Is the composition of living things similar to that of their surroundings? And why aren’t we made mostly of metals?

Observing the organisms around us, we see that they resemble each other in composition, and the differences between them lie in the combination of their components. For example, organisms are made of various proteins, but most proteins’ element composition is very similar. Here we might ask, does this composition reflect the variety of elements that exist in our habitats?

To answer this question, we need to examine the elemental composition in living organisms and that of their environment. This composition is measured in relation to the total weight of an element in the body or the environment, not the number of atoms, though usually there is no significant difference between those methods. Like other living things, our body is mostly made of water and organic compounds, thus, it has a large amount of hydrogen, oxygen, and carbon. The body and cells of all living creatures also contain nitrogen and phosphorus in abundance, as these are essential components in proteins and nucleic acids – DNA and RNA.

Calcium is also ubiquitous in living organisms – especially in bones, teeth, and the external skeleton of such organisms as corals and crabs. Additional elements, including sodium, potassium, chlorine, and sulfur are fairly prevalent in most organisms, including humans. Sulfur is a component of two essential amino acids – methionine and cysteine, which are present in many proteins; sodium, potassium, and chlorine affect the ionic composition of cells and the extracellular fluid, and are essential for regulating the activity of many cells.

Additional elements are essential to the human body, and some – to most life forms, but present in minute quantities. Cobalt, for example, is a main component of vitamin B12 needed by cells, yet accounts for only one-millionth of one percent of our body weight. Some elements are needed in small amounts for some species and not at all for others, but these are the exceptions. It’s safe to say that the main elements that make up our bodies also compose all other living organisms.

 

החומרים החיוניים ביותר לקיום החיים: תרכובות פשוטות של מימן, חמצן, פחמן וחנקן | איור: Shutterstock

The essential materials needed for life: Simple compounds of hydrogen, oxygen, carbon, and nitrogen.

The world we live in

As for the composition of the environment, the first difficulty is defining what exactly is the environment: After all, the composition of the stars and space, which represent the frequency of elements in the entire universe, is not similar to that of the Earth, including its core, oceans, and atmosphere. We will therefore consider Earth’s crust, with its oceans and ocean floors, as the environment because these constitute the habitats of most living things. 

The environment’s composition is quite different from that of living organisms. The environment, in contrast with the previously discussed elements, is rich in metals, which create minerals and salts. Silicon, aluminum and iron, in this order, are prevalent in the Earth’s crust. Of these, only iron is critical to the human body, although in relatively small quantities. Aluminum and silicon, which together are responsible for 35% of the Earth’s crust, also have some biological importance, but we need only tiny amounts of either.

And on it goes. Titanium, the ninth most common element in the Earth’s crust, has no known biological role and is very rare in our bodies. Carbon, abundant in all the organic compounds of all living matter, is only the 15th most frequent element, constituting one-hundredth to one-tenth of a percent of the Earth’s crust.

הסביבה שלנו מכילה מתכות רבות, אך חשיבותן בקיום חיים קטנה מבחינה כמותית. מתכות | צילום: Shutterstock

Our environment is made of many metals, which, in terms of quantities, have little significance for sustaining life. Photo: Shutterstock

The uniqueness of living materials

Where, then, do these significant differences stem from? Why are living organisms not made of the same elements that abound in the environment?

The answer lies in the various elements’ properties. Living organisms need, among other things, complex molecules in various forms, with a complex structure and function, and the ability to self-replicate. Organic molecules, like those based on hydrogen-carbon bonds, are obvious candidates. This is because carbon, with its ability to bond with four different atoms and tendency to create long and complex chains, can create complex molecules. Metals, on the other hand, typically do not create long and complex atomic chains, like those that make up proteins and nucleic acids. 

Other chemical differences between the metals (which include the most common elements in Earth’s crust), and nonmetals (which include the most common elements in living organisms) make the latter more suitable for life than metals. As a result, nonmetals compose most of our body.

Another point is that even if a certain element is abundant in Earth’s crust, it cannot always be easily located in concentrated deposits: Rubidium, for example, is prevalent in nature but invariably comes with potassium, and in low concentrations. Since their properties are quite similar, life evolved so that potassium is essential for living organisms yet rubidium apparently has no biological role.

In other cases, it’s hard to extract elements from the environment to use in building cells or key molecules. Aluminum, for example, is the third most common element in Earth’s crust, but ordinarily appears as an oxide or a silicate, which are very stable materials, so that it is difficult to produce aluminum from natural sources. Therefore, only few living organisms probably use it, and possibly none.

Finally, the ubiquitous elements in nature do exist in the human body, even if they serve no use. Although they don’t tend to be absorbed in our bodies, as  they serve no function in it, their high environmental concentration causes them to be present in it in small quantities. Some of them, like silicon and rubidium, are more prevalent in the body than other elements we require to maintain its function, such as iodine, needed in the thyroid, or cobalt.

 

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