PHOTOSYNTHESIS

Photosynthesis is the most important anabolic process by which green plants / autotrophic organisms synthesize complex carbohydrates from CO2 and H2O with the help of light energy. Green pigment chlorophyll is required for this process. Photosynthesis is performed by chloroplast. 3.1 HISTORY OF PHOTOSYNTHESIS : - Aristotle and Theophrastus (320 B.C.) believed that plants obtain all their nourishment from the soil. - Jan Baptista Van Helmont (Belgian Physician) in 1648, concluded by his experiment that weight of plant increases due to water absorbed and not due to soil. - Stephen Hales (1727) discovered that green plants require sunlight and air for their nutrition. - Joseph Priestley (1733 - 1804) showed that plants take up CO2 from the atmosphere and release O2 - Jan Ingenhousz (1730-1799) confirmed Priesteey's work. He discovered that O2 was released only in sunlight and only by green parts of plant. - Theodore de Saussure (1804) found that water is essential for photosynthesis. - Pelletier and Caventon (1818) discovered and named the green pigment 'chlorophyll' EQUATION FOR PHOTOSYNTHESIS : It was given by C.B. Van Niel of Stanford University, U.S.A. The above simple equation is now written as : 3.2 SITE FOR PHOTOSYNTHESIS : It occurs in green parts of the plant, mostly leaves. Leaves have specialized cells - Mesophyll cells which contain Chloroplasts. Chloroplasts are the actual sites of photosynthesis. The thylakoids in the chloroplast contain most of the machinery for photosynthetic reactions, like pigment- chlorophyll. A pigment is a substance which absorbs light of different wavelengths. Chlorophyll absorbs light in violet, blue and red wavelengths of visible spectrum of light. This portion of spectrum ranges from 400 nm - 700 nm, and is called as 'Photosynthetically active radiation' (PAR). STRUCTURE OF CHLOROPHYLL :  It is main photosynthetic pigment, is green in colour and is of many types. It can be chl-a, b, c, d, e bacteriochlorophyll a, b, c, d, e and bacterioviridin. Bacteriochlorophyll is found in bacteria. Chl-a & b are widely found. Chl - c, d, e occur in certain algae. Chl -a is made up of four Pyrrole rings and a central core of Mg. Each pyrrole has 4 carbon atoms and 1 Nitrogen. This tetrapyrrole ring is called 'Porphyrin Head' 'Phytol tail' is attached to porphyrin head. Phytol tail is made of 20 carbon alcohol attached to carbon 7 position of pyrrole ring 4. So one pyrrole ring gets esterified with alcohol (C20H39,OH). When chl - b absorbs light, it gets excited and transfers its energy to chl - a molecule. Chl - a converts light energy to energy. Thus chl - a acts as Reaction Centre. Accessory Pigments like carotenoids also capture light and transfer it to reaction centers for conversion to electrical energy. Reaction centres and Accessory pigments, together, form 'Photosystem' 3.3 PHOTCHEMICAL & BIOSYNTHETIC PHASES :  A photosynthetic unit is the smallest group of pigment molecules which take part in a photochemical reaction. It has a 'reaction centre' which is fed by 'pigment molecules'. Main pigment is chlorophyll (chl) which occurs in many forms like chl-a. Chl -a has further many forms depending upon the wavelength which shows absorption peak. eg : chl-a 673 (shows absorption peak at 673 nm), chl-a 683 (P680) - shows absorption peak at 683 nm : chl-a 703 (P700) -shows absorption peak at 703 nm. 'P' stands for pigment. These pigments occur in thylakoids as units called 'Photosystems'. A photosystem has 250-400 pigment molecules. There are 2 types of photosystems: PSI and PS II. PSI is located in storma & grana of chloroplast, has chl-700 (P700) as reaction centre and comprises of 200-400 chlorophylls. PS.II is located in grana, has chl-683 (P683) as reaction centre, consists of about 200 chlorophyll molecules. Both systems, PSI & PSII interact with each other, trap light energy, convert it into chemical energy which is then stored as ATP. ELECTRON TRANSPORT CHAIN : Electron Transport chain includes the photosynthetic reactions initiated by light. It was formulated by Robert Hill in 1939. P680 (PSII) absorbs light and gets excited. It transfers its electrons to an electron accepting molecule and becomes an oxidizing agent. Then it splits a water molecule to release O2. This splitting is called Photolysis. Mn2+, Ca2+ and Cl - play a role in it. Electrons generated in this process are picked up by P680 again. Then it donates these electrons to downstream components in electron transport