Critical Thinking Activity - Chapter 3

Matter may be arranged in a generalized hierarchy from the very simple to the quite complex. We may begin with subatomic particles which compose atoms - molecules - compounds - organelles - cells - tissues - organs - organisms - populations - communities - ecosystems - Biosphere.

The Biosphere is the combination of the entire Earth and the life upon it. We could go further, to the Solar System - Milky Way Galaxy - The Visible Universe. Where in this hierarchy does life begin?

Philosophers have struggled for millennia to define life. What is life? The problem is like that of defining beauty. We all know what beauty is, but it is very difficult, probably impossible, to construct a satisfactory definition of it. As with beauty, we all know what life is, but we cannot adequately define it.

Regardless, science has come to a consensus as to where life begins in the organizational hierarchy of matter from subatomic particles to the Biosphere. A consensus is achieved when a majority supports a particular opinion. Not all biologists agree with the current consensus of exactly what is the smallest unit of life.

Some scientists believe that bits of matter smaller than those currently accepted as alive should be assumed to be living. Others emphatically deny that these structures are living.

There are still other structures that one could argue whether or not they are alive. Click here, look through the Web site that appears, and decide for yourself how many of the items mentioned might be considered as living.

FOR INVESTIGATION: For each inquiry, write your thoughts in a short paragraph for later comparison.

1. How might you define life?
2. Why do most scientists locate the smallest unit of life where they do in the organizational hierarchy of matter?
3. On what basis do some scientists disagree with this location?

The six URLs used in the discussion are here annotated in the order presented.

On Youth and Old Age, On Life And Death, On Breathing, by Aristotle, 350 B.C., translated by G. R. T. Ross

An introduction to cell biology, with graphics. Includes what is a cell, sizes of cells, differences among elements, what is living, and what is interesting about cell biology.

Cell basics, biological hierarchy, relative sizes of biological objects, elemental cell composition, the three most basic criteria that must be satisfied for something to be alive, why a rock is not alive.

The question as to why a virus is sometimes considered a living organism and sometimes not is dealt with.

Frequently asked questions about virology. What is virology? What is a virus? Is a virus alive?

Structure and Function of Organelles. A good graphic of a cell is given, with a list and discussion of the organelles found in cells.