LIGHT INDEPENDENT REACTIONS The CALVIN CYCLE Melvin Calvin

LIGHT INDEPENDENT REACTIONS (The CALVIN CYCLE) Melvin Calvin in the early 1960's; Nobel Prize in 1961; died in 1997. Occurs in the stroma of the chloroplast. ATP and NADPH from the light reactions go into the stroma and are used to create carbohydrate from atmospheric carbon. Glucose is created from water and carbon dioxide. Carbon Fixation: the attachment of atmospheric carbon dioxide to an organic molecule. Requires no light energy. 3 phases: http: //courses. cm. utexas. edu/emarcotte/ch 339 k/fall 2005/Lecture-Ch 19 -3/Slide. Calvin. Cycle. jpg

I: CARBON FIXATION Calvin Cycle begins and ends with a 5 C molecule called Ribulose 1, 5 Bisphosphate (Ru. BP) 3 Ru. BP capture 3 atmospheric carbon dioxides to form 3 six carbon compounds. This reaction is catalyzed by rubisco, a slow enzyme that catalyzes 3 molecules per second. To make up for its slow speed, it makes up 20 -50% of the protein in a chloroplast…it may be the most abundant protein in the planet! Each of the 3 - 6 C compounds immediately react with water to form 2 – 3 C molecules of phosphoglyceric acid (PGA). Therefore 6 PGA in total. For 3 carbon dioxide molecules you would get 6 PGA molecules, therefore a total of 12 are created for 6 CO 2 in one chemical reaction of photosynthesis.

II: REDUCTION REACTIONS (essentially the reverse of glycolysis) Energy (6 ATP) and reducing power (6 NADPH) are used to convert 6 PGA into 6 G 3 P (also known as phosphoglyceraldehyde (PGAL) ). Each of the 6 molecules of PGA are phosphorylated by an ATP to form six molecules of 1, 3 bisphoglycerate (1, 3 -BPG). A pair of electrons from each of the six NADPH molecules reduces six molecules of 1, 3 -BPG to 6 molecules of glyceraldehyde-3 phosphate (G 3 P or PGAL), a sugar. One PGAL leaves the cycle at this point.

III: Ru. BP REGENERATION The remaining molecules of PGAL go through a series of reactions to regenerate 3 molecules of Ru. BP. This uses 3 ATP in the process.

END RESULT The overall equation for the Calvin Cycle (per G 3 P produced) is: 3 Ru. BP + 3 CO 2 + 9 ATP + 6 NADPH + 5 H 2 O --> 9 ADP + 9 P + 6 NADP + G 3 P + 3 Ru. BP The G 3 P can be: combined with another G 3 P to form glucose. (requires 6 turns – hence 6 CO 2) converted to starch and stored in the chloroplast as a reserve for when light is not present. converted into sucrose and translocated to other parts of the plant. Both glucose and sucrose can be made into many things essential for the plant (cellulose, lipids, amino acids. . . ) 6 CO 2 (g) + 12 H 2 O (I) + light energy -----> C 6 H 12 O 6 (aq) + 6 O 2 (g)

SUMMARY http: //www. ualr. edu/botany/photosynthesis. gi f