Spring arrives in a New England forest, and, almost overnight, bare branches and barren ground sprout bright green. On the Great Plains, the brown prairie becomes an emerald patchwork of wheat, corn, and hay. Even the southwestern desert shimmers with green life after a drenching spring rain.
This lush growth is fueled by a phenomenon called photosynthesis, the process by which plants use the energy of sunlight to create the foods on which we all depend. The word itself means "putting together" (synthesis) with "light" (photo). Specifically, photosynthesis assembles carbohydrates (sugars and starches) out of carbon dioxide (a gas) and water. The power fueling this reaction is the energy of the Sun. The by-product, or "waste" material, of photosynthesis is pure oxygen.
It would be difficult to overestimate the tremendous importance of photosynthesis. Without it, there would be no plants, and so no plant-eating animals, nor any animals that eat plant-eating animals. Without photosynthesis, oxygen would all but disappear from our atmosphere. The only living things able to survive in such a world would be a few types of bacteria that could take energy from inorganic chemicals.
Although few people stop to think about it, even much of modern industry is powered by photosynthesis. Our technological society all but runs on "fossil fuels"—coal, oil, and natural gas—which come from the piled-up remains of plants that flourished, died, and were buried many millions of years ago.
In one way or another, most everyone today has at least a basic appreciation of photosynthesis and the wondrous "green machines" that we call plants. Even young schoolchildren, who may not yet know the term photosynthesis, understand that plants grow, thanks to sunlight.
So it may seem especially odd that, in the long history of science, photosynthesis was discovered less than 200 years ago. Not even in the mid-1700s, a time of great scientific renaissance and discovery, did botanists have the slightest idea that plants drew their energy from sunlight. Indeed, they still believed that plants took all the nourishment they needed from soil and water.
THE STUDY OF PHOTOSYNTHESIS
The first glimmering of understanding came in 1771, when the English chemist Joseph Priestley noticed that a candle in an airtight container burned longer if the container also included a plant. Likewise, he found that a small animal had less difficulty breathing when its airtight container also housed a living plant. Priestley concluded that plants produced some substance (which we now know to be oxygen) that "restored," or refreshed, air that had been "injured," or depleted, by a burning candle or a breathing animal. Soon after, Priestley received a medal for his discovery. It read, in part: "For these discoveries we are assured that no vegetable grows in vain … but cleanses and purifies our atmosphere."
What Priestley did not realize was that sunlight was the energy fueling the plant's production of oxygen. In fact, he was baffled when his experiments failed in a shaded corner of his lab.
It was the Dutch physician Jan Ingenhousz who, in 1779, recognized that plants "purify air" only in sunlight and only with their green parts. In 1796, Ingenhousz further suggested that plants are not just exchanging "good air" for "bad," but are also absorbing carbon from the air (in the form of carbon dioxide) and using it for nourishment. Ingenhousz had the basic idea.
In 1804, the Swiss botanist Nicholas Theodore de Saussure confirmed Ingenhousz's ideas with experiments showing that a growing plant gains weight equal to the amount of carbon it draws from the air plus the amount of water taken up by its roots.
But it would be nearly half a century later (1845) before scientists finally put all the pieces together and realized that light energy from the Sun is stored as chemical energy in the carbon products, or carbohydrates, created as an outcome of photosynthesis.
The result was the formula that summarizes this important process:
6CO2 + 6H2O + light energy → C6H12O6 + 6O2.
In this equation, the letters C, H, and O stand for the elements carbon, hydrogen, and oxygen. In other words, carbon dioxide plus water plus light energy yields carbohydrate (such as a sugar) plus oxygen. The sugar is then available as fuel for growth and other life processes. Extra sugars are stored in plant tissues as starch.
In recent years, scientists have discovered that carbohydrate is not the only type of plant "food" that results from the process of photosynthesis. In smaller amounts, photosynthesis also produces amino acids, proteins, fats, and other organic (carbon-based) products. The creation of these and other "building materials" requires a number of additional elements such as nitrogen, phosphorus, and sulfur. Yet the photosynthetic process that produces them remains essentially the same. Indeed, it can be said, quite literally, that a plant builds itself through the process of photosynthesis.
