Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Excitation of Chlorophyll by Light When a pigment absorbs light, it goes from a ground state to an excited state, which is unstable When excited electrons fall back to the ground state, photons are given off, an afterglow called fluorescence If illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heat

Excited state Heat Photon (fluorescence) Ground state Chlorophyll molecule Photon Excitation of isolated chlorophyll molecule Fluorescence Energy of electron e–e–

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A Photosystem: A Reaction Center Associated with Light-Harvesting Complexes A photosystem consists of a reaction center surrounded by light-harvesting complexes The light-harvesting complexes (pigment molecules bound to proteins) funnel the energy of photons to the reaction center

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A primary electron acceptor in the reaction center accepts an excited electron from chlorophyll a Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions

Thylakoid Photon Light-harvesting complexes Photosystem Reaction center STROMA Primary electron acceptor e–e– Transfer of energy Special chlorophyll a molecules Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID) Thylakoid membrane

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings There are two types of photosystems in the thylakoid membrane Photosystem II functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm Photosystem I is best at absorbing a wavelength of 700 nm The two photosystems work together to use light energy to generate ATP and NADPH

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Noncyclic Electron Flow During the light reactions, there are two possible routes for electron flow: cyclic and noncyclic Noncyclic electron flow, the primary pathway, involves both photosystems and produces ATP and NADPH

Light P680 e–e– Photosystem II (PS II) Primary acceptor [CH 2 O] (sugar) NADPH ATP ADP CALVIN CYCLE LIGHT REACTIONS NADP + Light H2OH2O CO 2 Energy of electrons O2O2

Light P680 e–e– Photosystem II (PS II) Primary acceptor [CH 2 O] (sugar) NADPH ATP ADP CALVIN CYCLE LIGHT REACTIONS NADP + Light H2OH2O CO 2 Energy of electrons O2O2 e–e– e–e– + 2 H + H2OH2O O2O2 1/21/2

Light P680 e–e– Photosystem II (PS II) Primary acceptor [CH 2 O] (sugar) NADPH ATP ADP CALVIN CYCLE LIGHT REACTIONS NADP + Light H2OH2O CO 2 Energy of electrons O2O2 e–e– e–e– + 2 H + H2OH2O O2O2 1/21/2 Pq Cytochrome complex Electron transport chain Pc ATP

Light P680 e–e– Photosystem II (PS II) Primary acceptor [CH 2 O] (sugar) NADPH ATP ADP CALVIN CYCLE LIGHT REACTIONS NADP + Light H2OH2O CO 2 Energy of electrons O2O2 e–e– e–e– + 2 H + H2OH2O O2O2 1/21/2 Pq Cytochrome complex Electron transport chain Pc ATP P700 e–e– Primary acceptor Photosystem I (PS I) Light

P680 e–e– Photosystem II (PS II) Primary acceptor [CH 2 O] (sugar) NADPH ATP ADP CALVIN CYCLE LIGHT REACTIONS NADP + Light H2OH2O CO 2 Energy of electrons O2O2 e–e– e–e– + 2 H + H2OH2O O2O2 1/21/2 Pq Cytochrome complex Electron transport chain Pc ATP P700 e–e– Primary acceptor Photosystem I (PS I) e–e– e–e– Electron Transport chain NADP + reductase Fd NADP + NADPH + H H + Light

ATP Photosystem II e–e– e–e– e–e– e–e– Mill makes ATP e–e– e–e– e–e– Photon Photosystem I Photon NADPH

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cyclic Electron Flow Cyclic electron flow uses only photosystem I and produces only ATP Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle

Photosystem I Photosystem II ATP Pc Fd Cytochrome complex Pq Primary acceptor Fd NADP + reductase NADP + NADPH Primary acceptor

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A Comparison of Chemiosmosis in Chloroplasts and Mitochondria Chloroplasts and mitochondria generate ATP by chemiosmosis (generation of ATP by the movement of hydrogen ions across a membrane), but use different sources of energyATPhydrogen membrane Mitochondria transfer chemical energy from food to ATP; chloroplasts transform light energy into the chemical energy of ATP The spatial organization of chemiosmosis differs in chloroplasts and mitochondria

MITOCHONDRION STRUCTURE Intermembrane space Membrane Electron transport chain Mitochondrion Chloroplast CHLOROPLAST STRUCTURE Thylakoid space Stroma ATP Matrix ATP synthase Key H+H+ Diffusion ADP +P H+H+ i Higher [H + ] Lower [H + ]

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The current model for the thylakoid membrane is based on studies in several laboratories Water is split by photosystem II on the side of the membrane facing the thylakoid space The diffusion of H + from the thylakoid space back to the stroma powers ATP synthase ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place

STROMA (Low H + concentration) Light Photosystem II Cytochrome complex 2 H + Light Photosystem I NADP + reductase Fd Pc Pq H2OH2O O2O2 +2 H + 1/21/2 2 H + NADP + + 2H + + H + NADPH To Calvin cycle THYLAKOID SPACE (High H + concentration) STROMA (Low H + concentration) Thylakoid membrane ATP synthase ATP ADP + P H+H+ i [CH 2 O] (sugar) O2O2 NADPH ATP ADP NADP + CO 2 H2OH2O LIGHT REACTIONS CALVIN CYCLE Light

