And using two ATP molecules, we form a high energy molecule, a molecule that has a high transfer potential known as carbon oil phosphate. Now, this takes place in the matrix of the mitochondria. We have an amino acid called ornithine that actually formed in step five of the urea cycle that moves into the matrix of the mitochondria. And the ornithine is combined with the carbon acid or the carbon phosphate to form the citrulline. And citrulline is yet another amino acid. So ultimately, even though these two are amino acids, they're not actually used in protein synthesis.