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FEATURE ARTICLE

How Plants Produce Dioxygen

At its core, oxygen production comes down to the chemistry of a poorly understood manganese-containing complex in the membranes of plant chloroplasts

Veronika Szalai, Gary Brudvig

The Bell Jar

In the late 18th century, Joseph Priestley, an English Unitarian minister, used glass bell jars to trap various gaseous components of air. In 1772, he devised an experiment to determine whether plants and mice were similarly affected by air that contained carbon dioxide (CO₂) but not O₂. To his surprise, he found that a mouse quickly died in the bell jar lacking O₂, whereas a sprig of mint continued to live. Perhaps more important, when a mouse was placed in a jar from which mint had just been removed, the mouse remained alive. The plant had changed the CO₂ atmosphere by producing O₂. It was another seven years before the Dutch physician Jan Ingen-Housz showed that plants need to be placed in direct sunlight in order for O₂ to be generated. Twenty-five years after Ingen-Housz's discovery, Swiss scholar Théodore de Saussure was studying the source of matter in growing plants. He reported that the combined weights of the O₂ produced by plants and the organic matter contained in plants was too large to be derived from CO₂ alone. Therefore, he reasoned that water (H₂O), the only other material he had added, must also be absorbed by plants. Combining these earlier observations, in 1842 the German physiologist Julius von Mayer surmised that plants convert light energy from the sun into chemical energy through the process now called photosynthesis.

Putting all of this information together, scientists concluded that plant photosynthesis is a process that uses light energy to convert CO₂ and H₂O into carbohydrates and dioxygen. The overall reaction for plant photosynthesis is shown in the equation in Figure 2.

One of the most enduring questions, however, has been the exact source for the oxygen. During the first few decades of the 20th century, it was believed that CO₂ and water combined to produce carbohydrates and that the oxygen atoms in O₂ were derived directly from CO₂. In the early 1930s, Cornelis van Niel of Stanford University was studying anaerobic bacteria that use hydrogen sulfide (H₂S) instead of H₂O in photosynthesis and found that the bacteria generate sulfur (S₈) as a byproduct. Along with other studies of plant and bacterial photosynthesis, van Niel's result led him to propose that photosynthesis consists of two separate reactions. The first of these reactions is oxidation of a compound with the general formula H₂A (for example, H₂S or H₂O) with the concomitant generation of protons (H⁺) and electrons (e^–), as seen in the first equation of Figure 3. This reaction requires light to proceed and is now referred to as the light reaction. The second reaction uses the protons and electrons generated by the first reaction to reduce CO₂ and produce carbohydrates and water (second equation of Figure 3). Because CO₂ reduction does not depend directly on light, it is called the dark reaction.

Dividing photosynthesis into two elementary steps helps classify photosynthetic organisms based on the source of electrons (H₂A) for the light reaction. The anaerobic bacteria studied by van Niel belong to a larger order of photosynthetic bacteria, Rhodospirillales, which use reduced sulfur compounds like hydrogen sulfide, sulfur or other sulfide compounds (S^2–), or hydrogen (H₂) in the light reaction. Plants, algae and cyanobacteria (previously called blue-green algae) use only H₂O as the electron donor in the light reaction and generate O₂ as the light-reaction product.

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