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Photosynthesis:

Photosynthesis:. The Light Reactions & The Calvin Cycle. Photosynthesis: Overview. 4.3 The Light Reactions. General Summary Info Conversion of visible light into chemical energy needed to produce sugars in the Calvin cycle

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Photosynthesis:

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  1. Photosynthesis: The Light Reactions & The Calvin Cycle

  2. Photosynthesis: Overview

  3. 4.3 The Light Reactions General Summary Info • Conversion of visible light into chemical energy needed to produce sugars in the Calvin cycle • Chlorophyll in the thylakoid absorb the light, water is split into Hydrogen and Oxygen, & light energy is converted into chemical energy.

  4. 4.3 The Light Reactions • Light absorbing pigments form 1 of 2 kinds of clusters: Photosystem (PS) I and II. • The PS absorbs light energy & this energy gets transferred to different molecules until it ends up at a reaction center • Reaction center – a specific chlorophyll a molecule • The reaction center accumulates so much energy that some of its e- jump to electron carriers

  5. 4.3 The Light Reactions

  6. 4.3 The Light Reactions • These electron carriers form an electron transport system between the 2 photosystems. • Electrons from PS II replace electrons lost from PS I. • PS II receives replacements from an enzyme near its reaction center that splits water into protons, electrons, & oxygen. 2 H2O  4 H+ + 4 e- + O2

  7. 4.3 The Light Reactions • When the enzyme oxidizes the water, oxygen is released as a gas & protons build up in the thylakoid. • The e- replace the lost e- in PS II.

  8. 4.3 The Light Reactions • When e- from H2O reach PS I, they receive an energy boost from the reaction center & this energy is used to reduce NADP+ into NADPH. • Both the protons and electrons from water are used to convert NADP+ • The protons & electrons of NADPH is then used later to reduce CO2 in the Calvin Cycle.

  9. 4.3 The Light Reactions

  10. 4.3 The Light Reactions • As electrons are flowing, some of the solar energy powers the active transport of H+ across the thylakoid membrane. • So a large number of H+ build up inside the thylakoid causing a difference in charge, which creates a difference in potential energy (think of a battery). • Just like a battery, this potential energy can do work.

  11. 4.3 The Light Reactions • The protons diffuse out of the thylakoid through an enzyme complex (ATP synthetase) & as they pass through, they transfer energy to the ATP synthetase. • ATP synthetase uses the energy to synthesize ATP from ADP & a phosphate.

  12. 4.3 The Light Reactions A Summary • Energy from light forces electrons to flow from water to NADP+. • The electrons retain the energy in NADPH, which is then used to synthesize ATP. • So the light reactions, convert light energy into the chemical energy found in ATP and NADPH, with the overall products being O2, ATP, & NADPH.

  13. Light Reactions Animation http://www.stolaf.edu/people/giannini/flashanimat/metabolism/photosynthesis.swf

  14. 4.4 The Calvin Cycle General Summary Intro • Saves the chemical energy produced in the light reactions in the form of sugars • The Calvin Cycle occurs in the stroma of the chloroplasts.

  15. 4.4 The Calvin Cycle 1. CO2 combines with a 5 carbon phosphate-sugar, called ribulose biphosphate (RuBP). This is called carbon fixation because carbon dioxide gas is “fixed” into an organic molcule. This produces an unstable 6 carbon molecule, which instantly breaks down into two 3 carbon molecules called phosphoglyceric acid (PGA). Catalyzed by Rubisco

  16. 4.4 The Calvin Cycle 2. Each molecule of PGA is reduced to the 3 carbon sugar-phosphate molecule, phoshpoglyceraldehyde (PGAL). This requires 1 ATP and 1 NADPH (from the light reactions).

  17. 4.4 The Calvin Cycle 3. A series of enzymes catalyzes the combination and rearrangement of the PGAL, producing a 5 carbon sugar-phosphate, ribulose 5-phosphate.

  18. 4.4 The Calvin Cycle 4. An ATP molecule is used to add a 2nd phosphate group to the Ribulose 5-phosphate, producing a molecule of the starting product, RuBP (completing the cycle).

  19. 4.4 The Calvin Cycle • Three turns of the cycle (which uses 3 CO2 molecules) results in 6 PGAL molecules. • 5 of those PGAL are used to regenerate RuBP & the last one is available to the organism to use for maintenance & growth. • The PGAL is removed from the Calvin Cycle to synthesize other compounds like complex carbohydrates or amino acids.

  20. Animation of The Calvin Cycle http://www.sinauer.com/cooper/4e/animations0305.html

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