Title PHOTOSYNTHESIS IN HIGHER PLANTS.pdf ✅

 Digital www.allendigital.in [ 89 ] 1. Introduction : • "Photosynthesis is a Physico - chemical or photo–biochemical process in which the organic compounds (carbohydrates) are synthesised from the inorganic raw material (H2O & CO2) in the presence of light & pigments. O2 is evolved as by product or one of the net products". • Light energy is converted into chemical energy by photosynthesis. • Photosynthesis is a redox reaction during which oxidation of H2O occurs (as it provides H+ and e–) during light reaction and reduction of CO2 occurs (as it accepts H+ & e–) during dark reaction (biosynthetic phase) • Photosynthesis is an Anabolic (synthesising) & Endergonic (Energy absorbing) process. 2. What do we know ? • Some simple experiments show that chlorophyll (green pigment of the leaf), light and CO2 are required for photosynthesis to occur. • Look for starch formation in two leaves – a variegated leaf or a leaf that was partially covered with black paper and one that was exposed to light. On testing these leaves for starch it was clear that photosynthesis occurred only in the green parts of the leaves in the presence of light. 6CO2 + 12H2O C6H12O6 + 6H2O + 6O2 Sunlight Chlorophyll Starch formation in two leaves Variegated leaf Leaf partially covered with black paper Leaf attached with plant Area of leaf unaffected with lodine Starch test Black paper strip Clip to hold paper Areas of leaf affected with Iodine Moll's Half leaf experiment Variegated leaf experiment Split cork KOH solution Wide mouth bottle Bluish part Colourless parts Potted plant (destarched) Photosynthesis in Higher 03 Plants
NEET : Biology [ 90 ] www.allendigital.in  Digital • Another experiment is the Moll's half-leaf experiment, 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 starch later in the two halves of the leaf, the exposed part of the leaf tested positive for starch while the portion that was in the tube, tested negative. This shows that CO2 is required for photosynthesis. (i) J. Priestley : • Joseph Priestley (1733-1804) in 1770 performed a series of experiments that revealed the essential role of air in the growth of green plants. • 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 hypothesized as follows: plants restore to the air whatever breathing animals and burning candles remove. (ii) Jan Ingenhousz (1779) : • Using a similar setup as the one used by Priestley, but by placing it once in the dark and once in the sunlight, Jan Ingenhousz (1730 - 1799) showed that sunlight is essential to the plant process that somehow purifies the air fouled by burning candles or breathing animals. O2 Presence of light Absence of light CO2 OXYGEN BUBBLES Test tube Beaker Water Funnel Hydrilla plant
Photosynthesis in Higher Plants  Digital www.allendigital.in [ 91 ] 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. 3. Early Experiments : (i) 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. (ii) Cornelius van Niel (1897- 1985) : • A milestone contribution to the understanding of photosynthesis was that made by a microbiologist, Cornelius van Niel, 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. • H2A=suitable oxidizable compound or H Donor. • 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. (iii) Ruben & Kamen (1941) : • Used O18 (radioisotopic technique) to show experimentally that O2 in photosynthesis released from water. (iv) R. Hill : • Detailed study of light reaction in isolated chloroplast of Stellaria plant. He illuminated the isolated chloroplasts of Stellaria plant in the presence of hydrogen acceptors (ferricyanides) in the absence of carbon dioxide. The chloroplasts evolved oxygen. A=Hydrogen acceptor (Hill reagent) DCPIP (Dichlorophenol indophenol) is a blue colour dye, which become colourless on reduction. 2H2A + CO2 2A + CH2O + H2O Light 6CO2 + 12H2O18 C6H12O6 + 6H2O + 6O2 18 Light Chlorophyll 2A + 2H2O 2AH2 + O2 Light Chlorophyll Hill reagent Natural Artificial e.g. NADP e.g. Potassium ferricyanide, Benzoquinone, chromate and DCPIP etc.
NEET : Biology [ 92 ] www.allendigital.in  Digital (v) Robert Hill and Bendall : They are credited for : Detailed study of light reaction and proposed Z scheme. 4. Where Does Photosynthesis Take Place : • Photosynthesis does take place in the green leaves of plants but it does so also in other green parts of the plants. There is a clear-cut division of labour within the chloroplast. Chloroplast solution DCPIP Al foil Chloroplast solution Chloroplast solution + DCPIP (light) Chloroplast solution + DCPIP (covered with Al foil) Outer membrane Inner membrane Stromal lamella Grana Stroma Ribosomes Starch granule Lipid droplet Stroma Thylakoid H2O CO2 O2 sugar LIGHT REACTIONS NADP+ ADP P + i ATP NADPH e– H+ CALVIN CYCLE Chloroplast