Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings PowerPoint Lectures for Biology: Concepts and Connections, Fifth Edition – Campbell, Reece, Taylor, and Simon Lectures by Chris Romero Chapter 7 Photosynthesis: Using Light to Make Food

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Photosynthesis is the process by which certain organisms use light energy –To make sugar and oxygen gas from carbon dioxide and water Light energy PHOTOSYNTHESIS 6 CO 2 6+ H2OH2O Carbon dioxideWater C 6 H 12 O 6 6+ O2O2 GlucoseOxygen gas

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings As the human demand for energy grows –Fossil fuel supplies are dwindling Energy plantations –Are being planted to serve as a renewable energy source

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings AN OVERVIEW OF PHOTOSYNTHESIS 7.1 Autotrophs are the producers of the biosphere Plants are autotrophs –Producing their own food and sustaining themselves without eating other organisms

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Plants, algae, and some bacteria are photoautotrophs –Producers of food consumed by virtually all organisms Figure 7.1A–D

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings 7.2 Photosynthesis occurs in chloroplasts In plants, photosynthesis –Occurs primarily in the leaves, in the chloroplasts, which contain stroma, and stacks of thylakoids called grana Figure 7.2 Leaf Cross Section Leaf Mesophyll Cell Mesophyll Vein Stoma CO 2 O2O2 Chloroplast Grana Stroma TEM 9,750  Stroma Granum Thylakoid space Outer membrane Inner membrane Intermembrane space LM 2,600 

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings 7.3 Plants produce O 2 gas by splitting water The O 2 liberated by photosynthesis –Is made from the oxygen in water Reactants: Products: 6 CO 2 12 H 2 O C 6 H 12 O 6 6 H 2 O 6 O 2 Labeled Experiment 1 Experiment 2 6 CO 2 12 H 2 O 6 CO 2 12 H 2 O C 6 H 12 O 6 6 H 2 O6 O 2 Not labeled C 6 H 12 O 6 6 H 2 O6 O Figure 7.3A–C

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings 7.4 Photosynthesis is a redox process, as is cellular respiration In photosynthesis –H 2 O is oxidized and CO 2 is reduced Figure 7.4A, B Reduction Oxidation 6 O 2 6 H 2 O Reduction Oxidation 6 O 2 6 CO 2  6 H 2 OC 6 H 12 O 6   6 CO 2 

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings 7.5 Overview: Photosynthesis occurs in two stages linked by ATP and NADPH The complete process of photosynthesis consists of two linked sets of reactions –The light reactions and the Calvin cycle

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings The light reactions –Convert light energy to chemical energy and produce O 2 The Calvin cycle assembles sugar molecules from CO 2 –Using ATP and NADPH from the light reactions Figure 7.5 Light CO 2 H2OH2O Chloroplast LIGHT REACTIONS (in thylakoids) CALVIN CYCLE (in stroma) NADP + ADP +P ATP NADPH OSugar Electrons

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY 7.6 Visible radiation drives the light reactions Certain wavelengths of visible light, absorbed by pigments –Drive the light reactions of photosynthesis Figure 7.6A, B Increasing energy 10 –5 nm10 –3 nm 1 nm 10 3 nm10 6 nm 1 m 10 3 m Gamma rays X-raysUVInfrared Micro- waves Radio waves Visible light nm Wavelength (nm) Transmitted light Absorbed light Reflected light Light Chloroplast 380

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings 7.7 Photosystems capture solar power Thylakoid membranes contain multiple photosystems –That absorb light energy, which excites electrons Figure 7.7A

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Each photosystem consists of –Light-harvesting complexes of pigments –A reaction center with a primary electron acceptor that receives excited electrons from a reaction-center chlorophyll Figure 7.7B, C Energy of electron Photon Excited state Heat Photon (fluorescence) Ground state Chlorophyll molecule e–e– Photosystem Light-harvesting complexes Reaction center Primary electron acceptor e–e– To electron transport chain Pigment molecules Chlorophyll a molecule Transfer of energy Photon Thylakoid membrane

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings 7.8 In the light reactions, electron transport chains generate ATP and NADPH Two connected photosystems absorb photons of light –And transfer the energy to chlorophyll P680 and P700

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings The excited electrons –Are passed from the primary electron acceptor to electron transport chains Figure 7.8A Thylakoid space Photon Stroma Thylakoid membrane 1 Photosystem II e–e– P680 2 H2OH2O O2O2 H+H+ 3 ATPElectron transport chain Provides energy for synthesis of by chemiosmosis 4 Photosystem I Photon P700 e–e– 5 + NADP + H+H+ NADPH 6

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Electrons shuttle from photosystem II to I –Providing energy to make ATP Electrons from photosystem I –Reduce NADP + to NADPH Figure 7.8B e–e– ATP Mill makes ATP Photon Photosystem II Photosystem I NADPH e–e– e–e– e–e– e–e– e–e– e–e–

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Photosystem II regains electrons by splitting water –Releasing O 2

