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18.2: Chemistry of Life

The most important attributes of life are growth and reproduction. The smallest unit in which these life processes occur is the cell. All known living things are composed of one or more cells, which in turn contain an intricate array of molecules. Most biologists think that the simplest organisms are the simplest living cells: bacteria. How far can we stretch the definition of life? Is a virus considered a life form? Although viruses are simpler than cells, they can reproduce themselves using materials from host cells. However, viruses cannot function independently of cells, and they appear to have evolved from living cells rather than the other way around. Similarly, can certain machines or computers be considered living entities? Technology has advanced to the point where machines and computers have taken on many of the attributes of life.

A key aspect of living things is their dynamic natures. We often make the mistake of thinking of ourselves as static beings, yet virtually every cell in the body is replaced over a seven-year period. Even our seemingly inert skeletons are living and changing, always replacing their cells. We must keep changing — our cells process new material to stay alive. When the processing stops, we call it death. The dynamic nature of living beings is illustrated by an analogy from the Russian biochemist A.I. Oparin. Consider a bucket that has water pouring in at the top from a tap and flowing out at the same rate through a tap at the bottom. The water level in the bucket stays constant, and a casual observer would call it "a bucket full of water." But it is not simply a bucket standing full of water. The water at any instant is not the same water at any other instant, yet the outward appearance is constant. We are like buckets with water, nutrients, and air flowing through us. We also have more complex attributes such as the ability to reproduce and be affected by structural changes that let us evolve from generation to generation.

What is life made of? The ten elements most common in living human tissue are listed below:

  • 65% Oxygen
  • 18% Carbon
  • 10% Hydrogen
  • 3% Nitrogen
  • 2% Calcium
  • 1.1% Phosphorus
  • 0.35% Potassium
  • 0.25% Sulfur
  • 0.15% Sodium
  • 0.15% Chlorine

Contrast this composition of biological material with the ten most common elements in the Earth's crust below your feet:

  • 46% Oxygen
  • 28% Silico
  • 8.3% Aluminum
  • 5.6% Iron
  • 4.2% Calcium
  • 2.4% Sodium
  • 2.3% Magnesium
  • 2.1% Potassium
  • 0.006% Titanium
  • 0.001% Hydrogen

Compared to these, the Earth's atmosphere is relatively simple in a chemical sense:

  • 75% Nitrogen
  • 23% Oxygen
  • 1.3% Argon
  • 0.00013% Carbon
  • 0.000013% Neon
  • 0.000003% Krypton
  • 0.0000007% Helium
  • 0.0000004% Xenon
  • 0.00000004% Hydrogen
  • 0.000000001% Sulfur

Finally, the overall chemical composition of the universe is represented well by the Sun, which has these ten most common ingredients:

  • 75% Hydrogen
  • 23% Helium
  • 0.009% Oxygen
  • 0.004% Carbon
  • 0.0014% Iron
  • 0.0010% Silicon
  • 0.0009% Nitrogen
  • 0.0008% Magnesium
  • 0.0006% Neon
  • 0.0004% Sulfur

There are several interesting aspects to the list of chemical ingredients in the human body. First, over 99% of the atoms in your body are from just four elements: hydrogen, oxygen, carbon, and nitrogen. The large amount of hydrogen and oxygen in living organisms (and its ratio of two hydrogen atoms to every oxygen atom) indicates the high percentage of water that all life contains. Nitrogen is the most common element in the Earth's atmosphere, as well as being important in all living things.

Carbon is the critical element for life. Organic chemistry is defined as the set of processes that involve molecules containing carbon, regardless of whether or not a living organism is involved. All the life elements (except hydrogen) are created inside stars and are widely distributed through the cosmos. It is striking that the composition of life resembles a star more than it does the Earth below our feet. For example, carbon and nitrogen are essential to life, but they are more common in the Sun than they are in the Earth. Apart from oxygen, the most common elements in the Earth — iron, silicon, and magnesium — play only small roles in organic chemistry.

Why does life employ so few of the 85 stable elements that exist in nature? The answer lies in carbon's unique ability to build complex molecules. Helium is the second most abundant element in the universe yet it cannot form chemical bonds with other elements to build complex molecules. This trait makes it useless for creating life forms. Among the most common elements in living organisms, hydrogen can combine with oxygen to form only two molecules: water (H2O) and hydrogen peroxide (H2O2). Similarly, hydrogen can combine with nitrogen to form only two molecules: ammonia (NH3) and hydrazine (N2H2). On the other hand, the number of ways that hydrogen can combine with carbon is so large that it is unknown! The largest molecule listed in the Handbook of Chemistry and Physics has a chemical formula of C90H154. Carbon is thus the perfect building block for complex structures.

You can think of the chemical processes of life as a form of information storage. Organic chemistry uses only a small number of ingredients. Yet these ingredients can combine with great complexity and therefore have the potential to store large amounts of information. Is carbon-based chemistry the only way to store information? Chemists (and science fiction writers) have speculated on life chemistry based on silicon, or some other element. Even more speculative is the idea of life based on some other organizing principle, such as electric or magnetic fields. Nobody has ever observed such life forms, so we can say nothing substantive about them. However, the chemistry of the universal elements is well understood. In terms of a basis for life, carbon is superior to any other element in its ability to form complex chains and thereby store information.

Earth is a watery planet. We might therefore wonder if water is essential to life. First, we note that water is potentially the most abundant liquid in the universe. Oxygen is far more abundant than silicon, the main rock-forming element. Therefore a rocky planet that uses up all its silicon by combining it with oxygen to make rocks will still have plenty of oxygen to combine with the most abundant element, hydrogen, to make water, or ice. Because it is liquid over a wide range of temperatures, water acts as a solvent: it dissolves other materials to form a solution. In this role, water maintains many vital cell functions: it dissolves and transports nutrients and waste products within a cell, regulates an organism's temperature, and even plays a role in shielding life from harmful UV radiation. It is thus not surprising that complex life forms originated in the oceans of the Earth. Even on the land, a large fraction of the weight of plants (40%) and animals (70%) consists of water. Other solvents exist, such as ammonia and ethyl alcohol. However, water is the most abundant liquid, and it has special advantages for facilitating life processes.

We can also place the chemical basis for life in a cosmological setting. The early universe contained only the lightest elements: hydrogen, helium, and trace amounts of lithium and beryllium. Such a universe is chemically inert, or has no basis for complex chemistry. The carbon, nitrogen, and oxygen required to make organic molecules do not exist until a generation of massive stars has created these elements by fusion. Moreover, several generations of star birth and death are required before these elements return to the interstellar medium (by mass loss from giant stars, novae, and supernovae). In other words, the ingredients for life increase in abundance and become more widespread as the universe gets older. Also, we have good evidence that the CNO cycle (the means by which stars make elements heavier than helium) operates in stars across our galaxy and in stars in other galaxies too. It follows that the chemical basis for life exists across the universe and is not due to a particular set of circumstances in our stellar neighborhood. This has important implications for the probability that life formed elsewhere.