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Photosynthesis in Higher Plants

  • All living organisms require continuous use of energy to carry out their different activities. This energy directly or indirectly comes from sun.

  • Photosynthesis is the only process on earth by which solar energy is trapped by autotrophic organisms and converted into food for the rest of organisms.

  • In photosynthesis process, 'energy rich compounds like carbohydrates are synthesized from simple inorganic compounds like carbon dioxide and water in the presence of chlorophyll and sunlight with liberation of oxygen'. The process of photosynthesis can also be defined as "transformation of photonic energy (i.e. light or radiant energy) into chemical energy".

  • Earlier, photosynthesis was considered to be reverse of respiration, i.e.,

  • Above reaction gives an idea that O2 comes from CO2. But Ruben and Kamen (1941) experimentally verified that source of liberated O2 in photosynthesis is H2O, not CO2.

  • Thus, overall reaction can be corrected as given below :

  • About 90% of total photosynthesis in world is done by algae in oceans and in freshwater. More than 170 billion tonnes of dry matter are produced annually by this process. Further CO2 fixed annually through photosynthesis is about 7.0 × 1013kg. Photosynthesis is an anabolic and endothermic reaction. Photosynthesis helps to maintain the equilibrium position of O2 and CO2 in the atmosphere.

Historical background

Before seventeenth century it was considered that plants take their food from the soil.

  • Van Helmont (1648) concluded that all food of the plant is derived from water and not from soil.

  • Stephen Hales (Father of Plant Physiology) (1727) reported that plants obtain a part of their nutrition from air and light may also play a role in this process.

  • Joseph Priestley (1772) demonstrated that green plants purify the foul air (i.e., Phlogiston), produced by burning of candle, and convert it into pure air (i.e., Dephlogiston).

  • Jan Ingen-Housz (1779) concluded by his experiment that purification of air was done by green parts of plant only and that too in the presence of sunlight. Green leaves and stalks liberate dephlogisticated air (Having O2) during sunlight and phlogisticated air (Having CO2) during dark.

  • Jean Senebier (1782) proved that plants absorb CO2 and release O2 in presence of light. He also showed that the rate of O2 evolution depends upon the rate of CO2 consumption.

  • Lavoisier (1783) identified the pure air (i.e., dephlogiston) as oxygen (O2) and noxious air (i.e., Phlogiston) produced by the burning of candle as carbon dioxide (CO2).

  • Nicolus de Saussure (1804) showed the importance of water in the process of photosynthesis. He further showed that the amount of CO2 absorbed is equal to the amount of O2 released.

  • Pelletier and Caventou (1818) discovered chlorophyll. It could be separated from leaf by boiling in alcohol.

  • Dutrochet (1837) showed the importance of green pigment chlorophyll in photosynthesis.

  • Julius Robert Mayer (1845) proposed that light has radiant energy and this radiant energy is converted to chemical energy by plants, which serves to maintain life of the plants and also animals.

  • Liebig (1845) indicated that main source of carbon in plants is CO2.

  • Bousingault (1860) reported that the volume of CO2 absorbed is equal to volume of O2 evolved and that CO2 absorption and O2 evolution get start immediately after the plant was exposed to sunlight.

  • Julius Von Sachs (1862) demonstrated that first visible product of photosynthesis is starch. He also showed that chlorophyll is confined to the chloroplasts.

  • J.C. Maxwell (1864) developed 'wave model of light', leading to recognition that light is source of energy in photosynthesis.

  • Theodore Engelmann (1884, 88) showed that chloroplast as the site of photosynthesis in the cell and also discovered the role of different wave lengths of light on photosynthesis and plotted the action spectrum.

  • F.F. Blackmann (1905) proposed the 'law of limiting factor' and also discovered two steps of photosynthesis i.e., light dependent and temperature independent steps and a light independent and temperature dependent step.

He proved that photosynthesis is a photochemical and biochemical reaction. Photochemical reaction is light reaction and biochemical reaction is dark reaction or carbon dioxide fixation.

  • Willstatter and Stoll (1912) studied structure of photosynthetic pigments.

  • Warburg (1919) performed flashing light experiment using green alga-Chlorella as a suitable material for the study of photosynthesis.

  • Van Niel (1931) demonstrated that some bacteria use H2S instead of H2O in the process of photosynthesis.

  • Emerson and Arnold (1932) proved the existance of light and dark reactions by flashing of light experiment in photosynthesis.

  • Robert Hill (1937) demonstrated photolysis of water by isolated chloroplast in the presence of suitable electron acceptor.

  • S. Ruben and M. Kamen (1941) used heavy isotope 18O and confirmed that oxygen evolved in photosynthesis comes from water and not from CO2.

  • Melvin Calvin (1954) traced the path of carbon in photosynthesis (Associated with dark reactions) and gave the C3 cycle (Now named Calvin cycle). He was awarded Nobel prize in 1961 for the technique to trace metabolic pathway by using radioactive isotope.

  • Emerson, Chalmers and Cederstrand (1957) discovered Emerson effect.

  • Hill and Bendall (1960) proposed Z scheme and suggested that two photosystems operate in series.

  • Arnon (1961) discovered photophosphorylation and gave the term 'assimilatory powers'.

  • Peter Mitchell (1961) proposed chemi-osmotic coupling hypothesis.

  • Kortschak (1965) discovered the formation of C4 dicarboxylic acid in sugarcane leaves.

  • Hatch and Slack (1966) reported the C4 pathway for CO2 fixation in certain tropical grasses.

  • Huber, Michel and Deisenhofer (1985) crystallised the photosynthetic reaction center from the purple photosynthetic bacterium, Rhodopseudomonas viridis. They analysed its structure by X-ray diffraction technique. In 1988 they were awarded Nobel prize in chemistry for this work.

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