The production of ATP in the chloroplast or in the other membranes during light reaction is called photophosphorylation. Photophosphorylation occurs in ATP-synthase complex or coupling factor (CF) located in stroma thylakoid membranes. The coupling factor is also responsible for transport of H+ from the thylakoid channel to the stroma.
The electron transfer during photophosphorylation takes place firstly when the quinone of PS II picks up two protons from the stromal side of the membrane and move these to cyt b6 complex and secondly when proton uptake occurs on the periphery of the PS I complex when NADP+ is reduced to NADPH2.
The H+ ions in thylakoid channels arise from oxidation of water and PQH2. These oxidations cause the H+ concentration in the channel (pH 5) to become 1000 times as great as in the stroma (pH 8) when photosynthesis is occurring. There is thus a strong H+ and other ions gradient toward the stroma, but thylakoids are quite impermeable to the H+ and other ions except when transported by coupling factor. This pH gradient across the membrane provides a powerful form of chemical potential energy largely responsible for driving photophosphorylation.
ATP synthesis can take place via two processes: non-cyclic photophosphorylation and cyclic photophosphorylation.
Non Cyclic Photophosphorylation
In non-cyclic photophosphorylation, the electrons removed from water do not cycle back to the water molecule. The electrons originating in water are passed by PS II and PS I to NADP+ and NADPH2 and ATP are formed together with the evolution of O2. This can be simply expressed as:
NADP + H2O ————————– NADPH2 + ½ O2
The electrons and protons are chlorophyll a, pheophytin, quinone, cytochrome b and f, Fe-S canters, plastocyanin, ferredoxin and enzyme FNR. The electrons are removed from H2O under the influence of light and donated to NADP with the coupled formation of ATP. This overall reaction can be expressed as:
2NADP + 2 H2O + 2 ADP + 2 Pi ————————– 2 NADPH2 + 2 ATP + O2
In cyclic photophosphorylation, the electron take a cyclic path from PS I and return to photosystem I. ATP is formed but not NADPH2. It may be very simple represented by the following equation:
ADP + Pi ————————– ATP
The electron removed from P700 is donated to Fe-S centers and subsequently to ferredoxin, which becomes reduced. The ferredoxin instead of transferring its electron to NADP donates its electron to cyt b6 and then through electron transport cycles back to P700.
The additional ATP molecules generated during cyclic photophosphorylation are used to convert CO2 into complex compounds and other processes in the cell.
Photophosphorylation can occur only because light energy somehow drives it. Peter Mitchell in 1961, proposed a chemiosmotic hypothesis to account for ATP formation accompanying electron transport in mitochondria and chloroplasts. The chemiosmotic theory suggests that:
- Intact membrane is also impermeable to the passive flow of protons.
- The electrons donors and acceptors of electron and protons are arranged in the membranes; and
- During photosynthetic electron transport, protons are transported from stroma (external medium) into the osmotic space of the intra-thylakoid membrane.
Thus, when the chloroplasts area illuminated, for each quantum transferred to reaction Centre (P680 or P700), an electron is excited and transported through the membrane; simultaneously a proton is taken up from outside to the inside of the membrane. As a result of the movement of protons (and electrons) across the membrane, he membranes become energized and generate proton motive force (electrochemical potential) which promote coupling of ADP and Pi.
Photosynthesis in C-4 Plants
It was found that in tropical and sub-tropical plants (growing in hot climate) the initial products of carbon dioxide fixation are both 3-PGA and malic acid (malate), oxaloacetate acid (oxaloacetate) and aspartic acids (aspartate). Malate is 4-C compound; therefore, these plants are called C-4 plants. The formation of C-4 acid as initial CO2 reduction product was first observed in sugar-cane leaves by H. P. Kortschak, C. E. Hartt and G. O. Burr (1965). Later, it was found that most grasses including maize fix CO2 in the same way.
Leaves of C-4 plants have a distinctive sheath of cells around the vascular bundles of the leaf, known as bundle sheath cells. These cells are packed with chloroplasts. This distinctive arrangement of chloroplast containing cells around the vascular bundles is referred to as Krantz anatomy.
The mesophyll cells of C-4 plants are not differentiated into palisade and spongy parenchyma, their chloroplasts are without RuBP, carboxylase/oxygenase and there is a high concentration of PEP carboxylase and malate dehydrogenase. The stomata of C-4 plants are usually located in such a way that the substomatal cavity is immediately adjacent to the mesophyll cell chloroplasts.
The C-4 Pathway of CO2 Fixation
The following steps are involved in C-4 pathway of CO2 fixation:
(a). The CO2 which diffuses into the leaf through the stomata enters the mesophyll cytoplasm where it reacts with phosphoenol pyruvic acid (PEP) to form oxaloacetic acid in the presence of the enzymes phosphoenolpyruvate carboxylase.
(b). The malate is transported to the bundle-sheath cells where it is carboxylated to pyruvate (pyruvic acid) and CO2. The CO2 released is used for sugar and starch production via the Calvin cycle (C-3 cycle).
- (c). C-3 acid (3-PGA) is transported back to mesophyll cells where it is used to regenerate phosphoenolpyruvate.
The higher carbon dioxide level in the cells inhibit photorespiration increasing the production of carbohydrate.