Since the beginning of April, I’ve been keenly aware that the days are rapidly getting longer. Like a spool unraveling. The Sun rises and sets farther north. Now even in the upper Midwest on the first day of spring, the Sun feels positively hot on your face. Plants have sensed this too and are bursting.
That energy, be it rising prices, shortages, or new sources, is constantly in the news should be no surprise. Our civilization would grind to a halt without energy; the universe and life itself, in fact, would cease. Since the middle of the last century, we’ve used nuclear fission to generate energy. For several thousand years, we’ve harnessed rivers and streams to turns wheels to grind grain and now to turn turbines to produce electricity. We are currently exploring and utilizing “green” energy sources—wind, thermal, and solar. But all are mere dalliances compared with the energy harvested today by plants and that housed in the fossil fuels captured from our Sun 300 million years ago in the Carboniferous Period.
Astrophysically speaking, the Earth is in a “sweet spot” relative to the Sun, not too close and not too far away. Light intensity drops sharply with increasing distance from the Sun (decreasing with the inverse of the square of the distance). Mars, for example, is quite Earth like and just 1.6 times farther from the Sun than Earth, but it receives not quite 50% of the Earth’s solar radiation.
Every living thing on Earth is a biochemical machine that requires constant energy inputs to survive and thrive, and life on Earth evolved to exploit solar energy. It’s unclear when exactly the ability to harvest the Sun’s energy evolved, but by 2.4 billion years ago, the concentration of oxygen in Earth’s atmosphere had risen dramatically from a ten-thousandth of today’s concentration to perhaps a one-hundredth. This is known as the “Great Oxidation Event”. Photosynthesis clearly must have evolved earlier than this, possibly as far back as 3.7 billion years ago, as the fossil record suggests.
In the simplest sense, photosynthesis is harnessing sunlight to drive the conversion of inorganic carbon (carbon dioxide) to organic carbon (sugar, chemical energy). The process requires in addition to sunlight and carbon dioxide, a light-harvesting mechanism. This is accomplished by pigments embedded in membranes. In plants, chlorophyll is the light-harvesting pigment; it’s chlorophyll that makes plants green. Chlorophyll is contained within the membranes of specialized bodies (chloroplasts) in plant cells. The energy captured by chlorophyll is used to remove electrons from a substance such as water (a reductant). The power of these electrons is then used in the reactions that turn carbon dioxide into organic compounds. In the process, oxygen is released as a by-product.
The predominant form of carbon on Earth is inorganic, mostly in the form of carbon dioxide and carbon monoxide, both “greenhouse” gases. This carbon is derived from atomic collisions deep within long-dead, giant stars and spread across the universe by supernova explosions. Today’s Earth contains in the neighborhood of 38,000 billion tons of inorganic carbon, and at the beginning, there was of course more. All this carbon and the potential energy locked within it created strong selective pressure for our carbon-based, life-form cohorts to find ways to use it. Photosynthesis was one way, and by means of photosynthesis, a portion of the primordial carbon reserve has been converted to the chemical energy that supports life on Earth. There are some 8000 billion tons of living biomass on Earth (ourselves included), and photosynthesis is essential to maintaining this.
Commonly, we think of photosynthesis as being the domain of plants alone. But when photosynthesis first arose, there were no plants; there were relatively simple, primitive organisms only. Diverse groups of bacteria and marine archaea (a group similar to but distinct from common bacteria) are representatives of these. In these groups, photosynthesis evolved independently. There are four main biochemical pathways that use different enzymes but share the same basic mechanisms, and there are subpathways within the main pathways.
Cyanobacteria (blue-green algae) are photosynthetic bacteria (not archaea) and are thought to be largely responsible for the Great Oxidation Event; so they are ancient. Of all the photosynthetic pathways, theirs is dominant. It is the same as is used in chloroplasts in plants and algae, the most prominent and successful photosynthesizers. Chloroplasts are in fact evolved cyanobacteria cells. They are thought to have resulted from an “endosymbiotic” relationship in which cyanobacteria were engulfed by protoplant cells, accepted certain favors, and in return supplied their hosts with a continuous stream of chemical energy (organic carbon).
For the purposes of the Earth’s living things, solar energy is limitless. But capturing solar energy, converting it to chemical energy, and redistributing it is not free. The first law of thermodynamics says that energy can be neither created nor destroyed. A portion of the solar energy involved in photosynthesis is stored as sugar; the rest is dissipated. The total amount of energy—before and after photosynthesis—though remains the same. It is the second law of thermodynamics that dictates and constrains how much energy can be conserved, and any process that uses, conserves, or transforms energy is constrained by this law. This means that there is an energetic cost associated with these activities. Photosynthesis is efficient in comparison with mechanical devices, but in the process of harvesting and storing energy, most of that energy is lost. It turns out to be slightly less than 10% of the energy harvested is stored as chemical energy.
“Nature abhors a vacuum” is often attributed to Rabelas, the French monk and satirist. Regardless of who coined the phrase originally, the notion is plain to see in evolution. With the advent of photosynthesis, another group of living things emerged to exploit those that exploit the Sun—the herbivores, who sopped up the organic soup and thrived. After the herbivores were established, came creatures to prey on them (predators like ourselves). That’s as far as the sequence of energy dependency goes. As the erstwhile solar energy moves through this food chain, from photosynthesizer to herbivore to predator, energy is lost—roughly 90% at each step. If 1000 units of energy are captured and stored through photosynthesis, only 10 remain by the time it reaches the predator. So think twice about the viability of gigantic science fiction creatures like Godzilla.
Our efforts to use solar energy to produce electricity are constrained not only by the physical laws but by space as well; there are only so many places that solar panels can be placed. For us, utilizing solar energy will probably never be much more than a supplement to a supplement. Plants, algae, and the other photosynthesizers, in contrast, cover most of the Earth and much of the sea; they support and sustain all life on Earth. But then they have spent billions of years perfecting their craft.
Soon here in the Northern Hemisphere, it will be summer. Plants will reach for their arc of growth. As you begin work in your garden—planting, weeding, fertilizing, mulching—consider that a sense of privilege and honor is perhaps in order to have the opportunity to propagate and care for these organisms that support us and have maintained life on Earth for so long.