Active transport is the net movement of a substance up its concentration gradient by use of energy from respiration.

Active transport and plantsEdit

When a plant needs to absorb nitrate ions into its roots from the soil, there is a concentration gradient. However, it is the wrong way for diffusion - there is a high concentration of nitrate ions inside the cells, and a low concentration in the soil. Therefore, the roots need to make the nitrate ions go up the concentration gradient. This is done using transport proteins, which combine with the nitrate ions and 'push' them into the cell cytoplasm. This is active transport, and uses up energy from respiration.