What is the difference between diffusion and active transport?

Diffusion is a process in which ions or molecules move down their concentration gradient (i.e. from an area of higher to an area of lower concentration). The concentration gradient drives the movement, so no energy is required. Because no energy is required, diffusion is described as a passive process. If they have the appropriate chemical structure, molecules can dive views directly through the cell membrane. Because of the chemical nature ions can't directly diffuse through the cell membrane they can diffuse through membrane channels. Larger molecules such as glucose need to be assisted through the cell membrane and this assistance is given by proteins carriers that physically pick molecule up on one side of the membrane and release it on the other. Because this is occurring down the concentration gradient and how energy is required this is still an example of diffusion, but in this case the carrier is helping (or facilitating) the process so it is known as facilitated diffusion.

Active transport occurs when carriers pump ions against their concentration gradient (i.e. from their lower to higher concentration). Any pump needs a source of energy to function and the cell membrane carriers are no different. They are using the molecule ATP to power the transport, so this process is known as active transport.