Photosynthesis: The Power of Light
Photosynthesis powers life on Earth. The word has Greek roots, with Photo being “light” and Synthesis being “put together.” Barb Shaw puts together everything you need to know about the process that converts light energy into chemical energy.
A couple of billion years back, on a very different Earth, pigment molecules evolved in cyanobacteria that could grab and make use of a photon, light energy from the sun. Now, life’s fuel could come from the sky, not just from molecules already present in the sea, the only fuel useful to archaic anaerobic bacteria.
Chlorophyll and related pigments in cyanobacteria, blue-green algae, got very busy capturing sunlight and using it to make their own simple sugars from the oxygen and hydrogen atoms in water and the carbon dioxide in air. The sugars could be used for energy or linked together to make polysaccharides, starches, and larger molecules.
With the ability to use solar energy, always available in vast amounts on Earth, cyanobacteria multiplied rapidly. Carbon dioxide, water, and sunlight became an unbeatable combo for the proliferation of life based on chlorophyll. The waste product was oxygen.
But, after a few million years, a huge problem arose. Minerals that oxidize when exposed to oxygen were saturated. They could no longer absorb enough, and the atmosphere became oxygen-rich.
The Oxygen Holocaust
A molecule of two oxygen atoms is quite energetic and ready to break up to combine with just about any available atom, so it can damage fragile molecules. Oxygen is toxic to anaerobic bacteria. The poisoning of these archaic life forms led to the era known as the Oxygen Holocaust.
Things got much worse. Busy oxygen combined with and destroyed the methane in the atmosphere, the primary greenhouse gas at the time. The world grew much colder. Even cyanobacteria were at risk. The Earth became an icy, frozen snowball. Thousands of species of bacteria died off. All life stood to lose in this mighty experiment gone wrong. It was the planet’s first mass extinction. Earth was just not ready to use photosynthesis to its full advantage.
The Rise of Mitochondria
About the time it looked like the game was over, in a hidden pocket of snowball Earth, some cells captured and protected inside their cells simple bacteria that could metabolize oxygen. The formerly free-living bacteria became symbiotic, forming cell organelles we call mitochondria, which have DNA different from that of their host. Inside them, the process—now called oxidative phosphorylation—was highly efficient. Given a secure place to live and guaranteed food, mitochondria specialized to produce energy for their hosts. Soon enough, oxygen-powered metabolism produced far more competitive forms of life. The worst was over—at least until the next mass extinction.
Chloroplasts in Captivity
Just as mitochondria were once free-living, the chloroplasts that hold chlorophyll and carotenoid pigments in the cells of plants were once free-living cyanobacteria. Their “captivity” turned out to be an immense advantage for them and they evolved into little green sugar factories. A typical leaf cell holds 40 or 50 chloroplasts, with half a million in a square millimeter of leaf surface. And they are proactive captives now. Inside a cell, chloroplasts move around a bit to better orient to the incoming light.
In our world, all but the most primitive cells depend on the reactions inside mitochondria to produce life energy, reactions that have CO2 as a waste product. With atmospheric oxygen now a great resource and plenty of CO2 being produced, the world warmed and photosynthesis could reach its full potential. Today, chloroplasts and mitochondria work together to power the natural world.
So, what big magic occurs when a photon hits a molecule of chlorophyll? The full story of photosynthesis may not be what curious growers want to know. The short version is that chlorophyll traps light energy. The energy that bumps out an electron then becomes available to interact with the molecules of metabolism as chemical energy.
The Light Reaction
Six water molecules plus six carbon dioxide molecules, powered by photons, combine to form a single molecule of glucose with six atoms of carbon, 12 of hydrogen, and six of oxygen. Six oxygen molecules, each with two linked oxygen atoms, are set free into the atmosphere, available for us and animals to use.
This part is known as the “light reaction.” (A second set of metabolic changes named the “dark reaction” can occur without, though are not halted by, light.)
In the process of making the glucose, one molecule after another passes the electron along. More than 50 Nobel Prizes went to the geniuses who elucidated how metabolism actually works at the molecular and quantum levels, so even readers adept at chemistry won’t want all the details just now. But before we go further, let’s look more closely at chlorophyll.
The Absorption Spectrum
By far the most common type of chlorophyll is chlorophyll A, a green pigment present in cyanobacteria and higher plants. Its chemical formula is C55H72MgN4O5. The large molecule is structurally similar to hemoglobin that makes blood red, but at its center is a single atom of magnesium. That spot is occupied by iron in hemoglobin.
Chlorophyll B is a helper molecule, a yellow accessory pigment quite similar to chlorophyll A. It comprises about a quarter of the chlorophyll and allows a plant to utilize more wavelengths of light by absorbing in the blue part of the spectrum, then transferring its energy to a chlorophyll A molecule. Chlorophyll C occurs in a few bacteria and chlorophyll D is found in some deep-water red algae.
A pigment is any molecule that absorbs light, usually when light breaks a double chemical bond. If a pigment absorbs all wavelengths, it looks black. Most pigments selectively absorb certain wavelengths or energy levels, reflecting the colors we see. Chlorophyll A absorbs principally in the violet and red wavelengths, and reflects green. A graph of how much is absorbed in each wavelength is called the absorption spectrum.
The visual wavelengths are a very small part of the electromagnetic spectrum, made up of photons released when electrons that have been raised to high energy levels spontaneously fall back to their resting level in an atom. Each atom has its unique possible energy states, and energy of the photons it releases are a very accurate way to identify the presence of those atoms. The full electromagnetic spectrum encompasses the highest energy, shortest wavelength cosmic rays, then X-rays, all the way down to low energy radio waves many kilometers in length.
Color is how our eyes and brains perceive the energy level of visible light, a narrow zone somewhere near the middle of the vast spectrum. Higher energy light waves are oscillating faster, and the light appears violet. Decreasing energies are indigo, blue, green, yellow, orange, and red. Our eyes can’t detect ultra-violet (UV) or infra-red (IR) light, but UV can harm living tissue and IR is felt as heat.
So, why can we see only a small part of the electromagnetic spectrum? Most other wavelengths are simply not very useful for life on our planet. Most UV in sunlight is screened out by oxygen and ozone in the atmosphere, and most IR is absorbed by water vapor and carbon dioxide, so mainly visible wavelengths are available to plants for photosynthesis and for our eyes to perceive.
An understanding of photosynthesis can lead to a better comprehension of the physics of keeping plants happy and healthy. Each plant has its ideal light regime that usually mimics nature but in some cases, can be improved upon with grow lights that add more or less light in specific wavelengths and at specific times of day and season. For high tech growers, lighting to maximize photosynthesis will produce larger, healthier plants.