Maybe it would help the earlier poster to lay it out more explicitly. The bulk of an animal's structure is made of things like hair, and skin, and muscle, and

Maybe it would help the earlier poster to lay it out more explicitly. The bulk of an animal's structure is made of things like hair, and skin, and muscle, and cartilage, which are made largely of proteins, which in turn contain a lot of nitrogen. The bulk of a plant's structure is made of things like cellulose and lignin, which contain no nitrogen.