Sulfur is a structural component of some amino acids (including cystein and methionine) and vitamins, and is essential for chloroplast growth and function; it is found in the iron-sulfur complexes of the electron transport chains in photosynthesis. It is needed for N2 fixation by legumes, and the conversion of nitrate into amino acids and then into protein. In plants, sulfur cannot be mobilized from older leaves for new growth, so deficiency symptoms are seen in the youngest tissues first. Symptoms of deficiency include yellowing of leaves and stunted growth.