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings 7.9 Chemiosmosis powers ATP synthesis in the light reactions The electron transport chain –Pumps H + into the thylakoid space

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Chloroplast Stroma (low H + concentration) Light NADP + + H+H+ NADPH H+H+ H+H+ H+H+ H+H+ ATP P ADP + Thylakoid membrane H2OH2O 1 2 O2O2 2 H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ Photosystem II Electron transport chain Photosystem I ATP synthase Thylakoid space (high H + concentration) + The diffusion of H + back across the membrane through ATP synthase –Powers the phosphorylation of ADP to produce ATP (photophosphorylation) Figure 7.9

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings THE CALVIN CYCLE: CONVERTING CO 2 TO SUGARS 7.10 ATP and NADPH power sugar synthesis in the Calvin cycle The Calvin cycle –Occurs in the chloroplast’s stroma –Consists of carbon fixation, reduction, release of G3P, and regeneration of RuBP Figure 7.10A Input CO 2 ATP NADPH CALVIN CYCLE G3POutput:

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Using carbon from CO 2, electrons from NADPH, and energy from ATP –The cycle constructs G3P, which is used to build glucose and other organic molecules Figure 7.10B CALVIN CYCLE 3 3 P CO 2 Step Carbon fixation. An enzyme called rubisco combines CO 2 with a five-carbon sugar called ribulose bisphosphate (abbreviated RuBP). The unstable product splits into two molecules of the three-carbon organic acid, 3-phosphoglyceric acid (3-PGA). For three CO 2 entering, six 3-PGA result. 1 Input: In a reaction catalyzed by rubisco, CO 2 is added to RuBP. P 6 P RuBP 3-PGA 1 G3P 6P 2 Step Reduction. Two che- mical reactions (indicated by the two blue arrows) consume energy from six molecules of ATP and oxidize six molecules of NADPH. Six molecules of 3- PGA are reduced, producing six molecules of the energy- rich three-carbon sugar, G3P 6 ATP 6ADP +P 6NADPH 6 NADP Step Release of one molecule of G3P. Five of the G3Ps from step 2 remain in the cycle. The single molecule of G3P you see leaving the cycle is the net product of photosynthesis. A plant cell uses two G3P molecules to make one molecule of glucose. Output: 1 P G3P Glucose and other compounds ADP ATP 4 Step Regeneration of RuBP. A series of chemical reactions uses energy from ATP to rearrange the atoms in the five G3P molecules (15 carbons total), forming three RuBP molecules (15 carbons).These can start another turn of the cycle. 5P G3P 4

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings PHOTOSYNTHESIS REVIEWED AND EXTENDED 7.11 Review: Photosynthesis uses light energy to make food molecules Figure 7.11 Light H2OH2OCO 2 NADP + Photosystem II Photosystem I Electron transport chains ADP P + RUBP CALVIN CYCLE (in stroma) 3-PGA Stroma G3P NADPH ATP O2O2 LIGHT REACTIONSCALVIN CYCLE Sugars Cellular respiration Cellulose Starch Other organic compounds Thylakoid membranes Chloroplast

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings 7.12 C 4 and CAM plants have special adaptations that save water In C 3 plants a drop in CO 2 and rise in O 2 when stomata close on hot dry days –Divert the Calvin cycle to photorespiration

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings C 4 plants first fix CO 2 into a four-carbon compound –That provides CO 2 to the Calvin cycle Figure 7.12 (left half) Sugarcane C 4 plant CALVIN CYCLE 3-C sugar CO 2 4-C compound CO 2 Mesophyll cell Bundle-sheath cell

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings CAM plants open their stomata at night –Making a four-carbon compound used as a CO 2 source during the day CO 2 Figure 7.12 (right half) CAM plant Day CALVIN CYCLE 3-C sugar CO 2 4-C compound Night Pineapple CO 2

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings PHOTOSYNTHESIS, SOLAR RADIATION, AND EARTH ’ S ATMOSPHERE CONNECTION 7.13 Photosynthesis moderates global warming Greenhouses used to grow plants – Trap solar radiation, raising the temperature inside Figure 7.13A

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Excess CO 2 in the atmosphere –Is contributing to global warming Figure 7.13B Sunlight ATMOSPHERE Some heat energy escapes into space Radiant heat trapped by CO 2 and other gases

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Photosynthesis, which removes CO 2 from the atmosphere –Moderates this warming

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings TALKING ABOUT SCIENCE 7.14 Mario Molina talks about Earth’s protective ozone layer Figure 7.14A

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Solar radiation converts O 2 high in the atmosphere to ozone (O 3 ) –Which shields organisms on the Earth’s surface from the damaging UV radiation

Copyright © 2005 Pearson Education, Inc. Publishing as Benjamin Cummings Industrial chemicals called CFCs have caused dangerous thinning of the ozone layer –But international restrictions on CFC use are allowing recovery Figure 7.14B Southern tip of South America Antarctica