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Photosynthesis: The Process That Nourishes the Biosphere

Learn about the process of photosynthesis, where solar energy is converted into chemical energy, sustaining the entire living world. Discover how autotrophs produce organic molecules from inorganic compounds, and how heterotrophs rely on autotrophs for food and oxygen. Explore the stages of photosynthesis and the role of chloroplasts in this essential process.

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Photosynthesis: The Process That Nourishes the Biosphere

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  1. Chapter 10 Photosynthesis

  2. Overview: The Process That Feeds the Biosphere • Photosynthesis is the process that converts solar energy into chemical energy • Directly or indirectly, photosynthesis nourishes almost the entire living world

  3. Autotrophs sustain themselves without eating anything derived from other organisms • Autotrophs are the producers of the biosphere, producing organic molecules from CO2 and other inorganic molecules • Almost all plants are photoautotrophs, using the energy of sunlight to make organic molecules from water and carbon dioxide

  4. Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes • These organisms feed not only themselves but also the entire living world

  5. LE 10-2 Plants Unicellular protist 10 µm Purple sulfur bacteria 1.5 µm Multicellular algae Cyanobacteria 40 µm

  6. 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

  7. Concept 10.1: 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

  8. Chloroplasts: The Sites of Photosynthesis in Plants • Leaves are the major locations of photosynthesis • Their green color 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, CO2 enters the leaf and O2 exits

  9. 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

  10. LE 10-3 Leaf cross section Vein Mesophyll Stomata O2 CO2 Mesophyll cell Chloroplast 5 µm Outer membrane Thylakoid Intermembrane space Thylakoid space Stroma Granum Innermembrane 1 µm

  11. Tracking Atoms Through Photosynthesis: Scientific Inquiry • Photosynthesis can be summarized as the following equation: 6 CO2 + 12 H2O + Light energy  C6H12O6 + 6 O2 + 6 H2 O

  12. The Splitting of Water • Chloroplasts split water into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules

  13. LE 10-4 12 H2O 6 CO2 Reactants: 6 O2 6 H2O C6H12O6 Products:

  14. Photosynthesis as a Redox Process • Photosynthesis is a redox process in which water is oxidized and carbon dioxide is reduced

  15. 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 O2, produce ATP, and form NADPH • The Calvin cycle (in the stroma) forms sugar from CO2, using ATP and NADPH • The Calvin cycle begins with carbon fixation, incorporating CO2 into organic molecules

  16. LE 10-5_1 H2O Light LIGHT REACTIONS Chloroplast

  17. LE 10-5_2 H2O Light LIGHT REACTIONS ATP NADPH Chloroplast O2

  18. LE 10-5_3 H2O CO2 Light NADP+ ADP + P i CALVIN CYCLE LIGHT REACTIONS ATP NADPH Chloroplast [CH2O] (sugar) O2

  19. Concept 10.2: The light reactions convert solar energy to the chemical energy of ATP and NADPH • Chloroplasts are solar-powered chemical factories • Their thylakoids transform light energy into the chemical energy of ATP and NADPH

  20. The Nature of Sunlight • Light 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

  21. 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

  22. LE 10-6 1 m (109 nm) 10–3 nm 103 nm 106 nm 10–5 nm 103 m 1 nm Gamma rays Micro- waves Radio waves X-rays Infrared UV Visible light 650 750 nm 500 550 600 700 450 380 Shorter wavelength Longer wavelength Higher energy Lower energy

  23. 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 Animation: Light and Pigments

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

  25. 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

  26. LE 10-8a Refracting prism White light Photoelectric tube Chlorophyll solution Galvanometer 0 100 The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light. Green light Slit moves to pass light of selected wavelength

  27. LE 10-8b White light Chlorophyll solution Photoelectric tube Refracting prism 0 100 The low transmittance (high absorption) reading indicates that chlorophyll absorbs most blue light. Slit moves to pass light of selected wavelength Blue light

  28. 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 • An action spectrum profiles the relative effectiveness of different wavelengths of radiation in driving a process

  29. LE 10-9a Chlorophyll a Chlorophyll b Carotenoids Absorption of light by chloroplast pigments 400 700 500 600 Wavelength of light (nm) Absorption spectra

  30. LE 10-9b Rate of photo- synthesis (measured by O2 release) Action spectrum

  31. 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 O2 • He used aerobic bacteria clustered along the alga as a measure of O2 production

  32. LE 10-9c Aerobic bacteria Filament of algae 600 500 700 400 Engelmann’s experiment

  33. 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

  34. LE 10-10 in chlorophyll a CH3 in chlorophyll b CHO 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

  35. 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

  36. LE 10-11 Excited state e– Heat Energy of electron Photon (fluorescence) Photon Ground state Chlorophyll molecule Fluorescence Excitation of isolated chlorophyll molecule

  37. 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

  38. 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

  39. LE 10-12 Thylakoid Photosystem STROMA Photon Light-harvesting complexes Reaction center Primary electron acceptor e– Thylakoid membrane Special chlorophyll a molecules Pigment molecules Transfer of energy THYLAKOID SPACE (INTERIOR OF THYLAKOID)

  40. 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

  41. 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

  42. LE 10-13_1 H2O CO2 Light NADP+ ADP CALVIN CYCLE LIGHT REACTIONS ATP NADPH O2 [CH2O] (sugar) Primary acceptor e– Energy of electrons Light P680 Photosystem II (PS II)

  43. LE 10-13_2 H2O CO2 Light NADP+ ADP CALVIN CYCLE LIGHT REACTIONS ATP NADPH O2 [CH2O] (sugar) Primary acceptor e– H2O 2 H+ + O2 1/2 e– e– Energy of electrons Light P680 Photosystem II (PS II)

  44. LE 10-13_3 H2O CO2 Light NADP+ ADP CALVIN CYCLE LIGHT REACTIONS ATP NADPH O2 [CH2O] (sugar) Primary acceptor Electron transport chain Pq e– H2O Cytochrome complex 2 H+ + O2 1/2 Pc e– e– Energy of electrons Light P680 ATP Photosystem II (PS II)

  45. LE 10-13_4 H2O CO2 Light NADP+ ADP CALVIN CYCLE LIGHT REACTIONS ATP NADPH O2 [CH2O] (sugar) Primary acceptor Primary acceptor Electron transport chain e– Pq e– H2O Cytochrome complex 2 H+ + O2 1/2 Pc e– P700 e– Energy of electrons Light P680 Light ATP Photosystem I (PS I) Photosystem II (PS II)

  46. LE 10-13_5 H2O CO2 Light NADP+ ADP CALVIN CYCLE LIGHT REACTIONS ATP NADPH Electron Transport chain O2 [CH2O] (sugar) Primary acceptor Primary acceptor Electron transport chain Fd e– Pq e– e– e– NADP+ H2O Cytochrome complex 2 H+ + 2 H+ NADP+ reductase + NADPH O2 1/2 Pc e– + H+ P700 Energy of electrons e– Light P680 Light ATP Photosystem I (PS I) Photosystem II (PS II)

  47. LE 10-14 e– ATP e– e– NADPH e– e– e– Mill makes ATP Photon e– Photon Photosystem II Photosystem I

  48. 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

  49. LE 10-15 Primary acceptor Primary acceptor Fd Fd NADP+ Pq NADP+ reductase Cytochrome complex NADPH Pc Photosystem I ATP Photosystem II

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