We learned that matter is made up of atoms. There are nearly 100 types of atoms that occur naturally on Earth and they are known as elements. There are only six elements that make up most of the human body. These and other elements are important for cell processes in all living things. We learned about molecules which are groups of atoms that are held together by chemical bonds. We discovered that molecules can be made up of only one type of atom as in oxygen gas which has two oxygen atoms or they can be made up of two types of atoms as in water. Next we learned that substances made up of atoms of two or more elements joined by chemical bonds are called compounds and most of the molecules found in cells are compounds. Organisms need molecules for growth, repair, and other life processes. Nutrients such as lipids, proteins, and carbohydrates are used for energy and building materials. The molecules that contain instructions for cell functions are nucleic acids.