What is the difference between passive and active transport?
Passive transport does not require ATP energy to be spent by the cell; active transport involves the cell spending energy to move materials.
Molecules naturally move from areas where they are abundant (high concentration) to where they are less common (low concentration) in a process called diffusion. Some molecules will cross in or out of a cell membrane in this way without the cell having to spend any energy to do so. This is passive transport.
When the cell wants to build up an excess of molecules on one side of the cell membrane (inside or outside) it must spend energy to counteract the tendency towards diffusion (described above). The cell must spend energy to do this and this is referred to as active transport.
To see a series of videos I've made on this topic follow these links:
#20 – Cell Transport part 1 – Simple diffusion ()
#21 – Cell Transport part 2 – Osmosis ()
#22 – Cell Transport part 3 – Active transport ()