The Calvin Cycle Lecture Notes PDF

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This is a brief lecture note about the Calvin Cycle, and Doctor Melvin Calvin....

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The Calvin Cycle (aka The Light-Independent Stage) Building block: (C3H6O3 G-3P) Key Points ● The Calvin cycle refers to the light-independent reactions in photosynthesis that take place in three key steps. ● Although the Calvin Cycle is not directly dependent on light, it is indirectly dependent on light since the necessary energy carriers (ATP and NADPH) are products of light-dependent reactions. ● In fixation, the first stage of the Calvin cycle, light-independent reactions are initiated; CO2 is fixed from an inorganic to an organic molecule. ● In the second stage, ATP and NADPH are used to reduce 3-PGA into G3P; then ATP and NADPH are converted to ADP and NADP+, respectively. ● In the last stage of the Calvin Cycle, RuBP is regenerated, which enables the system to prepare for more CO2 to be fixed.

Key Definitions ● light-independent reaction: chemical reactions during photosynthesis that convert carbon dioxide and other compounds into glucose, taking place in the stroma ● rubisco: every chloroplast must have it. (ribulose bisphosphate carboxylase) a plant enzyme which catalyzes the fixation of atmospheric carbon dioxide during photosynthesis by catalyzing the reaction between carbon dioxide and RuBP. (Takes CO2 and is able to attach it to RuBP) ● ribulose bisphosphate: an organic substance that is involved in photosynthesis, reacts with carbon dioxide to form 3-PGA

Stage 2: The Light-Independent Stage (Calvin Cycle) In this portion of photosynthesis, the ATP molecules and the H + ions stored in NADPH are used to reduce the carbon found in carbon dioxide to form sugar. The stage is sometimes mistakenly called the “dark reaction” which tends to suggest that it only occurs in the dark. Instead, it will occur both in the dark and in the light. It is not directly dependent on the presence of light but it does require the products of the light- dependent stage. As long as those products are present in sufficient quantities, the reaction will proceed. Unlike the light-dependent stage which takes place on the thylakoid membrane, this stage occurs in the stroma of the chloroplasts. The formation of the glucose sugar occurs in a series of cyclic reactions where some of the starting material is regenerated during the process. Every photosynthetic plant uses the Calvin cycle. It was named in honour of Melvin Calvin, the researcher who determined the steps involved in the process. The cycle itself can be broken down into three distinct stages: carbon fixation, reduction and regeneration. 1. Carbon Fixation

Initially, to incorporate carbon molecules into organic molecules, carbon dioxide brought into the plant cell through the stomata on the underside of the leaf, must first be attached to a 5-carbon compound called ribulose bisphosphate, RuBP which is already present in the stroma. The resulting 6-carbon molecule is highly unstable and is immediately split into two 3-carbon molecules of PGA or phosphoglycerate. Rubisco, a very large enzyme, catalyzes this reaction. This portion of the cycle must repeat three times before enough PGA molecules are generated to move onto the next stage. If three carbon dioxide molecules are fixed (1 C x 3 = 3C), they will join with three RuBP molecules (5 C x 3 = 15 C) and then split into 6 molecules of PGA (3 C x 6 = 18 C) which is the required amount. At this point, the 6 molecules can move onto the next step but this will only generate half of a glucose molecule. Therefore, in order to produce one full molecule of glucose, six carbon dioxide molecules need to enter the cycle so this portion of the pathway needs to be repeated six times in total. 2. Reduction of PGA In this phase, the six molecules of PGA are phosphorylated by six ATP molecules to form six higher energy molecules of 1,3-bisphosphoglycerate (1,3-BGP). The ATP molecules utilized in this step are the products of the light-dependent reaction. Next, six NADPH molecules, also produced in the light-dependent stage, donate a hydrogen ion and two electrons to 1,3-BGP, reducing it to six molecules of glyceraldehyde-3-phosphate (PGAL). The NADP is now free to return to the thylakoid membrane. One molecule of PGAL exits the cycle as a final product. It is capable of joining with other PGAL molecules to form the various sugars made within the plant, including, most importantly, glucose. 3. RuBP Regeneration The five remaining PGAL (3 C x 5 = 15 C) molecules are rearranged with the help of three ATPs to regenerate three molecules of RuBP (5C x 3 = 15 C). This regeneration of RuBP allows more carbon dioxide molecules to enter the Calvin cycle and start the process all over again....