Joseph Priestley (1733-1804) in 1770 performed a series of experiments that revealed the essential role of air in the growth of green plants, Using a similar setup as the one used by Priestley, but by placing it once in the dark and once in the sunlight, Jan Inqen-housz (1730-1799) showed that sunlight is essential to the plant process that somehow purifies the air fouled by burning candles or breathing animals.
Ingenhousz in an elegant experiment with an aquatic plant showed that in bright sunlight, small bubbles were formed around the green parts while in the dark they did not. Later he identified these bubbles to be of oxygen. Hence, he showed that it is only the green part of the plants that could release oxygen.
It was not until about 1854 that Julius von Sachs found that the green parts in plants is where glucose is made, and that the glucose is usually stored as starch.
The first action spectrum was studied by Engelmann (1843-1909) by using a green algae (Cladophora).
Van Niel(1897-1985)proposed that oxygenic photosynthesis of organisms is actually an oxidation reduction reaction involving action of hydrogen of water over carbon dioxide to from organic compounds by using purple and green bacteria. The hydrogen acceptors are often called Hill oxidants while the reaction involving the production of oxygen by the illuminated chloroplasts in the absence of CO2 fixation is called Hill reaction. Hill reaction was discovered by R.Hill.
Electron Transport chain was formulated by Hill (1939) while details were worked out later on. P680of photosystem II absorb light energy, gets excited but transfers its election to election acceptor molecule phaeophytin. Phaeophytin on accepting electrons becomes strong reducing agent. It donates its electrons to downstream component of election transport chain (Q, Cyt b - Cytf complex, plastocyanin).
Photophosphorylationis the process of formation of ATP form ADP and Pi in presence of sunlight in the electron transport chain. Photophosphorylation is of two types :
(i) Cycle photophosphorylation
(ii) Non-cyclic photophosphorylation
In cyclic photophosphorylation only PS I is functional and electron move in a closed circle as electrons free from chlorophyll after excitation to acceptors returns to chlorophyll only. The primary electron acceptor during cyclic photophosphorylationferredoxin.Ferredoxin is an iron containing non-heme red colouredprotein. In cyclic photophosphorylation water is not utilized, no evolution of O2 takes places. In cyclic photophosphorylationonly 2 ATP molecules are produced (one between ferredoxin and Cyt.b6 and another between Cyt.b6 and Cyt.f) and no NADPH2 is formed. As a result assimilation of CO2 is slowed down. Movements of electrons in cyclic photophosphorylationare as follows :
PS I ®FD ® Cyt.b6®Cyt.f®plastocyanin®PS I
In non-cyclic photophosphorylation both PS I and PS II arefunctional. In non-cyclic photophosphorylation, water is the ultimate source of electrons and NADP+ is the final acceptor. The oxygen is evolved as by-product. Non-cyclic photophosphorylationis dominant in green plants and process is stopped by use of DCMU. The herbicide DCMU kills plants by blocking electron flow at the quinine acceptors of photosystem II. In non-cyclic photophosphorylation one ATPmolecule and one NADPH2are produced. Non-cyclic photophosphorylation is also known as Z scheme. Z scheme was given by Hill and Bendall(1960). Electron donor in non-cyclic photophosphorylation is H2O.
The end products of light reaction are oxygen, water, ATP and NADPH2. Photochemical phase involves photolysis of water and production of assimilatory power (NADPH and ATP). NADP is reduced to NADPH2, by the enzyme Fd-NADP reductase(flavoprotein enzyme) by using the H+ ions released from photolysis of water. ATP and NADPH2 are utilized to reduce CO2 into organic compound (carbohydrate) in dark reaction.
ATP in photosynthesis is required at two places.
(a) ATP provides energy for the reduction of CO2 to carbohydrates by utilizing NADPAH2.
(b) Needed for conversion of Ribulose-5PO4 to Ribuose-1, 5-diphosphate (RuBP or RuDP).
Dark reaction is purely enzymatic reaction and is lower than light reaction of photosynthesis. Dark reaction was first of all established by F.F.Blackman (1905) and later on studied in detail by Dr. Calvin, Benson and J. Bassham. For this work they were given Nobel Prize (1961). The techniques used for studying different steps (carbon pathway) were radioactive tracer technique using 14C (half life-5720 years), Chromatography and Autoradiography.