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Calvin cycle uses ATP and NADPH to convert CO 2 to sugar The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle The cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH Carbon enters the cycle as CO 2 and leaves as a sugar named glyceraldehyde-3-phospate (G3P) For net synthesis of one G3P, the cycle must take place three times, fixing three molecules of CO 2

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Calvin cycle has three phases: – Carbon fixation (catalyzed by rubisco) – Reduction – Regeneration of the CO 2 acceptor (RuBP)

[CH 2 O] (sugar) O2O2 NADPH ATP ADP NADP + CO 2 H2OH2O LIGHT REACTIONS CALVIN CYCLE Light Input 3 CO 2 (Entering one at a time) Rubisco 3 P P Short-lived intermediate Phase 1: Carbon fixation 6 P 3-Phosphoglycerate 6 ATP 6 ADP CALVIN CYCLE 3 P P Ribulose bisphosphate (RuBP)

[CH 2 O] (sugar) O2O2 NADPH ATP ADP NADP + CO 2 H2OH2O LIGHT REACTIONS CALVIN CYCLE Light Input CO 2 (Entering one at a time) Rubisco 3PP Short-lived intermediate Phase 1: Carbon fixation 6 P 3-Phosphoglycerate 6 ATP 6 ADP CALVIN CYCLE 3 PP Ribulose bisphosphate (RuBP) 3 6 NADP NADPH P i 6P 1,3-Bisphosphoglycerate P 6 P Glyceraldehyde-3-phosphate (G3P) P1 G3P (a sugar) Output Phase 2: Reduction Glucose and other organic compounds

[CH 2 O] (sugar) O2O2 NADPH ATP ADP NADP + CO 2 H2OH2O LIGHT REACTIONS CALVIN CYCLE Light Input CO 2 (Entering one at a time) Rubisco 3PP Short-lived intermediate Phase 1: Carbon fixation 6 P 3-Phosphoglycerate 6 ATP 6 ADP CALVIN CYCLE 3 PP Ribulose bisphosphate (RuBP) 3 6 NADP NADPH P i 6P 1,3-Bisphosphoglycerate P 6 P Glyceraldehyde-3-phosphate (G3P) P1 G3P (a sugar) Output Phase 2: Reduction Glucose and other organic compounds 3 3 ADP ATP Phase 3: Regeneration of the CO 2 acceptor (RuBP) P 5 G3P

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Alternative mechanisms of carbon fixation have evolved in hot, arid climates Dehydration is a problem for plants, sometimes requiring tradeoffs with other metabolic processes, especially photosynthesis On hot, dry days, plants close stomata, which conserves water but also limits photosynthesis The closing of stomata reduces access to CO 2 and causes O 2 to build up These conditions favor a seemingly wasteful process called photorespiration

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Photorespiration: An Evolutionary Relic? In most plants (C 3 plants), initial fixation of CO 2, via rubisco, forms a three-carbon compound In photorespiration, rubisco adds O 2 to the Calvin cycle instead of CO 2 Photorespiration consumes O 2 and organic fuel and releases CO 2 without producing ATP or sugar

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Photorespiration may be an evolutionary relic because rubisco first evolved at a time when the atmosphere had far less O 2 and more CO 2 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

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings C 4 Plants C 4 plants minimize the cost of photorespiration by incorporating CO 2 into four-carbon compounds in mesophyll cells These four-carbon compounds are exported to bundle-sheath cells, where they release CO 2 that is then used in the Calvin cycle

Photosynthetic cells of C 4 plant leaf Mesophyll cell Bundle- sheath cell Vein (vascular tissue) C 4 leaf anatomy Stoma Bundle- sheath cell Pyruvate (3 C) CO 2 Sugar Vascular tissue CALVIN CYCLE PEP (3 C) ATP ADP Malate (4 C) Oxaloacetate (4 C) The C 4 pathway CO 2 PEP carboxylase Mesophyll cell

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings CAM Plants CAM plants open their stomata at night, incorporating CO 2 into organic acids Stomata close during the day, and CO 2 is released from organic acids and used in the Calvin cycle

Bundle- sheath cell Mesophyll cell Organic acid C4C4 CO 2 CALVIN CYCLE SugarcanePineapple Organic acids release CO 2 to Calvin cycle CO 2 incorporated into four-carbon organic acids (carbon fixation) Organic acid CAM CO 2 CALVIN CYCLE Sugar Spatial separation of stepsTemporal separation of steps Sugar Day Night

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Importance of Photosynthesis: A Review 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 In addition to food production, photosynthesis produces the oxygen in our atmosphere

Light CO 2 H2OH2O Light reactionsCalvin cycle NADP + RuBP G3P ATP Photosystem II Electron transport chain Photosystem I O2O2 Chloroplast NADPH ADP +P i 3-Phosphoglycerate Starch (storage) Amino acids Fatty acids Sucrose (export)