Nutrients are chemical substances required by an organism for growth. Not all nutrients are required in the same amounts. Essential elements required by organisms are hydrogen (H), oxygen (O), carbon (C), nitrogen (N), phosphorous (P), and sulfur (S). The estimated chemical formula for a cell is CH1.8O0.5N0.2 C H 1 point 8 O 0 point 5 N 0 point 2 which means that C, H, O, and N are the predominant nutrients required by an organism. Microorganisms also require many other elements but in smaller amounts. Phosphorous, generally obtained in the form of phosphate (PO43-), is a key element in nucleic acids and phospholipids. Microorganisms also require trace metals such as copper, iron, zinc, and cobalt to be present inside the culture medium. These trace metals are generally provided in the form of ions that will serve as cofactors in enzymatic reactions. Iron is required for the synthesis of heme-containing compounds such as cytochromes and the electron transport system.