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PHOTOSYNTHESIS

PHOTOSYNTHESIS. 1. Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes. 2. These organisms feed not only themselves but also the entire living world. LE 10-2. Plants. Unicellular protist. 10 µm. Purple sulfur bacteria. 1.5 µm. Multicellular algae.

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PHOTOSYNTHESIS

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  1. PHOTOSYNTHESIS

  2. 1. Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes. 2. These organisms feed not only themselves but also the entire living world.

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

  4. 3. Photosynthesis occurs in the chloroplasts of • green plants. 4. General equation: 6H2O + 6CO2 ----------------- C6H12O6 + 6O2 Light Chlorophyll

  5. 5. Recall that the chloroplast has a double outer membrane, a liquid stroma and an extensive inner membrane system made of thylakoids that occur in stacks (grana).

  6. 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

  7. 6. There are 2 basic parts of photosynthesis: light-dependent and light-independent reactions.

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

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

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

  11. 7. Embedded in the thylakoid membranes are the photosystems (PS I and PSII). 8. Photosystem = light harvesting complex + a reaction center. A reaction center = a chlorophyll molecule and a protein. 9. Both types of photosystems are involved in noncyclic photophosphorylation. Only PSI is involved in cyclic photophosphorylation.

  12. 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)

  13. -Each photosystem can absorb a different wavelength of light. PSI absorbs the far-red part of the spectrum best. PSII absorbs the not-so-far red part of the spectrum.

  14. 10. Noncyclic photophosphorylation • a. Light energizes chlorophyll in PS II exciting • an electron (e-) which moves to the electron • transport chain. Now we see how • photosynthesis uses light energy to get • started.

  15. 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)

  16. b. The chlorophyll that was oxidized (last slide) is reduced by the photolysis (breakdown) of water. H2O  2 e- + 2 H+ 1 oxygen These e- replace the ones lost from the chlorophyll. • Now we see why photosynthesis requires • water and why it releases oxygen!

  17. 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)

  18. c. Those protons (2 H+)from the split water are added to the lumen. Just like in cellular respiration, we are building a proton gradient. The free energy of the e- is used to power proton pumps transporting protons from the stroma to the lumen.

  19. LE 10-16 Mitochondrion Chloroplast CHLOROPLAST STRUCTURE MITOCHONDRION STRUCTURE Diffusion H+ Thylakoid space Intermembrane space Electron transport chain Membrane ATP synthase Key Stroma Matrix Higher [H+] Lower [H+] ADP + P i ATP H+

  20. d. Next, more light energizes electrons in PS I. These excited e- move to the electron transport system. e. The e- and the free protons in the stroma are used to reduce NADP+ to NADPH.

  21. 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)

  22. 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)

  23. 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)

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

  25. -Our products from noncyclic photophosphorylation are .. . ATP and NADPH -These products will be used in the dark reactions of photosynthesis.

  26. 11. Cyclic photophosphorylation occurs in PS I. It is not as common as noncyclic. Excited e- in PS I are cycled back and forth to the electron transport system. Their energy is used to pump protons in the lumen. -The only product of cyclic is ATP. The dark cycle use lots of ATP

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

  28. -For both cyclic and nocyclic: A proton gradient is then established. The protons pass down the gradient and go through ATP synthases embedded in the membrane. Their energy here is used to join ADP and Pi to form ATP (This process is called chemiosmosis).

  29. LE 10-17 H2O CO2 Light NADP+ ADP CALVIN CYCLE LIGHT REACTIONS ATP NADPH O2 [CH2O] (sugar) STROMA (Low H+ concentration) Cytochrome complex Photosystem I Photosystem II Light NADP+ reductase Light 2 H+ NADP+ + 2H+ Fd NADPH + H+ Pq Pc H2O O2 1/2 THYLAKOID SPACE (High H+ concentration) 2 H+ +2 H+ To Calvin cycle Thylakoid membrane ATP synthase STROMA (Low H+ concentration) ADP + ATP P i H+

  30. 12. Light Independent Reactions – A.K.A. – THE DARK REACTIONS! • The light dependent reactions produced • ATP’s and NADPH’s. These are used to • power the dark reactions. B. The dark reactions occur in the stroma. They occur in a cycle called the Calvin cycle.

  31. C. The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle. D. The cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH.

  32. E. Carbon enters the cycle as CO2 and leaves as a sugar named glyceraldehyde-3-phospate (G3P) F. For net synthesis of one G3P, the cycle must take place three times, fixing three molecules of CO2.

  33. G. The Calvin cycle has three phases: • *Carbon fixation (catalyzed by rubisco) • *Reduction • *Regeneration of the CO2 acceptor (RuBP)

  34. LE 10-18_1 H2O CO2 Input Light (Entering one at a time) 3 NADP+ CO2 ADP CALVIN CYCLE LIGHT REACTIONS ATP Phase 1: Carbon fixation NADPH Rubisco O2 [CH2O] (sugar) 3 P P Short-lived intermediate P 6 3 P P 3-Phosphoglycerate Ribulose bisphosphate (RuBP) 6 ATP 6 ADP CALVIN CYCLE

  35. LE 10-18_2 H2O CO2 Input Light (Entering one at a time) 3 NADP+ CO2 ADP CALVIN CYCLE LIGHT REACTIONS ATP Phase 1: Carbon fixation NADPH Rubisco O2 [CH2O] (sugar) 3 P P Short-lived intermediate P 6 3 P P 3-Phosphoglycerate Ribulose bisphosphate (RuBP) 6 ATP 6 ADP CALVIN CYCLE 6 P P 1,3-Bisphosphoglycerate 6 NADPH 6 NADP+ 6 P i P 6 Glyceraldehyde-3-phosphate (G3P) Phase 2: Reduction 1 P G3P (a sugar) Glucose and other organic compounds Output

  36. LE 10-18_3 H2O CO2 Input Light (Entering one at a time) 3 NADP+ CO2 ADP CALVIN CYCLE LIGHT REACTIONS ATP Phase 1: Carbon fixation NADPH Rubisco O2 [CH2O] (sugar) 3 P P Short-lived intermediate P 6 3 P P 3-Phosphoglycerate Ribulose bisphosphate (RuBP) 6 ATP 6 ADP 3 ADP CALVIN CYCLE 6 P P 3 ATP 1,3-Bisphosphoglycerate 6 NADPH Phase 3: Regeneration of the CO2 acceptor (RuBP) 6 NADP+ 6 P i P 5 G3P P 6 Glyceraldehyde-3-phosphate (G3P) Phase 2: Reduction 1 P G3P (a sugar) Glucose and other organic compounds Output

  37. -The majority of the 3-carbon molecules are actually recycled to keep the Calvin cycle going.

  38. The G3P are used to make sugars. These sugars are stored or used by the mitochondria in the process of cellular respiration.

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