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Photosynthesis

Photosynthesis. Chapter 4b. Objectives. Describe energy flow through the biosphere Understand the difference between autotroph and heterotroph Describe the location and structure of a chloroplast. Explain how chloroplast structure is related to its function

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Photosynthesis

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  1. Photosynthesis Chapter 4b

  2. Objectives • Describe energy flow through the biosphere • Understand the difference between autotroph and heterotroph • Describe the location and structure of a chloroplast. Explain how chloroplast structure is related to its function • Recognize and explain the summary equation for photosynthesis • Understand the properties of light discussed in class • Summarize the light reactions of photosynthesis • Summarize the Calvin cycle reactions of photosynthesis • Describe the role of NADPH and ATP in the dark reactions

  3. Heterotroph Autotroph Biosphere: region of the world that supports life

  4. Overview of Photosynthesis • Process by which chloroplast bearing organisms transform solar (light) energy into chemical bond energy (sugars) • Light Reactions: convert solar energy into cellular energy • Calvin Cycle: manufacture sugars by converting CO2 to CH2O

  5. Photosynthesis Equation • Photosynthesis6CO2 +6H20 + light  C6H1206 + 6O2 • Reduction of carbon dioxide into carbohydrate via the oxidation of energy carriers (ATP, NADPH) • Light reactions add energy to the carriers • Calvin Cycle reactions produce sugars in the form of glucose

  6. Where is all this happening?

  7. Structure of the Chloroplast • Thylakoid: membranous system within the chloroplast (site of light reactions). Segregates the chloroplast into thylakoid space and stroma. • Stroma: region of fluid between the thylakoids and inner membrane. Where Calvin Cycle reactions take place

  8. Light: The Source of Energy • Electromagnetic energy travelling in waves • Wavelength (): distance from peak of one wave to the peak of a second wave • inverse relationship between wavelength and energy   energy

  9. Visible Spectrum • The portion of the electromagnetic spectrum that our eyes can see • White light contains all  of the visible spectrum • Colors are the reflection of specific  within the visible spectrum •  not reflected are absorbed • Composition of pigments affects their absorption spectrum

  10. Why are plants green? • Pigments contained within the thylakoid membrane absorb most  of light but absorb the green  the least • Pigments include • Chlorophyll a • Chlorophyll b • Carotenoids • Carotenes • Xanthophylls

  11. Photosystems • Collection of pigments and proteins found associated with the thylakoid membrane that harness the energy of an excited electron to do work.Photosystem I and II • Captured energy (an elevated electron) is transferred between pigmented molecules until it reaches electron-accepting proteins of an electron transport chain

  12. Recall: Electron Transport Chain • An electron transport chain is a collection of proteins foundassociated with a membrane that extract energy from electronsby moving them to lower energy orbitals. • The extracted energy is used to create a concentration gradient of H+ that will be used to make ATP.

  13. What happens in the Light Reactions? • Electrons from photosystem chlorophylls are added to an ETC • the ETC transfers the electrons to each protein in the chain resulting in the electron moving to a lower energy position and giving some energy to the proteins of the ETC • One energized ETC proteins use the energy it received to move protons (H+) against a gradient (stores potential energy)

  14. What happens in the Electron Transport Chain? • Proton gradient energy is used to make ATP as it moves through ATP synthase • The electron reaches the last protein in the chain and is added to a reaction to make NADPH • By the ETC, the light energy captured by the pigments is converted to ATP and NADPH. These energy forms will supply the energy needed to make the sugars in the Calvin cycle reactions.

  15. Dark Reactions • Light reactions have produced 2 different forms of chemical energy: ATP and NADPH • These molecules are moved to the stroma where they will be used to fuel the Calvin cycle reactions

  16. Dark Reactions Three things happen during the Calvin cycle reactions • Fixation of Carbon dioxide • Reduction of Carbon Dioxide into Carbohydrate • Remaining molecules reorganize to form starting molecules

  17. 1) Carbon Fixation • Carbon dioxide (CO2) is converted from an abiotic gas into part of an organic molecule • This is assisted by an enzyme Rubisco

  18. 2) Change of CO2 toCH2O • Carbon dioxide (now as part of a three carbon sugar) will have its energy increased resulting in CO2 being converted to CH2O. • ATP and NADPH energies made from the light reactions accomplish this change • A molecule of sugar is exported from the Calvin cycle reactions

  19. Reform starting molecule of RuBP • After the export of a sugar, the remaining molecules of the dark reaction recombine to form fewer larger molecules of RuBP. • This requires energy in the form of ATP (made during the light reactions)

  20. Photosynthesis Summary • The reactions of photosynthesis produce sugars for most living organisms on this planet • These reactions involve the conversion of solar energy into chemical energy (ATP, NADPH)

  21. Photosynthesis Summary • Chemical energy made in the light reactions is transferred to carbon dioxide via the Calvin cycle reactions converting it to carbohydrate

  22. Variations Anyone? • In hot/arid regions plants may run short of CO2 as a result of water conservation mechanisms • Photorespiration results in a drain of energy from a plant • To avoid photorespiration C4Photosynthesis and CAM Photosynthesis evolved • These mechanisms allow CO2 to be captured and stored in forms that may liberate CO2 during times of reduced availability

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