The movement of substances across the cell membrane either into or out of the cell
For use in (aerobic) respiration, polypeptides would first be hydrolysed to their amino acid monomers. These are each deaminated, i.e. have their amino groups removed, producing an organic acid, which can be fed into the Kreb's Cycle, and a waste product, ammonia (NH₃), which is moved to the liver and removed from the blood.
