1. Deducing the Nature of Photosynthesis
Observed: Plants grow! Does the increase in mass of a plant come
from the air? …the soil? …the water? 1

2. Van Helmont 1643 • Massed a pot of soil • Planted a seed
• Watered for 5 years
• Weighed plant & soil
Plant Was 75 kg, soil mass unchanged
• Concluded
Mass came From Water 2

3. Priestley
• Burned candle in bell jar. Candle goes out, mouse dies.
• Placed sprig of mint in bell jar for a few days
• Candle relit; it burns, mouse survives
• Concluded Plants add something to the air (O2) necessary for burning (& for mice!)
A bell jar is vacuum sealed, the air removed 3

4. Ingenhousz 1779 Repeated Priestly experiment with & without sunlight
Concluded light is essential to plant respiration
Jan Ingenhousz was born in 1730 in the city of Breda in what is today the southern Netherlands, was trained as a doctor at the Catholic University of Louvain in Belgium.
After graduation, he practiced for several years in London ( ), where he became a proponent of variolation, an early method of smallpox vaccination that involved inoculating patients with smallpox virus taken from patients with a mild form of the disease.
In 1768 he traveled to Vienna and successfully inoculated the Austro-Hungarian Empress, Maria Theresa, and the rest of the imperial family, which gained him far more fame in his lifetime than did his discovery of photosynthesis. This feat secured his professional career since he became, as a result, Court Physician to the Empress.
Discovery of photosynthesis
Joseph Priestley
(1733–1804)
In 1779, Ingenhousz took a leave of absence from his position in the Austrian court and traveled to Calne, a small town in southwestern England. There, at Bowood House, a country manor, in the same laboratory where his friend and colleague Joseph Priestley had discovered oxygen itself only five years before, Ingenhousz carried out his research on photosynthesis.
In the experiment leading to his discovery, he placed plants underwater in a transparent container and saw that the undersides of their leaves made bubbles in sunlight. However, when the same plants were placed in darkness, the bubbles eventually stopped forming. He was also able to see that the leaves and other green (chlorophyll-containing) parts of the plants were the sites where the gas was produced.
He collected this gas and conducted a series of tests to determine its identity. He eventually found that a smoldering candle would burst into flame when exposed to the unknown gas, which showed it was oxygen.
In recognition of his discovery, Ingenhousz was elected to the Royal Society of London that same year.
candle burns 4

5. Ingenhousz 1779 In the presence of light, oxygen is produced
visible as bubbles on the leaves of aquatic plants
Jan Ingenhousz was born in 1730 in the city of Breda in what is today the southern Netherlands, was trained as a doctor at the Catholic University of Louvain in Belgium.
After graduation, he practiced for several years in London ( ), where he became a proponent of variolation, an early method of smallpox vaccination that involved inoculating patients with smallpox virus taken from patients with a mild form of the disease.
In 1768 he traveled to Vienna and successfully inoculated the Austro-Hungarian Empress, Maria Theresa, and the rest of the imperial family, which gained him far more fame in his lifetime than did his discovery of photosynthesis. This feat secured his professional career since he became, as a result, Court Physician to the Empress.
Discovery of photosynthesis
Joseph Priestley
(1733–1804)
In 1779, Ingenhousz took a leave of absence from his position in the Austrian court and traveled to Calne, a small town in southwestern England. There, at Bowood House, a country manor, in the same laboratory where his friend and colleague Joseph Priestley had discovered oxygen itself only five years before, Ingenhousz carried out his research on photosynthesis.
In the experiment leading to his discovery, he placed plants underwater in a transparent container and saw that the undersides of their leaves made bubbles in sunlight. However, when the same plants were placed in darkness, the bubbles eventually stopped forming. He was also able to see that the leaves and other green (chlorophyll-containing) parts of the plants were the sites where the gas was produced.
He collected this gas and conducted a series of tests to determine its identity. He eventually found that a smoldering candle would burst into flame when exposed to the unknown gas, which showed it was oxygen.
In recognition of his discovery, Ingenhousz was elected to the Royal Society of London that same year. 5

6. F. Blackman 1905 • Measured the effect of light, CO2, temp on psn
• Concluded there are 2 sets of rxns:
photo- light dependant, independent of temp, CO2
-synthesis - limited by CO2, not light (light-independent)
That photosynthesis does involve at least two quite distinct processes became apparent from the experiments of the British plant physiologist F. F. Blackman. His results can easily be duplicated by using the setup on the left. The green water plant Elodea (available wherever aquarium supplies are sold) is the test organism. When a sprig is placed upside down in a dilute solution of NaHCO3 (which serves as a source of CO2) and illuminated with a flood lamp, oxygen bubbles are soon given off from the cut portion of the stem. One then counts the number of bubbles given off in a fixed interval of time at each of several light intensities. Plotting these data produces a graph like the one below.
Since the rate of photosynthesis does not continue to increase indefinitely with increased illumination, Blackman concluded that at least two distinct processes are involved: one, a reaction that requires light and the other, a reaction that does not. This latter is called a "dark" reaction although it can go on in the light. Blackman theorized that at moderate light intensities, the "light" reaction limits or "paces" the entire process. In other words, at these intensities the dark reaction is capable of handling all the intermediate substances produced by the light reaction. With increasing light intensities, however, a point is eventually reached when the dark reaction is working at maximum capacity. Any further illumination is ineffective, and the process reaches a steady rate. 6

7. Ruben & Kamen 1941 Used Isotopes To trace Oxygen In Photosynthesis
Follow the atoms! 7

8. Following the Gases Proposed: CO2 + H2O → (CH2O) + O2
C of CO2 went into sugars & O is released
Senebier
1742
Showed: CO2 + H2S → (CH2O)
in purple sulfur bacteria no O2 is produced
therefore O2 comes from water
Proposed: split water also used to ‘fix’ C into organics
The growth of plants is accompanied by an increase in their carbon content. A Swiss minister, Jean Senebier, discovered that the source of this carbon is carbon dioxide and that the release of oxygen during photosynthesis accompanies the uptake of carbon dioxide.
Van Niel
1930 8

9. Melvin Calvin 1948 Traced the path that carbon (CO2) takes in forming
glucose
• Does NOT require
sunlight
• Calvin Cycle / Light
Independent Reaction
(not ‘Dark’ rxn) 9 9

10. Late 20th century contributions
• Light nrg can generate reducing power
DPIP visibly reduced in lab
• Chloroplasts are the site of psn
• ATP/NADPH reduce CO2, forming sugars
in absence of CO2, ATP accumulates, no glucose
• Electron transport chain described 10