B3

This occurs when substances move across a concentration gradient or across a partially permeable membrane. Active transport allows cells to move substances from an area of low concentration to an area of high concentration. This allows cells to move substances like sugars and ions to move from one place to another through the cell membrane. The energy from active transport comes from cellular respiration which happens in the mitochondria.

The lungs are made up of clusters of alveoli. These are tiny air sacs. They have a very large surface area, a rich blood supply and they are moist. Diffusion occurs here. They have very thin walls. Oxygen moves into the blood by diffusion then carbon dioxide passes out of the blood through diffusion.