Cracking the AP Biology Exam

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The Chemistry of Life

THE ESSENTIAL ELEMENTS OF LIFE

Although there are 92 natural elements, 96 percent of the mass of all organisms is made up of just four of them: oxygen (O), carbon (C), hydrogen (H), and nitrogen (N). Other elements such as calcium (Ca), phosphorus (P), potassium (K), and magnesium (Mg) are also present, but in smaller quantities. These elements make up most of the remaining 4 percent of an organism’s weight. Some elements are known as trace elements because they are only required by an organism in very small quantities. Trace elements include iron (Fe), iodine (I), and copper (Cu).