Primary and Secondary Reactions of Photosynthesis

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.