Division of Labor in Chloroplasts Green thylakoids Capture light Liberate O 2 from H 2 O Form ATP from ADP and phosphate Reduce NADP + to NADPH Colorless.

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  • Division of Labor in Chloroplasts

    Green thylakoidsCapture lightLiberate O2 from H2O Form ATP from ADP and phosphateReduce NADP+ to NADPH

    Colorless stromaContains water-soluble enzymesCaptures CO2Uses energy from ATP and NADPH for sugar synthesis

    Light reactionsDark reactions

  • Light(-dependent) reactions

  • Fig. 10-5, p. 152Wavelength (nm)400500600700020406080100chlorophyll bchlorophyll aPercent of light absorbedAbsorption spectra of Chlorophyll a and b

  • potential to transfer electrons (measured in volts)+0.8+0.40-0.6ADP + PieeeeeeNADPHH+ + NADP+P700*P700photosystem Iphotosystem IIreleased energy used to form ATPfrom ADP and phosphatesunlightenergyH2OsunlightenergyP680*NONCYCLIC ELECTRON TRANSPORTphotolysisP680: reaction center of photosystem II P700: reaction center of photosystem I Pigments from the light harvesting complex

  • ADP + PieeeeeeNADPHH+ + NADP+P700*P700photosystem Iphotosystem IIreleased energy used to form ATPfrom ADP and phosphatesunlightenergyH2OsunlightenergyP680*CYCLIC ELECTRON TRANSPORTphotolysis

  • Fig. 10-3, p. 151sunlight energyoxygen releasedH2O is splitH+eH+H+NADP+eH+H+carbohydrate end product (e.g. sucrose, starch, cellulose)carbon dioxide usedLight-independent reactionssugar phosphateLight-dependent reactionslumen(H+ reservoir)ADP + PiStromaelectron transport systemphotosystem IIphotosystem Ielectron transport system

  • Compare to respiration

  • Fig. 9-8c, p. 142inner membranepyruvate from cytoplasmCoenzymes give up electrons, hydrogen (H+) to transport systemNADHNADHFADH2acetyl-CoATCA cycleATPcarbon dioxide

    2As electrons pass through system, H+ is pumped out from matrixOxygen accepts electrons, joins with 2H+, forms wateroxygenINTERMEMBRANEspaceATPsynthesizedADPPiH+H+H+ flows inH+H+H+H+electron transport systemH+MATRIXeee

  • Dark reactions

    or

    Light-independent reactions

  • The Calvin cycle (C3 pathway of photosynthesis)

    PGA: phosphoglyceric acidPGAL: phosphoglyceraldehydeRuBP: ribulose bisphosphateRubisco: ribulose bisphosphate carboxylase

    The energy carriers ATP and NADPH (formed by photosystems I and II) are used to form high energy containing C-C and C-H bonds starting from H2O and CO2.

    Through the Calvin cycle, plants capture CO2 and H2O and transform low energy containing C=O and H-O bonds into the high energy containing C-C and C-H bonds of sugar.

    Rubisco is the worlds most abundant protein!

  • Using ATP and NADPH to generate high energy containing covalent bondsPGA: phosphoglyceric acidPGAL: phosphoglyceraldehydeCCOHHCOO HHHPCCOHHCOHHHPPGAPGALATP + NADPHLow energy electronsHigh energy electrons

  • Photorespiration

    When Rubisco uses O2, this will result in one molecule of PGA and one molecule of phosphoglycolate (a two-carbon molecule), instead of two PGA molecules (see the Calvin Cycle).Phosphoglycolate cannot be used in the calvin cycle and thus represents a loss of efficiency in photosynthesis.Photorespiration can cause up to a 25% reduction in photosynthesis in C3 plants.

  • Photorespiration

    C3 PlantsHigh rates of photorespiration (particularly on hot, bright days)Produce less sugar during hot, bright days of summerC4 PlantsShow little or no photorespirationProduce 2 or 3 times more sugar than C3 plants during hot, bright days of summer

  • Fig. 10-10, p. 158

    Corn, a C4 plant (right), is able to survive at a lower CO2 concentration than bean, a C3 plant (left), when they are grown together in a closed chamber in light for 10 days.

  • Fig. 10-12, p. 159C4 cycleAMPmesophyll cellsC3 cyclebundle sheath cells

    Interaction between the C4 cycle and the C3 cycle

    The C4 pathway concentrates CO2

  • Fig. 10-11, p. 159airspacemesophyll cellsvascular bundleCO2 movementguardcelllowerepidermisupperepidermisbundlesheath cellThe C4 pathway concentrates CO2

    In C4 plants, CO2 is first captured by PEP carboxylase in mesophyll cells to make oxaloacetate which is subsequently turned into malate. This malate then diffuses into the chloroplasts of bundle sheath cells where it releases CO2. Thus, bundle sheath chloroplasts contain higher CO2 concentrations compared to chloroplasts in mesophyll cells and therefore have higher photosynthesis and lower photorespiration rates.

  • However!The C4 pathway requires additional ATP for CO2 fixation.

    Thus, C4 plants only grow better than C3 plants under hot and dry environmental conditions.

  • Transforming CO2 and H2O into food

    Light energy is captured to make ATP and NADPH via the action of photosystems I and II.

    This ATP and NADPH is used via the Calvin cycle to transform the low energy containing C-O and H-O bonds of CO2 and H2O into the high energy containing C-C and C-H bonds of sugar.

