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The Sun

The Sun. The Visible Spectrum of Light. Photosynthesis uses only small visible portion of the electromagnetic spectrum Wavelengths of visible light most important for photosynthesis. The symbol for wavelength is λ. Fig. 8.3. How Electrons Capture Energy.

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The Sun

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  1. The Sun

  2. The Visible Spectrum of Light • Photosynthesis usesonly small visible portion of the electromagneticspectrum • Wavelengths of visible light most important forphotosynthesis. • The symbol for wavelength is λ

  3. Fig. 8.3

  4. How Electrons Capture Energy • Electrons can absorb radiant energy. • Radiant energy comes in parcels called photons • When electrons absorb energy, they hop to a higher shell. • When electrons release energy, drop back to the lower shell. • The energy released is a kind of light energy called fluorescence.

  5. Absorption of Light Energy • Light energy is absorbed by electrons • The energy causes electrons to jump shells; the more energy absorbed, the further away electrons move from the nucleus • The energy may be shed as fluorescence • Or transferred in the form of an electron to another molecule

  6. Leaf Pigments Absorb Light Energy • Leaf Pigments absorb light energy • Chlorophylls a and b • Carotenoids

  7. Chlorophylls are Leaf Pigments • Chlorophylls collect light energy (absorbs it) in a resonant porphyrin group that hangs out like a kite on the surface of the thylakoid • Chlorophyll a initiates the light-dependent reactions • Chlorophyll b is an accessory pigment • Carotenoids are yellow and orange pigments that capture light energy and pass electrons to chlorophyll

  8. The Structure of Chlorophyll • Note the double bonds and the Mg2+ ion • The positive charges on the Mg2+ ion attract electrons • The electrons bounce around the porphyrin ring

  9. Photosynthesis 6CO2 + 12H2O + light energy → C6H12O6 + 6O2

  10. Leaf Structure • Leaves have a layered organization • The mesophyll tissue (middle layers of cells) is the main site of photosynthesis • Sap flows through the veins

  11. Internal Structure of a Leaf

  12. The Chloroplast • The site of light harvesting or energy capture • The site of the start of carbohydrate synthesis

  13. Photosynthesis in the Chloroplast • The light-dependent reactions (the harvesting of light) occur on thylakoid membranes • The carbon fixation reactions (formation of carbohydrate) occur in the stroma

  14. 6 CO + 12H O C H O + 6O 6 12 6 2 Photosynthesis 6CO2 + 12H2O + light energy → C6H12O6 + 6O2 • Carbon dioxide is reduced to sugar • Water is converted to oxygen 2 2

  15. Three Energy Carriers of Photosynthesis • NADH • NADPH: • Much like NADH except that it bears a phosphate • Phosphate is attached to the sugar group • ATP

  16. Light-Dependant Reactions Cyclic photophosphorylation Non-cyclic photophosphorylation Light-Independent reactions Calvin cycle (C3) Two Major Steps in Photosynthesis

  17. Understand this slide!

  18. The Light Dependant Reactions • Water molecules are split apart, producing electrons and hydrogen ions, and O2 gas is released. • Electrons from the split water are passed along an electron transport chain • Energy storing ATP molecules are produced • Hydrogen from the split water is transferred from • NADP  NADPH and used in the light independent reactions

  19. Capturing Light Energy • Photons are absorbed by chlorophyll • Energy (as an electron) “falls” from one chlorophyll to the next.

  20. Two Different ways to Photosynthesize in the Light-Dependant Reactions 1. Cyclic photophosphorylation • e- run in a cycle • makes ATP • No carbohydrate made • Uses only P700 2. Noncyclic photophosphorylation • e- derived from splitting of water • Releases O2 • Makes lots of ATP • Makes carbohydrate • Uses P700 and P680

  21. Cyclic Photophosphorylation • Photosystem I • P700 reaction center • Energized e- hops to an e- acceptor protein • e- “falls” from protein complex to protein complex, losing energy as it cascades down

  22. Cyclic Photophosphorylation • Does not produce any carbohydrate • But still makes some ATP • Found in certain primitive plants • Found also in bacteria • Also seen in plants that have sufficient carbohydrate but need ATP:

  23. Noncyclic Photophosphorylation • Plants that need a lot of energy must make a large quantities of carbohydrate. • They use noncyclic photophosphorylation • Both PII (P680) and PI (P700) are used. • The 680/700 designations indicate the peak wavelengths absorbed by each chlorophyll reaction center

  24. Steps of Non-cyclic Photophosporylation • Photosystem II (P680) uses light energy to split a water molecule– rips it apart into electrons, protons and oxygen • e- cascades from protein complex to protein complex losing energy. • The e- gets re-energized by photosystem I (P700) and “falls” down another cascade of protein complexes. • The energy from the e- cascades is used to pump H+ from the split water molecule into the intermembrane space of the thylakoid. • The H+ from the water ultimately is used to make ATP and NADPH

  25. Noncyclic Photophosphorylation

  26. Non-cyclic Photophosphorylation

  27. The Electron Transport Chain

  28. The Chloroplast ATP Synthase • The H+ forms a proton gradient. • The H+ moves from [H]→ [L] • H+ are transported through the ATP synthase • Makes ATP by combining ADP and phosphate • The H+ that have been pumped into the stroma are used to make NADPH

  29. The Light Independent Reactions • So-called because they do not directly need light (radiant energy) • They occur in the stroma of the chloroplast • They fix carbon to make carbohydrate • Named theCalvin Cycle

  30. The Light Independent Reactions • CO2 is combined with RuBP to yield a 6C sugar • Enzyme RuBP carboxylase • 6C sugar is broken into TWO 3C sugars • NADPH and ATP supply the energy for the conversion • Most of the 3C sugar gets recycled into the Calvin cycle • The remainder gets converted into sucrose Takes place in the stroma of the chloroplast

  31. RuBisCO • Ribulose bisphosphate carboxylase/oxygenase • Most abundant enzyme in the whole world • Carbon fixation enzyme

  32. Fig. 8.13

  33. The PCR (Calvin) Cycle: • CO2 uptake • Catalyzed by the enzyme RUBISCO; ribulose bisphosphate carboxylase • ATP and NADPH from the light-dependant reactions are used as energy sources to rearrange the 3-C sugars

  34. Most of the 3-C sugars are sent to the cytoplasm to make glucose • The remaining 3-C sugars are recycled back into the Calvin cycle to regenerate RUBP

  35. The PCRCycle is very expensive in terms of Energy • CO2 is the carbon source • For each glucose to be formed, 18 ATP and 12 NADPH are used!

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