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Photosynthesis in Higher Plants 1 1 Photosynthesis in Higher Plants What Do We Know? • Some simple experiments you may have done in the earlier classes have shown that chlorophyll (green pigment of the leaf), light and CO2 are required for photosynthesis to occur. • You may have carried out the experiment to look for starch formation in two leaves – a variegated leaf or a leaf that was partially covered with black paper, and exposed to light. On testing these leaves for the presence of starch, it was clear that photosynthesis occurred only in the green parts of the leaves in the presence of light. • Another experiment you may have carried out where a part of a leaf is enclosed in a test tube containing some KOH soaked cotton (which absorbs CO2), while the other half is exposed to air. The setup is then placed in light for some time. On testing for the presence of starch later in the two parts of the leaf, you must have found that the exposed part of the leaf tested positive for starch while the portion that was in the tube, tested negative. This showed that CO2 was required for photosynthesis. Early Experiments • J. Priestley (1733-1804) : He carried out very interesting experiment on Bell jar, mice, mint & candle. In 1770 Priestley performed a series of experiments that revealed the essential role of air in the growth of green plants. Priestley, you may recall, discovered oxygen in 1774. Priestley observed that a candle burning in a closed space–a bell jar, soon gets extinguished. Similarly, a mouse would soon suffocate in a closed space. He concluded that a burning candle or an animal that breathe the air, both somehow, damage the air. But when he placed a mint plant in the same bell jar, he found that the mouse stayed alive and the candle continued to burn. Priestley hypothesised as follows: Plants restore to the air whatever breathing animals and burning candles remove. • Jan Ingenhousz (1730-1799) : He explained the importance of light and green colour and also suggested the O2 is released in the presence of light by green parts. He 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 • J. V. Sachs (1854) : It was not until about 1854 that Julius von Sachs provided evidence for production of glucose when plants grow. Glucose is usually stored as starch. His later studies showed that the green substance in plants (chlorophyll as we know it now) is located in special bodies (later called chloroplasts) within plant cells. He found that the green parts in plants is where glucose is made, and that the glucose is usually stored as starch.
2 Photosynthesis in Higher Plants • T.W. Engelmann (1843-1909) : Described action spectrum of photosynthesis with the help of Spirogyra/Cladophora and aerobic bacteria experiment. Now consider the interesting experiments done by T.W Engelmann (1843 – 1909). Using a prism he split light into its spectral components and then illuminated a green alga, Cladophora, placed in a suspension of aerobic bacteria. The bacteria were used to detect the sites of O2 evolution. He observed that the bacteria accumulated mainly in the region of blue and red light of the split spectrum. A first action spectrum of photosynthesis was thus described. It resembles roughly the absorption spectra of chlorophyll a and b. • Cornelius Van Niel (1897-1985) : O2 releases from water. studies of purple and green bacteria. A milestone contribution to the understanding of photosynthesis was that made by a microbiologist, Cornelius van Niel (1897-1985), who, based on his studies of purple and green bacteria, demonstrated that photosynthesis is essentially a light-dependent reaction in which hydrogen from a suitable oxidisable compound reduces carbon dioxide to carbohydrates. It is a redox process In green plants H2O is the hydrogen donor and is oxidised to O2. Some organisms do not release O2 during photosynthesis. When H2S, instead is the hydrogen donor for purple and green sulphur bacteria, the ‘oxidation’ product is sulphur or sulphate depending on the organism and not O2. Hence, he inferred that the O2 evolved by the green plant comes from H2O, not from carbon dioxide. This was later proved by using radio isotopic techniques. "Photosynthesis is a physico-chemical (photo–biochemical) process (anabolic & endergonic) in which organic compounds (carbohydrates) are synthesised from the inorganic raw material (H2O & CO2) in presence of light & pigments. O2 is evolved as a by product". • Light energy is conserved into chemical energy by photosynthesis. • First true & oxygenic photosynthesis started in Cyanobacteria. (BGA) Significance of photosynthesis : • Photosynthesis is vital process for life on planet earth as it is the only process, that links the physical and biological world by conservation of solar energy into organic matter, which make bulk of the dry matter of any organism. • Presence of O2 in the atmosphere is also an outcome of photosynthesis. This oxygen is helpful to living organisms in two ways.

4 Photosynthesis in Higher Plants • Action spectrum : Graph plotted between the rate of photosynthesis against the wavelength of light. First action spectrum was plotted by Engelmann (Experiment on Cladophora). These graphs, together, show that most of the photosynthesis takes place in the blue and red regions of the spectrum; some photosynthesis does take place at the other wavelengths of the visible spectrum. Let us see how this happens. Though chlorophyll is the major pigment responsible for trapping light, other thylakoid pigments like chlorophyll b, xanthophylls and carotenoids, which are called accessory pigments, also absorb light and transfer the energy to chlorophyll a. Indeed, they not only enable a wider range of wavelength of incoming light to be utilised for photosyntesis but also protect chlorophyll a from photo-oxidation. WHAT IS LIGHT REACTION? Light reactions or the ‘Photochemical’ phase include light absorption, water splitting, oxygen release, and the formation of high-energy chemical intermediates, ATP and NADPH. Several protein complexes are involved in the process. The pigments are organised into two discrete photochemical light harvesting complexes (LHC) within the Photosystem I (PSI) and Photosystem II (PS II). These are named in the sequence of their discovery, and not in the sequence in which they function during the light reaction. The LHC are made up of hundreds of pigment molecules bound to proteins. Each photosystem has all the pigments (except one molecule of chlorophyll a) forming a light harvesting system also called antennae. These pigments help to make photosynthesis more efficient by absorbing different wavelengths of light. The single chlorophyll a molecule forms the reaction centre. The reaction centre is different in both the photosystems. In PS I the reaction centre chlorophyll a has an absorption peak at 700 nm, hence is called P700, while in PS II it has absorption maxima at 680 nm, and is called P680. a: Graph showing the absorption spectrum of chlorophyll a, b and the carotenoids b: Graph showing action spectrum of photosynthesis c: Graph showing action spectrum of photosynthesis superimposed on absorption spectrum of chlorophyll a