Photosynthesis

 Under natural conditions, the acidity of green shoots of some non-halophytic succulents and semi-succulent plants increase at night and decrease during the daytime. This diurnal change in the acidity was first discovered in Bryophyllum belonging to the family Crassulaceae. Therefore, it is called Crassulacean Acid Metabolism (CAM) . This metabolism occurs only in green organs and it is quite common in the plants belonging to the families like Crassulaceae, Cactaceae, Orchidaceae, Bromeliaceae, Liliaceae, Asclepiadaceae, Vitaceae, and Euphorbiaceae. All such plants are called Crassulacean Acid Metabolism plants(CAM plants). Most CAM plants possess the succulent habit. A typical example of a commercial crop possessing a CAM pathway is pineapple. Mechanism of CAM cycle The sequence of reactions taking place in the CAM pathway is shown in Fig.20. 1. During the night when stomata are open, CO2 is fixed through the action of PEP carboxylase to Malic acid. This is accomplised in two steps :

 

 They are more than one pathway of dark reactions of CO2fixation leading to the synthesis of carbohydrate. Following are the three types of pathways, which are well established now. 1. Calvin cycle (C3 plants) 2. Hatch and Slack cycle (C4 plants) 3. CAM cycle (CAM plants) 1. C3 CYCLE (or) CALVIN CYCLE This dark reaction process of photosynthesis has been named variously such as -Calvin Cycle, Bassham and Calvin Cycle, Bassham and Calvin Cycle, Blackman reaction, Carbon assimilation, Path of carbon in photosynthesis, Reductive Pentose Phosphate Cycle. Calvin cycle consists of two important steps : i. Synthesis of carbohydrate ii. Regeneration of RuDP Sequences of reaction taking place in Calvin (C3) cycle are furnished below : i. Synthesis of Carbohydrate 1. CO2 is first accepted by RuDP and forms an unstable 6-carbon compound from which two molecules of phosphoglyceric acid (PGA) are formed. The reaction is regulated by an enzyme called carboxydismutase or RuDP carboxylase ( Rubisco ).

 Amon et al.(1954) demonstrated yet another kind of phosphorylation. They observed that: 1. In the absence of CO2 and NADP, it can produce ATP from ADP and PI in the presence of FMN (Flavine Mono Nucleotide) or vitamin K and O2. 2. The process is very simple. 3. It requires no chemical change except for the formation of ATP and water. 4. This is also called Oxygen-dependent FMN catalyzed photophosphorylation or pseudocyclic photophosphorylation involving reduction of FMN with the production of O2. II. PATH OF CARBON IN PHOTOSYNTHESIS (OR) DARK REACTION (OR) BLACKMAN’S REACTION (Activities found in stroma). The dark reaction of photosynthesis is purely enzymatic and slower than the primary photochemical reaction. It takes place in the stroma portion of chloroplast and is independent of light, i.e., it can occur either in the presence or absence of light. The main credit for investigating the sequences of dark reactions in photosynthesis goes to Melvin Calvin who was awarded Nobel Prize

S.No. Cyclic photophosphorylation                              Non-cyclic Photophosphorylation 1. Associated with PS I                                                     Associated with both PS I and PSII 2. Electron expelled from chl molecule                            Electron expelled from chl molecule is cycled back.                                                                   is not cycled back. Its loss is compensated                                                                                        by electrons coming from the photolysis of water. 3. Photolysis of water and O2                                         Photolysis of water and O2 evolution evaluation does not take place                                        take place. 4. Phosphorylation takes place at two places                Phosphorylation takes place only at one place. 5. Two ATP molecules are produced.                             one ATP and one NADPH2 are produced. 6. NADP is not reduced           

 1. Light energy has been converted into chemical energy during the primary photochemical reaction and is trapped in ATP and NADPH2 molecules. 2. This chemical energy is finally stored in carbohydrate molecules when ATP and NADPH2 (assimilatory powers) are utilized in the dark reaction of photosynthesis in reducing CO2 to carbohydrates.

