Tiêu đề Med-RM_Bot_SP-2_Ch-10-Photosynthesis in Higher Plants.pdf ✅

Chapter Contents Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 Contributions of Some Scientists Photosynthetic Pigments Photosynthetic units/Pigment systems Mechanism of photosynthesis The Electron Transport Non-cyclic Photophosphorylation Splitting of water Cyclic Photophosphorylation Chemiosmotic Hypothesis Where are ATP and NADPH used? The C4 Pathway (Hatch and Slack pathway) Photorespiration Factors affecting Photosynthesis It is a process in which green parts of the plants synthesize or manufacture complex organic food substances (carbohydrates) using carbon dioxide and water in the presence of sunlight and release oxygen as a by-product. In this process, energy from the sun is converted into chemical energy. It is an anabolic, endergonic (requiring energy) and oxido-reduction process. A simple equation representing the process is: Light 2 2 6 12 6 2 2 Chlorophyll 6CO + 12H O C H O + 6H O + 6O  CONTRIBUTIONS OF SOME SCIENTISTS Moll’s half leaf : Showed that CO2 is required for photosynthesis. experiment Joseph Priestley : He revealed the essential role of air in the growth of green plants through several experiments and discovered oxygen. Jan Ingenhousz : He showed that sunlight is essential for the plants. Julius Von Sachs : Found that glucose is made in the green parts of plants. T.W. Engelmann : Described first action spectrum of photosynthesis by using Cladophora. Cornelius Van Niel : Inferred that oxygen evolved by the green plants comes from H2O and not from carbon dioxide. Ruben, Kamen et. al : Proved that O2 evolve during light reaction comes from H2O not from CO2. Chapter 10 Photosynthesis in Higher Plants
102 Photosynthesis in Higher Plants NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 PHOTOSYNTHETIC PIGMENTS Photosynthesis takes place in chloroplasts. Pigments are substances that have an ability to absorb light, at specific wavelengths. A chromatographic separation of the leaf pigments shows that the colour of leaves is due to four pigments : (i) Chlorophyll a – Bright or blue green (ii) Chlorophyll b – Yellow-green (iii) Xanthophylls – Yellow (iv) Carotene – Yellow to yellow-orange Of these, chlorophyll-a is the primary photosynthetic pigment. Chlorophyll Pigments Chlorophyll has a tadpole like structure. It consists of a porphyrin head and a phytol tail. Porphyrin head Phytol tail Porphyrin head : (i) All chlorophylls have a complex ring structure chemically related to the porphyrin-like groups found in haemoglobin and cytochromes. (ii) Site of the electrons rearrangements when the chlorophyll is excited. (iii) A cyclic tetrapyrrolic structure with non-ionic magnesium atom. Phytol tail : (i) A long hydrocarbon tail is almost always attached to the ring structure. (ii) Anchors the chlorophyll to the hydrophobic portion of the thylakoids. Major types of chlorophylls are chlorophyll a, b, c, d, e; bacteriochlorophyll a and b etc. Accessory Pigments All pigments other than chlorophyll a are called accessory pigments. These have two major roles in photosynthesis : (i) They absorb light of different wavelengths and transfer the energy to chlorophyll molecules, thus they are also called antenna molecules. This enables a wider range of wavelength of incoming light to be utilised for photosynthesis. Chlorophyll b accounts for about one-fourth of total chlorophyll content. (ii) Carotenoids protect plant from excessive heat and prevent photo-oxidation (oxidative destruction by light) of chlorophyll pigments. Thus, they are also called “Shield Pigments”. Let us study the graph showing ability of pigments to absorb lights of different wavelengths. Absorption spectrum : The graphic curve showing the amount of energy of different wavelengths of light absorbed by a substance/pigments.
NEET Photosynthesis in Higher Plants 103 Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 Chlorophyll b Carotenoids Chlorophyll a Absorbance of light by chloroplast pigments Fig. : Graph showing the absorption spectrum of chlorophyll a, b and the carotenoids Action spectrum : The graphic curve showing the relative rates of photosynthesis at different wavelengths of light. Fig. : Graph showing action spectrum of photosynthesis Action spectrum of photosynthesis corresponds closely to absorption spectra of chlorophyll a showing that chlorophyll a is the chief pigment associated with photosynthesis. Rate of photosynthesis Absorption Light absorbed 400 500 600 700 Wavelength of light in nanometres (nm) Fig. : Graph showing action spectrum of photosynthesis superimposed on absorption spectrum of chlorophyll a 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. These graphs depict that maximum photosynthesis occurs at the wavelength at which there is maximum absorption by chlorophyll a i.e., in the blue and red regions. PHOTOSYNTHETIC UNITS / PIGMENT SYSTEMS These are group of pigments molecules which take part in the conversion of light energy into the chemical energy. The photosynthetic units are called Photosystem I (PS-I) and Photosystem II (PS-II). Each unit has a reaction centre of a specific chlorophyll a molecule which absorbs light energy of long wavelength.These center can release electron upon absorption of energy. In PS-I, the reaction centre chlorophyll a has an absorption peak at 700 nm, hence is called P700, while in PS-II, reaction centre has an absorption maxima at 680 nm and is called P680.
104 Photosynthesis in Higher Plants NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 Photon Reaction centre Pigment molecules Primary acceptor Fig. : The light harvesting complex Reaction centre is sorrounded by number of light harvesting pigment (LHP) molecules. These are also called antenna molecules. These absorb photons of different wavelength and transfer this energy to reaction centre. Harvesting molecules occur in form of specific complexes called light-harvesting complexes (LHC) called LHC-I and LHC-II. The pigment molecules of these complexes are bound to proteins. These help to make photosynthesis more efficient. PS-I PS-II Reaction centre (P ) 700 Reaction centre (P ) 680 LHC-I LHC-II LHP Proteins LHP Proteins Some of the important differences between the two photosystems are : The reaction centre is P . PS I lies on the outer surface of the thylakoids. Found in both grana and stroma lamellae. Participates in both cyclic as well as non-cyclic flow of electrons. Not associated with splitting of water. 700 The reaction centre is P . PS II occurs on the inner surface of the thylakoids. Found in grana lamellae only. It is involved only in non-cyclic flow of electrons. Associated with splitting of water and release of O . 680 2 1. 2. 3. 4. 5. 1. 2. 3. 4. 5. Photosystem I / Pigment system I Photosystem II / Pigment system II MECHANISM OF PHOTOSYNTHESIS Photosynthesis occurs in two phases (a) light reaction : Solar energy is trapped by chlorophyll and is stored in the form of chemical energy (ATP) and as reducing power (NADPH). (b) Dark reaction: Reducing capacity of NADPH and the energy of ATP are utilised in the conversion of CO2 to carbohydrates. THE ELECTRON TRANSPORT Electron transport chain is a series of electron carriers over which electrons pass in a downhill journey releasing energy at every step that is used in generating an electrochemical proton gradient which helps in synthesising ATP. Based on path of electron, associated photophosphorylation can be identified as non-cyclic and cyclic photophosphorylation.