Experiments on photosynthesis - gases - class-X

Jan Ingenhousz (1730-1799), in his experiment with an aquatic plant, showed that in bright sunlight, small bubbles were formed around the green parts while in the dark, no such bubbles formed. He found out these bubbles to be of oxygen. He thus, confirmed that purification of air or formation of dephlogiston is carried out by green plants only in presence of sunlight. 

Isotope labeling experiments (using O-18) have demonstrated that the oxygen released during photosynthesis comes from the splitting of water molecules (photolysis), not from carbon dioxide.

Potassium bicarbonate (KHCO3) is commonly used in photosynthesis experiments to provide a controlled source of carbon dioxide for aquatic plants. When the plant performs photosynthesis, the oxygen evolved can be collected and observed.

To test that oxygen is evolved during photosynthesis few fresh Hydrilla plants in a glass beaker filled with water. A pinch of KHCO$ _{3}$ is added to it. It is kept in sunlight. After some time some bubbles (oxygen) escapes from the plant. This proves that oxygen is produced during the photosynthesis. It is evolved during the light reaction of the photosynthesis.

The balanced equation for photosynthesis is 6CO2 + 12H2O -> C6H12O6 + 6O2 + 6H2O. To produce one molecule of hexose (C6H12O6) from 6 molecules of CO2, 12 molecules of water are consumed, but 6 are released, resulting in a net requirement of 6 molecules of water.

Oxygen evolution in green plants is driven by Photosystem II, which performs the photolysis of water. Photosynthetic bacteria lack Photosystem II and possess only Photosystem I, utilizing other electron donors like hydrogen sulfide instead of water, and therefore do not release oxygen.

Richmond-Lang effect refers to senescence retardation in leaves by cytokinins (4). Emerson effect is the enhancement of photosynthesis (reversal of red-drop) (1). Pasteur effect is the inhibition of glycolysis by oxygen (2). Warburg effect is the inhibition of photosynthesis by high oxygen concentration (3).

Jan Ingenhousz (1730-1799), in his experiment with an aquatic plant, showed that in bright sunlight, small bubbles were formed around the green parts while in the dark, no such bubbles formed. He found out these bubbles to be of oxygen. He thus, confirmed that purification of air or formation of dephlogiston is carried out by green plants only in presence of sunlight. 

If boiled water is used in the test tube, there will be no air bubbles. This is hydrilla cannot survive the high temperature of boiled water. 

CO$ _{2}$ is needed for photosynthesis is shown by Moll's half leaf experiment. In this experiment, only that part of the leaf will show blue color which was present outside the bottle because they are able to take carbon dioxide from the stomata of the leaf.

In Moll's half-leaf experiment, the part of the leaf inside the bottle is deprived of CO2, preventing photosynthesis. The part outside has access to CO2, allowing photosynthesis to occur and starch to be produced, which turns blue with iodine.

Calvin cycle is used for the fixation of carbon dioxide to produce glucose. Radioactive isotopes of carbon (C-14) and phosphorus (P-32) was used to trace the path for the carbon assimilation. A photosynthesizing plant does not strongly discriminate between the most abundant natural carbon isotope C-12 and C-14. During photosynthesis in the presence of 14$CO _2$, the compounds formed become labeled with the radioisotope.

During the day, CAM plants decarboxylate malic acid that was stored in vacuoles overnight, releasing CO2 internally for the Calvin cycle. This allows them to fix carbon while keeping their stomata closed to conserve water. Thus, the procured CO2 comes directly from malic acid.

In Moll's half leaf experiment a destarched plant is used.Half part of one of the leaves from the plant is inserted into a corked bottle containing either KOH solution or KOH palletes. The plant is then placed in sunlight for a few hours. The leaf is then tested for starch with iodine solution. Part of the leaf that was inserted in the bottle does not turn blue or test positive for starch. This is because $CO _2$ present in the bottle was aborbed by KOH and the bottle was corked to prevent any entry of air.Proving that $CO _2$ is essential for photosynthesis.

Molls experiment evidently shows that plants do not form starch when carbon dioxide is not at their disposal, all the carbon dioxide contained in the bottle having been absorbed by the caustic potash solution.

The complete process of photosynthesis is studied under two phases - photochemical phase and biosynthetic phase. Photochemical phase involves absorption of light energy, photolysis of water with the release of $O _{2}$ and production of assimilatory power (i.e. ATP + NADPH).$CO _{2} $ fixation occurs during biosynthetic phase of photosynthesis.