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Chapter 7 Photosynthesis: Using Light to Make Food.

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Presentation on theme: "Chapter 7 Photosynthesis: Using Light to Make Food."— Presentation transcript:

1 Chapter 7 Photosynthesis: Using Light to Make Food

2 Plant Power Photosynthesis makes sugar and oxygen gas from carbon dioxide and water – Carried out by plants, algae, and some bacteria – The ultimate source of all food eaten by animals – A major source of heat, light, and fuel


4 Using photosynthesis to meet growing energy demands – Energy plantations of trees Renewable energy source Productive Environmentally safer than fossil fuels


6 AN OVERVIEW OF PHOTOSYNTHESIS 7.1 Autotrophs are the producers of the biosphere Autotrophs produce their own food Autotrophs produce the food supply for the global ecosystem Photoautotrophs produce organic molecules using light energy – Plants, algae, and photosynthetic bacteria Video: Cyanobacteria (Oscillatoria) Video: Cyanobacteria (Oscillatoria)





11 7.2 Photosynthesis occurs in chloroplasts In plants, photosynthesis occurs primarily in leaves – CO 2 enters and O 2 exits through stomata Chloroplasts are the site of photosynthesis – Concentrated in mesophyll cells of leaves

12 Chloroplast structure – Stroma Fluid enclosed by inner membrane Location of sugar synthesis – Thylakoids Suspended in stroma Interconnected sacs Stacked in grana Contain chlorophyll in membranes

13 LE 7-2 Leaf Leaf Cross Section Mesophyll Cell Vein Mesophyll Chloroplast Stoma Stroma Chloroplast CO 2 O2O2 Granum Grana Stroma TEM 9,750 Thylakoid Thylakoid space Intermembrane space Inner membrane Outer membrane LM 2,600

14 7.3 Plants produce O 2 gas by splitting water Experiments have made it possible to follow all the atoms in photosynthesis O 2 is both a product and a reactant in photosynthesis The O 2 liberated by photosynthesis is made from the oxygen in water



17 LE 7-3c Reactants: Products: 6 CO 2 12 H 2 O C 6 H 12 O 6 6 H 2 O6 O 2

18 7.4 Photosynthesis is a redox process, as is cellular respiration Photosynthesis – H 2 O is oxidized – CO 2 is reduced – Electrons gain energy Cellular respiration – Glucose is oxidized – O 2 is reduced – Electrons lose potential energy



21 7.5 Overview: 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 O 2 as a waste product Calvin cycle – Occurs in stroma – Assembles sugar molecules from CO 2 using ATP and NADPH from light reactions

22 LE 7-5 Light Chloroplast H2OH2O LIGHT REACTIONS (in thylakoids) ATP NADPH Electrons O2O2 CO 2 Sugar P ADP NADP CALVIN CYCLE (in stroma)

23 THE LIGHT REACTIONS: CONVERTING SOLAR ENERGY TO CHEMICAL ENERGY 7.6 Visible radiation drives the light reactions Sunlight is radiation, or electromagnetic energy – Radiation travels as waves – The distance between waves is a wavelength – Visible light is only a small part of the electromagnetic spectrum – Light also behaves as discrete photons Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Animation: Light and Pigments Animation: Light and Pigments

24 LE 7-6a Increasing energy Gamma rays X-rays UV Infrared Micro- waves Radio waves Visible light Wavelength (nm) 650 nm 10 –5 10 –3 nm 1 nmnm10 3 nm10 6 nm1 m10 3 m

25 Several pigments are built into the thylakoids of chloroplasts – Absorb some wavelengths of light; reflect or transmit others Chlorophyll a – Absorbs blue-violet and red light, reflects green light – Participates directly in the light reactions

26 Chlorophyll b – Absorbs blue and orange light, reflects yellow-green – Conveys absorbed energy to chlorophyll a Carotenoids – Yellow-orange pigments that absorb mainly blue-green light – May pass energy to chlorophyll a or protect it by dissipating excessive light energy

27 LE 7-6b Light Reflected light Absorbed light Chloroplast Transmitted light

28 Light also behaves as photons – A fixed quantity of energy – Specific amounts of energy in photons absorbed by different pigments

29 7.7 Photosystems capture solar power When a pigment molecule absorbs a photon – An electron is raised in energy from a ground state to an unstable excited state – The electron rapidly drops back to the ground state – The electron releases excess energy as heat – Some pigments also emit light (fluorescence)


31 LE 7-7b Excited state Ground state Photon (fluorescence) Photon Energy of electron Heat Chlorophyll molecule

32 The thylakoid membrane contains two types of photosystems, which consist of – Light-harvesting complexes of chlorophyll and other pigments and proteins – A reaction center that contains A chlorophyll a molecule A primary electron acceptor molecule – Receives excited electrons from the chlorophyll and transfers them to an electron transport chain

33 LE 7-7c Photosystem Light-harvesting complexes Reaction center Photon Primary electron acceptor To electron transport chain Pigment molecules Chlorophyll a molecule Transfer of energy Thylakoid membrane

34 7.8 In the light reactions, electron transport chains generate ATP and NADPH Photosystem II – A pigment molecule absorbs a photon – The energy is ultimately transferred to chlorophyll P680 – The excited electrons from P680 are passed to the primary electron acceptor, then to an electron transport chain

