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Wong_Katrina_Block5_MM5 (chemiosmosis: how chloroplasts and mitochhondria…

- - - - different pigments absorb different wavelengths, the color we see is reflected
      - chloropyll a suggests violet-blue, red light = best for photosynthesis, since they're absorbed, green is the least effective, is blue-green
        
        action spectrum: relative effectiveness of different wavelengths
      - chlorophyll b differs w a by 1 functional group bonded to porphyrin ring, is yellow-green
      - carotenoids: hydrocarbons that absorb violet/blue-green light...yellow and orange
        
        photoprotection: absorbs and dissipates excessive light energy that would damage chlorophyll, or interact w oxygen, forming a dangerous oxidative molecule
    - - photon absorption --> one of the molecule's electrons is elevated to an orbital w higher EA. when in normal orbital = ground state
        
        excited state = unstable, quickly drop back down, excess energy is released as heat (no electron acceptor), photons are also given off (fluorescence)
        
        this drop is prevented in a chloroplast
      - the only photons absorbed are = to energy difference between the ground state and excited state...varies...particular compound absorbs only photons corresponding to specific wavelengths...each pigment has a unique absorption spectrum
    - - electron flow
        
        noncyclic: predominant route, light reactions: solar power--> ATP+NADPH --> chem. energy + reducing power --> Calvin cycle
        
        photon strikes pigment molecule in light harvesting complex --> other pigment molecules until it reaches one of 2 P680 chlorophyll a molecules in PSII reaction center...excites one of P680 electrons to a higher energy state
        
        electron is captured by the primary electron acceptor
        
        enzyme splits H2O into 2 electrons, 2H+, 1 oxygen. electrons are supplied one by one to the P680 molecules, each replacing an electron lost to the primary electron acceptor (strongest oxidizing agent). Oxygen combines w another oxygen, forming O2
        
        each photoexcited electron: PSII --> PSI via ETC. ETC between PSII and PSI = electron carrier plastoquinone (Pq) + cytochrome complex + protein called plastocyanin (Pc)
        
        exergonic "fall" of electrons to a lower energy level provides energy for the synthesis of ATP
        
        meanwhile, light energy was transferred via a light-harvesting complex to the PSI reaction center, exciting an electron of 1 of 2 P700 chlorophyll a molecules located there. The photoexcited electron was captured by PSI's primary electron acceptor, creating an electron "hole" in P700, later filled by an electron that reaches the bottom of the electron transport chain from PSII
        
        photoexcited electrons: PSI's primary electron acceptor --> second ETC through the protein ferrodoxin (Fd)
        
        the enzyme NADP+ reductase transfers electrons: Fd --> NADP+. 2 electrons are required for its reduction to NADPH
        
        cyclic: alternate path using PSI only, short circuit
        
        electrons cycle back from Fd to cytochrome complex --> P700 chlorophyll in PSI reaction center
        
        no NADPH produced, no release of oxygen
        
        produces ATP. noncyclic produces = [ATP]+[NADPH], but Calvin cycle consumes more ATP, cyclic flow makes up the difference
        
        [NADPH] regulates which pathway electrons will take.
        
        low ATP for Calvin--> NADPH accumalates --> noncyclic to cyclic
      - light harvesting complex: consists of pigment molecules bound to particular proteins
        the number and variety of pigment molecules enable a photosystem to harness light over a larger surface&portion of the spectrum
        
        when a pigment molecule absorbs a photon, energy: pigment molecule --> pigment molecule within a light-harvesting complex until its funneled into the reaction center: protein complex that includes 2 special chlorphyll a molecules + primary electron acceptor
        
        these chlorophyll a molecules = special bc microenvironment --> lets them use energy from light to boost 1 of their electrons to a higher energy level
        
        = first step of the light reactions, primary electron acceptor captures (redox)
        
        reaction center + light harvesting complex = 1 unit (light --> chem)
      - photosystem II (PS II) and photosystem I (PSI)
        
        each has a reaction center - a particular kind of primary electron acceptor next to a pair of chlorophyll a molecules
        
        P680: reaction center chlorophyll a of PSII (red)
        
