By Ron B. Davis Jr., Georgetown University
The nonmetal elements of the p-block increase in their complexity as we take a step to the left and down on the table. The last of the nonmetals, namely carbon, sulfur, phosphorus, and selenium, are unique in their properties that render them valuable to the human race. These polyatomic nonmetals play a significant biological role in human health.
Polyatomic Nature of Nonmetals
Nonmetals like oxygen, and particularly nitrogen, are very reluctant to form larger molecules as pure elements, only forming larger allotropes under extreme conditions.
But, the elements carbon, phosphorous, sulfur, and selenium are different. These elements, sometimes collectively referred to as the polyatomic nonmetals, actually prefer to form larger, more complex elemental molecules using many atoms of the same element—polyatomic.
In this region of the p-block, we are beginning to see an even greater shift in behavior toward larger, more complex molecules. And with this tendency to form more complex molecules comes some amazingly versatile chemistry that has been exploited both in technology and in nature.
All of this becomes clear when we consider the biological roles of these four special elements.
Carbon-based Life
Carbon, phosphorus, and sulfur are biochemical heavyweights. Along with hydrogen, nitrogen, and oxygen, they complete the acronym CHNOPS—a group of six elements that together make up almost 99% of the human body.
We speak of a ‘carbon-based life’ because it is carbon that provides an irreplaceable scaffold for the many biological molecules necessary for life to exist. A case in point is some of the nucleic acids of DNA, or the amino acids that make up enzymes and proteins—or the fats, sugars, hormones, and many other classes of biological molecules.
Each of them contains chains, branches, and loops of interconnected carbon atoms. It is carbon whose strong bonds to one another hold the molecule together and gives it the unique shape that in part gives each molecule its specific function. This is why carbon-based life gets its name.
This article comes directly from content in the video series Understanding the Periodic Table. Watch it now, on Wondrium.
Carbon, the Backbone
Moreover, carbon is concentrated in biomass. This element only makes up about 0.5% of all the matter in the universe and only 0.02% of Earth’s crust. Yet our bodies are about 18% carbon by mass. Nature and evolution have gone out of their way to concentrate this element about 900-fold in our bodies compared to our environment.
The other elements in these molecules are often the site where they interact with the world chemically, but without carbon to hold them all in just the right place to do so, these molecules could never exist or function at all. That tough, sturdy carbon backbone serves as the base on which other elements are strategically placed to create the molecules of life.
Sulfur and Phosphorus in the Human Body
Sulfur is found in the proteins and enzymes that provide structure and moderate chemical reactions in the body. The amino acids methionine and cysteine both contain sulfur atoms. It can influence protein structure through sulfur bridges, just like those in vulcanized rubber.
Meanwhile, phosphorus serves a crucial role in energy management, where it combines with oxygen in the form of phosphates to make molecules like ATP, which is adenosine triphosphate.
One of the phosphates can be removed from this compound to release biochemical energy when and where our bodies need it.
Selenium Compound for Medicinal Uses
By comparison, selenium is much rarer, both in our environment and in the human body. It does, however, play some interesting roles, most of which are linked to its structural similarity to sulfur just above it on the table.
When combined with sulfur, the compound selenium disulfide can be made. This cyclic compound looks a great deal like S8 molecules, thanks to the fact that selenium’s similar valence and size allow it to take the place of one or more sulfur atoms. This gives selenium sulfide antifungal properties, making it useful in topical medications to treat infections of the skin.
But selenium’s ability to impersonate sulfur also plays a role in human health itself. The amino acid selenocysteine forms when a cysteine amino acid has had its sulfur atom replaced by a selenium atom, and it appears in as many as 25 different human proteins.
Perhaps equally remarkable is that even though we know selenium is essential to our biochemistry in small amounts, we still don’t know the function of many selenocysteine-containing enzymes.
Why Nonmetals Are Important
Both nature and humanity have found uses for all four of these unique elements. Carbon provides the tough scaffolding of biomolecules of all sizes and helps regulate nuclear reactors. Sulfur provides bridges that maintain structure in proteins, or it can toughen rubber products.
Phosphorus shuttles energy in our bodies, and also puts fire on-demand at our fingertips. Even selenium appears to play a role in human health in the compound selenocysteine, even as it can be used to produce light detection systems or even new metal alloys.
Common Questions about the Biological Roles of Nonmetals
When combined with sulfur, the compound selenium disulfide can be made. This cyclic compound looks a great deal like S8 molecules, thanks to the fact that selenium’s similar valence and size allow it to take the place of one or more sulfur atoms. This gives selenium sulfide antifungal properties, making it useful in topical medications to treat infections of the skin
Phosphorus serves a crucial role in energy management, where it combines with oxygen in the form of phosphates to make molecules like ATP, which is adenosine triphosphate.
Carbon, phosphorus, and sulfur are biochemical heavyweights. Along with hydrogen, nitrogen, and oxygen, they complete the acronym CHNOPS—a group of six elements that together make up almost 99% of the human body.
