Daily Newsletter

August 12, 2012 What is life?

In order to hold a productive conversation, all parties must be speaking in the "same language," that is to say, we must hold a common frame of reference. Words like life have a tremendous number of definitions, and have been debated since before recorded history. For our discussions this semester, we need to have a definition of LIFE.
The best way to start on this is to look at what scientists consider the basic characteristics of living systems.
First off, notice I used the phrase living system. A simple definition of system is a collection of interacting components that produces a whole. Within the definition is the idea that the relationship between the components is critical. The components themselves do not make the whole, and missing one or more components degrades the whole. A living system then is an interacting set of component whose relationship results in the characteristic we think of as life.
The characteristics:

Living systems are composed of cells.

The cell is the basic unit of life (remember this statement).
All organisms are composed of cells, and all cells come from preexisting cells.

Living systems show organization of components

This is actually a property of systems, but for nearly two centuries, the idea of biological order and organization has been central to how biologists describe life.
Molecular components make up cells; cells make up tissues, tissues make up organs; organs make up organ systems; organ systems make up organisms; organisms make up populations; populations make up communities; communities with the abiotic (non-biological) environment make up ecosystems; and the ecosystems make up the ecosphere (aka biosphere).
As you will see, it goes further than this, and will include the organization of metabolic pathways and genetic information.

Living systems require energy for maintenance.

You will recall the first and second laws of thermodynamics: energy can neither be created nor destroyed, only changed; and energy always moves from a state of order to disorder (entropy).
Every metabolic action of your body must follow this, so we always require inputs of energy to maintain the living system.
We will come back to this later in the semester, but there is only point when a living system actually reaches chemical equilibrium: death. Consider that statement as we discuss metabolism later in the semester.

Living systems respond to their environment.

All organisms, from the most primative prokaryotic cell to the most sophisticated multicelluar organism must have the ability to respond to environmental stimuli.
At the most basic level, cells respond to chemical signals.
Organisms can also respond to light (normal, UV and IR), electrical changes, touch, smell (chemical), magnetism and even gravity.
Many times this is described as the organisms ability to maintain homeostasis, but it has broder implications than just the maintenance of self.

Living systems grow and repair.

Cells produce biomass, and as they live they will accumulate biomass.
- At the most basic level, we have cell enlargement; when a cell get's big, it divides producing two cells.
- As I mentioned, that was the most basic level.
- As we move through biology, you will see that there are other means of expressing growth than just cell enlargement (remember the word biomass).

The second half of this is that living systems repair themselves.
- This also touchs on the idea of self-maintenance (homeostasis).
- At the most basic level, cells must be able to build and repair their cell membranes.

Living systems must be able to reproduce.

Originally this was considered in terms of macroscopic organisms: mamals had young, plants produced seeds, molds produced spores, etc....
In modern biology, we generally think of this as organisms must be able to replicate their genetic information and pass it to the next generation.
So the binary fission of bacteria as well as the fertilization of ovum by sprm are both seen as examples of reproduction.

Living systems are adaptable.

While this is a critical characteristic, it is one that is often neglected or outright ignored.
Every population of organisms (or species) has a given adaptation range (environmental conditions).
Take for instance the different zones we have for ornamental and vegetable plants. There are some plant that can't survive Georgia heat, and some plants that can't survive a Michigan winter. The same is true of all organisms.
When it gets colder, we metabolicly adapt. Same happens when it gets warmer (think about how long it takes you to get use to those high 90^o days).
Adaptation though goes further. As we pass genetic material to the next generation, we can also pass mutations and epigenetic markers.
If these provide our children with a greater chance of reproductive success, then we will see subsequent generations with more offspring with these traits.
This then can lead to an evolutionary event (when we start seeing the new trait in more individuals, and/or when we see a group becoming a new species).

Thinking in terms of this, how can we create a definition as to what is life?

Reflection: Throughout the next few days, consider these characteristics when you look at living organisms. Can you see what is meant by the term "living system". If you watch a "nature" documentary, can you see the organization inherent in living system?