Carnivore and herbivore diets are contrasting, with basic nitrogen and carbon proportions vary for their particular foods. "The nitrogen content of plant tissues averages about 2%, while in fungi, animals, and bacteria it averages about 5% to 10%." Many herbivores rely on bacterial fermentation to create digestible nutrients from indigestible plant cellulose, while obligate carnivores must eat animal meats to obtain certain vitamins or nutrients their bodies cannot otherwise synthesize. All animals' diets must provide sufficient amounts of the basic building blocks they need, up to the point where their particular biology can synthesize the rest. Animal tissue contains chemical compounds, such as water, carbohydrates (sugar, starch, and fiber), amino acids (in proteins), fatty acids (in lipids), and nucleic acids (DNA and RNA). These compounds in turn consist of elements such as carbon, hydrogen, oxygen, nitrogen, phosphorus, calcium, iron, zinc, magnesium, manganese, and so on. All of these chemical compounds and elements occur in various forms and combinations (e.g. hormones, vitamins, phospholipids, hydroxyapatite).