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Photosynthesis- conversion of light energy to chemical energy Where does it occur

Photosynthesis- conversion of light energy to chemical energy Where does it occur What are the pathways What are the adaptations to hot, dry climates. Autotrophs require only CO 2 , water and light (Where does the CO 2 come from?) Terrestrial plants Algae Some protists (e.g., Euglena )

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Photosynthesis- conversion of light energy to chemical energy Where does it occur

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  1. Photosynthesis- conversion of light energy to chemical energy Where does it occur What are the pathways What are the adaptations to hot, dry climates

  2. Autotrophs require only CO2, water and light (Where does the CO2 come from?) Terrestrial plants Algae Some protists (e.g., Euglena) Cyanobacteria Purple sulfur bacteria Many kingdoms represented! How does photosynthesis occur in plants?

  3. Primarily in leaves, in chloroplasts Interior tissue of leaf: mesophyll dozens of chloroplasts per cell Stomata let CO2 in and O2 out Veins provide transport throughout plant water from roots to leaves sugar from leaves to other structures

  4. How does photosynthesis happen? 6CO2 + 12H2O + light  C6H12O6 + 6O2 + 6H2O Glucose is eventually formed from 3-carbon products The oxygen is produced from the splitting of water, not carbon dioxide Confirmed using heavy oxygen

  5. Water is oxidized, CO2 is reduced

  6. Localization of photosynthesis reactions What does NADPH do? ATP?

  7. How is light energy captured? Different wavelengths of light contain different amounts of energy (photons) Visible light ranges from approx. 380-750 nm Pigments absorb some wavelengths of light and reflect others (refer to pp. 183-4 in anticipation of lab!)

  8. Chlorophyll a directly initiates light reactions Other pigments absorb light, transfer energy to chlorophyll a chlorophyll b carotenoids May have a “dampening effect” (photoprotection)

  9. When photons are absorbed by pigments their energy is transferred to an electron Electron falls back to ground state, giving off heat Some pigments (such as chlorophyll) also give off light (fluorescence; chlorophyll fluoresces red) Fluorescent “tags” have many applications Photosystems are specialized to harvest energy from light

  10. Two types of photosystems Photosystem I- P700 Photosystem II- P680 Electron flow: cyclic or (predominantly) non-cyclic

  11. Summary: when photosystem II absorbs light it becomes an oxidizing agent- oxidizes water and replaces electrons Electrons fall down the electron transport chain and energy is used to form ATP (photophosphorylation) Light hits photosystem I and releases electrons electrons from photosystem II “replace them NADPH is generated

  12. Cyclic phosphorylation

  13. When is this pathway used? When ATP is scarce (Calvin cycle uses more ATP than NADPH) How are these reactions organized in the thylakoid membrane?

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