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Ch.10 Photosynthesis (1st Stage of Photosynthesis (Properties of Light…

- - - - 1) The Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules
      - 2) Calvin Cycle then reduces the fixed carbon to carbohydrate by the addition of electrons (reducing power is provided by NADPH which acquired its cargo of electrons in the light reactions)
      - 3) Clavin Cycle also requires chemical energy in the form of ATP (which is also generated by the light reactions)
      - Thus, it is the Calvin Cycle that makes sugar but only by having the NADPH produced by the light reactions
  - - - Neither the light reactions nor the Calvin cycle alone can make sugar from CO2
      - Photosynthesis is an emergent property of the intact chloroplast, which integrates the 2 stages of photosynthesis
  - - - Incorporation of each CO2 molecule, one at a time, by attaching it to a five-carbon sugar named ribulose bisphosphate (abbreviated RuBP)
      - The enzyme that catalyzes this first step is RuBP carboxylase-oxygenase, or rubisco
      - The product of the reaction is energetically unstable that it immediately splits the six-carbon intermediate (the product) in half, forming 2 molecules of 3-phosphoglycerate
        (for each CO2 fixed)
    - - Each molecule of 3-phosphogylcerate receives an additional phosphate group from ATP, becoming 1, 3-bisphosphoglycerate
      - Next, a pair of electrons donated from NADPH reduces 1, 3-bisphosphoglycerate, which also loose a phosphate group in the process, becoming glyceral-dehyde 3-phosphate (G3P)
      - The cycle began with 15 carbons' worth of carbohydrate in the form of 3 molecules of the five-carbon sugar RuBP. And ended with 18 carbons' worth of carbohydrate in the form of 6 molecules of G3P
      - 1 molecule exits the cycle to be used by the plant cell, but the other 5 molecules must be recycled to regenerate the 3 molecules of RuBP
    - - The carbon skeletons of 5 molecules of G3P are rearranged by the last steps of the Calvin Cycle into 3 molecules of RuBP
      - To accomplish this, the cycle spends 3 more molecules of ATP. The RuBP is now prepared to receive CO2 again, and the cycle continues
      - The G3P spun off from the Calvin cycle becomes the starting material for metabolic pathways that synthesize other organic compounds including glucose (from 2 molecules of G3P), the disaccharide sucrose, and other carbohydrates
- - - - Segregates the stroma from the thylakoid space inside these sacs
      - Stacks of thylakoids form grana
      - Chlorophyll, the green pigment that gives leaves their color, resides in the thylakoid membranes of the chloroplast
        
        The internal photosynthetic membranes of some prokaryotes are also called thylakoid membranes
        
        It is light energy absorbed by chlorophyll that drives the synthesis of organic molecules in the chloroplast
    - - 6 CO2 + 12 H2O + Light energy ---> C6H12O6 + 6O2 + 6H20
      - In the presence of light, the green parts of plants produce organic compounds & oxygen from carbon dioxide and water
      - Overall chemical change during photosynthesis is the reverse of the one that occurs during cellular respiration
      - The Splitting of Water
        
        Researchers tracked the elements C, H, and O from the reactants of photosynthesis to the products
        
        The discovery that the O2 given off by plants is derived from H2O and NOT from CO2
        
        The chloroplasts splits water into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules
        
        Niel discovered this and hypothesized that plants split H20 as a source of electrons from hydrogen atoms, releasing O2 as a by-product
        
        Years later it was confirmed in an experiment using oxygen-18 (18^O), a heavy isotope as a tracer to follow the path of oxygen atoms during photosynthesis
        
        Experiments showed that the O2 from plants was labeled with 18^O only if water was the source of the tracer (experiment 1)
        
        If the 18^O was introduced to the plant in the form of CO2, the label did not turn up in the released O2 (experiment 2)
        
        Photosynthesis as a Redox Process
        
        Photosynthesis reverses the direction of electron flow compared to cellular 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 (b/c endergonic process requires energy)
        
        Preview: The 2 Stages of Photosynthesis
        
        Calvin Cycle
        
        Photosynthesis consists of 2 processes, each with multiple steps
        
        Light Reactions
- - - - 1) Split H2O
      - 2) Release O2
      - 3) Reduce the electron acceptor NADP+ to NADPH
      - 4) Generate ATP from ADP by photophosphorylation
      - Thus, light energy is converted to chemical energy in the form of 2 compounds:
        
        NADPH
        
        ATP
  - - - Range from less than a nanometer (for gamma rays) to more than a kilometer (for radio waves)
      - The entire range of radiation is known as the electromagnetic spectrum
        
        Important to life is the radiation known as visible light because it can be detected as various colors by the human eye
        
        Visible light also includes the wavelengths that drive photosynthesis
        
        Light also behaves as though it consists of discrete particles, called photons
        
        Photons act like objects, but they act like objects in that each of them has a fixed quantity of energy
        
