Life, What Is

"Is it alive?" Children ask that question when they see a bug or perhaps a mouse that is very quiet. Then they poke it, and if it moves they say, "It's alive!" But single-celled organisms such as yeast cannot move. How does one know if yeast is alive? If one puts a few yeast cells into a clear solution of sugar and comes back the next day, the solution is cloudy and full of yeast cells. Maybe a better definition of life is that living things reproduce themselves. But what about an old pet cat? She is too old to reproduce, yet there is no doubt that she is alive. There is more to a definition of life than just reproduction.

Instead of starting with a complex organism like a bug or a cat, or even a yeast cell, think about an even simpler system. Scientists know that all living things are composed of molecules such as proteins , nucleic acids, carbohydrates , and lipids . Could a scientist (or a student in his high-school biology class) put together a mixture of molecules in the laboratory that is alive?

Using this approach, a definition of life can be created. A "thought experiment" with the yeast cells might garner such a definition. First, use a soapy detergent to dissolve the yeast cell membranes so that all the cell components fall out into the detergent solution. No matter how one recombines the parts, one cannot regenerate the living yeast cell even though all the components that were in the yeast are present in the mixture. Why not? Because the components became disorganized when they were dumped out of the original yeast cells. To prove this, add a single yeast cell that has all the same components but organized within its cell membrane. When one returns the next day, that single yeast has used the nutrients and energy available in the growth medium to produce millions of new yeast cells. It is definitely alive.

Living things are complex, and the definition of life must also be complicated. In fact, the definition used here has three parts. The first is that something is alive if it is an organized system of molecules that can use energy and nutrients (a process called metabolism ) to grow by linking smaller molecules to make larger molecules. This energy-dependent process is called polymerization, and all life grows by making polymers from smaller molecules.

The second part of the definition is that a living organism also has the potential to reproduce itself at some point in its life cycle. The reason that an old cat is considered to be alive is that she can use energy and nutrients (cat food) to grow. And, of course, when she was younger she could reproduce by giving birth to kittens, so during her life cycle the cat had the potential to reproduce, even though it may no longer be possible.

The third part of the definition of life takes into account the fact that populations of living organisms can change over time from generation to generation and thereby respond to changes in their environment. This process is called evolution. Living organisms do not need to evolve to be alive, but as populations they must have the potential to evolve, and this potential is part of the definition of life.

In summary, life can be defined as an organized system of molecules that captures energy and nutrients to grow by polymerization reactions, has the ability to reproduce at some point in its life cycle, and has the potential to evolve in response to changes in the environment.

see also Evolution; Fungi; Origin of Life

David W. Deamer

Bibliography

Hazen, Robert. "Life's Rocky Start." Scientific American (April 2001): 77–85.

Holland, John H. Emergence: From Chaos to Order. Reading, MA: Helix Books, 1998.

Morowitz, Harold J. Beginnings of Cellular Life. New Haven, CT: Yale University Press, 1992.