Before a plant (or other photosynthetic organism) can use the energy of the Sun, it must "catch" it. The plant does so with a pigment, a substance that absorbs visible light. Some pigments absorb all wavelengths of visible light and so appear black. But most absorb only certain wavelengths, or colors.
Chlorophylls, the most important photosynthetic pigments, absorb light primarily in the violet, blue, and red range of the spectrum. They reflect green wavelengths—and so give plants their characteristic color.
We now know that there is not just one form of chlorophyll, but at least five. All photosynthetic organisms appear to use chlorophyll a. True plants and green algae also use chlorophyll b. Other photosynthetic algae and cyanobacteria employ chlorophylls c, d, and e. Purple bacteria and sulfur bacteria use still other types.
Chlorophylls are by far the most abundant and important photosynthetic pigments. But they do not work alone. They are "assisted" by other pigments, which funnel additional energy into the process of photosynthesis. In true plants, these "helper pigments" include carotene (yellow), xanthophyll (pale yellow), betacyanin (red), and anthocyanin (reddish to purple). Typically, their colors are hidden by the green mask of chlorophyll. They become most visible in autumn, when chlorophyll breaks down. Some red algae and cyanobacteria (blue-green algae) also contain accessory pigments, called phycobilins.
Energy Traps. What enables these pigments to function as little energy traps? All are complex molecules made, in part, of a ring of carbon atoms. It is this ring that is able to absorb light energy—and hold onto it, if only for a moment.
Chlorophyll consists of a large circular molecule with a single metallic atom—magnesium—embedded in its heart. Around this magnesium atom is a ring made of nitrogen and carbon atoms. A long chain of carbon atoms serves as an anchor, securing the pigment in its proper place in a photosynthetic cell.
When a pigment molecule absorbs light, the electrons in its carbon ring temporarily rise to a higher, or more excited, level. When they drop back to a lower level, the extra energy channels into one of three forms:
- It may be lost as heat;
- It may produce an energetic glow, called fluorescence; or
- It may be captured and used in the formation of a chemical bond.
Indeed, when chlorophyll pigments are separated from plant tissue and placed in a test tube, light will cause them to fluoresce, or glow. In other words, an isolated pigment quickly reemits the energy it captures. None of the light it absorbs is converted into a form useful to living things. For the latter to happen, a photosynthetic pigment must be associated with special proteins and embedded in a living cell.
THE EVOLUTION OF PHOTOSYNTHETIC ORGANISMS
The first living things on Earth surely lacked the ability to photosynthesize. Most likely, these simple one-celled creatures merely absorbed the nutrients they needed from the "organic soup" in which they lived. As best as scientists can reconstruct it, early Earth was buffeted by powerful electric storms and solar radiation. This intense energy would have produced an abundance of complex, carbon-based (organic) molecules in the atmosphere. These would then rain down into the seas to supply early life with simple nutrients, or food.
By chance, some of these organic molecules may have formed carbon rings similar to those seen in photosynthetic pigments. Then some of these primitive pigments may have found their way inside simple one-celled organisms. At first, these simple pigments may have acted as "spark plugs," emitting flashes of energy that drove a useful reaction or two. Then, through evolution, a chemical pathway for storing this energy may have developed.
The earliest known photosynthetic organisms were the cyanobacteria, or blue-green algae, which appeared some 3 billion years ago. Then, as today, they produced their own energy using the pigment chlorophyll. Their photosynthetic activities filled the atmosphere with oxygen (O2) and threw a protective blanket of ozone (O3) around the planet. The oxygen made possible the appearance of aerobic (oxygen-breathing) organisms. The ozone blunted the onslaught of intense solar radiation.
These first photosynthetic organisms were simple, one-celled organisms—more complex than the first living cells, but far simpler than any true plant. Like other early life, they were prokaryotic. That is, their single-celled bodies were not divided into membrane-bound structures such as a nucleus and chloroplasts.
