Calculate the net ATP yield (ATP equivalents) for the combined complete oxidation of 1 molecule of serine and 1 molecule of alanine to CO2 in the liver, taking into consideration the energy cost of nitrogen excretion as urea. Show your reasoning. (Assume GTP = ATP) Work out a pathway that uses the nitrogen in each amino acid for the nitrogens in the excreted urea without the input of additional nitrogen.