Chromoproteins Proteins plus a pigmented prosthetic group e.g. hemoglobin, myoglobin 4. But while fats are made up only of carbon, hydrogen, and oxygen atoms, all available in the sugar and water of the cell, amino acids also contain nitrogen. Most of the earth's supply of nitrogen exists in the form of gas in the atmosphere. Hence, the proportion of the earth's nitrogen supply available to the living world is very small. Plants incorporate the nitrogen in ammonia, nitrites, and nitrates into carbon-hydrogen compounds to form amino acids.
Animals are able to synthesize some of their amino acids, using ammonia as a nitrogen source.
People who eat meat usually get enough protein and the correct balance of amino acids. People who are vegetarians, whether for philosophical, esthetic, or economic reasons, have to be careful that they get enough protein and, in particular, the essential amino acids. Until recently, agricultural scientists concerned with the world's hungry people concentrated on developing plants with a high caloric yield. Another approach to the right balance of amino acids is to combine certain foods. Beans, for instance, are likely to be deficient in tryptophan and in the sulfur containing amino acids, but they are a good to excellent source of isoleucine and lysine.