Cells as Units of Life
The Fabric of Life
It is a remarkable fact that living forms, from amebas and
unicellular algae to whales and giant redwood trees, are
formed from a single type of building unit: the cell. All animals
and plants are composed of cells and cell products.
Thus the cell theory is another of the great unifying concepts
New cells come from division of preexisting cells, and
the activity of a multicellular organism as a whole is the sum
of the activities of its constituent cells and their interactions.
The energy to support virtually all of life’s activities flows
from sunlight that is captured by green plants and algae and
transformed by photosynthesis into chemical bond energy.
Chemical bond energy is a form of potential energy that can
be released when the bond is broken; the energy is used to
perform electrical, mechanical, and osmotic tasks in the cell.
Ultimately, all energy is dissipated, little by little, into heat.
This is in accord with the second law of thermodynamics,
which states that there is a tendency in nature to proceed
toward a state of greater molecular disorder, or entropy.
Thus the high degree of molecular organization in living
cells is attained and maintained only as long as energy fuels