Photosynthesis

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  • Photosynthesis
    • The light dependent stage
      • Takes place on the thylakoid membrane
      • Light photon hits a chlorophyll molecule in PSII  in the primary pigment reaction centre and excites electrons
        • Electrons are accepted by an electron acceptor and passed along electron transport chain
          • Energy used to pump protons into thylakoid space, conc. gradient made and protons diffuse back down to stroma through ATPsynthase making ATP
            • Flow of protons called Chemiosmosis
            • Kinetic energy from proton flow converted to chemical energy in ATP to be used in LI stage
          • Light photons hit PSI and electrons excited and accepted by electron acceptor
            • 2H+ (from photolysis) and 2e- (from PSI) react with NADP and make redNADP (used in LI stage)
        • Water splits by Photolysis, producing Oxygen, 2H+ and 2e-
          • 2e- reduce the oxidises PSII
          • 2H+ (from photolysis) and 2e- (from PSI) react with NADP and make redNADP (used in LI stage)
        • 2e- reduce the oxidises PSII
      • Cyclic Photo phosphorylation- excited PSI electron accepted by 2 electron acceptors then back to PSI
    • How the structure of chloroplasts enables them to carry out their functions
      • INNER MEMBRANE- Less permeable, has transport proteins for entry into stroma, folded into lamellae
        • Lamellae is stacked up, each stack called granum, between each grana is intergranal lamellae
          • GRANA-  stacks called thylakoids, sites for light absorption and ATP synthesis in LD stage
            • THYLAKOID MEMBRANE- have photosynthetic pigments arranged into photosystems for maximum absorption of a range of light wavelengths
              • PHOTO SYNTHETIC PIGMENTS- absorb certain wavelengths of light and reflect others
                • Chlorophyll A (450nm) is always the primary pigment
                • Chlorophyll B (500-640nm) and Carotenoid (and Xanthophyll) are accessory pigments
              • Photosystem II = P680/ nm + Photosystem I = P700/nm
      • OUTER MEMBRANE- Permeable to many small ions
      • STROMA- Fluid filled matrix for LI stage , contains RUBISCO and ATPase
      • DNA and RIBOSOMES- for coding and synthesising proteins needed for photosynthesis
    • Limiting factors and the Calvin cycle
      • Light Intensity
        • Measure of how much energy is associated with the light
        • L= 1/d^2  -- as the light spreads so if distance is doubled, light intensity is quartered
        • Increase of it affects the rate of the LD stage
          • More light energy to excite more electrons
          • More electrons in Photo phosphorylation so more ATP and redNADP
          • More ATP and redNADP means more H and energy for LI stage (GP to TP) and more ATP for TP to RuBP
        • Decrease of it stops LD stage and then LI stage as it need LD products
          • No GP to TP - GP accumulates and levels of TP fall
          • Less RuBP, so less carbon fixation and more GP formation
      • Temperature
        • At first, increase of temperature increases kinetic energy, faster enzyme activity so rate of LI stage increased
      • CO2 concentration
        • Leads to an increase in Carbon dioxide fixation (if light not a limiting factor)
          • More GP and more TP and more RuBP so more products of GP and RuBP
          • BUT more stomata open for gas exchange- increased transpiration and plant wilts due to transpiration exceeding soil water uptake- lower rate of photo synthesis
          • CO2 conc decreases below 0.01% means RuBP accumulates and levels of TP and GP drop
    • The light independent stage
      • CO2 from air diffuse into the leaf through open stomata into stroma in chloroplast
        • CO2 (1C) combines with RuBP (5C) using enzyme RUBISCO
      • Occurs in stroma of chloroplasts
      • 3 cycles for 1 glucose
    • Limiting factors
      • Factors present in the environment which may affect the rate of photosynthesis
      • Factors present in the least favourable level is the one that limits the process
      • When light intensity gets higher that the optimum, its no longer the limiting factor as it doesn't alter the rate, CO2 must be
      • Increasing CO2 conc. increases the rate until temp. becomes a limiting factor
      • Zero light intensity means no photosynthesis- its the limiting factor as when it increases the rate does too
      • Light intensity causes stomata to open so CO2 can enter
      • Light photon are trapped by chlorophyll A to excite electrons
      • Light photons split water molecules (photolysis) to make protons
      • Increasing the temp. increases the rate of photosynthesis till another factor limits the process
        • When temp. gets to 25 degrees, RUBISCO starts to catalyse O2 and RuBP making Phosphoglycolate that needs to be actively transported ->mitochondria-> redNADP-> ->GP (photorespiration)-> back to chloroplast
          • Called photorespiration as it occurs in both the chloroplast and the mitochondria
    • The importance of photosynthesis
      • Autotrophs are organisms that synthesise complex organic molecules from inorganic molecules and energy
        • Chemoautotrophs use energy from exergonic chemical reactions
        • Photoautotrophs use light energy for photosynthesis, using CO2 and H2O as inorganic molecules e.g.plants, some bacteria + some prokaryotes
          • Make carbs, proteins, nucleic acids and vitamins
            • Chemoautotrophs use energy from exergonic chemical reactions
          • Can both release chemical potential energy in complex organic molecules (that are made in photosyn.) in respiration
            • Heterotrophs are organisms that ingest and digest complex organic molecules, releasing the chemical potential energy stored in them
              • Synthesise lipids, proteins and nucleic acids
      • Heterotrophs are organisms that ingest and digest complex organic molecules, releasing the chemical potential energy stored in them
        • Synthesise lipids, proteins and nucleic acids

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