By contrast, all multicellular (many celled) organisms consist of eukaryotic cells—cells whose contents are organized into membrane-bound structures such as a nucleus, chloroplasts, and mitochondria. All plants, animals, algae, and fungi are eukaryotic organisms.
There is a theory that the first photosynthetic eukaryotic organism was created when a simple (prokaryotic) photosynthetic bacterium infected a larger, nonphotosynthetic, eukaryotic organism. Or perhaps the bacterium was engulfed by the larger organism. No doubt, such things occurred frequently. But at least once—the theory goes—the union of the two cells proved symbiotic: the larger organism gave the smaller, photosynthetic bacterium a safe haven; in return, the bacterium supplied its host with energy.
Within their host cells, these photosynthetic bacteria are believed to have evolved into chloroplasts. A chloroplast is a kind of plastid, a membrane-bound cell structure unique to plants. Specifically, it is the plastid, or cell structure, in which all plant photosynthesis takes place.
The development of the chloroplast was one of life's great evolutionary breakthroughs. It is, in essence, a highly efficient combination power plant and factory. In it, the pigments are arranged to maximize their capture of sunlight and production of energy. By contrast, in photosynthetic organisms that lack chloroplasts, photosynthesis is far less efficient. In these organisms, the photosynthetic pigments simply float freely through the cell.
In shape, a chloroplast more or less resembles a football. In size, it is about 2,500 nanometers thick and 5,000 nanometers long (1 nanometer equals 0.00000004 inch). This is large enough to fill over half the "body" of a certain single-celled alga. By comparison, a typical leaf cell contains up to 200 chloroplasts.
Separating the chloroplast from the rest of the cell is its double-layered outer membrane. Several striking features can be seen within the chloroplast (as viewed with an electron microscope). Lacing throughout the chloroplast is a folded network of internal membranes called lamellae. Under very high magnification, one can see that the lamellae are organized into flattened disks called thylakoids.
The thylakoids, in turn, are stacked throughout the chloroplast like so many piles of poker chips. These stacks of chlorophyll-packed "chips" were visible as green specks to early scientists using ordinary light microscopes. They called them grana, meaning "grains," a term still used today. The grana are interconnected by bridgelike extensions that stretch between some of their thylakoid disks.
The light-capturing reactions of photosynthesis (described below) take place in the grana, where chlorophylls and other pigments lie embedded in the lamellae. These pigments are arranged in special units called photosystems. Each photosystem contains a cluster of 250 to 400 pigments linked together and held in place to maximize the efficient capture of light.
All of the pigments in the photosystem can absorb light energy. But only one pair of chlorophyll molecules can actually use this energy to fuel a photosynthetic reaction. This special pair sits at the core of the photosystem—the reaction center. The other pigments are called antenna pigments. Arranged like spokes around the reaction center, they pass their captured light energy along to the reaction center. The two chlorophyll molecules sitting in the center bundle this energy in the form of an electron. Then they pass it to a carrier molecule, which carries the electron away from the grana.
Surrounding the grana is a dense solution called the stroma. It is packed with the enzymes and small molecules needed for the "dark reactions" of photosynthesis. In these reactions, energy produced in the grana are used to convert carbon dioxide into sugar and other organic (carbon-containing) compounds. The double membrane that surrounds the chloroplast keeps the stroma from leaking out into the cell's thinner cytoplasm. If the membrane breaks, the stroma enzymes leak out and photosynthesis comes to a halt.
STAGES OF PHOTOSYNTHESIS
The grana and the stroma, described above, are the separate sites of the two stages of photosynthesis. Stage one, in the grana, involves the steps in which light energy gets captured. These steps are therefore called the light reactions. Stage two, in the stroma, uses the energy produced in stage one (the light reactions) to build carbon products. The reactions in this second stage do not require light, so they are often called the dark reactions. It should be noted, however, that the dark reactions do not require the absence of light. Indeed, the term "light-independent" more accurately describes the second-stage reactions than does the term "dark reactions." In fact, these reactions typically occur during the daytime in living plants.
