in the toolbar onto the lipid bilayer. Uniporters act through what is called “facilitated diffusion” because they allow molecules to move according to their concentration gradient and do not use energy.

Pump proteins are found in the lipid bilayer of cells. Pumps act through what is called "active transport" because they allow particular molecules to move against their concentration gradient by using energy in the form of ATP.

A new molecule has been introduced outside
of the cell. Place the various proteins in the lipid bilayer to see how they impact the concentration of this molecule and the others. Toggle the direction of the pump proteins by clicking on their icons in the toolbox.

In order to move the new molecule you will need to use antiporters and symporters, which are membrane proteins that move two molecules at once through facilitated diffusion.