My photosynthesis professor, Dr. Leland Jahnke loved leafy conundrums. On the first day of Biochemistry of Photosynthesis, Jahnke told his 6 graduate students about Julius von Mayer, a 19th century scientist who proposed a radical definition for photosynthesis. “The fleeting sun’s rays fixed and skillfully stored for future use,” Dr. Jahnke quoted Mayer. That poetic language hooked me, the old English major. I scribbled down every word Jahnke said.
Dr. Jahnke stretched our minds with puzzles. He asked us to consider the speed of light and how fast a single molecule of chlorophyll has to be to capture the speeding photon. The action happens in a femtosecond, a billion millionth of one second.
All of us bent to our notebooks and I dutifully wrote, “10-15”.
“It’s a temporal solipsism,” Dr. Jahnke chuckled. His eyes twinkled. “A solipsism is an interesting concept. In philosophy.”
Five botany students and I squinted at our professor, straining to recall any memory of that word and I wondered how an algae research scientist could veer off into philosophy. Jahnke laughed out loud and defined the word. “Because the only consciousness is mine, everything else is a guess.” His voice changed into a casual confiding tone and he shared a joke, “Two solipsists are walking across campus and one wonders to himself, ‘Should I say ‘Hi’ to that solipsist over there?’”
I had no idea what that meant but I scribbled on because Jahnke had leapt back to leaf and light. One photon is absorbed by one chlorophyll molecule in 10-15th of a second, a million billionth of a second, he repeated. “There are more femtoseconds in one second than there are seconds in 30 million years. It’s tiny. A 10-15 second. We can’t relate. We can see maybe one-tenth of a second. But we can’t see photosynthesis,” Jahnke said.
The photo-solipsism beguiled me into Jahnke’s class. We would learn how a chlorophyll molecule and an array of assisting pigments capture the photon in what is called the light reaction. Humans are still exploring last secrets of how it works. We still don’t know how a plant breaks apart not one but two molecules of water in the femtosecond splash down of each photon.
The hydrogen and oxygen in one molecule of H2O are a stable threesome. Break them apart and the oxygen becomes a hysterical entity desperate for another electron. In our bodies, we call these free radicals.
“It’s impressive it even evolved without self-damage,” Jahnke said quietly. In other words, how does a chloroplast capture a photon from the sun without exploding? Why don’t my sugar maples catch fire or blow up, I wondered. Researchers still don’t have an explanation for how the leaf does it.
Photons are captured in a light harvesting unit, something like a satellite dish, a funnel of pigments that bounce the photon down a spiral to one green chlorophyll molecule at the base. Its name is chlorophyll a, the bull’s eye of photosynthesis. It sits at the bottom of the light harvesting unit atop a mysterious machine called the oxygen evolving center. Two water molecules sit in this OEC.
When a photon strikes the leaf, its energy resonates like a pinball down the cone of pigments, down, down past 300 yellow, green and orange molecules to chlorophyll a. The photon’s energy is absorbed by this one molecule at the base of the antenna. Excited by sunlight, chlorophyll a loses an electron. That electron goes off to help its leaf make sugar. But chlorophyll a needs to replace that electron or it will be a used-up useless molecule. Chlorophyll a reaches down beneath its feet, so to speak, into the OEC and gets help from one of the waiting water molecules.
Water molecules are extremely stable. That’s because oxygen, one of the most reactive elements on Earth, has a huge attraction for two hydrogen atoms. Oxygen is like a powerful iron magnet gripping a few iron filings. It isn’t easy to pull a hydrogen atom away from oxygen. Yet chlorophyll a does it with ease. It grabs a hydrogen atom and is now a stable molecule ready to receive another incoming photon.
But the water molecule is now destabilized. Why doesn’t it grab a hydrogen atom from a tender cell in the leaf? Why doesn’t it set off a chain reaction of atoms, each one stealing hydrogen ions from their leafy neighbors? Why doesn’t the whole maple tree explode?
But the oxygen sits quietly and safely next to its partner water molecule. Scientists do not know how or why.
Then, in another femtosecond, another photon’s energy reaches down into the cone of pigments to touch that same chlorophyll a molecule. The second water molecule is split apart. Now the two unstable oxygens join one another. Now stabilized, the O2 floats out of the chloroplast benignly, adding to an atmosphere that we humans are grateful to breathe.
Janhke’s solipsism, the idea that we can’t be sure anything exists outside our own mind, is this mystery of photosynthesis. We can’t be sure it happens, though it does.
Human beings do not know how to break apart water molecules, unless they use enormous and inefficient amounts of energy. We call it nuclear fission. Trees and their chloroplasts do it millions upon billions of times each day, at ambient temperature, quietly, with the energy of the sun. Trees do not explode.
Every photosynthesis class with Dr. Jahnke left his six students reeling with impossible ideas. We tried to keep up with his good humor. At the bus stop each day, we challenged each other to make jokes about photosynthesis.
Chlorophyll a goes into the pub and takes a seat at the bar. The bartender greets him, “Hey, Chlor. What’ll it be.”
“Oh, just water. But make mine a double.”
Love this!!! Keep writing to us! You have made my morning!
You have beguiled without deceiving us this morning with magnificent writing containing even more magnificent (awesome) information.
Thank you,
Anne