    In other words: Light energy from the sun is used by plants to increase the potential energy of electrons in the bonding orbitals of covalent bonds. This is done by replacing oxygen in C-O and H-O bonds by carbon or hydrogen, leading to the production of O2 and carbohydrates (sugars, starch, etc).

    SUMMARY: Transforming Light Energy into Chemical Energy

  • Consumption of photosynthesis products

    AgricultureAnnual accumulation of light energy as C-H and C-C bonds (FOOD).

    2. Fossil fuelsAccumulation of light energy as C-C and C-H bonds over millions of years (accumulation of photosynthesis products over millions of years).

    3. Energy intensive agriculture use of fossil fuels to increase agricultural yields (fertilizer and pesticide production, irrigation, harvest, storage, transportation, etc). Use of photosynthesis products of the past to increase FOOD yields (present photosynthesis productivity).

    How do we maintain present levels of food production when fossil fuel sources become depleted?

    ***Figure 10.5: Absorption spectra of chlorophylls a and b at different wavelengths of light. Graph shows the fraction of received light that is absorbed when the pigment is exposed to various wavelengths of light. The relation between wavelength and color is also shown.*Figure 10.7: The pathway of noncyclic electron transport from water to reduced nicotinamide adenine dinucleotide phosphate (NADPH), with the associated adenosine triphosphate (ATP) synthesis. Pi, Inorganic phosphate.*Figure 10.7: The pathway of noncyclic electron transport from water to reduced nicotinamide adenine dinucleotide phosphate (NADPH), with the associated adenosine triphosphate (ATP) synthesis. Pi, Inorganic phosphate.*Figure 10.3: Diagram of a section of chloroplast granum showing where reactions take place. ADP, Adenosine diphosphate.**Figure 9.8c: Detail of membranes, showing location of electron transport system and adenosine triphosphate (ATP) synthesis.

    **Figure 10.9: Some major steps in the C3 pathway of photosynthesis. Carbon dioxide enters the cycle when the enzyme rubisco combines CO2 with ribulose bisphosphate (RuBP) to produce two molecules of phosphoglyceric acid (PGA). Carbon atoms of the key molecules are shown in red. All of the intermediates have one or two phosphate groups attached. For simplicity, only the phosphates on the resulting sugar phosphate are shown. ADP, Adenosine diphosphate; ATP, adenosine triphosphate; PGAL, phosphoglyceraldehyde; Pi, inorganic phosphate.*Figure 10.9: Some major steps in the C3 pathway of photosynthesis. Carbon dioxide enters the cycle when the enzyme rubisco combines CO2 with ribulose bisphosphate (RuBP) to produce two molecules of phosphoglyceric acid (PGA). Carbon atoms of the key molecules are shown in red. All of the intermediates have one or two phosphate groups attached. For simplicity, only the phosphates on the resulting sugar phosphate are shown. ADP, Adenosine diphosphate; ATP, adenosine triphosphate; PGAL, phosphoglyceraldehyde; Pi, inorganic phosphate.*Figure 10.9: Some major steps in the C3 pathway of photosynthesis. Carbon dioxide enters the cycle when the enzyme rubisco combines CO2 with ribulose bisphosphate (RuBP) to produce two molecules of phosphoglyceric acid (PGA). Carbon atoms of the key molecules are shown in red. All of the intermediates have one or two phosphate groups attached. For simplicity, only the phosphates on the resulting sugar phosphate are shown. ADP, Adenosine diphosphate; ATP, adenosine triphosphate; PGAL, phosphoglyceraldehyde; Pi, inorganic phosphate.*Figure 10.10: Corn (Zea mays), a C4 plant (right), with its low CO2 compensation point is able to survive at a lower CO2 concentration than bean (Phaseolus vulgaris), a C3 plant (left), when they are grown together in a closed chamber in light for 10 days.*Figure 10.12: Interaction between the C4 cycle in mesophyll cells and the C3 cycle in bundle sheath cells.*Figure 10.11: Photosynthesis in corn (Zea mays). A section through a leaf shows the concentric arrangement of bundle sheath and mesophyll cells. Compare this diagram with Figure 6.10a.**Figure 10.9: Some major steps in the C3 pathway of photosynthesis. Carbon dioxide enters the cycle when the enzyme rubisco combines CO2 with ribulose bisphosphate (RuBP) to produce two molecules of phosphoglyceric acid (PGA). Carbon atoms of the key molecules are shown in red. All of the intermediates have one or two phosphate groups attached. For simplicity, only the phosphates on the resulting sugar phosphate are shown. ADP, Adenosine diphosphate; ATP, adenosine triphosphate; PGAL, phosphoglyceraldehyde; Pi, inorganic phosphate.*Figure 10.9: Some major steps in the C3 pathway of photosynthesis. Carbon dioxide enters the cycle when the enzyme rubisco combines CO2 with ribulose bisphosphate (RuBP) to produce two molecules of phosphoglyceric acid (PGA). Carbon atoms of the key molecules are shown in red. All of the intermediates have one or two phosphate groups attached. For simplicity, only the phosphates on the resulting sugar phosphate are shown. ADP, Adenosine diphosphate; ATP, adenosine triphosphate; PGAL, phosphoglyceraldehyde; Pi, inorganic phosphate.