1. Visualizing Biochemical Pathways: Discoveries in photosynthesis
Benjamin Heyne
Visualizing Biochemical Pathways:
Discoveries in photosynthesis
An example of a Crassulacean plant

2. Jean Senebier
Jean Senebier was a Swiss pastor and naturalist born in 1742.
Although Jan Ingenhousz proved the simultaneous disappearance of carbonic acid, Senebier demonstrated that this was confined to the green parts of the plant, and only in sunlight.
He proved that “fixed air” (carbon dioxide) is indeed essential in the processes of photosynthesis.
He demonstrated that the green parts of plants consume carbon dioxide from the air and release combustion-supporting oxygen while under influence of light.

3. Theodore de Saussure
Theodore de Saussure was a Swiss chemist who made many important advances in the field of phytochemistry.
Studied fermentation and conversion of starches into sugars.
He suggested that water participates in photosynthesis as a reactant, and that the increase in mass during plant growth was not only due to the uptake of carbon dioxide, but also due to the uptake of water. In doing so, he outlined the basic reaction photosynthesis uses to produce glucose.
His results implied that respiratory carbon dioxide and external carbon dioxide were being taken up as a result of CAM (Crassulacean Acid Metabolism).

4. B. Heyne’s “Experiment”
He described the taste of the leaves of the plant Byrophyllum calycinum during different times of the day. He said that in general it had a herbaceous taste.
Results:
In the morning, he described the plant as having a taste as acid as sorrel. Further on in the day, he noted that they tended to lose their acidic taste, and instead by around noon become rather tasteless. He then described that during the evening the leaves of the plant began to have a sort of bitter taste to them.

5. What these results mean…
Although Benjamin Heyne himself did not prove this, his observations on the changes in acidity throughout the day of the plant Byrophyllum calycinum helped to explain the Crassulacean acid metabolism, also known as CAM photosynthesis.

6. CAM photosynthesis
Benjamin Heyne’s observations led into further research in Crassulacean plants. CAM is a carbon fixation pathway that is present in some types of plants. It is mostly found in plants that occur in arid conditions and have adapted to life by conserving water.
During the night, the stomata of the plants are kept open. Carbon dioxide enters through the stomata and are stored as the four carbon organic acid malate. This happens in the spongy mesophyll cell’s cytoplasm because of a phosphoenolpyruvate (PEP) reaction. It is then stored in the vacuoles. This happens because it cannot be immediately passed on for use in the Calvin cycle as the light reactions that provide it with ATP and NADPH cannot take place.

7. CAM photosynthesis - continued
During the day, the organic acids are freed from the vacuoles and enter the stroma, cytoplasm of the chloroplasts, and into the Calvin cycle. The carbon dioxide is released, where it becomes concentrated around RuBisCO (an enzyme) and therefore increases the efficiency of photosynthesis. This pathway allows the plants to close their stomata during the day which helps in the conservation of water in arid conditions.
As the carbon dioxide is released, the concentration of the organic acid malate that is used to store the carbon dioxide in the vacuoles would decrease as the day goes on. This would explain why Benjamin Heyne noted a progressively less acidic taste in the plant as the day progressed.

8. Pineapple is an example of a plant that performs CAM photosynthesis

9. References