Friday, February 5, 2016
What does it mean to say something is alive?
Note: This program first aired January 30, 2016.
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.
Biological Science, 5/E
Scott Freeman, Kim Quillin, Lizabeth Allison,
ISBN-10: 0321743679 • ISBN-13: 9780321743671
©2014 • Benjamin Cummings • Cloth, 1416 pp