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 to ATP.
Thus, the electrons return to the chlorophyll molecule in a low-energy state, and the oxidized chlorophyll is in a condition to be reduced again. The ATP thus formed may be used as the phosphorylating agent in the synthetic reactions of photosynthesis.
The cyclic pathways of electron transport outlined above constitute only a part of the light phase of photosynthesis, and it should be noted that the electrons removed initially from chlorophyll are eventually replaced. It should also be noted that in these cyclic pathways the water represented in our initial equation is not involved. Rather, flavins and cytochromes as well as a substance known as vitamin K transport the electrons.
These cyclic pathways leading to the production of ATP are considered to be minor when compared to the entire process of photosynthesis. Furthermore, green plant cells also can on respiration, discussed in a alter topic, in which a relatively high yield of ATP is achieved. Thus, ATP production in the cells is by no means limited to the light phase of photosynthesis.
At this point, let us return to a consideration of the overall photosynthetic equation. It should be apparent now that the products of the reaction, oxygen and carbohydrates, are formed by the splitting of water molecules.
Thus, the hydrogen so released reduces the carbon dioxide to carbohydrate, and molecular oxygen is produced. Here again, we are involved in an oxidation-reduction, but one which is highly unlikely from a thermodynamic viewpoint. The problem is this: a weak oxidant must oxidize a weak reluctant producing a strong oxidant and a strong reluctant. In other words, the C02 and H20 are much more stable than the 02 and the carbohydrate.
Carbon dioxide joins a five-carbon sugar, rib lose diphosphate, which is already present in the cell, to form a very unstable six-carbon compound. The six-carbon compound has a very brief existence; almost immediately it breaks down spontaneously into two molecules of a three- carbon compound, 3-phosphoglyeerie acid. Each molecule of PGA is then reduced to the aldehyde form, phosphogylceraldehyde, by NADP.H2 with the aid of a molecule of ATP.
Thus, it is at this point that the products of the light phase enter into the reduction of carbon dioxide. PGAL is now at the reduction level of a carbohydrate which corresponds to that of an aldehyde, and it may travel any of several different pathways.
It may undergo a series of reactions and eventually be transformed to RDP, it may become modified into glycerol, or it may undergo condensation to form the six-carbon sugar fructose diphosphate, which can undergo dephosphioiylation and certain internal transformations to become glucose. Glucose may then serve as a building block for such saccharine sugars as sunrise or such polysaccharides as starch.
Although PGAL might justly be considered the end product of photosynthesis. PGA is frequently involved in transformation. It may proceed along a pathway leading to the formation of amino acids, which subsequently become involved in protein synthesis, or it may become involved in the formation of fatty acids, which join with glycerol in the formation of fats.
Notice that this scheme shows the entrance of carbon dioxide and the products of the light phase into a cycle involving the compounds we have discussed. Although we have mentioned only a few of the many possible synthetic pathways taken by PGA and PGAL, it should be obvious that the basic organic molecules which serve as nutrient materials for cells of green plants themselves and for the cells of other organisms are produced in photosynthesis.
In summary, photosynthesis is an extremely complex process involving many separate reactions. Like virtually all reactions which within occur living systems, they are catalyzed by a complex of specific enzymes. Although the light and dark phases of photosynthesis can be separated experimentally, they are closely interrelated in the overall metabolism of any given photiosynthetic cell.
In addition to photosynthesis, the plant cell carriers on respiration, during which large amounts of ATP are formed, and this ATP supplies energy for many of the synthetic reactions we have mentioned. In other words, the ATP formed during the light phase of photosynthesis is not nearly sufficient to drive the many endergonic reactions carried on in the plant.
Nevertheless, our original equation is accurate as a summary equation, because every energetic reaction is driven by energy which is ultimately supplied by sunlight.
Synthesis common to all cells there are numerous compounds not obtained by cells as prefabricated nutrients; rather, they are synthesized within the cells themselves. These are primarily the organic macromolecules which constitute the bulk of cell contents exclusive of water.
In all cells except the photo synthetic cells discussed above, and the chemosynthetic bacterial cells mentioned previously, the raw materials employed in synthetic reactions comes from the digestion of prefabricated materials taken into the organism. The energy necessary to drive these endergonic reactions also comes from these prefabricated materials, in their respiration.
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