By Robert Hazen, Ph.D., George Mason University
Many biologists would argue that it’s impossible to define life in any simple way. Nevertheless, there are six characteristics shared by all living organisms on the Earth. If you set down those six traits and understand them, it is possible then to come up with a useful, working, or operational definition for what life is.
Living Organisms and Metabolism
First of all, all known organisms are highly complex chemical systems. They have thousands of interdependent molecular components. The simplest life form is far more complex than the most advanced products of any human technology. In terms of the chemistry of life, there is a term called metabolism; it’s the collective chemical repertoire of a living organism.
Even the simplest life forms are capable of hundreds of different chemical reactions, and these reactions wouldn’t occur otherwise in nature. They don’t occur spontaneously; they only occur in living things. Most of life’s chemical reactions require enzymes: these are molecular catalysts. They greatly increase the efficiency of reactions, and the catalyst, or the enzyme itself is a molecule that doesn’t change in the process of facilitating that chemical reaction.
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Single-celled to Multicellular Organisms
The second characteristic of all life is that all organisms are composed of cells, which is the unit in which metabolism occurs.
Many organisms, including bacteria and yeast, are single-celled organisms. But most known species, including all the plants and all the animals, are multicellular, composed of many cells, sometimes tens or hundreds of trillions of cells collected together in a single organism. A human being, for example, contains about 100 trillion cells.
Obtaining and Using Energy
The third characteristic is that all organisms obtain and use energy. Energy is the ability to do work, to exert a force over a distance.
Every organism needs to feed, to grow, and to reproduce, and so it has to have energy. Plants, and many single-celled organisms, obtain their energy directly from the Sun, through a process called photosynthesis.
Most animals, on the other hand, obtain energy, directly or indirectly, from plants. They eat plants, or they eat organisms that have eaten plants, and so forth. There are a few simple one-celled organisms that have been found to get their energy directly from Earth’s inner heat. They get the energy from chemistry; in fact, they eat rocks, essentially.
Learn more about the genetic code.
DNA: The Genetic Mechanism
The fourth characteristic of all life on Earth is that all organisms reproduce using the same genetic mechanism. A genetic mechanism is a way of processing information, passing that information from one generation to the next. It turns out that this aspect of information passing from one generation to another is key to all organisms.
There are three characteristics about this genetic code that should be remembered. The first is that each individual reproduces its own species. The second is that offspring inherit genetic instructions from their parents through a molecule called DNA; DNA carries information that passes from one generation to the next.
Finally, the genetic information of offspring can differ from that of the parents. This gives you a way of reproduction with variation, and that variation is key to living things, the fact that one generation can differ slightly from the previous.
Living Organisms Grow and Develop
The fifth characteristic of all life is that all organisms grow and develop. Most organisms change form and capabilities as they get larger.
For example, a seed develops into a flower. A fertilized human egg, which is a single cell, develops into the 100 trillion cells that are a human being.
This implies that all organisms have to gather atoms, and they have to gather energy. There’s no way you can go from a single cell to 100 trillion cells unless you use a lot of energy, and a lot of new atoms and molecules in the process.
Learn more about the atomic theory.
Response to Changes
The sixth characteristic and final characteristic is that all organisms respond to changes in their external environment while maintaining a relatively constant internal environment. This point has a couple of subtexts.
First of all, there has to be an inside and an outside to all living things. The inside is in some way controlled, chemically and physically. Furthermore, organisms can respond in significant ways to the external influence.
Larger animals, for example, may cause smaller animals to scurry away; that’s a kind of response to your environment. But plants also respond to environmental changes. For example, the direction of the flowers following the Sun; the fact that roots, as they penetrate deep into the ground, will change their direction, seeking water or going around obstacles.
Common Questions about the Six Characteristics Shared by Living Organisms on Earth
All living organisms are highly complex chemical systems; they’re all composed of cells; they all obtain and use energy; they all reproduce using the same genetic mechanisms; they all grow and develop; and finally, they all respond to changes in their environment.
Bacteria and yeasts are examples of single-celled organisms. At the same time, the rest of the living things, such as animals and plants, consist of hundreds of trillions of cells and fall into the category of multicellular living organisms.
Living organisms reproduce utilizing the same genetic mechanism. The genetic mechanism carries data and passes it on from one generation to the next. An essential fact about the genetic mechanism is that one generation can be slightly different from the previous one, and this develops diversity in living organisms.