 Significance of Cyclic Photophosphorylation 1. When cyclic photophosphorylation alone operates, the CO2 assimilation drops down because of the shortage of reduced NADP (NADPH2). 2. It generates only ATP molecules and as such can not drive dark reactions of photosynthesis. 3. It is an important system in providing ATP for synthetic processes (other than photosynthesis)like a synthesis of protein, lipids, nucleic acids, and pigments within chloroplasts. Limitations of Cyclic Photophosphorylation 1. This system operates if the activity of the PSII is blocked. 2. Under these conditions : a. only PS I remains active b. Photolysis of water does not take place c. blockage of non-cyclic ATP formation causes a drop in CO2 assimilation in dark reaction and, therefore, d. there is a shortage of reduced NADP. (i.e., NADPH2) 2. Non-cyclic Photophosphorylation It occurs in green plants and involves both PS I and PSII * During this process, the electron is excited by the absorption of a photon (

 The process of Photosynthesis is a complicated oxidation-reduction process ultimately resulting in the oxidation of water and reduction of CO2 to carbohydrates. The mechanism of photosynthesis can be studied with the following two processes : I. Primary Photochemical Reaction or Light reaction or Hill’s reaction, and II. Dark reaction or Blackman’s reaction or Path of carbon in photosynthesis Importance of two processes photosynthesis In the primary photochemical reaction, assimilatory powers (NADPH2 + ATP) are generated and O2 is released. These assimilatory powers are utilized in the dark reaction during which process CO2 is reduced to carbohydrates I. PRIMARY PHOTOCHEMICAL REACTION (OR) LIGHT REACTION (OR) HILL’S REACTION (Activities found in thylakoids or grana) In 1937, Robert Hill demonstrated that isolated chloroplasts evolved O2 when they were illuminated in the presence of suitable electron acceptors, such as ferricyanide. The ferricyanide is reduced to ferrocyanide by pho

Chlorophylls : * They are magnesium porphyrin compounds * The porphyrin ring consists of four pyrrole rings joined together by CH bridges. * A long chain of C atoms called as Phytol chain is attached to the porphyrin ring. * Molecular formulae :                         Both the chlorophylls (a and b) consist of : Mg-Porphyrin head which is hydrophilic in nature and a Phytol tail possessing lipophilic properly. Difference between two chlorophylls In chlorophyll b there is -CHO (aldehyde) group, instead of a _CH3 (acetyl) group in chlorophyll a molecule. Synthesis of Chlorophyll * Chlorophyll is formed from protochlorophyllide in the presence of light with the loss of two hydrogen atoms as follows :                       Protochlorophyll               →                Chlorophyll                                                                -2H Location of Chlorophyll Pigments in Chloroplast The photosynthetic pigments are located in grana portions of the chloroplasts of higher plan

  Each chloroplast is surrounded by a double membrane system. The external surface of the outer membrane is smooth whereas the inner membrane is thrown into lamellated structure showing two distinct regions, called Stroma and Grana Lamellae. Stroma Stroma is the main site for the dark reaction of photosynthesis. It forms the matrix (ground substance) of the chloroplast. In the matrix, the lamellae are loosely arranged. The lamellae found in the stroma are called as Stroma Lamellae . Besides these structures, ribosomes and osmophilic granules are also found in the stroma. Besides photosynthesis, lipid, protein, and nucleic acid metabolisms are also found to take place in the stroma region of the chloroplast. Grana Granum (= singular) is the most but complicated portion of the chloroplast. Light reaction of photosynthesis takes place only in the grana region of chloroplasts. About 40-60 grana are present in each chloroplast. In the granum region, the lamellae are compactly arranged just

 The simplest type of photosynthetic apparatus is observed in prokaryotic cells such as bacteria and blue-green algae in which they are represented by isolated and freely lying photosynthetic lamellae. In other algae, there is no distinction between grana and stroma regions. In these organisms, the photosynthetic lamellae are found closely arranged running parallel to each other; and carotenoid pigments dominate in the photosynthetic apparatus. These are called Chromatophores. The pigments are evenly distributed in chromatophores and are primitive in nature. In higher plants, well-developed photosynthetic apparatus is found which is commonly called as Chloroplasts . A chloroplast is an advanced, well-organized, and complicated photosynthetic apparatus.

 Photosynthesis is an oxidation-reduction process in which water is oxidized and CO2 is reduced to carbohydrates and water, and O2 molecules are by-products. The reduction of CO2 to carbohydrate level needs assimilatory powers such as ATP and NADPH2 . Reduction of CO2 occurs in dark ( light-independent ) but the production of assimilatory powers takes place in the presence of light and the process is light-dependent. Thus, Photosynthesis consists of two phases : 1. Light-dependent phase (light reaction or Hill reaction) and ` 2. Light-independent phase (dark reaction or Blackman’s reaction).