The Fires of Life - cell metabolism

How Cells "Combust" Metabolic Fuels Invisibly, Internally, and Biospherically

The grand physical description of life on a global scale is "energy flows and matter cycles." Energy from very energetic solar photons enters the biosphere as visible light and exits as heat that radiates back into the deep cold of outer space. Between the energy's coming and going, the photons captured by photosynthesis power the intermediate processes that organize the structures of life and generate the fire of life.

Photosynthesis generates biofuels in solar-powered cellular factories. During photosynthesis, the captured photons split water into hydrogen and oxygen. The oxygen is released into the air to join the pool of atmospheric oxygen gas, while the hydrogen combines with carbon dioxide to give rise to organic molecules like edible sugars.

Subsequent metabolism uses these organic molecules for structure building, and as fuel for doing work. During metabolism, biomolecules recombine with oxygen to form water and carbon dioxide, while the energy, created does all manner of biological work and is finally released as heat. This slow combustion of living molecules--energizing the performance of work and the release of heat--is the fire of life.

In this sense, all fire is the fire of life, for the oxygen that combusts with wood and fossil fuels has also been made by life during photosynthesis. Energy flows and matter cycles: The photons from the fiery sun--captured by plants, algae, and cyanobacteria--store solar energy as hydrogen compounds and release oxygen, which becomes part of the atmosphere. Oxygen made by life is the energizer of life, for fast or slow combustion.

Fast versus Slow Burns

In each living cell is a series of enzymes that govern its chemistry. These remarkable molecules guide reactions, allowing a slow, low-temperature "burning" that converts the stored energy of foodstuffs into chemical potential with the least possible energy loss as heat.

In an ordinary steam engine, fire from wood, coal, or oil converts all the stored chemical energy of the fuel to ash and heat. A small amount of the heat converts water to steam, and the engine does its work. However, most of the heat energy is dissipated; steam engines are not very efficient, and indeed may cause excessive local heating or thermal pollution. (In a typical car engine, 25 percent of the combustion energy is turned into useful work; 75 percent is lost as heat.)

In a cellular system, much less of the energy is thermalized; a substantial amount is stored as chemical potential in molecules of adenosine triphosphate (ATP). These molecules are the biological batteries that supply the energy for the mechanical work of muscles, the electrical work of nerves, and the chemical work of synthesizing new cells. In a cell--significantly more efficient than the car--40 percent of the combustion energy is conserved, and 60 percent lost as heat.

Firewood versus Bread

Let's look at the difference between the hot fire of a burning wood pile and the slow fire "burning" within us when we eat a piece of bread. Both the wood and the bread are complex molecules (polymers) of sugars, so both reactions can be described as: Sugars + Oxygen yields Carbon Dioxide + Water + Energy or, more precisely:

[C.sub.6]H10[O.sub.5] + 6[O.sub.2] yields 6[CO.sub.2] + 5[H.sub.2]O + Energy.

In the case of the wood fire, the energy is entirely released in the form of heat. In the case of the metabolism of bread, an appreciable fraction of the released energy ends up in the chemical potential of ATP, whence it can. go on to do the work of life.

In the wood fire, the heated wood combines directly and somewhat randomly with the oxygen in the air, to yield a wide variety of breakdown products (such as soot), all of which ultimately go all the way to carbon dioxide and water. The energy emerges as heat--the kinetic energy of the moving atoms and molecules. Because the wood's polymers are being destroyed by heat at high temperatures, we see flames.

The bread, of course, must enter the cells in order to be metabolized. Pieces of bread cannot pass through the cell membrane, but sugar molecules can. So step one is the conversion of bread to sugar, beginning with the action of the enzyme amylase in the saliva. Enzymes are specific-cell catalysts, meaning they can speed up designated cell reactions without being altered themselves. (An approximate metaphor would be an ax that splits wood into pieces without itself changing.) Because a cell operates at low temperature, the reactions that take place are determined by which enzymes are present.

The secret of slow combustion is that the complex network of chemicals in the cell breaks up the combustion process into a large number of elementary steps, each of which involves relatively small energy changes. Small incremental energy change permits the cell to operate nearer to equilibrium, and permits a higher conversion of energy into chemical potential. (Contrast this to wood fire, which takes just one or two steps to violently raise the temperature in one big energy change.)

Starting with one sugar glucose molecule, it takes eleven steps and eight enzymes to bring us to two molecules of acetic acid. Each acid then enters a cell cycle where it is completely "burned" (oxidized) to carbon dioxide and a spectacular product of energy-rich, hydrogen-rich molecules. The carbon dioxide is released to the environment and the molecules of the energy-rich ATP are made available to the cell.

At this stage, the hydrogen-rich compounds go through still another series often reactions and enzymes, ultimately combining the hydrogen with oxygen to form water. The sugar has now been completely combusted to carbon dioxide and water. The energy from the final oxidation is used to charge up a proton storage cell. This stored energy is used to make many more ATPs, the ubiquitous energy-transfer molecules. So the slow burning of sugar in cells converts a good deal of the energy of the reaction into a form to do biological work.

Without the Inner Fire, No Life

The central reactions of cellular metabolism go back some four billion years, to all life's universal ancestor--which originally operated in the absence of oxygen. The ancestor's energy came from chemicals bubbling up from the magma just below the surface of the earth. Then, about two billion years ago, photosynthetic organisms started to build up our rich oxygen atmosphere. At this point some cells learned how to use the oxygen for energy, employing familiar chemical pathways but running them in the opposite direction. Invisible, internal, cellular fire entered the world. As the biosphere's oxygen concentration increased, the flash point lowered and ordinary, visible biomass fire appeared. After photosynthesis took over as the primary energy source for biomass fuels, all earthly combustion became a gift from the fiery sun. And so it has been for the last two billion years.

The earth, water, air, and fire of the ancients are the lithosphere, hydrosphere, atmosphere, and biosphere of today's geochemists. Without the last of these, there would be no fire.

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