Why do cells need to use active transport?

Active transport is when substances are moved from one place to another with the expense of energy ( usually against the concentration gradient ). Cells use active transport when they need to move substances against the concentration gradient or electro-chemical gradient and to do so they must use energy. For example:

In plants, active transport is used to obtain nitrates and other minerals required minerals by the root cells. The plant cannot rely on diffusion as the nitrates would diffuse out of root cell into the soil. In animals, active transport occurs in the small intestine. After the food has been absorbed by the villi, the concentration of glucose molecules inside the villi increases to the point where no more food can diffuse in. As more food is still required simple sugars, amino acids, vitamins and minerals are actively transported into the villi against the concentration gradient.

3 is treated as 12/4 
12/4 + 1/4 = 13/4
 Hope this helps