Science to Live By: Photosynthesis: The Economy of Green Plants (Part Two)


© J. Dirk Nies, Ph.D.

It’s summertime.  The sun is at its zenith, having just completed its annual, six-month northward trek from the Tropic of Capricorn across the equator to the Tropic of Cancer.  Days are long and the weather hot.  Wouldn’t it be nice to capture and store this profuse, abundant solar energy for use toward practical purposes? But that’s the trick isn’t it: how to efficiently collect and store energy in a form that can be used in a controlled and directed way, perhaps months or years later?  This is exactly what photosynthesis accomplishes. By capturing the sun’s energy that arrives in the form of photons (the “photo” part of photosynthesis) and storing this energy by making energy-rich molecules from simple starting materials (the “synthesis” part of photosynthesis), green plants efficiently collect and store solar energy for future use, powering the economy of life on earth.

Last month we learned that green plants synthesize sugar (glucose) by combining (in a complex, multi-step process) atmospheric carbon dioxide (CO2) with hydrogen obtained by splitting apart water (H2O). Plants use glucose as a versatile building block to construct their cellulosic infrastructure of roots, trunks, branches, stems and leaves (cellulose is the most common organic compound on earth and is made of thousands of glucose molecules linked together). Plants also use glucose to assemble complex carbohydrate foodstuffs, such as starch found in potatoes, wheat, corn and rice (starch also is made of a large number of glucose molecules linked together, but in a different, more digestible way).

Let’s turn our attention to the “photo” (energy) side of the photosynthetic process.

For work to be done, for life to flourish, energy must be available in a form that can be directed and controlled, and it must be expended in a useful, purposeful way. For more than a century, we have powered our industrial economy chiefly by extracting, refining and burning fossil fuels. Our technical prowess converts the chemical energy present in the bonds of natural gas, oil and coal into useful work such as generating electricity, heating our homes, cooking our food, and powering our cars.  In this respect, our economy is similar to that of animals, fungi, yeast, and many bacteria that derive energy from pre-existing materials in a non-renewable way.

In contrast to this purely consumptive energy model, green plants accomplish the self-sustaining task of making their food. Using only locally available materials and energy from the sun, green plants endow us all with biofuel for life.

Julius Robert von Mayer, a German physician and physicist, proposed in 1845 that the sun is the originating source of energy utilized by living organisms, and he helped pioneer the concept that photosynthesis converts light energy into chemical energy. To help us envision this process, imagine a solar powered Ferris wheel where CO2 is pictured at the bottom and food at the top. As the wheel rotates clockwise, CO2 is lifted up on the left side of the wheel and converted to high-energy food by photosynthetic plants. Oxygen is given off in the process. On the right side, food descends as it is digested by plants, animals and microorganisms, ultimately regenerating CO2. Oxygen is consumed in the process. The energy-releasing, downhill ride of consuming food and oxygen is possible only because the energy-storing, uphill climb of photosynthesis regenerates food and oxygen.

Plants capture solar energy in their leaves using the green pigment called chlorophyll (meaning “green leaf”). This pigment was first isolated in 1817 by two French chemists, Pierre-Joseph Pelletier and Joseph Bienaimé Caventou. Eighty-nine years later in 1906, Richard Willstätter, a German organic chemist, found that chlorophyll actually was a pair of compounds he called (somewhat unimaginatively) chlorophyll a and chlorophyll b. His analysis showed that they contained the common and expected elements of carbon, hydrogen and oxygen. He also found that nitrogen was present within pyrrole rings that were part of the structure of chlorophyll. His most surprising discovery was finding magnesium; chlorophyll was the first compound of living tissue found to contain that element.  For this work on chlorophyll and other colored substances in plants, he received the Nobel Prize for chemistry in 1915.

Two decades later, Hans Fischer, a fellow German chemist and physician, provided more surprising insights into the composition of chlorophyll. When he turned his attention to chlorophyll, Fischer had already won the 1930 Nobel Prize in chemistry for elucidating the structure of the heme molecule of hemoglobin, the iron-containing, red pigment in blood. Heme contains four pyrrole rings linked in a circle to form a larger ring that can hold in its center positively charged metal ions such as iron(II) and magnesium(II). He discovered that chlorophyll also possesses a four-pyrrole ring, with a long hydrocarbon tail as an appendage. Who would have thought that the study of the red pigment that transports oxygen necessary to release food energy in animals would be so pertinent to the study of the green pigment that helps power the formation of food and oxygen in plants?

Scientists have since discovered another astonishing parallel between photosynthesis (the upward turning side of the Ferris wheel) and respiration (the downward turning side of the Ferris wheel).  Photosynthesis begins within microscopic structures (organelles) called chloroplasts, while respiration occurs at the molecular level within organelles called mitochondria. Both these organelles have their own sets of membranes and their own genetic material that are separate and distinct from the membranes and DNA of the cells in which they reside. Scientists postulate that chloroplasts (which store energy as food) and mitochondria (which release energy from food) are modified descendants of ancient bacteria that were taken up whole within larger plant and animal cells.

Through the process of photosynthesis, green plants convert sunlight’s electromagnetic energy into usable energy stored in chemical bonds of sugars and other energy-rich compounds. And in summer, they take advantage of the heat to run their photosynthetic machinery in high gear.  Throughout these processes, plants use environmentally benign compounds and locally available raw materials. This is the way nature generates and stockpiles fuel.

Apropos to this natural pattern, Nancy Young, vice president of environmental affairs at Airlines for America, a trade group for U.S. airlines, said at the BIO World Congress this spring: “Our industry is under the tyranny of petroleum-based crude.” She expressed concern not only about the rising cost and price volatility of traditional jet fuel, but also the environmental concerns associated with its emissions. “We need another commodity to compete with fossil fuels. Only a drop-in replacement will do, as passenger planes powered by solar energy, electricity, or compressed gas are not going to be part of the solution.”  Perhaps by mimicking nature’s way of making fuels photosynthetically, we will provide the answer she is looking for.

Next we will learn about more recent advances in photosynthesis, explore intricate cycles and balances that are in play, and contemplate their relevance to our human economy and the environment.