Biology is an interdisciplinary science in which the concepts of chemistry and physics are frequently applied. Life is defined by many different elements and specific characteristics, stemming from a long history of scientists seeking to answer that simple question: What is life, anyway?
All biology websites or textbooks begin with this supposedly simple question. The scope of it seems large at first, but when looking to define such a complex, multitudinous series of systems, the lens of study becomes smaller and smaller, straight down to the elemental level.
Looking at the Big What-is-Life Picture
All living (and nonliving, for that matter) things are first and foremost composed of matter. Matter is a very broad term, applying to anything that has mass and takes up space in the world. There are of course different types of matter, each with its own particular characteristics. Plants, beetles, gasoline, the family dog, river rocks, the living room couch, and the obnoxious uncle are all made up of matter. But this doesn’t seem to get us any closer to answering the big question; on the contrary, matter is where to start when looking at life as a whole.
From matter, the lens zooms in on elements, of which all matter is comprised. Elements are defined as pieces of matter that cannot be broken down any further into separate entities. As of today, chemists through the ages have identified 92 naturally occurring elements (though some argue that number as low as 88), each with its own unique properties.
Elements join together to form compounds, where two or more different elements combine into a fixed ratio. Without getting too far into the chemistry part of this discussion, when compounds form, their union unleashes emergent properties, which can make two previously dangerous elements into a harmless compound.
For instance, common table salt (NaCl in chemist-speak), which most Northern Americans consume on a daily basis, is a compound made up of the elements sodium (Na) and chlorine (Cl). Sodium, on its own, is a heavy metal, and chlorine a toxic gas. Of course, when the two form their ionic bond and become a compound, they are harmless in reasonable doses (consuming extremely large amounts is not a good idea, though). This is an example of the emergent properties of a compound.
The Actual Elements of Life
Out of all those naturally occurring elements, only about 25 or so of the elements are actually known to be essential to life. And out of this smaller group, only four elements make up over 95 percent of living matter, the others being trace elements: carbon (C), hydrogen (H), nitrogen (N) and of course, oxygen (O). Other important and not-quite-trace elements that make up most of the other five percent are phosphorous (P), calcium (Ca), potassium (K), and sulfur (S).
Trace elements are only required in, well, trace amounts. Some are universally needed by the various life forms in trace amounts, such as iron (Fe). Other trace elements are needed on a species-to-species basis.
Identified Indicators and Characteristics of Life
One essential characteristic of life is, quite simply, order and complexity. This does sound vague and almost cliché, but all types of life are highly ordered and diverse, in spite of having very basic similarities. This order and structure is brought about, at the most basic level, by the organizing of elements into compounds and so on down the line.
Next, life adapts to ensure its survival. This is easiest understood when looking at a camouflaged animal. Take the chameleon, for instance. In order to blend in to its environment and avoid being picked off by predators or seen by its prey, it has evolved special pigment cells called chromatophores that allow the lizard to change color.
Life responds to its environment. When the weather is cold, humans put on coats. The arctic fox’s fur changes its brown color to a nice, blending-in white. Plants navigate towards sunlight. They all respond.
All life must process energy, through intake of food and other types of energy sources (like the sun and heat for plants and types of bacteria).
Reproduction is also a necessary characteristic to call something “living.” It must also be able to grow and develop, in order to carry on the genetic information into future generations.
These are only a few of the essential properties of life; this list is far from comprehensive. And with that said, possessing one or a few of these qualities doesn’t necessarily mean something is alive (organelles in cells, for instance, fulfill some of these requirements, but are not considered to be alive). For more information on the properties of life, visit the University of Miami’s colorful and useful 10 Properties of Life page.