Photosynthesis Case

The Process of Photosynthesis

All organisms depend ultimately upon green plants as a source of organic nutrients.. This process occurs whenever radiant energy is received by chlorophyll molecules. By a series of energy- transformations carbon dioxide and water are combined in the production of carbohydrates and oxygen. A net reaction can be written as follows, using glucose as the carbohydrate.

For many years after the raw materials and end products of photosynthesis were known, it was thought that carbon and oxygen separated during the process, in such a reaction, carbon would attach to water, and oxygen would be released.

It was supposed further that the unit was "multiplied" in some fashion to form sugars. As is so often the case in scientific matters, however, the most attractive, plausible, or popular hypothesis does not always turn out to a be a fruitful one.

During the 1930's, it was demonstrated that some bacteria carried on photosynthesis without the liberation of oxygen, and in the early 1940's, studies using readiosotopic tracers indicated that in green plants liberated oxygen did not come from CO.

This was accomplished by incorporating "heavy" oxygen into water molecules and tracing it throughout the process. Contrary to the earlier idea, it was found that the oxygen of H20, not that of COz, became the O, liberated during photosynthesis. About the same time, the biochemist Robert Hill found that exposure to light of green cells in a test tube in the presence of hydrogen acceptors resulted in the liberation of oxygen, but no carbohydrate was synthesized. A little later, it was shown that carbohydrate synthesis would occur in the dark within green ells if they had previously been exposed to light.

Thus, it became obvious that photosynthesis involved two phases: the light, or photo, phase and the dark, or synthetic, phase. We shall discuss these in order. The chlorophyll molecule is so constructed that it can absorb "packets" of light. In the process of doing so, certain of its electrons become energized and actually leave the molecule. The energized state of the electrons represents the transferred radiant energy.

This process of electron separation leaves the chlorophyll molecule in an ionic state. Eventually, electrons will return to the molecule, but only after their energy of excitation has been transferred elsewhere.

Apparently, there are two possible pathways, or cycles, by means of which the electrons may get back to the chlorophyll molecules. Both of these cycles involve oxidation-reduction reactions, and as the electrons are transferred from one acceptor to another, they pass to lower energy levels. In the process, the "excess" energy of the electrons is transferred into high-energy phosphate bonds when ADP is phosphorylated