Thomas Brennan

Department of Biology
Dickinson College, Carlisle, PA 17013

c350 BC
Aristotle proposes that plants, like animals, require food. Anticipates Priestley's work 2000 years later by asserting that plants do not require animals but animals require plants.

c300 BC
Theophrastus writes that plants obtain their nourishment through the roots.

Nicholas of Cusa proposes (but apparently never performs) an experiment in which a plant is weighed and then planted in a container containing a weighed amount of soil. After a period of growth, the final weights of plant and soil, as well as the total weight of water applied, are determined and compared to the initial values. He speculates this will demonstrate that the mass of the plant was derived from water rather than soil.

Jean Baptiste van Helmont performs the experiment proposed by Nicholas of Cusa nearly 200 years earlier. He concludes that the entire mass of the plant came from water, but ignores a very slight decrease in the weight of the soil.

Edme Mariotte proposes that plants obtain part of their nourishment from the atmosphere.

Stephen Hales writes that plant leaves "very probably" take in nourishment from the air, and that light may also be involved.

Charles Bonnet observes the emission of gas bubbles by a submerged illuminated leaf.

Joseph Priestley finds that air which has been made "noxious" by the breathing of animals or burning of a candle can be restored (i.e., made to support breathing or combustion again) by the presence of a green plant. He isolates the gas later identified as oxygen.

Antoine Lavoisier begins to investigate and later names oxygen. He recognizes that it is consumed in both animal respiration and combustion. His work discredits the theory of "phlogiston," a hypothetical substance then believed to be emitted during respiration or combustion, and lays the foundations of modern chemistry.

Jan Ingenhousz discovers that only the green parts of plants release oxygen and that this occurs only when they are illuminated by sunlight.

Jean Senebier demonstrates that green plants take in carbon dioxide from the air and emit oxygen under the influence of sunlight.

Comparetti observes green granules in plant tissues, later identified as chloroplasts.

Nicolas de Saussure shows that the carbon assimilated from atmospheric carbon dioxide cannot fully account for the increase in dry weight of a plant. He hypothesized that the additional weight was derived from water. At this point, therefore, the basic equation of photosynthesis was established. It was understood as a process in which a green plant illuminated by sunlight takes in carbon dioxide and water and converts them into organic material and oxygen.

Pierre Joseph Pelletier and Joseph Bienaime Caventou give the name "chlorophyll" to the green pigment in plants.

Rene Dutrochet makes the connection between chlorophyll and the ability of plants to assimilate carbon dioxide. Also identifies stomata on leaf surfaces.

Matthias Schleiden postulates that the water molecule is split during photosynthesis.

Hugo von Mohl makes detailed observations of the structure of chloroplasts.

Julius Robert von Mayer proposes that the sun is the ultimate source of energy utilized by living organisms, and introduces the concept that photosynthesis is a conversion of light energy into chemical energy.

Julius von Sachs demonstrates light-dependent starch formation in chloroplasts.

Jean Baptiste Boussingault makes accurate quantitative measurements of carbon dioxide uptake and oxygen production, a step leading to a balanced equation for photosynthesis: 6CO2 + 12H2O + light energy ----> C6H12O6 + 6O2 + 6H2O

Emil Godlewski confirms that atmospheric carbon dioxide is the source of carbon in photosynthesis by showing that starch formation in illuminated leaves depends upon the presence of carbon dioxide.

Theodor Wilhelm Engelmann illuminates a filamentous alga with light dispersed through a prism. He finds that motile aerobic bacteria congregate near the portions illuminated by red and blue wavelengths, thus producing the first action spectrum for photosynthetic oxygen evolution.

Arthur Meyer describes the chloroplast grana.

Charles Barnes suggests that the process by which illuminated green plants manufacture carbon compounds be called either "photosyntax" or "photosynthesis." Although Barnes prefers the former, "photosynthesis" is adopted into common usage.

F. F. Blackman develops the concept of limiting factors, showing that photosynthesis consists of two types of reactions: a rapid light-dependent photochemical process and a slower temperature-dependent biochemical process. These are later termed "light reactions" and "dark reactions," respectively.

Richard Willstatter and Arthur Stoll publish studies on the structure and chemistry of chlorophyll. Willstatter awarded Nobel Prize, 1915.

Robert (Robin) Hill demonstrates that in the presence of an artificial electron acceptor isolated chloroplasts can evolve oxygen in the absence of carbon dioxide.

Cornelis van Niel publishes a summary of his work showing that photosynthetic bacteria which use H2S as an electron donor produce elemental sulfur instead of oxygen. He suggests by analogy that the O2 released in plant photosynthesis is derived from H2O rather than CO2.

Samuel Ruben and Martin Kaman use water labeled with the heavy isotope 18O to confirm that the oxygen produced in photosynthesis comes from H2O.

Daniel Arnon demonstrates light-dependent ATP formation in chloroplasts.

Daniel Arnon demonstrates that isolated chloroplasts are capable of carrying out complete photosynthesis.

Melvin Calvin and coworkers use radioactively labeled 14CO2 to elucidate the pathway of carbon assimilation in photosynthesis. Calvin awarded Nobel Prize in 1961.

Robert Emerson describes the "red drop" and "enhancement" effects, the first indication that the light reactions of photosynthesis consist of two separate photochemical systems.

Robert Woodward synthesizes chlorophyll. Awarded Nobel Prize, 1965.

Robin Hill and Fay Bendall, based on the work of Emerson and others, propose the "Z scheme" model for the photosynthetic light reactions. According to this model, the light reactions consist of two separate photosystems operating in tandem, each activated by slightly different wavelengths of light. One photosystem oxidizes water and reduces cytochrome f, while the other oxidizes cytochrome f and reduces NADP+.

Louis Duysens provides evidence in support of the Z scheme by demonstrating that exposure to alternating wavelengths of light causes cytochrome f to switch between oxidized and reduced states.

Roderick Clayton isolates reaction center complexes.

Bessel Kok proposes the "S-states" model of charge accumulation to explain the stepwise oxidation of H2O and release of O2.

Hans Deisenhofer, Hartmut Michel, and Robert Huber crystallize the photosynthetic reaction center from a purple bacterium and use X-ray diffraction techniques to determine its detailed structure. The three share Nobel Prize, 1988.

Junko Yano, Vittal Yachandra, and co-workers determine the structure of the manganese-calcium water-splitting complex of Photosystem II.


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