<p>Photosynthesis</p><p>3CO2 + 6H2O C3H6O3 + 3O2 + 3H2O</p><p>Photosynthesis • Two major steps: – Energy transduction reactions or light reactions – Carbon-fixation reactions or dark reactions or Calvin Cycle</p><p>Light reactions • Light energy forms ATP and NADPH • Water molecule is split and oxygen is released</p><p>Dark reactions • Energy from ATP and NADPH used to synthesize organic molecules</p><p>What is light? • Visible light-portion of electromagnetic spectrum that causes sensation of vision in humans • ~400-700 nm </p><p>What is light? • Light has wave properties - it has measurable wavelengths  , measured in nanometers (nm), or in frequency of peaks (Hz)</p><p>Light • Light also has particle properties-hits earth as "packets" of energy called photons; the energy of 1 photon=1 quantum; quantum energy is inversely proportional to  </p><p>What is light? • Light sources vary: • Sunlight varies in distribution of wavelengths throughout the day; enriched in red at noon, far-red at sunset or in shade • Fluorescent lights are enriched in "cool" blue • Incandescent lights are enriched in "warm" red How is light characterized and measured? • Light quantity-how many total photons impact a surface over time • Photon Fluence Rate=mol photons/m2*sec, or in energy equivalents= J/m2*sec, • Quantum sensors commonly measure quantity of PAR (400-700nm) striking a surface (commonly 2000  mol/m2*sec PAR on a clear summer day) </p><p>Light characterization • Light quality - distribution of different  s impacting a surface = mol photons/m2*sec* • Quantum radiometers measure light quality • Both quantity and quality vary daily with changes in atmosphere, clouds, shading, sun angle </p><p>Light quality effects • Red/far-red light ratio (R/FR)-important signal for plant development, ratio lower under shade or at end of day </p><p>• Infrared-long wavelengths largely reflected by earth, but absorbed by CO2 and water vapor in atmosphere, leading to heat buildup and the Greenhouse Effect </p><p>Light quality effects • Ultraviolet (UV)-short wavelengths, higher energy, mutagenic • Most uv is absorbed by ozone in stratosphere but required for some plant processes • Hole in stratospheric ozone layer means more UV radiation for the earth = skin cancer, reduced plant productivity</p><p>How is light energy captured and transferred? • Photoreceptors (pigments)-required to capture light energy • Chlorophyll-main photosynthetic pigment and best studied pigment</p><p>Chlorophyll • Synthesized as protochlorophyll in the dark- requires light for conversion to chlorophyll • Porphyrin head (N and Mg-containing), plus long hydrocarbon tail • Loss of Mg upon extraction forms pheophytin • Insoluble in water, found in chloroplast membrane lipids, associated w/proteins </p><p>Chlorophyll • Several forms: • a (all plants, algae, cyanobacteria) abs. peaks at 420 (blue) and 670 (red) – ¾ of chlorophyll in leaves of green plants • b (plants, green algae) abs. peaks at 490 and 650 • c (brown algae) </p><p>Accessory pigments • A pigment not directly involved in photosynthetic energy transduction • Serves to broaden the range of light that can be used in photosynthesis</p><p>Other pigments • Carotenoids – Red, orange or yellow lipid-soluble pigments found in all chloroplasts • Carotene • Xanthaophylls</p><p>Energy capture and transfer • Pigments (e.g. chlorophyll) rapidly absorb particular  ; energy transferred to an electron in pigment • Excites molecule from low energy stable ground state to a higher energy, unstable excited state </p><p>Energy transfer • Energy dissipates from unstable excited state: • 1. Some energy lost as combination of heat and as a lower energy photon = fluorescence • 3. Some energy transferred between pigments = resonance energy transfer • 4. Electron given up to an acceptor molecule as pigment is photooxidized = main act of photosynthesis </p><p>Light-dependent reactions of photosynthesis • Light energy (captured by chlorophyll) is used to extract low energy electrons from water, transferring them through