        P700: chlorophyll a at PSI's reaction center (far red)
        
        identical, but association w diff proteins in thylakoid membrane affects electron distribution --> slight diff in light-absorbing properties
    - - water is split...byproduct = O2
      - NAD+ = electron carrier in cellular respiration
        
        solar power--> reduces NADP+ to NADPH by adding electron pair along w H+ /// solar power --> ADP to ATP
  - - - the produced carbohydrate is a 3 carbon sugar: glyceraldehyde-3-phosphate (G3P)
        
        cycle must repeat 3 times, fixing 3 CO2 molecules to make 1 molecule of this sugar
    - - rubisco: RuBP carboxylase: catalyzes...incorporates each CO2 molecule, one at a time, by attaching a 5 carbon sugar: ribulose biphosphate (RuBP)
        
        product: very unstable 6-carbon intermediate that immediately splits in half, forming 2 molecules of 3-phosphoglycerate (per CO2)
    - - each molecule of 3-phosphoglycerate + phosphate group from ATP --> 1,3-bisphosphoglycerate
        
        NADPH donates electrons, reduces 1,3-bisphosphoglycerate --> G3P (a sugar, same as the 3 carbon sugar formed in glycolysis by splitting glucose
        
        for every 3 molecules of CO2, 6 molecules of G3P
        
        1 molecule exits cycle to be used by plant cell, 5 recycled to regenerate the 3 molecules of RuBP
    - - carbon skeletons of 5 G3P are rearranged by the last steps into 3 RuBP by spending 3 molecules of ATP--> RuBP is now prepared to receive CO2 again...cycle continues
        
        FOR NET SYNTHESIS OF 1 G3P, calvin cycle consumes 9 ATP, 6 NADPH
        
        G3P = starting material for metabolic pathways
  - - - 12 water molecules consumed, 6 newly formed
        
        --> 6CO2 + 6H2O + light energy ---> C6H12O6 + 6O2.
        
        --> CO2 + H20 ---> [CH2O] + O2
        
        6 repetitions = 1 glucose molecule
      - C6H12O6 = 3 carbon sugar
    - - CO2 + 2H2X + 2 H2X ---> [CH2O] + H2O + 2X
      - plants split water as a source of electrons from hydrogen atoms, releasing oxygen as a byproduct
    - - cellular respiration: energy is released from sugar when electrons associated w hydrogen are transported by carriers to oxygen, forming water...electrons lose EA as they fall down electron transport chain towards electronegative oxygen
      - photosynthesis: water is split --> electrons are transferred along w hydrogen ions from H2O to CO2, reducing it to sugar. electrons increase in EA from water to sugar, requiring energy
- - - - C3 plants, because first organic product of carbon fixation is a 3 carbon compound...3-phosphoglycerate
        
        when their stomata partially closes, C3 plants produce less sugar (because less CO2 in the leaf starves Calvin cycle
        
        rubisco can bind O2 instead of CO2. As CO2 = scarce, rubisco adds O2 to Calvin cycle instead of CO2
        
        product splits, compound leaves chloroplast--> rearranged and split by peroxisomes and mitochondria--> released = photorespiration
        
        consumes ATP, produces no sugar...decreases photosynthetic output by taking material from the Calvin Cycle
        
        an ancient relic, but modern rubisco retains affinity for O2, which is v concentrated in atmosphere...photorespiration is inevitable
  - - - PEP carboxylase--> PEP (lose CO2)
        
        PEP carboxylase carries it out: CO2 + phosphoenolpyruvate (PEP), forming 4 carbon product oxaloacetate
        
        has a higher affinity for CO2 than rubisco and no affinity for O2,...PEP carboxylase can fix carbon efficiently when rubisco cannot (stomata is partially closed)
        
        mesophyll cells export 4 carbon products to bundle-sheath cells
        
        within bundle-sheath cells, 4 carbon compounds release CO2, which is reincorporated into organic material by rubisco and the calvin cycle
        
        pyruvate is also regenerated in this coversion
      - mesophyll cells of a C4 plant pump CO2 back into the bundle sheath, keeping [CO2] high enough for rubisco to bind CO2 instead of oxygen
    - - arranged into tightly packed sheats around the veins of the leaf
      - calvin cycle is confined to bundle sheath's chloroplasts
    - - between bundle sheath and leaf surface
      - loosely arranged
      - CO2 is incorporated into organic compounds
  - - - closing conserves water, prevents CO2 from entering
- - - - 2 membranes enclose the stroma: dense fluid within the chloroplast
        
        outermembrane: selectively permeable
        
        inner membrane: embedded transport proteins that allow certain proteins and glucose to enter and leave the chloroplasts
      - thylakoids: elaborate system of interconnected membranous sacs, segregates stroma from the interior of the thylakoids (thylakoid space)
        
        thylakoid sacs stacked in columns = grana
        
        thylakoid membrane separates lumen, embedded w chlorophyll