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C10 photosynthesis (The light reactions convert solar energy to the…

- - - - Chloroplasts and mitochondria generate ATP by chemiosmosis, but use different sources of energy
      - Mitochondria transfer chemical energy from food to ATP; chloroplasts transform light energy into the chemical energy of ATP
      - Spatial organization of chemiosmosis differs between chloroplasts and mitochondria but also shows similarities
      - In mitochondria, protons are pumped to the intermembrane space and drive ATP synthesis as they diffuse back into the mitochondrial matrix
        In chloroplasts, protons are pumped into the thylakoid space and drive ATP synthesis as they diffuse back into the stroma
      - ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place
  - - - electrons cycle back from Fd to the PS I reaction center via a plastocyanin molecule (Pc)
        
        Cyclic electron flow uses only photosystem I and produces ATP, but not NADPH
        No oxygen is released
        
        Some organisms such as purple sulfur bacteria have PS I but not PS II
        
        Cyclic electron flow is thought to have evolved before linear electron flow
        Cyclic electron flow may protect cells from light-induced damage
    - - the primary pathway, involves both photosystems and produces ATP and NADPH using light energy
      - 8 steps
        
        A photon hits a pigment in a light-harvesting complex of PS II, and its energy is passed among pigment molecules until it excites P680
        An excited electron from P680 is transferred to the primary electron acceptor (we now call it P680+)
        
        H2O is split by enzymes, and the electrons are transferred from the hydrogen atoms to P680+, thus reducing it to P680
        
        P680+ is the strongest known biological oxidizing agent
        
        The H+ are released into the thylakoid space
        
        O2 is released as a by-product of this reaction
        
        Each electron “falls” down an electron transport chain from the primary electron acceptor of PS II to PS I. Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane
        Potential energy stored in the proton gradient drives production of ATP by chemiosmosis
        
        In PS I (like PS II), transferred light energy excites P700, which loses an electron to the primary electron acceptor
        
        Each electron “falls” down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd)
        NADP+ reductase catalyzes the transfer of electrons to NADP+, reducing it to NADPH
  - - - transfer the energy of photons to the chlorophyll a molecules in the reaction-center complex
      - These chlorophyll a molecules are special because they can transfer an excited electron to a different molecule
    - - an association of proteins holding a special pair of chlorophyll a molecules and a primary electron acceptor
      - consists of pigment molecules bound to proteins
      - primary electron acceptor
        
        accepts excited electrons and is reduced as a result
        
        Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions
  - - - Pigments are substances that absorb visible light
      - Different pigments absorb different wavelengths
      - Wavelengths that are not absorbed are reflected or transmitted
      - Leaves appear green because chlorophyll reflects and transmits green light
      - When a pigment absorbs light, it goes from a ground state to an excited state, which is unstable
        When excited electrons fall back to the ground state, excess energy is released as heat
- - - - The light reactions (in the thylakoids)
        
        Split H2O
        
        Release O2
        
        Reduce the electron acceptor NADP+ to NADPH
        
        Generate ATP from ADP by photophosphorylation
    - - The Calvin cycle (in the stroma) forms sugar from CO2, using ATP and NADPH
      - The Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules
  - - - 6 CO2 + 12 H2O + Light energy → C6H12O6 + 6 O2 + 6 H2O
      - Chloroplasts split H2O into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules and releasing oxygen as a by-product
  - - - Photosynthesis reverses the direction of electron flow compared to respiration
      - Photosynthesis is a redox process in which H2O is oxidized and CO2 is reduced
      - Photosynthesis is an endergonic process; the energy boost is provided by light
  - - - Chlorophyll, the pigment that gives leaves their green color, resides in the thylakoid membranes
  - - - The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds
      - Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells
      - Plants store excess sugar as starch in chloroplasts and other structures such as roots, tubers, seeds, and fruits
- - - - 1.The production of the four-carbon precursors is catalyzed by the enzyme PEP carboxylase in the mesophyll cells
      - 2.These four-carbon compounds are exported to bundle-sheath cells
        3.Within the bundle-sheath cells, they release CO2 that is then used in the Calvin cycle
  - - - C4 photosynthesis uses less water and resources than C3 photosynthesis
        
        Scientists have genetically modified rice, a C3 plant, to carry out C4 photosynthesis
        
        They estimate 30–50% increase in yield compared to C3 rice
  - - - Some plants, including succulents, use crassulacean acid metabolism (CAM) tofix carbon
      - CAM plants open their stomata at night, incorporating CO2 into organic acids that are stored in the vacuoles
      - Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle
- - - - Reduction
        
        Regeneration of the CO2 acceptor (RuBP)
        
        Carbon fixation (catalyzed by rubisco)