“Some part of our being knows this is where we came from. We long to return. And we can. Because the cosmos is also within us. We're made of star-stuff.” – Carl Sagan, “Cosmos”
We were born in thermonuclear fire, our very essence owes its existence to the great stellar furnaces of the early universe. But that is not the end of the story. Far be it from me to correct Professor Sagan: of course he was right. At the atomic level, the heavy elements which give us life were forged in long dead stars, but heavy elements alone do not an apple pie make. From the very beginning of life on this planet, a far more familiar star has devoted a part of its vast energies to enabling and nourishing life on earth. In a very real way, from the vast boreal forests of the taiga to your daughter’s goldfish, life is made of stellar fire. Photons frozen in place and sapped of their energies, waiting to be released in every sprinter’s burst of speed and every forest fire’s flame.
But we can’t eat photons. Even on a sunny day in California, as invigorating as the sun feels on our skin, we’re not gaining any energy or sustenance from it. We’ve evolved to depend on the immense base of life’s pyramid, consisting of most of the biomass on planet earth, and spending all of its time doing nothing but look up at the sun and feast on its rays. This feasting is known as photosynthesis, and effectively converts the bombardment of photons constantly blanketing the earth into useful building blocks and energy containers useable by bacteria, fish, trees and cosmologists.
The process begins with a photon of light falling upon a plant. Plant cells are chock full of coloured pigments called chlorophylls. Being colourful molecules, these chlorophylls naturally interact with certain colours (or wavelengths) of light, and in most plants, a number of types of chlorophyll (each a slightly different colour) come together to efficiently consume much of the incoming photons, with the exception of green ones (which is why plants appear green). Having absorbed a photon, the chlorophyll molecule becomes “excited”, which means that one of the electrons in the molecule has taken the energy of the photon and jumped up to a higher energy. This energetic electron is captured from the chlorophyll and through a series of temporary molecular rest stops, slowly loses its energy. This lost energy is harnessed by molecular machinery surrounding the photosynthetic center, and helps to create a voltage drop across a special membrane, much like a battery recharging by forming a voltage across its two terminals. Once the electron reaches the end of its journey, it is contained in a molecule called NADPH; a more stable energy carrier that will be used to make sugar in the next step.
The chlorophyll we left behind is now lacking an electron, and this is not a good thing— so called “oxidized” chlorophylls cannot absorb light, and having one-time use light receptors is hardly an efficient design. So other molecular machinery in the cell steals an electron from a stray water molecule, and regenerates the chlorophyll. Lacking an electron, water is split and turns into a molecule of oxygen and a two hydrogen ions. While the oxygen is released into the air (much to our benefit), the hydrogen ions are herded to the cell’s battery membrane to bolster the population of ions making the voltage.
All the while, at the same time as the battery membrane is charging by light, it is being discharged by the flow of ions through special molecular turbines in the membrane. Much like water through a dam, these ions turn the turbine and create useful energy in the form of ATP molecules bristling with chemical energy.
The bulk of the process is now complete. The energy from incident light has been converted into chemical energy in the form of ATP and NADPH. But this energy must now be fixed into a useful building block that life can use. As a material source, plants gather carbon dioxide (CO2) from the atmosphere and by burning ATP and NADPH, more molecular machines weld carbon atoms one at a time together to form six-carbon rings – known as sugars.
These sugars (that make you and trees up, just the same) are truly fixed stellar fire. They are sometimes harvested by people and converted into ethanol to burn in a car, others are eaten by livestock and converted into larger, more complicated biomolecules called proteins to be later consumed as a steak dinner and burned by your brain even as you read this article. Photons generated in a burning star, fixed by complex molecular machinery on earth, and then released to burn again and power a mind that can understand this whole process.
Moss chloroplasts image: https://commons.wikimedia.org/wiki/File:Moss_chloroplasts_100%C3%97_objective_oblique.jpg