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BIOLOGY CH-10 (Photosynthesis is a process by which green plants and some…

- - - - Directly or indirectly, photosynthesis nourishes almost the entire living world.
        
        Heterotrophs obtain organic material from other organisms
        
        Heterotrophs are the consumers of the biosphere
        
        Some eat other living organisms; others called decomposes, consume dead organic material or faces.
        
        Almost all heterotrophs, including humans, depend on photoautotrophs for food and O2
        
        Earth's supply of fossil fuels was formed from the remains of organisms that died hundreds of millions of years ago
        
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  - - - Photosynthesis as a redox process
        
        Photosynthesis is a redox process in which H20 is oxidized and CO2 is reduced
        
        Photosynthesis is an endergonic process; the energy boost is provided by light
        
        Photosynthesis reverses the direction of electron flow compared to respiration
      - Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part)
        
        The light interactions (in the thylakoids)
        
        Release O2
        
        Reduce the electron acceptor NADP+ to NADPH
        
        Split H2O
        
        Generate ATP from ADP by photophosphorylation
        
        The calvin cycle begins with carbon fixation, incorporationg CO2 into organic molecules
        
        The calvin cycle (in the stroma) forms sugar from CO2 into organic molecules
        
        Chloroplasts are solar-powered chemical factories
        
        Their thylakoids transform light energy into the chemical energy of ATP and NADPH
        
        The nature of sunlight
        
        The electromagnetic spectrum is the entire range of electromagnetic energy, or radiation.
        
        Visisble light consists of wavelengths (380 nm to 750nm)
        
        Wavelengh determines the type of electromagnetic energy
        
        Visisble light also includes the wavelengths that drive photosynthesis
        
        Wavelengh is the distance between crests of electromagnetic waves
        
        Light also behaves as though it consists of discrete particles, called photons.
        
        Electromagnetic energy travels in rhythmic waves
        
        Light is electromagnetic energy, also called electromagnetic radiation
  - - - This machine sends light through pigments and measures the fraction pf light transmitted at watch wavelength
      - An absorption spectrum is a graph plotting a pigment's light absorption versus wavelength
        
        There are three types of pigments in chloroplasts
        
        Chlorophyll b, an accessory pigment
        
        Crotenoids, a seprate group of accesory pigments
        
        Chlorophyll a, the key light-capturing pigment
        
        The absorption spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesis
        
        An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process
        
        The action spectrum of photosynthesis was first demonstrated in 1883 by Theodor W. Engelmann
        
        He used the growth of aerobic bacteria clustered along the alga as a measure of O2 production
        
        Areas receiving wavelengths favorable to photosynthesis produced excess O2
        
        In this experiment, he exposed different segments of a filamentous alga to different wavelengths
        
        The action spectrum for photosynthesis is broader than the absorption spectrum of chlorophyll
        
        Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis
        
        The difference in the absorption spectrum between chlorophyll a and b is due to a slight structural difference between the pigment molecules
    - - When excited electrons fall back to the ground state, excess energy is releases as heat
      - In isolation, some pigments also emit light, an afeterglow called flourescence
      - When a pigment absorbs light, it goes from a ground state to an excited state, which is unstable
      - A photosysthem consists of a reaction-center complex surrounded by light-harvesting complexes
        
        The reaction-center complex is an association of proteins holding a special pair of chlorophyll a molecules and primary electron acceptor.
        
        The light-harvesting complex consists of pigment molecules bound to proteins
        
        Light-harvesting complexes transfer the enrgy 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
        
        A primary electron accept-or in the reaction center 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
- - - - There are eight steps in linear electron flow
        
        8- NADP+ reductase catalyzes the transfer of electrons to NADP+, reducing it to NADPH
        
        The electrons of NADPH are available for the reactions of the Calvin cycle
        
        This process also removes an H+ from the stroma
        
        4- Each electron falls down an electron transport chain from the primary electron acceptor of PS II to the PS I . Energy released by the fall drives the creation of a proton gradient across the thylakoid membrane
        
        5- Potential energy stored in the proton gradient drives production of ATP by chemiosmosis
        
        2- An excited electron from P680 is transferred to the primary electron acceptor (we now call it P680+)
        
        7- Each electron falls down an electron transport chain from the primary electron acceptor of PS I to protein ferredoxin (Fd)
        
        1- 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
        
        The enrgy changes of electrons during linear flow through the light reactions can be shown in a mechanical analogy
        
        Cyclic electron flow
        
        Cyclic electron flow uses only photosystem I and produces ATP, but not NADPH
        
        No oxygen is released
        
        In cyclic electron flow, electrons cycle back from Fd to the PS I reaction center via a plastocyanin molecule (Pc)
        
        Some organisms such as purple sulfur bacteria have PS I but not PS II
        
        Cyclic electron flow is thought to have envolved before linear electron flow
        
        Cyclic electron flow may protect cells from light induced damage
        
        In PS 1 (like PS II), transferred light energy excites P700, which loses an electron to the primary electron acceptor
        
        P700+ (P700 that is missing an electron) accepts an electron passed down from PS II via electron transport chain
        
        3- H20 is split by enzymes, and the electrons are transferred from the hydrogen atoms to P680+, thus reducing it to P630
        
        P680+ is the strongest known biological oxidizing agent
        
        O2 is released as a by-product of this reaction
        
        The H+ are released into the thylakoid space
  - - - 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 inetrmembrane 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
        
        In summary, light reactions generate ATP and increase the potential energy of electrons by moving them from H2O to NADPH.
    - - The Calvin cycle like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle
        
        The Calvin cycle is anabolic; it builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH
        
        Carbon enters the cycle as CP2 and leaves as a sugar named glyceraldehyde 3-phosphate (G3P)
        
        The calcin cycle has three phases
        
        Reduction
        
        Regeneration of the co2 acceptor (RuBP)
        
        Dehydration is a problem for plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesis
        
        In hot dry days, plants close stomata, which conserves H2O but also limits photosynthesis
        
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        1- Carbon fixatation (catalyzed by rubisco)
      - An evolutionary relic
        
        In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound (3-phosphoglycerate)
        
        In photorespiration, rubisco adds O2 instead of Co2
        
        Photorespiration consumes O2 and organic fuel and releases CO2 without producing ATP or sugar
        
        Life depends on photosynthesis
        
        Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthsize the organic molecules of cells
        
        Plants store excess sugar as starch in chloroplasts and other structires such as roots, tubers, seeds, aand fruits
        
        Ther energy enetering chloroplasts as sunlight gets stored as chemical energy in organic compounds