Understanding of the reactivity of bio-organic molecules and their interaction with metal surfaces is of fundamental importance. Using the simplest amino acid, glycine, chemisorbed on Cu(110) and the related model adsorbates formate, acetate, and ammonia, we demonstrate how X-ray emission spectroscopy (XES) can be applied in combination with theoretical calculations to investigate the contribution to the valence molecular orbitals from each atom in the molecule. The orbitals in glycine that interact with the substrate are strongly localized on the functional groups,; making the bonding similar to what is found for the model molecules.