All organisms require a source of raw materials for the synthesis of various chemical components of the body as well as for the production of energy for all biological activities. Animals get both materials and energy from the food they eat. Plants and many microorganisms, however, obtain the raw materials from air and the soil. Photosynthetic plants and microorganisms receive energy from sunlight. These raw materials that organisms need for various synthetic activities and for production of energy, are called nutrients. The term nutrient includes any substance that is used in the metabolism of organisms. Organisms may differ markedly in their specific requirements of certain nutrients. However, they show a great deal of similarity in their basic nutritional requirements. This similarity is essentially due to the fundamental similarity in the composition of living material. All living things contain substantial amounts of the elements, carbon (C), nitrogen (N), oxygen (O) and hydrogen (H). In addition to these, the proteins of organisms contain sulphur (S) and the nucleic acids contain phosphorous (P). There is magnesium (Mg) in chlorophyll and iron (Fe) in many important compounds such as cytochromes.