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  • 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.