Early Experiments and the Historical Discovery of Photosynthesis

The modern picture of photosynthesis was assembled gradually, experiment by experiment, over nearly two hundred years, and the sequence of discoveries is itself instructive: each experiment resolved exactly one open question, and the questions grew steadily more precise.

The story conventionally begins with the English clergyman-scientist Joseph Priestley, who in the 1770s performed a now-famous set of bell-jar experiments. Priestley found that a burning candle, or a mouse, placed under a sealed glass jar would eventually go out, or die, having apparently "injured" or used up something in the enclosed air. But when he placed a sprig of mint in the same sealed jar beforehand, the candle burned, or the mouse survived, for much longer -- as if the plant had somehow "restored" the air. This was the first experimental hint that green plants release a gas (later identified as oxygen) that supports both combustion and respiration.

Priestley's results, however, were not always reproducible by others, and it was the Dutch physician Jan Ingenhousz who, working through the 1770s and 1780s, refined the experiment and resolved why. Ingenhousz showed conclusively that this air-purifying effect occurred only when the plant was kept in sunlight, not in darkness, and only from the green parts of the plant -- establishing, for the first time, that light and chlorophyll (or "greenness," as it was understood then) are both essential requirements for the process, not incidental details.

Later, the German botanist Julius von Sachs, working in the 1860s, provided direct evidence that photosynthesis produces a storable carbohydrate. By covering part of a green leaf to keep it in darkness while leaving the rest exposed to light, and then testing the whole leaf with iodine solution (which turns blue-black in the presence of starch), Sachs showed that starch accumulates only in the illuminated parts of the leaf -- and, crucially, that this starch appears specifically within the chloroplasts, correctly localising the site of CO2 fixation within the cell for the first time.

A more elegant and quantitative demonstration came from the German botanist T.W. Engelmann in 1882. Engelmann passed light through a prism to split it into its spectral colours, and directed this spectrum of light across a single, elongated filamentous green alga, Cladophora, immersed in a suspension of aerobic, motile bacteria that are strongly attracted to oxygen. He observed that the bacteria congregated far more densely around the parts of the alga illuminated by blue and red light than around the parts illuminated by green light -- demonstrating, without any chemical apparatus at all, that oxygen evolution (and therefore photosynthetic activity) is strongest exactly where chlorophyll absorbs light most strongly. This experiment effectively established the action spectrum of photosynthesis and its close correspondence to the absorption spectrum of chlorophyll, a relationship developed further in the section on pigments.

F.F. Blackman's work in the early 1900s contributed a different but equally important insight: by studying how the rate of photosynthesis responded to light intensity, CO2 concentration and temperature, Blackman showed that the overall process cannot be a single reaction, since some steps (later identified as the light reaction) are sped up by light and largely insensitive to temperature, while other steps (the dark/biosynthetic reaction) are sped up by higher temperature but are essentially independent of further increases in light intensity once saturated. This is the basis of his 1905 Law of Limiting Factors, discussed further in the section on factors affecting photosynthesis, and it was the clearest early evidence that photosynthesis consists of at least two mechanistically distinct phases.

The clinching separation of these two phases came from the English biochemist Robert Hill in 1937. Hill isolated chloroplasts from leaf cells, broke them free of the rest of the cell's machinery, and supplied them with light and an artificial electron acceptor in place of CO2. He found that these isolated chloroplasts still evolved oxygen, even with no CO2 present at all -- proving definitively that O2 evolution and CO2 reduction are two separate events, and that the light-dependent, oxygen-evolving machinery can function entirely on its own. This landmark result is still known today as the Hill reaction. …