Oxygen, Water and Light, Oh My!
The Toxicity of Life's Basic Necessities
By Joe W. Francis
Every living creature is made of amazingly small and complex units called
cells. Cells viewed under the microscope do not appear to do much, yet
they are full of microscale machines involved in tremendously complex
reactions. Most of life's processes are so small and transparent that we cannot
see them in action with microscopes. But the chemistry of life is constantly in
motion in living cells. College level biochemistry textbooks typically contain
over a thousand pages and describe hundreds to thousands of complex reactions
which occur simultaneously within these tiny packets of life we call
cells.
Despite this immense complexity, living cells are made primarily of four
atoms: carbon, hydrogen, oxygen, and nitrogen. Two of these atoms, hydrogen and
oxygen, are bonded together to make water which is the most abundant molecule
in living organisms. The oxygen molecule itself plays a critical role in
regenerating energy in the cell. In addition, all living creatures need a
supply of energy. Energy in most ecosystems ultimately comes from light. For
instance, all of the food energy we consume can eventually be traced back to
the light energy captured in cells. So, it is no surprise that oxygen, water,
and light are very abundant on the earth. Living organisms are continually
exposed to these very important substances upon which life depends.
Origin-of-life researchers, who try to determine how life originated by natural
means, must incorporate water, oxygen, and light into their formulas for early
life. However, curiously enough, all three of these substances are toxic to
life. In fact, living cells fight a daily, moment by moment battle against the
toxicity of oxygen, water, and light. Let us examine the toxicity of each
of these substances.
Oxygen interacts with many atoms and molecules. This is evident in metal
structures all around us which tend to "oxidize" or rust over time. If the
oxygen content in our atmosphere was just a few percent higher than its current
21 percent, the potential for devastating forest fires and an unstable
explosive atmosphere would increase significantly making life less likely to
thrive on the earth. The toxic effects of elevated oxygen levels on life can be
directly observed in the damaged lungs of human patients who receive oxygen for
therapeutic reasons.
Oxygen is toxic to living organisms because when it interacts with living
cells the oxygen molecule itself breaks down into toxic intermediates. These
intermediates interact with and modify many essential molecules in cells.
Consequently, because we live in an oxygen environment, our cells and their
contents are constantly being threatened by toxic oxygen intermediates. If this
threat is not continually neutralized, life would cease to continue. Cells
handle this threat by making a variety of toxic-oxygen-binding enzymes,
including a major type called superoxide dismutase (SOD) which binds and
deactivates superoxide, the dominant toxic oxygen species. SOD is found within
the cell, outside the cell, and in the membranes of the cell. Our body cells
are literally surrounded by SOD. In fact, the concentration of SOD in a cell
environment can be 100,000 times greater than the concentration of toxic
superoxide.
Because oxygen appeared very early in the development of life, SOD or a
protection mechanism similar to it would be required to appear early in the
evolution of life. This is problematic for several reasons. One is that the SOD
would need to specifically bind superoxide and not oxygen. Superoxide and
oxygen are very similar in size and shape and if SOD reacted with oxygen and
prevented its entry into the cell, this could be life threatening. Cells also
possess essential enzymes which specialize in binding to oxygen. Fascinatingly,
the enzymes which bind oxygen and those that bind superoxide are similar in
that they use the same type of metal atoms to attract and bind oxygen. Thus, it
appears that very early in the evolution of life, two complex enzymes with very
similar but distinct binding properties would have to appear simultaneously to
allow cells to take up oxygen while at the same time protecting cells from the
damaging effects of toxic oxygen.
Many origin of life scenarios initially exclude molecular oxygen because of
its reactivity and toxicity. However, atmospheric oxygen plays a major role in
filtering out much of the harmful ultraviolet (UV) light rays from the sun. In
the present day atmosphere, which contains oxygen, some UV light does reach the
earth and it is harmful to living things. UV light alters DNA in cells
ultimately causing mutation, cancer, or cell death. In fact, it is very likely
that DNA damage occurs in our cells every time we are exposed to sunlight. It
is estimated that in warm blooded animals over 10,000 alterations in the DNA
can take place in each cell every day. However, we seldom notice the damage
because our cells possess elaborate DNA repair mechanisms which can repair the
damage caused by UV light and other agents. In humans more than 100
genes are involved in DNA repair. In fact all organisms, including bacteria,
possess complex repair mechanisms to repair DNA damaged by light. Many
organisms possess up to four different kinds of DNA repair mechanisms. In
bacteria, there is a backup repair mechanism called SOS which is activated if
the cell is overwhelmed with DNA damage. The repair mechanisms are complex and
involve many parts to accomplish this repair. Let us consider how the
repair of UV light damage is accomplished.
DNA is a double stranded fiberlike molecule. UV light typically causes the
double strand to stick together abnormally in one spot. The repair mechanisms
recognize the sticky abnormal spot, cut it out, and resynthesize what was lost.