photosynthetic electron transport to produce a strong reductant (NADPH); ATP synthesis is coupled to this process Photosystems • Higher plant photosynthesis consists of two Photosystems (PSII and PSI), which are big chlorophyll-protein complexes embedded in the thylakoid membranes • Each reaction center of PSII and PSI consists of 2 molecules of chlorophyll a complexed with protein and surrounded by "antennae“ molecules </p><p>Photosystems • Antennae consist of 250-400 pigment molecules, which capture energy and funnel energy to each reaction center through resonance energy transfer • Reaction center of PSII absorbs maximally at 680nm, PSI absorbs maximally at 700nm </p><p>Lateral heterogeneity • PSII is located mostly in membranes in the appressed regions of the thylakoids (within the grana) • PSI is located mostly on the regions of the thylakoid membrane exposed to the stroma • PQ, PC, and fd carry electrons and diffuse to where needed </p><p>Photosynthetic electron transport 1. Antennae funnel energy to Reaction Center P680 in PSII exciting P680 to a higher energy state 2. P680+ donates an electron to pheophytin (the first electron acceptor) 3. At the same time, the Water splitting complex oxidizes water into O2 and H+ ions and then donates electrons originating from the water to P680 to make up for the electron it just gave up </p><p>Photosynthetic electron transport 4. Pheophytin to Plastoquinone pool (PQ) to iron-sulfur molecule (Fe-S) on the cytochrome b6/f complex, where Fe-S donates electron to cyt f 5. cyt f donates electron to the copper binding protein Plastocyanin (PC) 6. PC diffuses along the lumen side of the membrane and donates an electron to Reaction Center P700 in PSI</p><p>Photosynthetic electron transport 7. To continue electron transport, P700 must be excited by energy captured by the antennae of PSI 8. P700+ donates an electron to ferredoxin-NADP+ reductase (iron-containing protein/enzyme) 9. Fd-NADP+ reductase donates an electron to the final electron acceptor NADP+, reducing it to NADPH Photosynthetic electron transport</p><p>• NADPH is a strong reductant used to reduce CO2 in the stroma (next lecture) • 692 kJ of light energy used to excite each pair of electrons at the start of the process (only a portion of that gets conserved in chemical bonds) • Total energy from light reactions is 6 ATP and 6 NADPH</p><p>Z-scheme</p><p>Photophosphorylation • As photosynthetic electron transport progresses, the proton gradient formed in the lumen is used to drive ATP synthesis • Oxidation of water increases H+ in the lumen; H+ are also pumped into the lumen from the stroma through the PQ-cytochrome pump • H+ can be 3-4 thousand times higher on the lumen side of the membrane creating an electrochemical gradient</p><p>Cyclic photophosphorylation • More primitive • Produces ATP for dark cycle (requires 3:2 ratio of ATP:NADPH)</p><p>Carotenoids • Carotenoids are closely associated with chlorophyll; may help to capture light • Also protect photosystems from photooxidative damage </p><p>Photooxidation • Anthocyanins (that accumulate during Fall) are thought to protect against photooxidation during sunny, but cool periods during the fall</p><p>The dark reactions </p><p>• The Calvin cycle - the C3 pathway for photosynthetic carbon fixation • Elucidation by Melvin Calvin led to Nobel Prize </p><p>• 3CO2 + 6NADPH + 9ATP Glyceraldehyde 3-phosphate + + 6NADP + 9ADP + 9Pi +3H2O</p><p>CO2 uptake • Diffuses in through stomata into air spaces • Diffuses freely through membranes into mesophyll cells • Diffuses through cytoplasm to chloroplast</p><p>Reaction 1 of the Calvin Cycle</p><p>• Carboxylation (adding a carboxyl group from CO2) of the 5 carbon ribulose-1,5-bisphosphate (RuBP) to form 2 molecules of 3-Phosphoglycerate (3- PGA) • First detectable product of Calvin Cycle is 3-PGA - C3 pathway</p><p>Rubisco • Catalyzed by ribulose bisphosphate carboxylase-oxygenase (Rubisco)-the most abundant protein on earth) </p><p>• Has a binding site for both CO2 and O2 </p><p>Reaction 2 of the