Six elements on the periodic table account for 97 percent of your body's mass: carbon, hydrogen, nitrogen, oxygen, sulfur and phosphorus. Not coincidentally, these elements exist in great abundance in the Milky Way galaxy and beyond. Human beings are, as a popular saying suggests, stardust.
The names of these six elements can be remembered using the acronym CHNOPS. They are not distributed uniformly throughout the body, but some of them concentrate preferentially in some tissues.
Carbon's ubiquitous nature on Earth and beyond lies in its ability to form different types of chemical bonds: single, double and triple. With this property, carbon can join with a wide range of other elements. Carbon is a major component of amino acids, the building blocks of proteins. Proteins, in turn, make up the structural components of most organs and tissues, including muscle, enzymes and neurons.
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Hydrogen, the lightest and simplest chemical element, can form only one type of bond – a single bond. Nevertheless, hydrogen can form a greater variety of compounds than any other element, even carbon. It is, as the name implies, found in carbohydrates but also in proteins in fats, which are structural in animals. In addition, the starchy components of plants that give them their shape are made up of carbohydrates. Water, which makes up more than two-thirds of the human body, contains hydrogen.
Although nitrogen may get comparatively little attention, it is abundant in nature. More than three-fourths of the Earth's atmosphere consists of nitrogen gas. Nitrogen is found in all amino acids and thus in all proteins. In chemical terms, an amino group consists of one nitrogen atom and two hydrogen atoms. While protein is often thought of mainly as a dietary component, proteins are the drivers of everyday life, catalyzing essential biochemical reactions that build the organs and tissues that keep living things growing, adapting and reproducing.
Oxygen is vital for respiration on a moment-to-moment basis. At the same time, it is found in water, all proteins and all foods. Fats, which even the leanest animals possess in significant quantities, include oxygen, which – like carbon – is a wondrously versatile molecule from a chemical standpoint. As the Earth has aged over the course of its four-billion-plus-year lifetime, the concentration of oxygen in the atmosphere has steadily climbed from trace amounts to about 20 percent, underscoring its crucial nature in the scheme of life.
Phosphorus is something of a background player in the life-maintenance drama. It is a critical part of every plant and animal cell, as it forms the bulk of the phospholipid bilayer that gives cell membranes their integrity while allowing them to be selectively permeable to other substances. Phosphorus is also found in bone, and chemical energy derived from metabolic processes is stored for immediate use in phosphorus-based compounds such as ADP (adenosine diphosphate) and ATP (adenosine diphosphate).
Sulfur is found in all proteins, most notably in cysteine and methionine. While its role in humans is perhaps not frequently celebrated, it is especially critical in cyclic processes in bacteria, which have been around for billions of years longer than people and will almost certainly be around after human beings are long gone. Sulfur is also essential for many bacteria to properly carry out their version of photosynthesis, a set of reactions most commonly associated with plants.