Cell membranes are composed of two layers of phospholipids (a phospholipid bilayer). Some molecules are capable of crossing this membrane directly, without use of specific membrane channels. Oxygen and carbon dioxide are two molecules that can freely cross the cell membrane. In aerobic cells, oxygen is necessary for cell functioning and carbon dioxide is produced as a waste molecule. Hence, the cell “wants” oxygen to enter and carbon dioxide to leave. But molecules don’t move only in one direction–they diffuse randomly across the membrane. Set up the model with high oxygen and low carbon dioxide outside the cell and low oxygen and high carbon dioxide inside the cell. In which direction do the oxygen and carbon dioxide molecules move?