While there are 118 elements in the periodic table, the molecules that form living matter are composed primarily of carbon (C), hydrogen (H), oxygen (O), and nitrogen (N). The human body, for example, is on average approximately 60% water (H2O) by weight, and not all of the H and O atoms in the body are present as water molecules. That means that the total weight percentage of hydrogen and oxygen combined is greater than 60%. A living tree, which is also made of biological molecules, consists of primarily water molecules. Water makes up two-thirds of the mass of a living tree, or more precisely, 65% to 75% oxygen and 9% to 10% hydrogen. A living tree's mass also contains about 15% to 18% carbon, with the combined carbon-hydrogen-oxygen mass making up more than 90% of a tree's mass in some species.The elements calcium (Ca) and phosphorus (P) make up a much smaller fraction of biological molecules, and potassium (K) and sulfur (S) are an even smaller fraction. It is important to remember that although an element accounts for only a small percentage of the mass of the biological molecules in a living organism, the element's value is not necessarily insignificant. Consider that cell membranes, which are the boundaries between the interior and exterior of a cell, are made up of a type of molecule called a phospholipid. Each molecule of phospholipid contains exactly one phosphorus atom. This phosphorus imparts a set of properties to the phospholipid, which is critical to the integrity of the cell membrane. Simply put, without phosphorus, cells would fall apart. An additional example of the importance of phosphorus in cellular makeup is phosphate found in DNA nucleotides. The phosphate group is essential for linking the elements of DNA together and for providing a backbone holding DNA and RNA together.