Calvin Cycle • Formation of a triose-phosphate called glyceraldehyde-3-phosphate (G3P) from 3-PGA via two reactions mediated by two enzymes, NADPH and ATP </p><p>RuBP regeneration • Regeneration of the 5-carbon acceptor RuBP is accomplished by various reactions to form Ru5P, which is then phosporylated to RuBP requiring ATP • Three turns of entire cycle regenerates the 3 molecules of RuBP, plus one additional G3P</p><p>• 3 ATP + 2 NADPH are needed per CO2 fixed</p><p>Fates of G3P • Stored as starch in the chloroplast • Exported to cytosol, converted to sucrose, then exported and partitioned to various tissues • Converted to pyruvate and respired, yielding ATP and carbon skeletons </p><p>Regulation of the </p><p>Calvin Cycle • Regulation of cycle keeps carbon fixation compatible with carbon metabolism </p><p>Autocatalysis of RuBP • At night, Calvin cycle shuts down, RuBP regeneration stops, and the number of acceptor molecules for CO2 becomes limiting • In morning, RuBP can autocatalyze from G3P • Other regulatory control points occur at key enzymatic steps Regulation of Rubisco • Light regulated; turns off in darkness, on in light • Stroma pH: increases from 7.1 in dark to 7.9 in light (as protons move across the thylakoid membrane), and Rubisco is more active at pH 7.9 • Stroma Mg 2+ concentration: increases from 1-2 mM in dark to 3-5 mM in light; Rubisco activity requires Mg+ </p><p>• CO2 presence activates Rubisco in morning through Rubisco activase</p><p>Photorespiration • The problem of photorespiration and potential solutions </p><p>• We know that photosynthesis evolves O2 and uses CO2, but it can also use O2 and evolve CO2 </p><p>Dual affinity of Rubisco</p><p>• Rubisco has O2 and CO2 binding sites</p><p>• O2 binding favored under high O2, low CO2, and high temperature • Can result in losses of 50% or more of recently fixed carbon</p><p>Evolutionary solutions to photorespiration • Affinity for oxygen may be an evolutionary "hangover” from plant evolution in high CO2 environments </p><p>The C4 pathway</p><p>• A CO2 concentrating mechanism offers a solution • Initial evidence for alternative pathway found by noticing incorporation of radiolabelled CO2 into C4 acids (e.g. oxaloacetate) rather than the C3 acid 3- PGA </p><p>C4 Pathway</p><p>• Insensitive to O2 levels, and can operate at low CO2 levels-favored under high light and high temperatures • Found in 18 families of tropical and subtropical plants (1500 spp.) </p><p>C4 Anatomy • Anatomical component - two distinct types of photosynthetic cells "Kranz anatomy" • A "wreath" of bundle sheath cells containing many chloroplasts surround vascular tissue in leaf veins • Mesophyll cells surround the bundle sheath cells Kranz anatomy C4 pathway • In mesophyll cells:</p><p>• CO2 is fixed to phosphoenolpyruvate (PEP) by the enzyme PEP carboxylasese to create oxaloacetate (OAA) • OAA reduced to malate • Malate moves from mesophyll cells to bundle sheath cells</p><p>C4 pathway • In bundle sheath cells: • Malate is decarboxylated to release CO2 and pyruvate, which then enters the Calvin cycle • Pyruvate moves back to mesophyll cells and regenerates PEP</p><p>C4 pathway • C4 pathway is more costly compared to C3 pathway • Why do it?</p><p>– High ratio of CO2/O2 in bundle sheath cells favors carboxylation by rubisco rather than photo respiration</p><p>– Any CO2 lost in photorespiration in interior bundle sheath cells can be captured by other cells</p><p>C4 pathway</p><p>• Due to more efficient use of CO2, stomata can be smaller and closed more often • An advantage in dry, hot areas • Some C4-C3 intermediates in higher plants</p><p>Crassulacean Acid Metabolism (CAM) • Specialized form of C4 photosynthesis • Found in all succulents-e.g. cacti, Aloe vera</p><p>CAM</p><p>• Stomata open only at night; CO2 uptake at night – conserves water loss </p><p>• CO2 is "stored" in organic acids (malate) in vacuole until they can be decarboxylated during the day releasing CO2 to be fixed using light energy • No special anatomy; timing separates functions </p>