chain. Similarly, PSI(P700) absorbs energy, gets excited and transfers its electron to primary electron acceptor. Thus P700 gets oxidised. Reduced electron acceptor transfers its electrons to Ferredoxin and Ferredoxin-NADP reductase to reduce NADP to NADPH2. NADPH2 reduces CO2 to carbohydrates in the presence of ATP which is produced during ETS. PHOTOPHOSPHORYLATION : During electron transfer in ETS, some energy released is used up in the synthesis of energy rich compounds like ATP from ADP and ip (inorganic Po4). The production of ATP from ADP in th presence of light is called Photophosphorylation. Photophosphorylation occurs by two ways : (i) Non - cyclic (ii) Cyclic (i) NON-CYCLIC PHOTOPHOSPHORYLATION : In this, the electron flow is unidirectional and electrons released by pigment of PSI and PSII are never received back. ATP is produced. Electrons flow from water to pigment of PSII, then to pigment of PSI and finally to NADP. (ii) CYCLIC PHOTOPHOSPHORYLATION : It involves only PSI and the electrons received from PSI are returned back to it. Cyclic phosphorylation takes place when non-cyclic photophosphorylation can be stopped by illuminating chloroplast of PSII with light of wavelength more than 680 nm. Thus electron from H2O to NADP stops. CO2 fixation stops. NADPH is not oxidised & so NADP is not available. As NADP is not available, electrons from PSI are not passed to NADP and instead transferred back to P700. BIOSYNTHETIC PHASE : ATP & NADPH2 are essential for conversion of CO2 to carbohydrates. These reactions occur in stroma of choloroplast and are called as 'Carbon Reactions' (or Dark Reactions). Dark reaction can occur by C3 pathway which is also known as Calvin cycle. CALVIN CYCLE (C3 pathway) : The complete sequence of reactions were given by Melvin Calvin and Benson. They worked on algae Chlorella. The steps are : (i) Carboxylation : 6 molecules of Ribulose -5-PO4 react with 6ATP. 6 Ribulose -5-PO4 + 6ATP  6 Ribulose 1-5 diPO4 + 6 ADP. - 6 Ribulose -1-5 diPO4 react with 6CO2 and 6H2O to form 3 Phosphoglycerate (PGA). PGA is the first 3-C stable product, so this pathway is called C3 pathway. - 6 Ribulose -1-5 diPO4 + 6CO2 + 6H2O  12 PGA - Enzyme acting is Rubisco (Ribulose bisphosphate carboxylase oxygenase) (ii) Reduction of PGA : - 12 molecules of 3 PGA are reduced to 12 molecules of 3 PGAL (Phosphoglyceraldehyde) by using 12 NADPH2 & 12 ATP. 12 3 PGA + 12 NADPH2 +12 ATP  12 3 PGAL + 12 ADP + 12 NADP + 12 iP +12 H2O. Enzyme - Dehydrogenase Kinase. - 3 PGAL is isomeric with dihydroxyacetone 3PO4. (iii) Formation of Hexose sugar : - 3 molecules of 3 PGAL and 3 molecules of dihydroxyacetone 3 PO4 combine to form 3 molecules of Fructose 1-6 diPO4. 3 PGAL + 3 dihydroxyacetone 3PO4  3 Fructose 1-6 diPO4. Enzyme - Aldolase. - Fructose 1-6 diPO4 converts to fructose -6PO4 by removing iP. (iv) Regeneration of Ribulose -5-PO4 : 1. Remaining 2 molecules of Fructose -6PO4 combine with 2 molecules of PGAL to form Erythrose -4PO4 and Xylulose -5PO4. 2 Fructose -6PO4 + 2 PGAL  2 Erythrose - 4PO4 + 2 xylulose 5 PO4 2.  3. Each Sedoheptulose 1-7 diPO4 looses its PO4. 4. 2 Sedpheptulose 7 PO4 combine with 2 remaining molecules of PGAL. 2 Sedpheptulose -7 PO4 + 2 PGAL  2 Ribose - 5PO4 + 2 Xylulose -5-PO4 5.  2 Ribose 5PO4 is isomeric with 2 Ribulose -5PO4. 6. 4 molecules of xylulose 5 PO4 are isonmerised Thus, 6 molecules of Ribulose 5 PO4 are regenerated during C3 cycle. 3.4 C4 PATHWAY : (Hatch and Slack Cycle) It was worked out by Hatch & Slack (1965, 67), so it is also called Hatch & Slack cycle. They found that in several plants like sugarcane, Maize, the first stable product of CO2 fixation is Oxaloacetic acid which is a 4-carbon compound. So it is also known as C4 pathway and such plants are called as C4 plants. C4 plants are found is tropical / sub tropical regions, their leaves have Kranz anatomy, these have dimorphic chloroplast (i.e. chloroplast in mesophyll cells are granal and in bundle sheath cells are agranal). Mechanism : The reactions are : 1. In mesophyll cells : (i) Phosphoenol pyruvic acid (PEP) combines with CO2 to form a stable 4-C compound - oxaloacetic acid. (ii) Oxaloacetic acid is reduced to Malic acid. 2. In bundle Sheath cells : (i) Malic acid is transported to bundle sheath. Here it is decarboxylsed to form Pyruvic acid. (ii) CO2 enters into calvin cycle, combines with Ribulose, 5 diPO4 to form Phosphoglyceric acid and hexose sugar. 