Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Photosynthesis: The Process That Feeds the Biosphere Photosynthesis is the process that converts solar energy into chemical energy Directly or indirectly, life on earth depends on solar energy

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Autotrophs are the producers of the biosphere, producing organic molecules from CO 2 and other inorganic molecules Photoautotrophs use the energy of sunlight to make organic molecules from water and carbon dioxide They include plants, some protists and prokaryotes

LE 10-2 Plants Unicellular protist Multicellular algaeCyanobacteria Purple sulfur bacteria 10 µm 1.5 µm 40 µm

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Heterotrophs obtain their organic material from other organisms Heterotrophs are the consumers of the biosphere Almost all heterotrophs, including humans, depend on photoautotrophs for food and oxygen

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Photosynthesis converts light energy to the chemical energy of food Chloroplasts are organelles that are responsible for feeding the vast majority of organisms Chloroplasts are present in a variety of photosynthesizing organisms

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chloroplasts: The Sites of Photosynthesis in Plants Leaves are the major locations of photosynthesis Through microscopic pores called stomata, CO 2 enters the leaf and O 2 exits Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf A typical mesophyll cell has chloroplasts The chlorophyll is in the membranes of thylakoids (connected sacs in the chloroplast) Chloroplasts also contain stroma, a dense fluid

LE 10-3 Leaf cross section Vein Mesophyll Stomata CO 2 O2O2 Mesophyll cell Chloroplast 5 µm Outer membrane Intermembrane space Inner membrane Thylakoid space Thylakoid GranumStroma 1 µm

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Tracking Atoms Through Photosynthesis Photosynthesis can be summarized as the following equation: 6 CO H 2 O + Light energy  C 6 H 12 O O H 2 O C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O + Energy Compare this to respiration: Water is oxidized and CO 2 is reduced

LE 10-4 Reactants: Products: 6 CO 2 12 H 2 O C 6 H 12 O 6 6 H 2 O 6 O 2

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Two Stages of Photosynthesis: A Preview Photosynthesis consists of the light reactions (the photo part) and Calvin cycle (the synthesis part) The light reactions (in the thylakoids) split water, release O 2, produce ATP, and form NADPH The Calvin cycle (in the stroma) forms sugar from CO 2, using ATP and NADPH Carbon fixation is the incorporation of CO 2 into organic molecules

LE 10-5_3 H2OH2O LIGHT REACTIONS Chloroplast Light ATP NADPH O2O2 NADP + CO 2 ADP P + i CALVIN CYCLE [CH 2 O] (sugar)

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The light reactions convert solar energy to the chemical energy of ATP and NADPH Light is a form of electromagnetic energy, also called electromagnetic radiation Light travels in waves Light also behaves as though it consists of discrete particles, called photons Visible light consists of colors we can see, including wavelengths that drive photosynthesis

LE 10-6 Visible light Gamma rays X-rays UV Infrared Micro- waves Radio waves 10 –5 nm 10 –3 nm 1 nm 10 3 nm10 6 nm 1 m (10 9 nm) 10 3 m nm Longer wavelength Lower energy Shorter wavelength Higher energy

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Photosynthetic Pigments: The Light Receptors Pigments are substances that absorb visible light Wavelengths that are not absorbed are reflected or transmitted Leaves appear green because chlorophyll reflects and transmits green light

LE 10-7 Chloroplast Light Reflected light Absorbed light Transmitted light Granum

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings An absorption spectrum is a graph plotting a pigment’s light absorption versus wavelength An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process

LE 10-9a Chlorophyll a Chlorophyll b Carotenoids Wavelength of light (nm) Absorption spectra Absorption of light by chloroplast pigments

LE 10-9b Action spectrum Rate of photo- synthesis (measured by O 2 release)

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chlorophyll a is the main photosynthetic pigment Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll

LE CH 3 CHO in chlorophyll a in chlorophyll b Porphyrin ring: light-absorbing “head” of molecule; note magnesium atom at center Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts; H atoms not shown

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 When a pigment molecule absorbs light, it goes from a ground state to an excited state, which is unstable 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 reaction center consists of proteins, one chlorophyll a and a primary electron acceptor A primary electron acceptor in the reaction center accepts an excited electron from chlorophyll a

LE 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

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

LE 10-13_5 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

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

LE 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 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 Photophosphorylation uses Chemiosmosis to provide free energy for ATP synthesis

LE 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 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)

LE 10-18_3 [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 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 Occurs When it is Hot and Dry In most plants (C 3 plants), initial fixation of CO 2, via rubisco, forms a three-carbon compound In photorespiration, there is not enough CO 2, and rubisco adds O 2 to the Calvin cycle 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 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

LE 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 PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Chu trình Hatch-Slack (C 4 )

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings CAM (Crassulacean Acid Metabolism)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

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

LE 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

LE 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)

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Next Lecture: Meiosis