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Photosynthesis: I Course: Plant Physiology and Biochemistry (M.Sc)

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Photosynthesis: I Course: Plant Physiology and Biochemistry (M.Sc) Photosynthesis: I Course: Plant Physiology and Biochemistry (M.Sc) Pratibha Singh Department of Botany Photosynthesis is a process by which some unique living organism converts light energy in to chemical energy. The light is used to produce reducing equivalents (NADPH) and these reducing equivalents are used in the process of reduction of CO2 to sugars. : algae, blue green algae, plants, sulfur bacteria (1.1) Here H2A is electron donor CO2 is electron acceptor H2A =H2O in oxygenic (Oxygen releasing) photosynthetic organism; algae and plants H2A=H2S in anoxygenic photosynthetic bacteria; purple sulfur bacteria Equation No. 1.1 was given by C.V Neil upon finding that in some bacteria H2S is used as substrate rather than H2O *why oxygen and sulfur?? as both belong to the same group in periodic table, however oxidation of H2S will release lesser energy as compared to H2O as sulfur is more electrpositive and donate electron easily as compared to oxygen. Equation 1.1. also suggest that oxygen evolved by oxygenic photosynthetic bacteria comes from Water molecule and not from CO2 This was further shown by Robert Hill (the scientist who was also involved in studying oxygen binding activity of hemoglobin )in 1937 Hill reaction: 2 H2O + 2 A + (light, chloroplasts) → 2 AH2 + O2 A: electron acceptor 1. Chloroplast isolated 2. Chloroplast treated with light 3. Recorded oxygen evolution using hemoglobin. Hill harnessed the change in spectral property of hemoglobin upon oxygen binding as a measure of oxygen evolution 4. Amount of oxygen released was higher in the presence and absence of artificial electron acceptor (ferric oxalate salts) Conclusion: Photosynthetic cells only evolve oxygen in light when in the presence of extracts of leaves or certain ferric salts, and do not evolve oxygen from carbon dioxide. Proc R Soc London Ser B 127: 192–210 So the overall reaction is: History: 1. Jan Baptist van Helmont (17th century): Plant take water in highest amount from soil as compared to other nutrient 2. Joseph Priestley (1772): Plant releases some gas which keeps the candle burning and mouse alive in a sealed jar. 3. Jan Ingenhousz and Jean Senebier found that the air is only reviving in the day time indicating the role of light in photosynthesis. 4. Antoine-Laurent Lavoisier called that “revived air” is a separate gas, oxygen. 5. Thomas Engelmann (1881): First action spectrum was made. He found that oxygen is evolved only in presence of blue and red light indicating that the pigment that utilizes light to perform photosynthesis is green in colour 6. Frederick Blackman (1905): Law of limiting fatcor. 7. Otto Warburg (1922-23): measured maximum quantum yield in chlorella. 8. Thunberg (1923): hypothesized that Photosynthesis is a redox process. 9. van Niel (1929): Photosynthesis is a redox process in which CO2 is reduced and H2O is oxidized. 10.Emerson and Arnold in 1932: gave the concept of chlorophyll containing photosynthetic unit 11. Robin Hill: Oxygen is evolved from illuminated chloroplast upon oxidation of water. Sam Ruben later confirmed that oxygen is released by oxidation of water using O18 labelled water as substrate 12. Warburg (1943): Concept of Red Drop 13. Emerson (): Enhancement effect Photosynthesis Research 38: 185-209, Role of light in photosynthesis?? General concept 1. Dual nature of light: Light behave as wave and particle both. A transverse wave of velocity 3X108 (C) C= νλ As a wave the photosythetic active radiation (PAR) is in visible light range (400- 700 nm)for higher plants. The particle nature of light suggest that Light is a discrete packet of quanta called photon. Each photon is associated with energy directly proportional to the frequency of light E=hν Here h is Planck’s constant=6.63X10-34 Js The einstein is a unit of energy in one mole of photons . Microenstein per second per square meter is generally used in photosynthesis for PAR 1. Absorption spectrum: An absorption spectrum depicts the amount of light energy taken up or absorbed by a molecule 2. Chlorophyll upon absorption of light change its electronic state Chl + hν → Chl* 3. Chlorophyll a and b is abundant in plants Absorption spectra of some photosynthetic pigments. 1: bacteriochlorophyll a; 2: chlorophyll a; 3:chlorophyll b; 4: phycoerythrobilin; 5:β-carotene. (After Avers 1985.) Action spectrum: It depicts the magnitude of a response of a biological system to light, as a function of wavelength. • First action spectra were measured by T. W. Engelmann in the late 1800s 1. He projected a spectrum of light onto the filamentous green alga Spirogyra 2. observed that oxygen-seeking bacteria introduced into the system collected in the region of the spectrum where chlorophyll 3. pigments absorb that is collection of bacteria was in red and blue region. As absorption and action spectra of chlorophyll a is similar it was proposed that chlorophyll is the molecule involved directly in photosynthesis Source: Taiz and zeiger Structure of chlorophyll The key feature of the chlorophyll structure: 1. Have porphyrin-like ring structure with a magnesium atom (Mg) coordinated in the center, plannar sheet structure which allow the head to burry itself to protein. 2. long hydrophobic hydrocarbon tail that anchors them in the photosynthetic membrane 3. Conjugate double bond that allow these molecule to absorb light Carotenoids : Carotenoids are linear polyenes that serve as both antenna pigments and photoprotective agents. Bilin pigments:are open-chain tetrapyrroles found in antenna structures known as phycobilisomes that occur in cyanobacteria and red algae Light absorption and emission by chlorophyll. 1. Chlorophyll absorbs light in the blue and red region 2. Upon light absorption, the electronic arrangement is changed and enters in to excited state. 3. The excited chlorophyll has to dissipates its energy in order to come in to ground state 4. There are different ways Source: Taiz and Zeiger to loose the energy a) Heat loss b) Fluorescence c) Energy transfer d) Photochemistry .
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