3. In mesophyll cells : Pyruvic acid returns to mesophyll cells where it is converted back to phosphoenol pyruvic acid. 3.5 CRASSULACEAN ACID METABOLISM : (CAM PATHWAYS) : Succulent plant fix CO2 in dark. These plants grow in semi arid conditions. It is called CAM pathway because it was observed first in plants of family Crassulaceae eg. Bryophyllum It occurs in 3 steps : 1. Acidification : During dark (night), CO2 is absorbed from outside. Primary acceptor is phosphoenol pyruvic acid (PEP). 2. De - acidification : In presence of light, organic acids move out of vacuoles and CO2 is released. High temperature favours it. 3. Calvin Cycle : Released CO2 is taken by Ribulose 1-5 diPO4 to perform Calvin cycle. CAM pathway is an adaptation in certain plants to carry out photosynthesis without much loss of water. 3.6 FACTORS AFFECTING PHOTOSYNTHESIS : Photosynthesis is influenced by environmental as well as genetic factors. But according to F.F. Blackman, photosynthesis is limited (influenced) by the slowest step in pathway. This became popularly known as Blackmans Law of Limiting Factors (FF Blackman, 1905) ( or Principle of Limiting Factors). It states that : "When a process is conditioned as to its rapidity by a number of separate factors, the rate of process is limited by the pace of the slowest factor". Various factors controlling photosynthesis are : 1. LIGHT : Light intensity, Light quality and Light duration effect photosynthesis rate. As the intensity of light increases, the rate of photosynthesis also increases. Light wavelength between 400nm-700nm is most effective. This is called Photosynthetically active radiation (PAR). At higher light intensities, rate of photosynthesis decreases because chlorophyll gets destroyed and other factors become limiting. Light quality : Blue & Red light are the best for photosynthesis Light duration : Average light of 10-12 hrs/day results in higher rate of photosynthesis. 2. CO2 : Conc. of CO2 in atmosphere is 0.03% or 360 ppm. Increase in conc. upto 0.1% increase the rate of photosynthesis Above 0.1%, the photosynthesis rate declines. When conc. of CO2 is reduced, then 'Compensation point' or 'Threshold value' is reached at a certain stage when CO2 fixed in photosynthesis becomes equal to CO2 evolved in respiration and photorespiration. At this point, plants stop absorbing CO2 from atmosphere. This value is 50-100 ppm in C3 plants and 0-10 ppm in C4 plants. 3. Soil Water : Water deficiency decreases rate of photosynthesis as plants undergo water stress, stomata close, CO2 supply decreases and leaf expansion decreases. This reduces the photosynthetic surface area. 4. Nutrient supply : Most important nutrient is Nitrogen as it is constituent of chlorophyll and many enzymes. All essential elements effect rate of photosynthesis. 5. Temperature : Optimum temp. is 20-350C. Above this temp, rate of photosynthesis shows initial increase and then it declines with time. Temp does not influence light reaction but affects enzyme-controlled dark reactions. At higher temperature, enzymes are destroyed. 6. Leaf factors : Rate of photosynthesis increases with the age of leaf till it becomes maximum at full maturity. After that, it declines. If leaf turns yellow (loss of chlorophyll), Photosynthesis stops. Hormones (Cytokinins, Giberellins) increase the rate and Abscisic acid reduces the rate of photosynthesis. 3.7 TRANSLOCATION OF PHOTOSYNTHESIS : Energy rich compounds formed during photosynthesis (Photosynthates/Photoassinilates) are transported out of leaf to roots, stem, developing seeds and grains. This is called Translocation. It occurs through phloem. Sucrose is the main form of carbohydrate which is translocated . It is non reducing sugar and is stable chemically (i.e. does not react with other substances during translocation). 3.9 SIGNIFICANCE OF PHOTOSYNTHESIS : 1. It links physical and biological world by converting solar energy into organic matter which is the bulk of a living organism. 2. It produces O2 which is vital for life. 3. Agricultural productivity depends on photosynthesis.  3.9 CHEMOSYNTHESIS : Some aerobic bacteria do not require light for carbohydrate synthesis. They do so by using energy from chemical reactions. The process of carbohydrate synthesis, in which organisms use chemical reactions to obtain energy from inorganic compounds is called Chemosynthesis. eg.Bacteria Nitrosomonas oxidizes NH3 to nitrite. Energy released during the reaction is used by bacteria to convert CO2 to carbohydrates. These bacteria are called Chemosynthetic autotrophs.