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Photosynthesis

This text explains the process of photosynthesis, including the two stages - light reactions and the Calvin cycle. It discusses the role of pigments, electron flow, and the production of ATP and NADPH. The text also covers the different wavelengths of light and the importance of water and carbon dioxide in the process.

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Photosynthesis

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  1. Photosynthesis

  2. 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

  3. Photosynthesis as a Redox Process • A simplified equation of photosynthesis that indicates only the net consumption of water is as follows: • Photosynthesis is a redox process in which water is oxidized and carbon dioxide is reduced 6 CO2 + 6 H2O + Light energy  C6H12O6 + 6 O2

  4. The Two Stages of Photosynthesis: A Preview • Photosynthesis consists of the: • light reactions (the photo part) and • Calvin cycle (the sugar synthesis part) • The light reactions split water, release O2, produce ATP, and form NADPH • The Calvin cycle forms sugar from CO2, using ATP and NADPH made in the light reactions.

  5. 1- The light reactions (in the thylakoids) split water, release O2, produce ATP, and form NADPH H2O → 2H+ + 2e- + ½ O2 H2O Light LIGHT REACTIONS ATP NADPH Chloroplast O2

  6. 2- The Calvin cycle (in the stroma) forms sugar from CO2, using ATP and NADPH made in the light reactions. H2O CO2 Light NADP+ ADP + P i CALVIN CYCLE LIGHT REACTIONS ATP NADPH Chloroplast [CH2O] (sugar) O2

  7. The electromagnetic spectrum is the entire range of electromagnetic energy, or radiation 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 consists of colors we can see,including wavelengths that drive photosynthesis Visible light 650 750 nm 500 550 600 700 450 380 Shorter wavelength Longer wavelength Higher energy Lower energy

  8. Comparing the Wavelengths of the Different Colors of the Visible Light

  9. 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

  10. Photosynthetic Pigments • The main photosynthetic pigment is called • Chlorophyll a. • The accessory pigments are: • chlorophyll b • carotenoids absorb excessive light that would damage chlorophyll. Types of carotenoids: • Carotenes (are pure hydrocarbons, made solely from C & H. For example: -carotene in carrots) • Xanthophylls(are oxygenated hydrocarbons, made of C, H & O. For example lycopene, the red pigment in tomatoes)

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

  12. The Light Reactions • During the light reactions, there are two possible routes for electron flow: 1- the linear (noncyclic) electron flow. 2- the cyclic electron flow.

  13. The Linear Electron Flow • The linear (noncyclic) electron flow: • Is the primary pathway of the light reactions. • Occurs in the thylakoid membranes. • Involves both photosystems (PSI & PSII). • Splits water (H2O → 2H+ + 2e- + ½ O2) • Uses light energy to produces ATP and NADPH

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

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

  16. LE 10-13_3 H2O CO2 Light NADP+ ADP CALVIN CYCLE LIGHT REACTIONS ATP NADPH O2 [CH2O] (sugar) The Linear Electron Flow 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)

  17. LE 10-13_4 H2O CO2 The Linear Electron Flow 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)

  18. LE 10-13_5 The Linear Electron Flow 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)

  19. A mechanical analogy for the linear electron flow of the light reactions e– ATP e– e– NADPH e– e– e– Mill makes ATP Photon e– Photon Photosystem II Photosystem I

  20. The Cyclic Electron Flow • The cyclic electron flow: • Occurs in the thylakoid membranes. • Uses only photosystem I • Excited electrons leave P700 and return to it. • Does not produce NADPH or O2 • Produces only ATP. • Cyclic electron flow generates surplus ATP, satisfying the higher demand in the Calvin cycle

  21. LE 10-15 The Cyclic Electron Flow Primary acceptor Primary acceptor Fd Fd NADP+ Pq NADP+ reductase Cytochrome complex NADPH P700 Pc Photosystem I ATP Photosystem II

  22. 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+

  23. The Phases of the Calvin Cycle • The Calvin cycle has three phases: • Carbon fixation phase catalyzed by the enzyme: ribulose bishosphate carboxylase oxygenase (rubisco) • Reduction: Making the sugar G3P. • Regeneration of the CO2 acceptor 1,5-Ribulose Bisphosphate (RuBP) http://www.science.smith.edu/departments/Biology/Bio231/calvin.html

  24. LE 10-21 Light reactions Calvin cycle H2O CO2 Light NADP+ ADP + P i RuBP 3-Phosphoglycerate Photosystem II Electron transport chain Photosystem I ATP G3P Starch (storage) NADPH Amino acids Fatty acids Chloroplast O2 Sucrose (export)

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