Photosynthesis continued... The Calvin Cycle pg 161-165

Summary of Light Reactions The light reactions of photosynthesis take place in the thylakoid. The light reactions generate ATP and NADPH which will be used in the final stage of photosynthesis.

The Calvin Cycle The 3rd stage of Photosynthesis is known as the CALVIN CYCLE. (It is also called the C3 Photosynthesis) The purpose of this stage is Carbon Fixation: the conversion of inorganic carbon (CO2) to organic carbon that can be used by the plant. This process takes place in the stroma of the chloroplast.

Melvin Calvin The Calvin Cycle was named after scientist Melvin Calvin who determined the details of the cycle in the 1960’s.

The Calvin Cycle can be divided into 3 Phases Carbon Fixation Reduction Reactions Ribulose 1,5-bisphosphate (RuBP) Regeneration

Calvin Cycle Pg 161-162

Phase 1: Carbon Fixation 3 molecules of CO2 are added to 3 molecules of the five-carbon sugar RuBP (Ribulose 1,5-bisphosphate ) 3 unstable six-carbon intermediates are formed. These intermediates instantly split and their molecules are reorganized to form 6 molecules called 3-PGA (3-phosphoglycerate) which have three carbons each.

Phase 1 continued These reactions are catalyzed by the enzyme Rubisco (ribulose bisphosphate caboxylase/oxygenase). *** The Calvin cycle is also known as C3 Photosynthesis because the first compound to be produced contains 3 carbons (3-PGA).

Phase 2: Reduction Reactions In the next two reactions, products from the light reactions, ATP and NADPH, are used to boost the energy of these three-carbon molecules.

Phase 2 continued Each of the 6 molecules of 3-PGA is phosphorylated by an ATP molecule to form 6 molecules of 1,3-bisphosphoglycerate. Next, 6 molecules of NADPH give up 2 electrons each to reduce the six 1,3-bisphosphorglcerate. (NADPH is oxidized; 1,3 bisphosphoglycerate is reduced)

Phase 2 continued 6 molecules of the sugar glyceraldehyde-3-phosphate (G3P) is produced. ONE molecule G3P now exits the cycle as a final product of the Calvin Cycle. The remaining 5 molecules of G3P are used to regenerate RuBP.

Phase 3: RuBP Regeneration A series of enzyme-catalyzed reactions rearranges the 5 remaining molecules of G3P to regenerate 3 molecules of RuBP. 3 more molecules of ATP are used in the process. With RuBP regenerated, the Cycle can continue to fix more molecules of CO2.

The G3P molecules that leave the cycle as final products are used to synthesize larger sugars such as glucose. Three molecules of CO2 must be fixed before one 3-carbon G3P molecule can leave the cycle.

The net equation for a single G3P produced in the Calvin Cycle is: 3 CO2 + 9 ATP + 6NADPH + 5 H2O  9 ADP + 8Pi + 6NADP + G3P The ATP and the NADPH come from the Light Reactions.

For every 4 electrons transferred in the Light Reaction, 2 ATP molecules and 2 NADPH molecules are produced. But a ratio of 3 ATP molecules and 2 NADPH molecules are required in the production of each G3P molecule. Where does the extra ATP molecule come from????

The extra ATP may come from cyclic electron flow in the Light Reactions. Remember, cyclic electron flow does not produce NADPH but will drive ADP phosphorylation by creating a H+ gradient in the thylakoid.

G3P The final product of the Calvin Cycle, G3P, acts as the raw material in the production of other carbohydrates including glucose. Sucrose and amylose are actually the main photosynthetic products.

The Fate of G3P

DID YOU KNOW?? Rubsico makes up 50% or more of the total proteins of plants leaves Therefore, rubisco is the most abundant protein in the world!

http://glencoe. mcgraw-hill http://glencoe.mcgraw-hill.com/sites/0003292010/student_view0/chapter8/animations_and_videos.html# http://vcell.ndsu.edu/animations/photosynthesis/movie.htm