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PHOTOSYNTHESIS IN HIGHER PLANTS

     Important Terms:

·      Absorption Spectrum: A curve showing the amount of different wavelengths of light absorbed by a substance.

·      Action Spectrum: A curve depicting the relative rates of photosynthesis at different wavelengths of light.

·      Accessory Pigments: The pigments which handover the energy absorbed by them to the chlorophyll.

·      CAM Plants: Refers to the succulent and some other plants which show crassulacean acid metabolism.

·      Chemosynthesis: The process of carbohydrate synthesis in which the organisms use chemical reactions to obtain energy from inorganic compounds.

·      Fluorescence: A property of emission of radiations absorbed by a substance.

·      Photocentre: (also known as reaction centre) Refers to a primary pigment (chlorophyll) molecule where light energy is converted into chemical or electron energy.

·      Photolysis: Light dependent splitting of water molecule in the chloroplasts.

·      Photon: A tiny particle of light containing energy called quantum (hv).

·      Photophosphorylation: A light driven or energized synthesis of ATP from ADP and inorganic phosphate.

·      Photosystem: Light harvesting systems of pigment molecules present in thylakoids of the chloroplast. They are photosystem I and photosystem II.

·      Photosynthetically Active Radiation (PAR): The visible spectrum of light between 400 nm to 700 nm which can be absorbed by the chlorophyll molecules.

·      Photosynthetic Carbon Oxidation Cycle: Photorespiration or C2 cycle. Light dependent utilization of O2 and release of CO2 by the photosynthetic organs of a plant.

·      Photosynthetic Carbon Reduction Cycle: C3 cycle.

·      Phosphorescence: Delayed emission of long wave radiations from an irradiated and activated substance.

·      Primary Pigments: Photosynthetic pigments, which convert light energy into chemical or electron energy.

·      Quantum Yield: Number of oxygen molecules released per photon or quantum of light. Its value is 1/8 to 1/10 (evolution of one molecule of O2 requires 8-10 quanta).

ADDITIONAL INFORMATION

·      Photosynthesis is an oxidation reduction process where water is oxidized to O2 and CO2 is reduced to (CH2O)n.

·      The Redox Potential of H2O/O2 is + 0.8V and that of CO2/(CH2O)n is -0.4V. The difference is 1.2 V per electron transferred. Redox potential is the tendency of the system (atom/molecule) to give or take up electrons. The symbol used to express the standard redox potential at pH 7 is E0. The compound having low redox potential tend to lose electrons and those having high redox potential gain electrons.

·       Herbicides such as CMU ( 3 p.chlorophenyl 1,1 dimethyl urea) and DCMC (3,4- Dichlorophenyl 1,1 dimethyl urea) block electron transport between quinone (Q) to plastoquinone (PQ), during non-cyclic electron transport.

·      Quantum Yield: Number of oxygen molecules  released per photon or quantum of light is called quantum yield. Its value is 1/8 to 1/10 (10-12.5%). In other words, evolution of one molecule of oxygen or consumption/ fixation of one molecule of CO2 requires 8-10 quanta. The number of light quanta used for producing one molecule of O2 or fixation of one molecule of CO2 is called quantum requirement.

·      Chemical Coupling  Hypothesis: Synthesis of ATP is connected to liberation of energy while electrons pass down hill over the electron transport chain.

·      Chemi-Osmotic Hypothesis (Mitchell 1961): There is accumulation of protons which pass through membrane bases ATPase (for coupling factor, elementary particle) catalyzing synthesis of ATP from ADP and inorganic phosphate.

·      P-protein: It is synthesized in companion cells ( absent in gymnosperms). In sieve tube cells it has a role as defence against phloem feeding insects and sealing of damaged sieve tubes by plugging sieve plates.

·      Thylakoid Reactions: Reactions of photochemical phase as they occur over the thylakoids.

·      Stroma Reactions: Reactions of biosynthetic phase, as they occur in stroma.

Mr. A. KINGSLIN M.Sc, BEd, Phd,(doing)
Post Graduate Teacher in Botany
St. Mary Goretty Hr. Sec School, Manalikarai 
Kanyakumari district





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