35 – Electrons shuttle down the chain from photosystem II to photosystem I, providing energy to make ATP – Water is split, replacing electrons lost by P680 and releasing O 2 Photosystem I – A photon is absorbed and excites an electron of chlorophyll P700 – The excited electron passes through a short electron transport chain, reducing NADP + to NADPH

36 LE 7-8a Photon Photosystem II Photon Photosystem I Electron transport chain Provides energy for synthesis of by chemiosmosis ATP P700 e–e– P680 e–e– Stroma Thylakoid space Thylakoid membrane H2OH2O O2O2 H 1 2 H NADP NADPH Animation: Light Reactions Animation: Light Reactions

37 LE 7-8b Mill makes ATP NADPH Photon e–e– e–e– e–e– e–e– Photosystem IIPhotosystem I e–e– Photon e–e– e–e– ATP

38 7.9 Chemiosmosis powers ATP synthesis in the light reactions The electron transport chain – Pumps H + from the stroma into the thylakoid space – Generates a concentration gradient across the thylakoid membrane

39 Photophosphorylation – Energy of the concentration gradient drives H + back across the membrane through ATP synthase – ATP synthase couples flow of H + to the phosphorylation of ADP to produce ATP – Light provides the initial energy for the phosphorylation

40 LE 7-9 Chloroplast Stroma (low H concentration) Light H H H H H H H H H H H H H H H ATP synthase Photosystem I Electron transport chain Photosystem II Thylakoid space (high H concentration) H NADP NADPH O2O2 2 H2OH2O 1212 Thylakoid membrane ATP

41 THE CALVIN CYCLE: CONVERTING CO 2 TO SUGARS 7.10 ATP and NADPH power sugar synthesis in the Calvin cycle The Calvin cycle makes sugar in the chloroplast – Inputs Carbon from CO 2 Energy from ATP High-energy electrons from NADPH – Output Energy-rich G3P Can be used to make organic molecules

42 LE 7-10a Input CO 2 ATP NADPH CALVIN CYCLE Output:G3P

43 Steps of the Calvin cycle 1.Carbon fixation 2.Reduction 3.Release of one molecule of G3P 4.Regeneration of RuBP Animation: Calvin Cycle Animation: Calvin Cycle

44 LE 7-10b Input: 3In a reaction catalyzed by rubisco, CO 2 is added to RuBP. CO 2 3 P 6 P P 3-PGARuBP Carbon fixation Reduction Release of one molecule of G3P CALVIN CYCLE 5 P G3P 6 P 1 P Glucose and other compounds 6 6 ADP 6 6 NADP NADPH ATP P Output: Regeneration of RuBP ATP 3 ADP 3

45 PHOTOSYNTHESIS REVIEWED AND EXTENDED 7.11 Review: Photosynthesis uses light energy to make food molecules The light reactions – Capture light energy – Generate NADPH and ATP – Release O 2 and water The Calvin cycle – Manufactures sugar Cells use many of the same mechanisms in photosynthesis and cellular respiration

46 LE 7-11 Light H2OH2O CO 2 Chloroplast NADP ADP P RUBP CALVIN CYCLE (in stroma) NADPH ATP G3P 3-PGA Stroma Photosystem II Electron transport chains Photosystem I Thylakoid membranes O2O2 LIGHT REACTIONS Sugars CALVIN CYCLE Cellular respiration Cellulose Starch Other organic compounds

47 7.12 C 4 and CAM plants have special adaptations that save water C 3 plants – Corn, soybeans, wheat, rice – Use CO 2 directly – Rate of photosynthesis decreases in dry weather Stomata close, CO 2 supply reduced Calvin cycle diverted to photorespiration

48 C 4 plants – Include corn and sugarcane – Stomata closed in hot, dry weather – CO 2 fixed into a 4-carbon compound that acts as a carbon shuttle Enables plant to continue making sugar Prevents photorespiration and water loss

49 CAM plants – Pineapple, cactus, succulents – Adapted to very dry climates Open stomata and admit CO 2 only at night Fix CO 2 into a 4-carbon compound that banks CO 2 for release to Calvin cycle during the day

50 LE 7-12 Mesophyll cell CO 2 4-C compound CO 2 4-C compound Night Day CAM plantC 4 plant CALVIN CYCLE CALVIN CYCLE 3-C sugar Bundle-sheath cell SugarcanePineapple

51 PHOTOSYNTHESIS, SOLAR RADIATION, AND EARTH'S ATMOSPHERE: CONNECTION 7.13 Photosynthesis moderates global warming The greenhouse effect – Like a greenhouse, the atmosphere traps CO 2 and other gases that absorb heat radiated from Earth's surface – Excess greenhouse gases in the atmosphere contribute to global warming Photosynthesis, which removes CO 2 from the atmosphere, moderates warming Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings


53 LE 7-13b Sunlight Radiant heat trapped by CO 2 and other gases Atmosphere Some heat energy escapes into space

54 TALKING ABOUT SCIENCE 7.14 Mario Molina talks about Earth's protective ozone layer Nobel Prize winner Mario Molina has studied how pollutants are affecting Earth's ozone layer – Solar radiation converts O 2 high in the atmosphere to ozone (O 3 ) – The ozone layer shields organisms on Earth's surface from damaging UV radiation – CFCs have caused dangerous thinning of the ozone layer – International restrictions on CFC use are allowing a slow recovery


56 LE 7-14b Southern tip of South America Antarctica

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