Daily Newsletter: August 29, 2014 - Characteristics of Life

Daily Newsletter
August 29, 2014 

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Characteristics of Life

In order to have a meaningful discussion about living systems, we must first identify that which we will consider "living."  To do this, we identify characteristics that will allow us to differentiate between living and non-living objects.  To be effective, these characteristics must be see in all examples of life.

As a first principle, we will start with the cell theory:  All living organisms are composed of cells, and all cells come from pre-existing cells.  Thus, the cell is the basic unit of life.  One of our goals this semester is to gain an understanding of the cell, and build a working model of how a cell functions.  As a frame four this semester long discussion, let's first look at how the characteristics of life are revealed in a cell.

1. Homeostasis:  i.e., the same state.  This refers to the ability of a cell to maintain a constant 'internal' environment.  The environment around the cell can change (e.g., hot to cold, low salt to high salt), but the cell it self remains stable.  [Caveat:  All organisms have a range of environmental conditions where they can live; if you exceed the range, then the cell can not maintain homeostasis].  We shall see that the cell membrane is the defining structure of all cells, as it creates a boundary (inside vs. outside).  It is from this basic idea, and how we move things across the membrane, that we shall build the concept of homeostasis.
• A corollary characteristic is adaptation.   Cells can change in response to the environment.  For example, bacterial cells can change the phospholipids in their cell membrane as temperature changes.  One way this is done is by changing the amount of saturated fatty acids used in making phospholipids.  
• In order to adapt, you need to know how the environment changed.  This is done through systems of cellular receptors.  Stimulus (environmental change) ---> Receptor -----> Adaptation.  So cells also have the ability to Respond to Stimuli.  These stimuli could be physical factors (temperature or pH), or they could be chemical.  The thing to remember is that cells (and all life) have some type of Stimulus-Response (Receptor or Communication) system.
2. Growth:  It is easy to see a human growing from infant, through adolescence, to adulthood.  But what about cells?  We could look for the growth in size, but that can be difficult when looking at a single cell.  Instead for cells, we look at Cell Number (population size) and/or Biomass (e.g., the amount of carbon or maybe the dry weight of the population).  Growth is based on two other characteristics of life.
• Metabolism:  i.e.,  all chemical reactions in the body.  Often down played to just energy acquisition, metabolism also involves the acquisition of Carbon (needed for all biochemical compounds), other essential elements (like nitrogen), and the building of new biomolecules.  As cells make more biomolecules, they grow, and eventually divide.
• Reproduction:  reproduction is essentially the creation of new individual organisms, and is easily seen when a baby is born.  But reproduction has its basis in cellular action.  A cell, when given the correct stimulus, can divide.  Possibly it has grown too large, or it may have received a chemical signal to divide.  In either case, a cell will divide into two daughter cells.  Terms that will come up with reproduction are: binary fision, mitosis, meiosis, and cytokineses.
3. You may remember from other courses that Organization is often given as a characteristic.  Instead of talking about tissues and organs, I would remind you that living organisms are composed of cells.  If you have active, working cells, then you are living.  Doesn't matter if it is one cell or a million.