1. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chapter 10: Photosynthesis The Process That Feeds the Biosphere Autotrophs (producers) vs. heterotrophs (consumers) Photosynthesis happens in the chloroplasts Leaf cell structure – Chloroplasts and organelle structure within leaf cells Photosynthesis – Chlorophyll and how it absorbs light and energy – Photosystems I and II are the ‘light’ reactions. ATP and NADH from these photosystems flow into the Calvin cycle. – Calvin cycle: The synthesis part of photosynthesis where sugar is produced from the light energy.

2. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Autotrophs vs. heterotrophs Photosynthesis is the process that converts solar energy into chemical energy Directly or indirectly, photosynthesis nourishes almost the entire living world Autotrophs sustain themselves without eating anything derived from other organisms Autotrophs are the producers of the biosphere, producing organic molecules from CO 2 and other inorganic molecules Almost all plants are photoautotrophs, using the energy of sunlight to make organic molecules from water and carbon dioxide Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes These organisms feed not only themselves but also the entire living world

3. Plants Unicellular protist Multicellular algaeCyanobacteria Purple sulfur bacteria 10 µm 1.5 µm 40 µm Diverse organisms carry out photosynthesis

4. 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 Heterotrophs depend on autotrophs for survival

5. 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 Chloroplasts are the sites of photosynthesis in plants Leaves are the major locations of photosynthesis Their green color of leaves is from chlorophyll, the green pigment within chloroplasts Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast Through microscopic pores called stomata, CO 2 enters the leaf and O 2 exits

6. 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 Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf A typical mesophyll cell has 30-40 chloroplasts The chlorophyll is in the membranes of thylakoids (connected sacs in the chloroplast); thylakoids may be stacked in columns called grana Chloroplasts also contain stroma, a dense fluid A typical leaf cell

7. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Tracking Atoms Through Photosynthesis: Scientific Inquiry Photosynthesis can be summarized as the following equation: 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O Reactants: Products: 6 CO 2 12 H 2 O C 6 H 12 O 6 6 H 2 O 6 O 2 Chloroplasts split water into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules Photosynthesis is a redox process in which water is oxidized and carbon dioxide is reduced

8. 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 The Calvin cycle begins with carbon fixation, incorporating CO 2 into organic molecules

9. H2OH2O LIGHT REACTIONS Chloroplast Light ATP NADPH O2O2 NADP + CO 2 ADP P + i CALVIN CYCLE [CH 2 O] (sugar) The Two Stages of Photosynthesis thykaloids stroma

10. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The light reactions convert solar energy to the chemical energy of ATP and NADPH Chloroplasts are solar-powered chemical factories Their thykaloids transform light energy into the chemical energy of ATP and NADPH Light from the sun is a form of electromagnetic energy, also called electromagnetic radiation Like other electromagnetic energy, light travels in rhythmic waves Wavelength = distance between crests of waves Wavelength determines the type of electromagnetic energy Light also behaves as though it consists of discrete particles, called photons The electromagnetic spectrum is the entire range of electromagnetic energy, or radiation Visible light consists of colors we can see, including wavelengths that drive photosynthesis

11. 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 380 450 500550600 650 700 750 nm Longer wavelength Lower energy Shorter wavelength Higher energy The electromagnetic spectrum

12. Chloroplast Light Reflected light Absorbed light Transmitted light Granum Photosynthetic Pigments: The Light Receptors Pigments are substances that absorb visible light Different pigments absorb different wavelengths Wavelengths that are not absorbed are reflected or transmitted Leaves appear green because chlorophyll reflects and transmits green light

13. White light Refracting prism Chlorophyll solution Photoelectric tube Galvanometer The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light. Green light Slit moves to pass light of selected wavelength 0 100 A spectrophotometer measures a pigment’s ability to absorb various wavelengths This machine sends light through pigments and measures the fraction of light transmitted at each wavelength Spectrophotometer: chlorophyll absorbs very little green light

14. White light Refracting prism Chlorophyll solution Photoelectric tube The low transmittance (high absorption) reading indicates that chlorophyll absorbs most blue light. Blue light Slit moves to pass light of selected wavelength 0 100 Spectrophotometer: chlorophyll absorbs LOTS of blue light

15. Chlorophyll a Chlorophyll b Carotenoids Wavelength of light (nm) Absorption spectra Absorption of light by chloroplast pigments 400 500600 700 An absorption spectrum is a graph plotting a pigment’s light absorption versus wavelength The absorption spectrum of chlorophyll a suggests that violet-blue and red light work best for photosynthesis

16. Action spectrum Rate of photo- synthesis (measured by O 2 release) An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process

17. Engelmann’s experiment 400 500 600 700 Aerobic bacteria Filament of algae The action spectrum of photosynthesis was first demonstrated in 1883 by Thomas Engelmann In his experiment, he exposed different segments of a filamentous alga to different wavelengths Areas receiving wavelengths favorable to photosynthesis produced excess O 2 He used aerobic bacteria clustered along the alga as a measure of O 2 production

18. 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 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 The structure of Chlorophyll

19. Excited state Heat Photon (fluorescence) Ground state Chlorophyll molecule Photon Excitation of isolated chlorophyll molecule Fluorescence Energy of electron e–e– Excitation of Chlorophyll by Light When a pigment absorbs light, it goes from a ground state to an unstable excited state When excited electrons fall back to the ground state, photons are given off, an afterglow called fluorescence If illuminated, a solution of chlorophyll will fluoresce, giving off light and heat

20. 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, such as chlorophyll, that are bound to proteins) funnel the energy of photons to the reaction center 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

21. 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 A Photosystem

22. 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 light at a wavelength of 680 nm Photosystem I is best at absorbing light at a wavelength of 700 nm The two photosystems work together to use light energy to generate ATP and NADPH Plants have two photosystems

23. 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 + + 2 H + Light The coupling of the two photosystems comprises photosynthesis

24. 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 An analogy of how high energy electrons generated by light photons drives photosynthesis

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

26. Photosystem I Photosystem II ATP Pc Fd Cytochrome complex Pq Primary acceptor Fd NADP + reductase NADP + NADPH Primary acceptor Cyclic and Noncyclic Electron Flow

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

28. 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 + ] A Comparison of Chemiosmosis in Chloroplasts and Mitochondria

29. 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 Where does photosynthesis occur?

30. 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 Where does photosynthesis occur?

31. 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 The Calvin cycle has three phases: – Carbon fixation (catalyzed by rubisco) – Reduction – Regeneration of the CO 2 acceptor

32. [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 + 6 6 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 The Calvin cycle

33. 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

34. 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 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

35. 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 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

36. 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

37. 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 C4 Plants Vs. CAM Plants

38. 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

39. 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) The Importance of Photosynthesis: A Review