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This semester I am teaching the first year biology course at
the small college where I work. Our first class met today and I thought it
right to address the elephant in the room first thing. Yes, we are studying
biology, but what does it actually mean to say something is alive?
When I asked the question out loud, there was silence in the
room. When it comes down to it, we can generally discern whether something is
alive or not, but most of us have never really thought about what it is that
allows us to make that judgment. What are the criteria that determine whether
or not something is alive? There is no universally agreed upon set of
characteristics that describe what life is, but most definitions contain some
version of the following:
First, things that are alive use energy. They have to get
energy from some where, energy is required to do what living things do, which
is combat entropy. Entropy is the tendency of a closed system to proceed from
highly ordered to disorder. Spontaneous chemical reactions move in this
direction, everything moves in this direction without the input of additional
energy. Life brings that energy into the system and maintains those higher order
molecules. Life is higher order. The energy comes either from the sun (for
photosynthesizers) or from a food source (for heterotrophs) or from chemical
bonds (for the rare chemosynthesizers).
Everything that is alive is made of cells. We didn’t know
this until we had microscopes. Cells are small enough that we were unaware of
their existence, until the technology existed to be able to see them. Then,
they were everywhere we looked. And by the 1800’s it was clear, everything that
is alive is made of cells. And just what is a cell? A cell is best defined by
it’s plasma membrane. The cell membrane is what separates that which is inside
the cell, the part that is alive, from that which is outside the cell, that
which is not alive (in the case of a single celled organism). It defines the
cell in space. The membrane controls what gets into the cell and what gets out.
The cell is the fundamental unit of life, and the membrane is what delineates
the shape of that unit.
Things that are alive contain and use information. They
contain genes, the genetic information which directs the cell to make the
protiens and enzymes that keep it functioning. Thing that are alive can also
sense information from the environment and can respond to that information with
actions that enable the organism to maintain homeostasis or regulate its
internal environment. This occurs at every level of life, from bacteria to
large complex multicellular organisms.
Living things can and must reproduce themselves, some would
say this is the imperative of life. Without reproduction, after the first
organic molecule formed or the first cell was created, life would have ended.
Without reproduction, we would have to rely on organic molecules having to
chemically evolve again and again, starting from scratch each time. And that
would not have gotten us very far.
Lastly things that are living evolve. Evolution is simply a
change in gene frequency in a population over time, frequencies that respond to
selective pressure. Versions of genes that increase an individual’s fitness
tend to be passed on, as fitter individuals get to reproduce more. Versions of
genes that don’t support fitness tend to go away, because less fit individuals
don’t get to reproduce. The genes in a population at any given time reflect the
reproductive capacity of the individuals in the population. And while it is
populations that evolve, not individuals, individuals are part of the process,
it is individuals that suffer the direct impact of selective pressure, and
individuals that do or do not get to pass on their genes. In saying that things
that are alive evolve really means then that individuals are part of a larger
process inherent to life.
In class we reviewed these criteria, and then evaluated
several different examples for whether or not we would consider them alive. The
tree was easy, so was the granite rock. The stack of firewood sparked some
debate. Yes, it was dead now, but it had been alive, and had some of the
attributes we covered. Then I showed them a picture of a foamy yellow pile of
material on the forest floor. It was a slime mold, but I didn’t tell them that.
No one knew what it was, but they had the tools to assess its “aliveness”, they
knew the questions to ask, the observations to make. Our criteria may not be
universally accepted, but it provides a framework for thinking systematically
and critically about the world we live in. My students are at the beginning of
their careers as scientists, and these are exactly the tools they need.
References:
Biological Science, 5/E
Scott Freeman, Kim Quillin, Lizabeth
Allison,
ISBN-10: 0321743679 • ISBN-13: 9780321743671
©2014 • Benjamin Cummings • Cloth, 1416 pp
Pearson Publishing
