The movement of substances(ions and molecules) against their concentration gradient is called Active transport.Since these substances are moving against whew they are already in abundance,extra energy is needed for them to move against this gradient.Therefore,ATP.,oxygen is needed by the cells undergoing this transport process to transfer substances into the cells.
It is useful for entry of substances in plants e.g glucose,and for transport of materials in intestine in man
Na-K pumps, exocytosis,endocytosis are examples of this transport process.
