In this article we will discuss about the primary and secondary reactions of photosynthesis.
Ruben and his co-workers in early 1943 used 18O and proved that oxygen evolved in photosynthesis came from water. Photosynthesis allowed to proceed in H2O18 and normal CO2, had molecular oxygen evolved with heavy isotopes.
When photosynthesis was carried out in the presence of normal H2O and CO218, normal molecular oxygen was evolved as follows:
Clearly, the oxygen of carbohydrate comes from CO2. Also, CO2 has two oxygen atoms and only one appeared in CH2O. Moreover, the two oxygen (O218) atoms in the output cannot result from a single O18 atom of the water molecule (H2018) in the input. Since nature operates via balanced reaction, the input clearly must be 2H2O18 for every CO2.
The summary equation is therefore rewritten as:
In other words, of the atoms contained in CH2O, the C and O are obtained from CO2 and hydrogen from water in the input. All the remaining atoms of input form by-products i.e., O2 and water. Thus, water is a by-product as well as input raw material. Twice as much water must be supplied as is actually used up.
The oxygen on the right hand comes from H2O and not from CO2 as shown below:
Hill reaction demonstrates evolution of oxygen in the absence of CO2
when isolated chloroplasts suspended in water are exposed to light and essentially supplied with iron salts such as ferricyanides which serve as hydrogen acceptors. Using water (H2O18), it has been shown that oxygen released in the over-all process of photosynthesis comes from the water molecule. Thus, the Hill reaction simulates the light reaction of photosynthesis. It can be represented by the following equation
Clearly water is the sole source of oxygen. It also provides hydrogen electron required for the reduction of CO2. This splitting of water (H2 and O) is also called photolysis of water and essentially requires light and chlorophyll.
Arnon (1951) showed that NADP could also serve as hydrogen acceptor and be reduced by isolated chloroplasts. “Light + H2O + NADP → ⅟2 O2 + NADPH + H”. He has also reported that NADPH2 is used in photosynthesis for the reduction of CO2.
Arnon (1954) also demonstrated that the isolated chloroplasts, free of mitochondria, could produce ATP from ADP and inorganic phosphate (Pi) in the presence of light and called it photosynthetic phosphorylation. ATP formation in chloroplasts differs from that in mitochondria in being light dependent and also independent of respiration oxidations. In other words, the light energy is converted into chemical energy.
Arnon illuminated chloroplasts in the presence of water containing ADP, inorganic phosphate and NADP but in the absence of CO2. Chloroplasts accumulated ATP and NADPH2 and O2 is evolved as follows:
2 ADP + 2Pi+ 2 NADP+ 4 H2 O→ 2 ATP + 2 NADPH2 + 2H2O + O2
The above equation shows that the evolution of 1 mole of O2 was accompanied by 2 moles each of ATP and NADPH2. ATP and NADPH2 together provide the energy required for CO2 assimilation. This is referred to as the assimilatory power (ATP + NADPH2).
Enzymes needed for the dark reactions of CO2 assimilation were extracted. When the chlorophyll was discarded and the ATP and NADPH2 produced previously in the light reactions were used, CO2 assimilation in the dark was carried out. The photosynthesis products are identified in the stroma.
From the above discussion we may infer that photosynthesis comprises primary and secondary processes (Fig. 13-3). In the primary processes water is cleaved or it undergoes photolysis. It also includes noncyclic electron transport and cyclic electron transport. Secondary processes of photosynthesis consist of reduction of CO2 to carbohydrates through the hydrogen set free by photolysis. This reduction occurs only after the CO2 is bound to an organic acceptor. Before describing the two processes we shall describe the photosynthetic pigments.