        Amount of energy is inversely related to the wavelength of the light: The shorter the wavelength = the greater the energy of each photon of that light
  - - - They absorb light of different wavelengths, & the wavelengths that are absorbed disappear
      - Wavelengths that are not absorbed are reflected or transmitted
      - Leaves appear green because chlorophyll reflects and transmits green light
    - - This machine sends light through pigments and measures the fraction of light transmitted at each wavelength
      - An absorption spectrum is a graph plotting a pigment’s light absorption versus wavelength
        
        There are 3 types of pigments in chloroplasts
        
        Chlorophyll a, the key light-capturing pigment
        
        The absorption spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesis
        
        Confirmed by an action spectrum which profiles the relative effectiveness of different wavelengths of radiation in driving a process
        
        The action spectrum for photosynthesis is broader than the absorption spectrum of chlorophyll
        
        Action spectrum experiment
        
        In his experiment, he exposed different segments of a filamentous alga to different wavelengths
        
        Areas receiving wavelengths favorable to photosynthesis produced excess O2
        
        He used the growth of aerobic bacteria clustered along the alga as a measure of O2 production
        
        Chlorophyll b, an accessory pigment
        
        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
        
        Carotenoids, a separate group of accessory pigments
        
        Accessory pigments called carotenoids may broaden the spectrum of colors that drive photosynthesis
        
        Some carotenoids function in photoprotection; they absorb excessive light that would damage chlorophyll or react with oxygen
        
        The absorption spectra of chloroplast pigments provide clues to effectiveness of diff. wavelengths for driving photosynthesis
  - - - The reaction-center complex is an association of proteins holding a special pair of chlorophyll a molecules and a primary electron acceptor
      - The light-harvesting complex consists of pigment molecules bound to proteins
        
        Light-harvesting complexes 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
    - - Functions 1st in light reactions:
        Photosystem II (PS II) contains P680 chlorophyll a molecules in the reaction-center because it is best at absorbing a wavelength of 680 nm
      - Functions 2nd in Light Reactions:
        Photosystem I (PS I) The reaction-center chlorophyll a of PS I is called P700 because is best at absorbing a wavelength of 700 nm
  - - - Linear electron flow, the primary pathway, involves both photosystems and produces ATP and NADPH using light energy
      - There are 8 Steps
        
        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
        
        2) An excited electron from P680 is transferred to the primary electron acceptor
        (we now call it P680+)
        
        3) 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
        
        4) 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
        
        5) Potential energy stored in the proton gradient drives production of ATP by chemiosmosis
        
        6) In PS I (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 the electron transport chain
        
        7) Each electron “falls” down an electron transport chain from the primary electron acceptor of PS I to the protein ferredoxin (Fd)
        
        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
    - - Cyclic electron flow, uses photosystem I (NOT photosystem II) and produces ATP but no NADPH or O2
      - The cyclic electron flow is a short circuit: The electrons cycle back from ferredoxin Fd to the cytochrome complex then via a plastocyanin molecule (Pc) to a P700 chlorophyll in the PS I reaction-center complex
- - - - Photorespiration consumes O2 and organic fuel and releases CO2 without producing ATP or sugar
      - Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O2 and more CO2
      - Photorespiration limits damaging products of light reactions that build up in the absence of the Calvin cycle
      - In many plants, photorespiration is a problem because on a hot, dry day it can drain as much as 50% of the carbon fixed by the Calvin cycle
    - - C4 plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds
      - There are 2 distinct types of cells in the leaves of C4 plants:
        
        Bundle-sheath cells are arranged in tightly packed sheaths around the veins of the leaf
        
        Mesophyll cells are loosely packed between the bundle sheath and the leaf surface
      - Sugar production in C4 plants occurs in a 3 step process:
        
        1) The production of the four-carbon precursors is catalyzed by the enzyme PEP carboxylase in the mesophyll cells.
        
        PEP carboxylase has a higher affinity for CO2 than rubisco does; it can fix CO2 even when CO2 concentrations are low
        
        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
      - CAM Plants
        
        Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon
        
        CAM plants open their stomata at night, incorporating CO2 into organic acids that are stored in the vacuoles
        
        The CAM pathway is similar to the C4 pathway in that they both incorporate CO2 into organic intermediates before it enters the Calvin cycle
        
        The C4 pathway structurally separates the initial steps of carbon fixation from the Calvin cycle
        
        In the CAM pathway, these steps occur in the same cell, but are separated in time
        
        Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle
        
        Life depends on Photosynthesis
        
        Directly or indirectly photosynthesis nourishes almost the entire world
        
        1st mode of nutrition:
        Autotrophs =Producers
        
        Autotrophs are "self feeders" they sustain themselves without eating anything derived from other living beings (EX: Plants)
        
        Produce organic molecules from CO2 and other inorganic molecules
        
        2nd mode of nutrition:
        Heterotrophs =Consumers
        
        Heterotrophs are unable to make their own food. They obtain material from other organisms (EX: Humans & Decomposers)
        
        Are "other-feeding" and occurs when an animal eats plants or other organisms
        
        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
        
        Organic compounds produced by photosynthesis provide the energy and building material for earth's ecosytems