Light Reactions. Recall from the previous sections how the grana contain light-capturing pigments carefully arranged into "photosystems" that maximize their efficiency. We now know there are two kinds of photosystems, linked to each other and operating side by side like twin power plants. The chlorophyll pigments at the core of Photosystem I best absorb light at a slightly different wavelength than those at the core of Photosystem II. They work together as follows:
Light excites the electrons in the chlorophyll of Photosystem II—so much so that two electrons leave the chlorophyll molecule. They are immediately replaced by two electrons from a water molecule. This reaction splits the water molecules into two protons (2H+) and an atom of oxygen (O). This process is called photolysis, meaning "light splitting"; it yields the first important by-product of photosynthesis—oxygen.
Meanwhile, the two superenergized electrons (just boosted out of the chlorophyll) get snatched up by a carrier molecule. It passes the electrons to another carrier molecule, which does the same. As the electrons get passed through this electron-transport chain, some of their extra energy is used to produce two molecules of adenosine triphosphate (ATP) from adenosine diphosphate (ADP). ATP is the major source of usable energy in all living cells. So here we have the second important by-product of photosynthesis—chemical energy. It will be used later, in the "light-independent" reactions described below.
But the two liberated electrons have not yet finished their work. The electron-transport chain delivers them to the chlorophyll reaction center in Photosystem I. Light energy absorbed by the chlorophyll there boosts the electrons back into a high-energy state and out of the chlorophyll. Snatched up by a second, shorter electron-transport chain, they end up in the electron acceptor molecule (NADP). The NADP temporarily "holds" the electrons in place by picking up one of the protons (H+) left over from the previous splitting of water (H2O), becoming NADPH. NADPH then ferries the electrons' energy out of the grana to the surrounding stroma. There the energy will be used to fuel the production of carbohydrates and other organic molecules in the "light-independent" reactions.
In summary, the light reactions of photosynthesis convert light energy into electrical energy (free electrons), which is then converted into chemical energy (in the molecular bonds of ATP and NADPH). In the words of the biochemist and author Albert Szent-Györgyi: "What drives life is … a little electric current, kept up by the sunshine."
Light-independent Reactions. At this stage of photosynthesis, the energy generated by the "power plants" in the grana flows to carbon assembly lines, or "factories," in the stroma. The raw materials used in this factory are carbon dioxide (CO2) and hydrogen atoms from water (H2O). Most plants absorb carbon dioxide from the air through special openings, called stomata, in their leaves and stems. They draw water primarily through their roots. The process of assembling simple carbon into more-complex carbon compounds is called carbon fixation. It takes place in a series of reactions called the Calvin cycle (named after biochemist Melvin Calvin). In essence, every turn of the Calvin cycle splices one molecule of carbon dioxide onto a carrier molecule, along with the hydrogen from one molecule of water. So six turns of the Calvin cycle will incorporate six carbon atoms. This produces one molecule of the sugar glucose (C6H12O).
Fueling this process is the ATP and NADPH produced in the light reactions. In total, six revolutions of the Calvin cycle consume, or "burn," the energy in 18 molecules of ATP and 12 molecules of NADPH.
The sugar produced by these reactions can then be "burned" by the plant as fuel, stored as starch, or used as a skeleton to build more-complicated carbon molecules such as amino acids.
Summary of Photosynthesis. Here, in summary, is what basically happens during photosynthesis:
- Light reaches the chlorophyll and other pigments.
- The chlorophyll absorbs the light energy and releases electrical energy (free electrons), which in turn is converted to chemical energy (ATP and NADPH).
- Water molecules (H2O) split to resupply the chlorophyll with electrons and the NADP with protons (H+). The by-product of this reaction is oxygen.
- The energy produced by the above steps fuels a series of reactions (the Calvin cycle), which assemble molecules of carbon dioxide into complex carbon compounds such as the sugar glucose.
- The resulting sugar can be consumed by the plant as fuel, stored by the plant as starch, or assembled into other building materials such as amino acids.
Through this process, virtually all life on Earth receives its nourishment—either directly, as is the case with plants, algae, and photosynthetic bacteria—or indirectly, as with plant-eating animals and the animals that eat them.
Jessica Snyder Sachs