Remember, these aren't part of the 20 amino acids that our body uses to synthesize proteins. Now, once we form citrulline in step two, that moves into the cytoplasm of the cell, and in a cytoplasm, we combine the citrulline with aspartate to form argininosuxnate. Now, an important point must be made about step three. So we saw that the first amino group that ends up in the urea came from this free ammonia. But the second amino group that ends up on the urea comes from aspartate. Now, once we form arginosuxanate, that is then broken down into arginine and fumarate.