Photosynthesis in Higher Plants
All green plants prepare their own food by the process of photosynthesis.
During this process, plants take carbon dioxide from the atmosphere through stomata and useit tomake carbohydrates.
In the earlier time, Greek philosophers believed that plants obtained all of their nutrients from the soil.
English chemist Joseph Priestley in 1771 performed a series of important experiments. He believed that a noxious substance (Phlogiston) was released into the air when a flame burned.
Jan Ingenhousz took Priestley’s work further by demonstrating that plants need light to make oxygen.
Julius von Sachs and Pfeffer verified that starch is a by-product of photosynthesis.
T.W.Englemann discovered purple bacteria that effect light-dependent synthesis of cellular materials without the release of oxygen.
Cornelius van Niel studied the process of photosynthesis in anaerobicbacteria rather than in higher plants.
Chloroplasts are the main site of photosynthesis. Its structure comprises of stroma, grana, stroma lamella and thylakoids.
Chloroplasts contain chlorophyll. Chlorophyll is of two main types – chlorophyll and chlorophyll b. There are some other accessory pigments like carotenoids and phycocyanins.
Action spectrumis the graphic representation of comparative effects of different wavelengths of light on living systems or their components.
An absorption spectrum is a spectrum of radiant energy that is measured by the means of spectrophotometer.
Photosynthesis is a two-stage process: light dependent process (Light reaction) and light independent process (Dark reaction).
Light reaction occurs on the thylakoid membrane and converts solar energy into the chemical energy of ATP and NADPH, evolving oxygen in the process.In the chloroplasts, photosystems harvest the light energy.
The pigments are organised into two discrete photochemical light harvesting complexes (LHC) within the Photosystem I (PS I) and Photosystem II (PS II).
The single chlorophyll a molecule forms the reaction centre.
The whole scheme of transfer of electrons, starting from the PS II, down the electron transport chain to PS I, excitation of electrons, transfer to another acceptor, and finally downhill to NADP+reducing it to NADPH + H+ is called the Z scheme.
It is proposed by Hill and Bendall in 1960.
The process of formation of ATP in the chloroplast while making use of light energy is called as Photophosphorylation.
It is of two types non-cyclic Photophosphorylation and cyclic Photophosphorylation.
Chemiosmotic hypothesis was first proposed by Peter Mitchell in 1961 and it was called as chemiosmotic hypothesis.The theory explains how uncouplers of photophosphorylation work.
In dark reaction or biosynthetic phase, carbon dioxide is reduced to carbohydrate by the process of carbon fixation.
ATP and NADPH produced in light reaction are used up in the stroma of chloroplasts.
Melvin Calvin traced the path of carbon in the dark reaction through autoradiography, 14C, so this pathway is termed as Calvin cycle after him.