Passive transport allows a cell to have materials that are needed, enter the cell without any energy having to be used. Passive transport occurs through channel proteins in the phospholipid membrane. The ability for a chemical to enter the cell depends on its size or shape. This is why only specific substances can go through the channel proteins. This is called selective transport. The cell is selecting, based on size or shape, the molecules that can easily pass into the cell. in passive transport, no energy is expended and the materials have to go down the concentration gradient (high to low).
Facilitated diffusion is a type of passive transport. It allows certain molecules, again based on size and / or shape to pass into the cell. An example is potassium and sodium. Even though the cell will have to pump these ions out later, sodium and potassium will follow the concentration gradient. Sodium will come into the cell and potassium will leave the cell. Later the cell will have to use ATP energy to pump these ions back to where they started.