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Source-to-sink interaction
The plant parts that are able to fixed carbon are called “sources”, whereas the parts that consume or store the assimilates are named “sinks”. Interactions between source and sink are important for the regulation of plant growth and development. The main compounds transported in plants from source to sink are carbohydrates
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Photosynthesis occurs in chloroplasts
In plants, photosynthesis occurs primarily in leaves CO2 enters and O2 exits through stomata Chloroplasts are the site of photosynthesis Concentrated in mesophyll cells of leaves
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Chloroplast structure
Thylakoids Suspended in stroma Interconnected sacs Stacked in grana Contain chlorophyll in membranes Stroma Fluid enclosed by inner membrane Location of sugar synthesis Leaf Leaf Cross Section Mesophyll Cell Vein Mesophyll Chloroplast Stoma Stroma CO2 O2 Granum Grana TEM 9,750 Thylakoid Thylakoid space Intermembrane space Inner membrane Outer LM 2,600
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Photosynthesis occurs in two stages linked by ATP and NADPH
Light reactions Occur in thylakoid membranes Convert light energy to chemical energy as ATP and NADPH Produce O2 as a waste product Calvin cycle Occurs in stroma Assembles sugar molecules from CO2 using ATP and NADPH from light reactions
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An overview of photosynthesis
Cooperation of the light reactions and the Calvin cycle Light H2O CO2 Chloroplast NADP+ ADP + P RUBP CALVIN CYCLE (in stroma) NADPH ATP G3P 3-PGA Stroma Photosystem II Electron transport chains Photosystem I Thylakoid membranes O2 LIGHT REACTIONS Sugars CALVIN CYCLE Cellular respiration Cellulose Starch Other organic compounds
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LIGHT REACTIONS Photosystem Light-harvesting complexes Reaction center
Primary electron acceptor Photon To electron transport chain Thylakoid membrane Transfer of energy Pigment molecules Chlorophyll a molecule
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Electron transport chain Provides energy for synthesis of
Linear Electron Flow Photon Photon Photosystem I Photosystem II NADP+ + H + NADPH Stroma e– e– Thylakoid membrane P700 P680 Thylakoid space Electron transport chain Provides energy for synthesis of by chemiosmosis ATP H2O 1 O2 + 2 H +
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A photon of light strikes a pigment molecule in a light harvesting complex, boosting one of its electrons to a higher energy level. The simultaneously raised electron reaches the P680 pair of chlorophyll a molecule In PSII. It excites an electron in this pair of chlorophylls to a higher energy state The electron is transferred from the excited P680 to the primary electron acceptor An enzyme catalyzed the splitting of a water molecule into two electrons, two hydrogen ions, and a single oxygen. Each electron replace one in P680 molecules transferred to the primary electron acceptor Each photoexcited electron passes from the primary electron acceptor of PSII to PSI via an electron transport chain. Pq: plastoquinone, Pc: Plastocyanin The ecergonic “fall” of electrons to a low energy level provides energy for the synthesis of ATP Light energy is also transferred via light-harvesting complex pigments to the PSI reaction-center complex, exciting an electron of the P700 pair of chlorophyll a molecules. And the electron is then transferred to PSI’s primary electron acceptor. Photoexcited electrons are passed in a series of redox reactions The enzyme NADP+ reductase catalyzes the transfer of electrons from Fd (ferredoxin) to NADP+. It generate NADPH [NADP+ + H NADPH]
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H2O ATP NADPH Mill makes ATP Photosystem II Photosystem I e– e– e– e–
LE 7-8b e– ATP e– e– NADPH H2O e– e– e– Mill makes ATP Photon e– Photon Photosystem II Photosystem I
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THE CALVIN CYCLE: CONVERTING CO2 TO SUGARS
ATP and NADPH power sugar synthesis in the Calvin cycle The Calvin cycle makes sugar in the chloroplast Inputs Carbon from CO2 Energy from ATP High-energy electrons from NADPH Output Energy-rich G3P (Glyceraldehyde-3-phosphate, C3H7O6P) Can be used to make organic molecules
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LE 7-10a CO2 Input ATP NADPH CALVIN CYCLE Output: G3P
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Photosynthesis uses light energy to make food molecules
The light reactions Capture light energy Generate NADPH and ATP Release O2 and water The Calvin cycle Manufactures sugar Cells use many of the same mechanisms in photosynthesis and cellular respiration
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H2O CO2 + Chloroplast NADP+ ADP P RUBP CALVIN CYCLE (in stroma) 3-PGA
Light H2O CO2 Chloroplast NADP+ ADP + P RUBP CALVIN CYCLE (in stroma) NADPH ATP G3P 3-PGA Stroma Photosystem II Electron transport chains Photosystem I Thylakoid membranes O2 LIGHT REACTIONS Sugars CALVIN CYCLE Cellular respiration Cellulose Starch Other organic compounds
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C4 and CAM plants have special adaptations that save water
C3 plants soybeans, wheat, rice Use CO2 directly CO2 fixed into 3-carbon, 3-phosphateglycerate Rate of photosynthesis decreases in dry weather Stomata close, CO2 supply reduced Calvin cycle diverted to photorespiration - Some plant species have evolved that minimize photorespiration and optimize the Calvin cycle-even in hot and arid climate
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CAM (Crassulacean Acid Metabolism) plants
C4 plants Include corn and sugarcane Stomata closed in hot, dry weather CO2 fixed into a 4-carbon compound that acts as a carbon shuttle Enables plant to continue making sugar Prevents photorespiration and water loss CAM (Crassulacean Acid Metabolism) plants Pineapple, cactus, succulents Adapted to very dry climates Open stomata and admit CO2 only at night Fix CO2 into a 4-carbon compound that banks CO2 for release to Calvin cycle during the day
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LE 7-12 Mesophyll cell CO2 CO2 Night 4-C compound 4-C compound CO2 CO2
CALVIN CYCLE CALVIN CYCLE Sugarcane Pineapple Bundle-sheath cell 3-C sugar 3-C sugar Day C4 plant CAM plant
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