This requires at a minimum, an enzyme to recognize the sticky spot, a cutting
enzyme, and a re-synthesis and resealing enzyme. In some organisms a single
enzyme can repair the UV light damage, but this single enzyme called photolyase
requires the assistance of two complex cofactor molecules, and surprisingly
must be exposed to a certain wavelength of light to function. Not only do we
find elaborate repair mechanisms in all cells, but in plants, algae, and some
bacteria very complex systems exist which interact intentionally and very
specifically with light. These photosynthesis systems supply carbon and oxygen
for most all living things on earth.
A type of photosynthetic bacteria, called cyanobacteria, in the ocean could
be responsible for mobilizing about 50 percent of the carbon for living things
on earth. Curiously, the photosynthetic machinery of these bacteria can suffer
from sunburn; some of the proteins are sunburned so badly they stop
functioning. However, researchers have discovered a virus in the ocean which
infect these bacteria and repair the defect. The existence of elegant and
essential repair mechanisms which counter the toxic effects of light and oxygen
highlights the fact that repair mechanisms would have to be in place early in
the evolution of life. In addition, because photosynthesis produces oxygen,
cells would have to possess oxygen protection mechanisms before the advent of
photosynthesis.
Not only must cells possess repair and protection mechanisms to prevent
oxygen and light damage, cells must also be designed to handle the detrimental
effects of water. The water molecule possesses many fascinating unique
life-supporting characteristics. Yet water is a tremendously destructive force
at the cell and molecular level. Water is destructive because it can break
apart molecules by a process called hydrolysis. During hydrolysis, water
molecules force their way into spaces between atoms within molecules breaking
apart or preventing the formation of large molecular structures like proteins.
In fact, protein synthesis in cells requires the removal of water, a
dehydration reaction. How does this dehydration reaction occur in the water
based environment of the cell? The interior of the cell is thick with
molecules, proteins, and enzymes which assist the making of a protein. A
similar low-water environment and mechanism to remove water or supply enzyme
catalysts during protein synthesis is not postulated to exist in the
dilute watery environment of the early earth. In fact, this problem has led
origin of life researchers to conclude that proteins and other large polymers
(chain-like molecules) were constructed in dry environments like clay or
sand.
Water also destroys cells by inducing uncontrollable swelling. This can be
easily observed in red blood cells placed in water: the cells swell and break
open rapidly. The cell bursts because water moves freely into cells by
diffusion, a process whereby water seeks places which are low in water content.
As we have noted, the inside of the typical cell is low in water content
compared to its surroundings. Thus, all cells on earth face a continual battle
against the influx of water.
Cells possess several mechanisms to handle the continual influx of water.
Plant and bacteria cells, for example, possess rigid cell wall structures which
resist cell swelling and breakage. These cell wall structures can be quite
elaborate and in the case of bacteria, involve an intricate precision made
quilt-like structure made of protein and sugar chains. Animal cells do
not possess rigid cell walls, but instead, constantly pump sodium out of
the cell to counter the movement of water into the cell. The pump is a
fascinating protein structure called the sodium-potassium pump. The pump sends
out three sodium ions in exchange for two potassium ions. The cell membrane
contains thousands of these pumps which constantly work to maintain cell volume
against the impending crushing force of water, utilizing up to one third of the
energy found in living cells. However, the pump is designed to work in an
environment which contains sodium and potassium in certain defined
concentrations, for instance, in the human body. Take one of these cells out of
this salty, watery environment and place it in a pure water environment and the
pump will not be able to prevent the cell from bursting. How then do
single-cell organisms which live in fresh water environments
survive?
Single celled pond organisms like paramecium, utilize a large bag-like
structure called a contractile vacuole which continually collects and excretes
excess water. Water moves into the vacuole because the paramecium actively
pumps salts into the vacuole utilizing proteins similar to the sodium-potassium
pump. Thus, it appears that paramecium and other single celled pond organisms
resist swelling and bursting by possessing both protein pumps and contractile
vacuoles.
One could argue that given enough time one of these protection mechanisms
could evolve, but the simultaneous evolution of several elaborate and complex
protection mechanisms which are required to protect cells from some of the very
basic necessities of life (namely, water, oxygen, and light), certainly
complicates the origin of life problem. On the other hand, how does this
observation fit with creation/design theories? The requirement of life for the
simultaneous existence of several complex protection mechanisms certainly is
consistent with a creation or design in nature that was premeditated and
constructed within a short period of time. However, one could ask why a
creator/designer would use toxic agents? Toxicity could be considered to
be a by-product of chemical reactivity. Reactivity is required in a world
whereby things are designed to move and interact. In addition, even the most
benign agents can be toxic under certain conditions. We know this from our
everyday experience. For instance, many beneficial and required food types can
be harmful if ingested in excessive amounts. We also know that potentially
toxic and destructive chemicals provide tremendous benefits if they are used
within certain parameters. For example, fuel in an engine is a marvelous and
tremendous technology which enhances life; however, placed in the wrong part of
the engine can lead to disaster and destruction.
In conclusion, water, oxygen, and light, three of the most basic necessary
requirements for life can be extremely toxic to living things. But, living
organisms possess complex protection mechanisms built into each living cell
which appear to have protected life from its very first appearance on
earth.