heterotrophs eat stuff, these stuffs most of the time are complex macromolecules like glucose etc. Heterotrophs then break down these complex macromolecules into simpler ones thru the process of respiration. in the process they produce ATP, these are energy molecules of the cell. and the end product is use to build up tissues and other parts of the heterotroph's body. So respiration breaks down large molecules into simpler ones which is a building block for another molecule needed by the heterotrophs. Also respiration is an efficient way to harness energy from the food stuffs the hetetrophs eat
