Expert: Florence M Rollwagen - 3/17/2009

Question
QUESTION: I found this definition at www.merriam-webster.com/dictionary/life

an organismic state characterized by capacity for metabolism, growth, reaction to stimuli, and reproduction

My question: Is this definition correct as far as science is concerned?

AND does this start at the fertilization off an egg?

Thank you.

ANSWER: Hi Paul:  Thanks for your question.

Biologists don’t usually use a dictionary for their definitions.  Generally the 7 aspects of life as we understand and teach to our students are:

1.  Homeostasis: Regulation of the internal environment to maintain a constant state; for example, electrolyte concentration or sweating to reduce temperature.

2.  Organization: Being structurally composed of one or more cells, which are the basic units of life.

3.  Metabolism: Consumption of energy by converting chemicals and energy into cellular components (anabolism) and decomposing organic matter (catabolism). Living things require energy to maintain internal organization (homeostasis) and to produce the other phenomena associated with life.

4.  Growth: Maintenance of a higher rate of synthesis than catabolism. A growing organism increases in size in all of its parts, rather than simply accumulating matter. The particular species begins to multiply and expand as the evolution continues to flourish.

5.  Adaptation: The ability to change over a period of time in response to the environment. This ability is fundamental to the process of evolution and is determined by the organism's heredity as well as the composition of metabolized substances, and external factors present.

6.  Response to stimuli: A response can take many forms, from the contraction of a unicellular organism to external chemicals, to complex reactions involving all the senses of higher animals. A response is often expressed by motion, for example, the leaves of a plant turning toward the sun (phototropism) and chemotaxis.

7.  Reproduction: The ability to produce new organisms. Reproduction can be the division of one cell to form two new cells. Usually the term is applied to the production of a new individual (either asexually, from a single parent organism, or sexually, from at least two differing parent organisms), although strictly speaking it also describes the production of new cells in the process of growth.


A fertilized egg is certainly alive.  If you're looking for a reason to consider a fertilized human egg a person, this definition won't cut it.

Hope this helps!  Write back if you have more questions.

FM Rollwagen, PhD



---------- FOLLOW-UP ----------

QUESTION: I have 3 more questions from your answer!

1. With regard to homeostasis, do reptiles do this?  Because I know they have to stay warm in the sun to regulate.

2. With regard to adaptation, is this talking about the species as a whole over time, or an individual? AND It looks to me not all things do adapt (animals going extinct), so which of these seven points are flexible and how flexible?

3. Does this definition mean a fertilized human egg is human life?

Answer
Hi again, Paul:

1.  Reptiles generally regulate their internal temperatures by behavior.  They seek out warm places at night (rocks, pavement) and hide during the heat of the day.

2.  When speaking about evolutionary adaptation, we mean that species adapt, not individuals.  And, yes, some don't adapt very well.  If you examine the five mass extinctions in evolution, you will find that they are related to rapid climate change.  Many of these can be attributed to volcanic events, meteor events, supernovae (flooding the planet with radiation) and plate tectonics or continental drift.  As to address the seven points above, it would depend on which extinction event we were examining.  A meteor strike, for example, could cause the collapse of the food chain by interfering with plant photosynthesis.  A supernova event could cause massive mutation and loss of variability in many species.

Many scientists think that we are in the middle of a mass extinction now, since many species are going extinct all at once.

3.  A fertilized human egg is certainly alive in the sense that a single celled organism (like an amoeba) is alive.  It can take in nutrients, reproduce (mitosis), it is organized, it can grow, etc.  Questions of "ensoulment" or "human life" are not scientific questions.

Hope this helps, write back if you have more questions.

FM Rollwagen, PhD